The distinct fate of smooth and rough Mycobacterium abscessus variants inside macrophages

Cell envelope polysaccharide modifications alter the surface properties and interactions of Mycobacterium abscessus with innate immune cells in a morphotype-dependent manner

Mycobacterium abscessus is one of the leading causes of pulmonary infections caused by non-tuberculous mycobacteria. The ability of M. abscessus to establish a chronic infection in the lung relies on a series of adaptive mutations impacting, in part, global regulators and cell envelope biosynthetic enzymes. One of the genes under strong evolutionary pressure during host adaptation is ubiA, which participates in the elaboration of the arabinan domains of two major cell envelope polysaccharides: arabinogalactan (AG) and lipoarabinomannan (LAM). We here show that patient-derived UbiA mutations not only cause alterations in the AG, LAM, and mycolic acid contents of M. abscessus but also tend to render the bacterium more prone to forming biofilms while evading uptake by innate immune cells and enhancing their pro-inflammatory properties. The fact that the effects of UbiA mutations on the physiology and pathogenicity of M. abscessus were impacted by the rough or smooth morphotype of the strain suggests that the timing of their selection relative to morphotype switching may be key to their ability to promote chronic persistence in the host.IMPORTANCEMultidrug-resistant pulmonary infections caused by Mycobacterium abscessus and subspecies are increasing in the U.S.A. and globally. Little is known of the mechanisms of pathogenicity of these microorganisms. We have identified single-nucleotide polymorphisms (SNPs) in a gene involved in the biosynthesis of two major cell envelope polysaccharides, arabinogalactan and lipoarabinomannan, in lung-adapted isolates from 13 patients. Introduction of these individual SNPs in a reference M. abscessus strain allowed us to study their impact on the physiology of the bacterium and its interactions with immune cells. The significance of our work is in identifying some of the mechanisms used by M. abscessus to colonize and persist in the human lung, which will facilitate the early detection of potentially more virulent clinical isolates and lead to new therapeutic strategies. Our findings may further have broader biomedical impacts, as the ubiA gene is conserved in other tuberculous and non-tuberculous mycobacterial pathogens.

A glycosylated lipooctapeptide promotes uptake and growth of Mycobacterium abscessus in the host

Pathogenic mycobacteria produce a wide array of lipids which participate in host cell interactions and virulence. While some of these are conserved across all mycobacteria, others, like glycopeptidolipids (GPL), are restricted to a few species. Mycobacterium abscessus, an emerging rapid-growing pathogen, transitions from a smooth to a virulent rough variant upon the loss of surface GPL. Here, we discovered that M. abscessus and phylogenetically-close species harbor a second GPL-related locus, comprising two adjacent non-ribosomal peptide synthetase genes, MAB_4690c and MAB_4691c. A MAB_4690c deletion mutant (ΔMAB_4690c) failed to produce a yet undescribed lipid, designated GL8P for glycosylated lipooctapeptide, sharing an acylated octapeptide core adorned by mono or di-O-rhamnosyl substituents. ΔMAB_4690c exhibited impaired uptake and survival in THP-1 cells and was attenuated in mice. Importantly, GL8P elicited a strong humoral response in patients infected with M. abscessus. These results highlight the role of GL8P in the pathophysiology of infection by rough M. abscessus and suggest its potential as a selective marker for M. abscessus infections.

Infection model of THP-1 cells, growth dynamics, and antimicrobial susceptibility of clinical Mycobacterium abscessus isolates from cystic fibrosis patients: Results from a multicentre study

Mycobacterium abscessus (MABS) is an emerging pathogen causing severe infections, particularly in cystic fibrosis (CF) patients. A prospective multicentre study included CF patients from four hospitals in Madrid between January 2022 and January 2024. Respiratory samples were collected, and MABS isolates were analysed to determine their antibiotic resistance profiles, growth dynamics, infection kinetics, intracellular behaviour, and pathogenicity. Intracellular bacterial growth and macrophage viability were evaluated through THP-1 cell infection experiments, with and without amikacin. Phenotypic susceptibility testing and genotypic susceptibility testing were also conducted. Among 148 patients, 28 MABS isolates were detected from 16 patients (10.8%), and the first isolate from each patient was analysed. Isolation was more prevalent in younger individuals (median age 24.4 vs. 28.4 years, p = 0.049), and most isolates (81.25%) were identified as M. abscessus subsp. abscessus (MABSa). MABS isolates exhibited high resistance rates (>85%) to doxycycline, tobramycin, ciprofloxacin, moxifloxacin (75%) and cotrimoxazole (56.3%). Amikacin resistance (18.8%) was higher than expected, and inducible (10/16 isolates) or acquired (1/16 isolate) macrolide resistance was found in 68.8% of strains. Phenotypic and genotypic testing results were fully concordant. Tigecycline demonstrated strong in vitro activity, and resistance to imipenem, linezolid, and cefoxitin remained low. Rough strains displayed lower optical density values in later growth stages, probably due to their increased aggregation. In THP-1 cell infection experiments, rough strains showed higher intracellular bacterial loads with statistically significant differences observed at 2 hours (both with and without amikacin) and at 72 hours (with amikacin) post infection. Notably, rough strains also exhibited a higher internalisation index and greater impact on THP-1 cell viability, especially in the absence of amikacin.

Divergent host humoral innate immune response to the smooth-to-rough adaptation of Mycobacterium abscessus in chronic infection

Mycobacterium abscessus is a nontuberculous mycobacterium emerging as a significant pathogen in individuals with chronic lung diseases, including cystic fibrosis and chronic obstructive pulmonary disease. Current therapeutics have poor efficacy. Strategies of bacterial control based on host defenses are appealing; however, antimycobacterial immunity remains poorly understood and is further complicated by the appearance of smooth and rough morphotypes, which elicit distinct host responses. We investigated the role of serum components in neutrophil-mediated clearance of M. abscessus morphotypes. M. abscessus opsonization with complement enhanced bacterial killing compared to complement-deficient opsonization. Killing of rough isolates was less reliant on complement. Complement C3 and mannose-binding lectin 2 (MBL2) were deposited on M. abscessus morphotypes in distinct patterns, with a greater association of MBL2 on rough M. abscessus. Killing was dependent on C3; however, depletion and competition experiments indicate that canonical complement activation pathways are not involved. Complement-mediated killing relied on natural IgG and IgM for smooth morphotypes and on IgG for rough morphotypes. Both morphotypes were recognized by complement receptor 3 in a carbohydrate- and calcium-dependent manner. These findings indicate a role for noncanonical C3 activation pathways for M. abscessus clearance by neutrophils and link smooth-to-rough adaptation to complement activation.

Zinc-limited Mycobacterium tuberculosis stimulate distinct responses in macrophages compared with standard zinc-replete bacteria

Tuberculosis (TB) is notoriously difficult to treat, likely due to the complex host-pathogen interactions driven by pathogen heterogeneity. An understudied area of TB pathogenesis is host responses to Mycobacterium tuberculosis bacteria (Mtb) that are limited in zinc ions. This distinct population resides in necrotic granulomas and sputum and could be the key player in tuberculosis pathogenicity. In this study, we tested the hypothesis that macrophages differentiate between Mtb grown under zinc limitation or in the standard zinc-replete medium. Using several macrophage infection models, such as murine RAW 264.7 and murine bone marrow-derived macrophages (BMDMs), as well as human THP-1-derived macrophages, we show that macrophages infected with zinc-limited Mtb have increased bacterial burden compared with macrophages infected with zinc-replete Mtb. We further demonstrate that macrophage infection with zinc-limited Mtb trigger higher production of reactive oxygen species (ROS) and cause more macrophage death. Furthermore, the increased ROS production is linked to the increased phagocytosis of zinc-limited Mtb, whereas cell death is not. Finally, transcriptional analysis of RAW 264.7 macrophages demonstrates that macrophages have more robust pro-inflammatory responses when infected with zinc-limited Mtb than zinc-replete Mtb. Together, our findings suggest that Mtb's access to zinc affects their interaction with macrophages and that zinc-limited Mtb may be influencing TB progression. Therefore, zinc availability in bacterial growth medium should be considered in TB drug and vaccine developments.

Stealth in non-tuberculous mycobacteria: clever challengers to the immune system

Non-tuberculous Mycobacteria (NTM) are found extensively in various environments, yet most are non-pathogenic. Only a limited number of these organisms can cause various infections, including those affecting the lungs, skin, and central nervous system, particularly when the host's autoimmune function is compromised. Among these, Non-tuberculous Mycobacteria Pulmonary Diseases (NTM-PD) are the most prevalent. Currently, there is a lack of effective treatments and preventive measures for NTM infections. This article aims to deepen the comprehension of the pathogenic mechanisms linked to NTM and to formulate new intervention strategies by synthesizing current research and detailing the different tactics used by NTM to avoid elimination by the host's immune response. These intricate mechanisms not only affect the innate immune response but also successfully oppose the adaptive immune response, establishing persistent infections within the host. This includes effects on the functions of macrophages, neutrophils, dendritic cells, and T lymphocytes, as well as modulation of cytokine production. The article particularly emphasizes the survival strategies of NTM within macrophages, such as inhibiting phagosome maturation and acidification, resisting intracellular killing mechanisms, and interfering with autophagy and cell death pathways. This review aims to deepen the understanding of NTM's immune evasion mechanisms, thereby facilitating efforts to inhibit its proliferation and spread within the host, ultimately providing new methods and strategies for NTM-related treatments.

The gene MAB_2362 is responsible for intrinsic resistance to various drugs and virulence in Mycobacterium abscessus by regulating cell division

Mycobacterium abscessus exhibits intrinsic resistance to most antibiotics, hence leading to infections that are difficult to treat. To address this issue, the identification of new molecular targets is essential for the development or repositioning of therapeutic agents. This study demonstrated that the MAB_2362-knockout strain, MabΔ2362, became significantly susceptible to a range of antibiotics, not only in vitro but also exhibited susceptibility to rifabutin, bedaquiline, and linezolid in vivo. While the bacterial burden of the wild-type M. abscessus (MabWt) increased by over 1 log10 CFU/lung in a murine infection model 16 days post-infection, that of MabΔ2362 strain decreased by more than 1 log10 CFU/lung, which suggests that the disruption leads to attenuation. Bioinformatics analysis revealed that MAB_2362 shares the highest similarity (41.35%) with SteA, a protein known to influence cell division in Corynebacterium glutamicum, suggesting that MAB_2362 might be involved in cell division. MabΔ2362 cells exhibited a median length of 2.62 µm, which was substantially longer than the 1.44 µm recorded for MabWt cells. Additionally, multiple cell division septa were observed in 42% of MabΔ2362 cells, whereas none were seen in MabWt cells. An ethidium bromide uptake assay further suggested a higher cell envelope permeability in MabΔ2362 compared to MabWt. Collectively, these findings underscore the role of MAB_2362 in intrinsic resistance and virulence of M. abscessus possibly through the regulation of cell division. Thus, MAB_2362 emerges as a promising candidate for targeted interventions in the pursuit of novel antimicrobials against M. abscessus.

PGC-1α activation to enhance macrophage immune function in mycobacterial infections

Nontuberculous Mycobacteria (NTM) are a heterogeneous group of environmental microorganisms with distinct human pathogenesis. Their incidence and prevalence are rising worldwide, due in part to elevated antimicrobial resistance which complicates treatment and potential successful outcomes. Although information exists on the clinical significance of NTMs, little is known about host immune response to infection. NTM infections alter macrophage mitochondrial capacity and decrease ATP production, efficient immune response, and bacterial clearance. Transcription factor peroxisome proliferator activated receptor (PPAR) γ coactivator-1α (PGC-1α) is a master regulator of mitochondrial biogenesis, influencing metabolism, mitochondrial pathways, and antioxidant response. Mitochondrial transcription factor A (TFAM) is a protein essential for mitochondrial DNA (mtDNA) genome stability, integrity, and metabolism. Both PGC-1α and TFAM regulate mitochondrial biogenesis and activity, and their disruption is linked to inflammatory signaling and altered macrophage function. We show that NTM causes macrophage mitochondrial damage and disrupted bioenergetics. Mechanistically we show that this is related to attenuation of expression of PGC-1α and TFAM in infected macrophages. Importantly, rescuing expression of PGC-1α and TFAM using pharmacologic approaches restored macrophage immune function. Our results suggest that pharmacologic approaches to enhance mitochondrial function provide a novel approach to target macrophage immune function and means to combat NTM infections.

Deletion of ESX-3 and ESX-4 secretion systems in Mycobacterium abscessus results in highly impaired pathogenicity

Type VII secretion systems participate in protein export, virulence, conjugation, and metabolic regulation. Five subtypes (ESX-1 to ESX-5) exist, each with specific roles and well-characterized secretion profiles in various mycobacterial species. Mycobacterium abscessus, encodes only ESX-3 and ESX-4. Here, single and double M. abscessus mutants lacking the main ATPases EccC3 and EccC4 were used to define ESX-3 and ESX-4 contributions to substrate secretion and virulence. Our results demonstrate that EsxG/H secretion depends entirely on ESX-3, whereas both ESX-3 and ESX-4 secrete EsxU/T. Furthermore, two newly identified PE/PPE substrates (MAB_0046/MAB_0047) require ESX-3 for secretion. Functional complementation restored secretion and revealed subpolar localization of these systems. Macrophage infections showed that ESX-3 and ESX-4 contribute to bacterial internalization, phagosomal escape, and intracellular survival. In mice, infections with eccC3- and/or eccC4-deletion mutants resulted in complete survival and reduced bacterial loads in the lungs. These findings demonstrate that both ESX systems drive M. abscessus pathogenicity.

FL058, a novel β-lactamase inhibitor, increases the anti-Mycobacterium abscessus activity of imipenem

BACKGROUND

β-lactams are crucial for anti-Mycobacterium abscessus complex (MABC) therapy. Treating infections is challenging since MABC produces a class A β-lactamase (BlaMab), which is capable of hydrolyzing β-lactams thus causing drug resistance. Diazabicyclooctane (DBO) β-lactamase inhibitors (BLIs) can inhibit BlaMab. FL058 is a novel DBO BLI; the anti-MABC activity of FL058 combined with β-lactams remains unknown.

METHODS

The activities of ten β-lactams (imipenem, meropenem, faropenem, tebipenem, cefoxitin, cefepime, ceftazidime, cefdinir, cefuroxime, and amoxicillin) combined with three DBO BLIs (FL058, avibactam, and relebactam) toward two MABC reference strains were determined by broth microdilution assay. The anti-MABC activities of imipenem combined with three BLIs against 193 clinical isolates were also evaluated. The activity of imipenem combined with FL058 was also tested against intracellular MABC residing in macrophages and in a mouse model. Finally, the BlaMab mutations in clinical isolates were analyzed using sequence alignment to determine whether BlaMab mutations are associated with DBO BLIs sensitivity.

RESULTS

FL058, avibactam and relebactam significantly increased the anti-MABC activity of β-lactams, especially imipenem, against reference strains and clinical isolates. The anti-MABC activity of imipenem combined with FL058 was superior to its activity when combined with either avibactam or relebactam. The combination of imipenem and FL058 significantly reduced the numbers of intracellular organisms in cultured macrophages, and of viable bacteria in the lungs of MABC-infected mice. Rough morphotypes tended to be more resistant than smooth morphotype. A BlaMab T141A mutation may reduce the susceptibility of MABC to imipenem-BLIs.

CONCLUSION

The elevated anti-MABC activity exhibited by imipenem combined with FL058 suggests a potential new approach to treating MABC infections.

Mechanisms of Virulence of Mycobacterium abscessus and Interaction with the Host Immune System

Mycobacterium abscessus is a non-tuberculosis fast-growing mycobacterium that has recently become a serious concern due to its rapidly increasing prevalence worldwide, mainly in individuals with a high susceptibility to pulmonary infections, for example, patients with cystic fibrosis, bronchiectasis, chronic obstructive pulmonary disease, and previous tuberculosis infection. According to present estimations, at least 20% of patients with cystic fibrosis are infected with M. abscessus. This bacterium is extremely resistant to most drugs, leading to a severe and difficult-to-treat infection. That is why M. abscessus, previously classified as a low-virulent opportunistic pathogen, is now reconsidered as a true pathogenic bacterium. There are no effective drugs for successful M. abscessus infection therapy, as well as no vaccines to prevent its spread. This review focuses on the molecular mechanisms ensuring M. abscessus resistance to immune response and its ability to survive in the aggressive intracellular environment of human immune cells, and describes virulence factors that can serve as potential targets for the development of innovative therapeutic approaches to combat the spread of infections caused by M. abscessus.

Remote disruption of intestinal homeostasis by Mycobacterium abscessus is detrimental to Drosophila survival

Mycobacterium abscessus (Mabs), an intracellular and opportunistic pathogen, is considered the most pathogenic fast-growing mycobacterium, and causes severe pulmonary infections in patients with cystic fibrosis. While bacterial factors contributing to its pathogenicity are well studied, the host factors and responses that worsen Mabs infection are not fully understood. Here, we report that Mabs systemic infection alters Drosophila melanogaster intestinal homeostasis. Mechanistically, Mabs remotely induces a self-damaging oxidative burst, leading to excessive differentiation of intestinal stem cells into enterocytes. We demonstrated that the subsequent increased intestinal renewal is mediated by both the Notch and JAK/STAT pathways and is deleterious to Drosophila survival. In conclusion, this work highlights that the ability of Mabs to induce an exacerbated and self-damaging response in the host contributes to its pathogenesis.

Trehalose polyphleates participate in Mycobacterium abscessus fitness and pathogenesis

Mycobacteria produce a large repertoire of surface-exposed lipids with major biological functions. Among these lipids, trehalose polyphleates (TPPs) are instrumental in the infection of Mycobacterium abscessus by the therapeutic phage BPs. However, while the biosynthesis and transport of TPPs across the membrane by MmpL10 have been reported, the role of TPPs in host infection remains enigmatic. Here, we addressed whether the loss of TPPs influences interactions with macrophages and the virulence of M. abscessus. As anticipated, the deletion of mmpL10 in smooth (S) and rough (R) variants of M. abscessus abrogated TPP production, which was rescued upon gene complementation. Importantly, infection of human THP-1 cells with the mmpL10 mutants was associated with decreased intramacrophage survival and a reduced proportion of infected cells. The rough mmpL10 mutant showed an impaired capacity to block phagosomal acidification and was unable to co-localize with Galectin-3, a marker of phagosomal membrane damage. This suggests that TPPs participate, directly or indirectly, in phagolysosomal fusion and in phagosomal membrane damage to establish cytosolic communication. The TPP defect that affects the fitness and virulence of M. abscessus was further demonstrated in zebrafish embryos using a rough clinical strain resistant to phage BPs and harboring a frameshift mutation in mmpL10. Infection with this strain was correlated with a slight decrease in embryo survival and a reduced bacterial burden as compared to the corresponding parental and complemented derivatives. Together, these results indicate that TPPs are important surface lipids contributing to the pathogenicity of M. abscessus.IMPORTANCETrehalose polyphleates (TPPs) are complex lipids associated with the mycobacterial cell surface and were identified 50 years ago. While the TPP biosynthetic pathway has been described recently, the role of these lipids in the biology of mycobacteria remains yet to be established. The wide distribution of TPPs across mycobacterial species suggests that they may exhibit important functions in these actinobacteria. Here, we demonstrate that Mycobacterium abscessus, an emerging multidrug-resistant pathogen that causes severe lung diseases in cystic fibrosis patients, requires TPPs for survival in macrophages and virulence in a zebrafish model of infection. These findings support the importance of this underexplored family of lipids in mycobacterial pathogenesis.

Polymicrobial interactions influence Mycobacterium abscessus co-existence and biofilm forming capabilities

The lungs of patients with cystic fibrosis (CF) are vulnerable to persistent polymicrobial colonization by bacterial pathogens including Pseudomonas aeruginosa, Staphylococcus aureus, and the non-tuberculous mycobacterium (NTM) Mycobacterium abscessus. The polymicrobial milieu within the CF lung impacts individual species fitness, influences biofilm-forming capabilities, pathogenicity, production of virulence factors and even antimicrobial responses, all potentially compromising therapeutic success. Interaction studies among these CF pathogens are very limited, especially studies on the influences of P. aeruginosa and S. aureus on M. abscessus co-existence and virulence. Based on the little known thus far about coinfection of these pathogens, we hypothesize that the co-existence of P. aeruginosa and S. aureus alters M. abscessus virulence and phenotypic characteristics. We evaluated the direct (co-culture) and indirect (using supernatant) effects of P. aeruginosa and S. aureus on M. abscessus growth rate, biofilm formation, macrophage internalization and glycopeptidolipids (GPL) expression. Our observations indicate that P. aeruginosa and S. aureus exert a competitive behavior toward M. abscessus during direct contact or indirect interaction in-vitro, probably as is the case of polymicrobial infections in the lungs of patients with CF. This is the first report that demonstrates S. aureus inhibitory effects on M. abscessus growth and biofilm forming capabilities. Collectively, co-culture studies enhance our understanding of polymicrobial interactions during coinfection and can guide to establish better management of coinfections and treatment strategies for M. abscessus.

Mycobacterium abscessus pulmonary infection and associated respiratory function in cystic fibrosis-like βENaC mice

Introduction

Chronic pulmonary infection with Mycobacterium abscessus (M. abscessus) is a significant cause of morbidity and mortality in people with cystic fibrosis (CF). Developing an animal model of M. abscessus pulmonary infection, especially under CF conditions, is essential to understanding clinical pulmonary M. abscessus infection. βENaC transgenic mice are known to develop spontaneous CF-like disease characterized by airway mucus obstruction and inflammation. The aim of this study was to evaluate the suitability of βENaC mice as a preclinical model and characterize their respiratory function during M. abscessus lung infection.

Methods

Mice received an intrapulmonary aerosol of M. abscessus using a high-pressure syringe device (Penn-Century) for subsequent characterization of disease progression and respiratory function. Whole body unrestrained plethysmography (WBP) data was collected to monitor lung function and endpoints determined organ bacterial burden and associated pathology.

Results

Endpoint CFU data in the lung and spleen showed that there was no significant difference in bacterial clearance between βENaC and WT mice. WBP data showed an impairment in overall respiratory function during and after M. abscessus infection in both strains of mice. Interestingly, even in wildtype control mice, lung dysfunction persisted after bacterial clearance.

Discussion

Even with CF-like features, the βENaC transgenic mice cleared M. abscessus at a similar rate than WT mice, however, the associated respiratory monitoring revealed that there are long-term implications of M. abscessus lung exposure. The clear decline in respiratory function, even after M. abscessus clearance, suggests that WBP coupled animal modeling provides important insight that is relevant to disease burden and treatment efficacy. The M. abscessus clearance in the βENaC mice may help improve the fields understanding of CF-modulated immune deficiencies in M. abscessus pulmonary infection.

A dTDP-L-rhamnose 4-epimerase required for glycopeptidolipid biosynthesis in Mycobacterium abscessus

Mycobacterium abscessus causes severe lung infections in cystic fibrosis patients and exhibits smooth (S) or rough (R) morphotypes. Disruption of glycopeptidolipid (GPL) production results in the S-to-R transition but the underlying molecular mechanisms of this transition remain incompletely understood. Herein, we characterized MAB_4111c in relation to GPL synthesis and investigated the effects of MAB_4111c deletion in M. abscessus pathogenicity. An enzymatic assay indicated that MAB_4111c, also designated Tle for Talose epimerase, is converting dTDP-L-Rhamnose into dTDP-6-deoxy-L-Talose. A tle deletion mutant was constructed in the S variant of M. abscessus and relative areas of Rhamnose and 6-deoxy-Talose and their methylated forms expressed as ratios of total monosaccharides, showed an altered GPL profile lacking 6-deoxy-Talose. Thus, Tle provides dTDP-6-deoxy-L-Talose, subsequently used by the glycosyltransferase Gtf1 to transfer 6-deoxy-Talose to the GPL backbone. Strikingly, the tle mutant exhibited an R morphotype, showed impaired sliding motility and biofilm formation, and these phenotypes were rescued upon functional complementation. Moreover, deletion of tle in M. abscessus results in increased pathogenicity and killing in zebrafish embryos. Together, our results underscore the importance of the dTDP-L-Rhamnose 4-epimerase activity in GPL biosynthesis and in influencing M. abscessus virulence.

Phosphatidylserine liposomes induce a phagosome acidification-dependent and ROS-mediated intracellular killing of Mycobacterium abscessus in human macrophages

Mycobacterium abscessus (Mab) is an opportunistic nontuberculous mycobacterium responsible of difficult-to-treat pulmonary infections in vulnerable patients, such as those suffering from Cystic Fibrosis (CF), where it represents a major cause of morbidity and mortality. Additionally, due to the intrinsic extensive antimicrobial resistance spectrum displayed by this species and the side effects reported for some available antibiotics, the therapeutic management of such infections remains extremely difficult. In the present study, we show that phosphatidylserine liposomes (PS-L) enhance intracellular mycobacterial killing of Mab infected human macrophages with functional or pharmacologically inhibited cystic fibrosis conductance regulator (CFTR), by a mechanism involving phagosome acidification and reactive oxygen species (ROS) production. Additionally, PS-L significantly reduce proinflammatory response of Mab infected macrophages in terms of NF-kB activation and TNF-α production, irrespective of CFTR inhibition. Altogether, these results represent the proof of concept for a possible future development of PS-L as a therapeutic strategy against difficult-to-treat Mab infection.

The diversity of clinical Mycobacterium abscessus isolates in morphology, glycopeptidolipids and infection rates in a macrophage model

Introduction. Mycobacterium abscessus (MABS) is a pathogenic bacterium that can cause severe lung infections, particularly in individuals with cystic fibrosis. MABS colonies can exhibit either a smooth (S) or rough (R) morphotype, influenced by the presence or absence of glycopeptidolipids (GPLs) on their surface, respectively. Despite the clinical significance of these morphotypes, the relationship between GPL levels, morphotype and the pathogenesis of MABS infections remains poorly understood.Gap statement. The mechanisms and implications of GPL production and morphotypes in clinical MABS infections are unclear. There is a gap in understanding their correlation with infectivity and pathogenicity, particularly in patients with underlying lung disease.Aim. This study aimed to investigate the correlation between MABS morphology, GPL and infectivity by analysing strains from cystic fibrosis patients' sputum samples.Methodology. MABS was isolated from patient sputum samples and categorized by morphotype, GPL profile and replication rate in macrophages. A high-content ex vivo infection model using THP-1 cells assessed the infectivity of both clinical and laboratory strains.Results. Our findings revealed that around 50 % of isolates displayed mixed morphologies. GPL analysis confirmed a consistent relationship between GPL content and morphotype that was only found in smooth isolates. Across morphotype groups, no differences were observed in vitro, yet clinical R strains were observed to replicate at higher levels in the THP-1 infection model. Moreover, the proportion of infected macrophages was notably higher among clinical R strains compared to their S counterparts at 72 h post-infection. Clinical variants also infected THP-1 cells at significantly higher rates compared to laboratory strains, highlighting the limited translatability of lab strain infection data to clinical contexts.Conclusion. Our study confirmed the general correlation between morphotype and GPL levels in smooth strains yet unveiled more variability within morphotype groups than previously recognized, particularly during intracellular infection. As the R morphotype is the highest clinical concern, these findings contribute to the expanding knowledge base surrounding MABS infections, offering insights that can steer diagnostic methodologies and treatment approaches.

Modeling nontuberculous mycobacterial infections in zebrafish

The incidence of infections due to nontuberculous mycobacteria (NTM) has increased rapidly in recent years, surpassing tuberculosis in developed countries. Due to inherent antimicrobial resistance, NTM infections are particularly difficult to treat with low cure rates. There is an urgent need to understand NTM pathogenesis and to develop novel therapeutic approaches for the treatment of NTM diseases. Zebrafish have emerged as an excellent animal model due to genetic amenability and optical transparency during embryonic development, allowing spatiotemporal visualization of host-pathogen interactions. Furthermore, adult zebrafish possess fully functional innate and adaptive immunity and recapitulate important pathophysiological hallmarks of mycobacterial infection. Here, we report recent breakthroughs in understanding the hallmarks of NTM infections using the zebrafish model.

Colony morphotype governs innate and adaptive pulmonary immune responses to Mycobacterium abscessus infection in C3HeB/FeJ mice

Mycobacterium abscessus is an emerging pathogen that causes chronic pulmonary infection. Treatment is challenging owing in part to our incomplete understanding of M. abscessus virulence mechanisms that enable pathogen persistence, such as the differing pathogenicity of M. abscessus smooth (S) and rough (R) colony morphotype. While R M. abscessus is associated with chronic infection and worse patient outcomes, it is unknown how immune responses to S and R M. abscessus differ in an acute pulmonary infection setting. In this study, immunological outcomes of M. abscessus infection with S and R morphotypes were examined in an immune-competent C3HeB/FeJ murine model. R M. abscessus infection was associated with the rapid production of inflammatory chemokines and recruitment of activated, MHC-II+ Ly6C+ macrophages to lungs and mediastinal LN (mLN). While both S and R M. abscessus increased T helper 1 (Th1) phenotype T cells in the lung, this was markedly delayed in mice infected with S M. abscessus. However, histopathological involvement and bacterial clearance were similar regardless of colony morphotype. These results demonstrate the importance of M. abscessus colony morphotype in shaping the development of pulmonary immune responses to M. abscessus, which further informs our understanding of M. abscessus host-pathogen interactions.

Mycobacterium abscessus, a complex of three fast-growing subspecies sharing virulence traits with slow-growing mycobacteria

BACKGROUND

Mycobacterium abscessus belongs to the largest group of mycobacteria, the rapid-growing saprophytic mycobacteria, and is one of the most difficult-to-treat opportunistic pathogen. Several features pertain to the high adaptability of M. abscessus to the host. These include the capacity to survive and persist within amoebae, to transition from a smooth to a rough morphotype that occurs during the course of the disease and to express of a wide array of virulence factors.

OBJECTIVES

The main objective of this narrative review consists to report major assets of M. abscessus that contribute to the virulence of these rapid-growing saprophytic mycobacteria. Strikingly, many of these determinants, whether they are from a mycobacterial origin or acquired by horizontal gene transfer, are known virulence factors found in slow-growing and strict pathogens for humans and animals.

SOURCES

In the light of recent published work in the field we attempted to highlight major features characterizing M. abscessus pathogenicity and to explain why this led to the emergence of this mycobacterial species in patients with cystic fibrosis.

CONTENT

M. abscessus genome plasticity, the smooth-to-rough transition, and the expression of a panel of enzymes associated with virulence in other bacteria are key players in M. abscessus virulence. In addition, the very large repertoire of lipid transporters, known as mycobacterial membrane protein large and small (MmpL and MmpS respectively), deeply influences the pathogenicity of M. abscessus, as exemplified here for some of them.

IMPLICATIONS

All these traits largely contribute to make M. abscessus a unique mycobacterium regarding to its pathophysiological processes, ranging from the early colonization steps to the establishment of severe and chronic pulmonary diseases.

Loss of LpqM proteins in Mycobacterium abscessus is associated with impaired intramacrophage survival

Mycobacterium abscessus, an emerging pathogen responsible for severe pulmonary infections in cystic fibrosis patients, displays either a smooth (S) or a rough (R) morphotype. Infections with M. abscessus R are associated with increased pathogenicity in animal models and humans. While the S-to-R transition correlating with reduced glycopeptidolipid (GPL) production is well-documented, the recent screening of a transposon library revealed additional gene candidates located outside of the GPL locus involved in this transition. These genes include MAB_1470c, encoding the putative lipoprotein peptidase LpqM. However, experimental confirmation of the implication of this gene in the morphotype switch is lacking. Herein, we re-examined the role of MAB_1470c, and its homolog MAB_1466c, in colonial morphotype changes by generating unmarked deletion mutants in M. abscessus S. Our results indicate that the morphotype of these mutants stayed smooth in different media. Unexpectedly, the intracellular growth of ΔMAB_1470c and ΔMAB_1466c in THP-1 macrophages was significantly reduced as compared to the parental S strain, and these defects were rescued upon complementation with their corresponding genes. Strikingly, the intracellular survival defect was further exacerbated in a mutant lacking both MAB_1470c and MAB_1466c genes. This implies that, despite their primary sequence relatedness, the two proteins are not functionally redundant. Collectively, this suggests that these two LpqM-related lipoproteins are unlikely to be involved in the S-to-R transition but are key players for intramacrophage survival of M. abscessus.

IMPORTANCE

Mycobacterium abscessus causes persistent infections in patients with underlying pulmonary diseases, resulting in progressive lung function deterioration. The rough (R) morphotype is well-established as associated with chronic and more aggressive infections in patients. In this study, we individually and simultaneously deleted the MAB_1470c and MAB_1466c genes in M. abscessus S, without observing changes in colony morphotypes. However, these mutants exhibited a severe impairment in their ability to survive within human macrophages, highlighting the critical role of these two lipoproteins in M. abscessus virulence.

Lipoarabinomannan modification as a source of phenotypic heterogeneity in host-adapted Mycobacterium abscessus isolates

Mycobacterium abscessus is increasingly recognized as the causative agent of chronic pulmonary infections in humans. One of the genes found to be under strong evolutionary pressure during adaptation of M. abscessus to the human lung is embC which encodes an arabinosyltransferase required for the biosynthesis of the cell envelope lipoglycan, lipoarabinomannan (LAM). To assess the impact of patient-derived embC mutations on the physiology and virulence of M. abscessus, mutations were introduced in the isogenic background of M. abscessus ATCC 19977 and the resulting strains probed for phenotypic changes in a variety of in vitro and host cell-based assays relevant to infection. We show that patient-derived mutational variations in EmbC result in an unexpectedly large number of changes in the physiology of M. abscessus, and its interactions with innate immune cells. Not only did the mutants produce previously unknown forms of LAM with a truncated arabinan domain and 3-linked oligomannoside chains, they also displayed significantly altered cording, sliding motility, and biofilm-forming capacities. The mutants further differed from wild-type M. abscessus in their ability to replicate and induce inflammatory responses in human monocyte-derived macrophages and epithelial cells. The fact that different embC mutations were associated with distinct physiologic and pathogenic outcomes indicates that structural alterations in LAM caused by nonsynonymous nucleotide polymorphisms in embC may be a rapid, one-step, way for M. abscessus to generate broad-spectrum diversity beneficial to survival within the heterogeneous and constantly evolving environment of the infected human airway.

Lsr2, a pleiotropic regulator at the core of the infectious strategy of Mycobacterium abscessus

Mycobacterium abscessus is a non-tuberculous mycobacterium, causing lung infections in cystic fibrosis patients. During pulmonary infection, M. abscessus switches from smooth (Mabs-S) to rough (Mabs-R) morphotypes, the latter being hyper-virulent. Previously, we isolated the lsr2 gene as differentially expressed during S-to-R transition. lsr2 encodes a pleiotropic transcription factor that falls under the superfamily of nucleoid-associated proteins. Here, we used two functional genomic methods, RNA-seq and chromatin immunoprecipitation-sequencing (ChIP-seq), to elucidate the molecular role of Lsr2 in the pathobiology of M. abscessus. Transcriptomic analysis shows that Lsr2 differentially regulates gene expression across both morphotypes, most of which are involved in several key cellular processes of M. abscessus, including host adaptation and antibiotic resistance. These results were confirmed through quantitative real-time PCR, as well as by minimum inhibitory concentration tests and infection tests on macrophages in the presence of antibiotics. ChIP-seq analysis revealed that Lsr2 extensively binds the M. abscessus genome at AT-rich sequences and appears to form long domains that participate in the repression of its target genes. Unexpectedly, the genomic distribution of Lsr2 revealed no distinctions between Mabs-S and Mabs-R, implying more intricate mechanisms at play for achieving target selectivity.IMPORTANCELsr2 is a crucial transcription factor and chromosome organizer involved in intracellular growth and virulence in the smooth and rough morphotypes of Mycobacterium abscessus. Using RNA-seq and chromatin immunoprecipitation-sequencing (ChIP-seq), we investigated the molecular role of Lsr2 in gene expression regulation along with its distribution on M. abscessus genome. Our study demonstrates the pleiotropic regulatory role of Lsr2, regulating the expression of many genes coordinating essential cellular and molecular processes in both morphotypes. In addition, we have elucidated the role of Lsr2 in antibiotic resistance both in vitro and in vivo, where lsr2 mutant strains display heightened sensitivity to antibiotics. Through ChIP-seq, we reported the widespread distribution of Lsr2 on M. abscessus genome, revealing a direct repressive effect due to its extensive binding on promoters or coding sequences of its targets. This study unveils the significant regulatory role of Lsr2, intricately intertwined with its function in shaping the organization of the M. abscessus genome.

An anti-mycobacterial conjugated oligoelectrolyte effective against Mycobacterium abscessus

Infections caused by nontuberculous mycobacteria have increased more than 50% in the past two decades and more than doubled in the elderly population. Mycobacterium abscessus (Mab), one of the most prevalent of these rapidly growing species, is intrinsically resistant to numerous antibiotics. Current standard-of-care treatments are not satisfactory, with high failure rate and notable adverse effects. We report here a potent anti-Mab compound from the flexible molecular framework afforded by conjugated oligoelectrolytes (COEs). A screen of structurally diverse, noncytotoxic COEs identified a lead compound, COE-PNH2, which was bactericidal against replicating, nonreplicating persisters and intracellular Mab.COE-PNH2 had low propensity for resistance development, with a frequency of resistance below 1.25 × 10-9 and showed no detectable resistance upon serial passaging. Mechanism of action studies were in line with COE-PNH2 affecting the physical and functional integrity of the bacterial envelope and disrupting the mycomembrane and associated essential bioenergetic pathways. Moreover, COE-PNH2 was well-tolerated and efficacious in a mouse model of Mab lung infection. This study highlights desirable in vitro and in vivo potency and safety index of this COE structure, which represents a promising anti-mycobacterial to tackle an unmet medical need.

Minimum Information for Conducting and Reporting In Vitro Intracellular Infection Assays

Bacterial pathogens are constantly evolving to outsmart the host immune system and antibiotics developed to eradicate them. One key strategy involves the ability of bacteria to survive and replicate within host cells, thereby causing intracellular infections. To address this unmet clinical need, researchers are adopting new approaches, such as the development of novel molecules that can penetrate host cells, thus exerting their antimicrobial activity intracellularly, or repurposing existing antibiotics using nanocarriers (i.e., nanoantibiotics) for site-specific delivery. However, inconsistency in information reported across published studies makes it challenging for scientific comparison and judgment of experiments for future direction by researchers. Together with the lack of reproducibility of experiments, these inconsistencies limit the translation of experimental results beyond pre-clinical evaluation. Minimum information guidelines have been instrumental in addressing such challenges in other fields of biomedical research. Guidelines and recommendations provided herein have been designed for researchers as essential parameters to be disclosed when publishing their methodology and results, divided into four main categories: (i) experimental design, (ii) establishing an in vitro model, (iii) assessment of efficacy of novel therapeutics, and (iv) statistical assessment. These guidelines have been designed with the intention to improve the reproducibility and rigor of future studies while enabling quantitative comparisons of published studies, ultimately facilitating translation of emerging antimicrobial technologies into clinically viable therapies that safely and effectively treat intracellular infections.

Exploring antibiotic resistance mechanisms in Mycobacterium abscessus for enhanced therapeutic approaches

Mycobacterium abscessus, a leading cause of severe lung infections in immunocompromised individuals, poses significant challenges for current therapeutic strategies due to resistance mechanisms. Therefore, understanding the intrinsic and acquired antibiotic resistance of M. abscessus is crucial for effective treatment. This review highlights the mechanisms employed by M. abscessus to sustain antibiotic resistance, encompassing not only conventional drugs but also newly discovered drug candidates. This comprehensive analysis aims to identify novel entities capable of overcoming the notorious resistance exhibited by M. abscessus, providing insights for the development of more effective therapeutic interventions.

Bacteriophage infection and killing of intracellular Mycobacterium abscessus

IMPORTANCE

As we rapidly approach a post-antibiotic era, bacteriophage (phage) therapy may offer a solution for treating drug-resistant bacteria. Mycobacterium abscessus is an emerging, multidrug-resistant pathogen that causes disease in people with cystic fibrosis, chronic obstructive pulmonary disease, and other underlying lung diseases. M. abscessus can survive inside host cells, a niche that can limit access to antibiotics. As current treatment options for M. abscessus infections often fail, there is an urgent need for alternative therapies. Phage therapy is being used to treat M. abscessus infections as an option of last resort. However, little is known about the ability of phages to kill bacteria in the host environment and specifically in an intracellular environment. Here, we demonstrate the ability of phages to enter mammalian cells and to infect and kill intracellular M. abscessus. These findings support the use of phages to treat intracellular bacterial pathogens.

Molecular Identification of Strains within the Mycobacterium abscessus Complex and Determination of Resistance to Macrolides and Aminoglycosides

One of the most relevant and pathogenic groups among the rapidly growing mycobacteria (RGM) is Mycobacterium abscessus complex (MABC) that includes three subspecies: M. abscessus subsp. abscessus, M. abscessus subsp. bolletii, and M. abscessus subsp. massiliense. The aim of this study was the analysis of prevalence of MABC among other non-tuberculous mycobacteria isolated from patients in the Malopolska Region of Poland, between 2018 and 2021, as well as determination of their subspecies and molecular mechanisms of resistance to macrolides and aminoglycosides. The incidence of MABC was 5,4% (12/223). Eight strains were classified as M. abscessus subsp. abscessus, three as M. abscessus subsp. massiliense and one M. abscessus subsp. bolletii. Molecular analysis showed resistance to macrolides for eight strains of M. abscessus subsp. abscessus associated with erm(41)T28 gene mutations. One strain of M. abscessus subsp. abscessus showed resistance to macrolides (two mutations simultaneously: in erm(41)T28 and rrl genes) and aminoglycosides (point mutation in rrs gene). One strain of M. abscessus subs. bolletii was resistant to macrolides (erm(41)T28 mutation), whereas presented no mutations for aminoglycosides. M. abscessus subsp. massiliense reveal no mutations. High clarithromycin resistance of M. abscessus, determines the urgent need for susceptibility-based treatment. Molecular determination of resistance mechanisms to aminoglycosides and macrolides enables fast and accurate targeted treatment implementation.

Real-World Data Collection from Expanded Access Case Studies for the Treatment of Nontuberculous Mycobacterial Infection with Clofazimine

Background

Due to its indolent nature, nontuberculous mycobacteria (NTM) are increasing in global prevalence as a cause of pulmonary infections and are difficult to treat with traditional antibiotics. Here, we study the repurposing of clofazimine (CFZ) to treat NTM through expanded access in a single health system. Our main objectives are to describe the feasibility of accessing and analyzing expanded access data and to generate hypotheses regarding CFZ use in NTM treatment.

Methods

A retrospective chart review was performed on patients within a single health system who had been approved for expanded access of clofazimine or who received it through an outside hospital for NTM treatment. Data were collected on patients' baseline demographics, details of their NTM infection, concomitant therapies, and results as of 30 June 2021.

Results

A total of 55 patients were identified upon initial review as potentially receiving CFZ for NTM infection. After excluding 19 patients who did not initiate CFZ, data from the remaining 36 patients were collected and summarized. The median age at which patients were diagnosed with NTM was 51.3 years old, with a median BMI of 21.2 kg/m2. Patients were more likely to be female (64%), have a baseline lung disease (72%), and 52% were current or former smokers at the time of their diagnosis. The most common species isolated was M. avium complex (47%) followed by M. abscessus (36%), with the most common site of infection being the lung (78%). The majority of patients presented with productive cough with excess sputum production followed by pulmonary nodules and bronchiectasis present on radiograph.

Conclusions

This study demonstrated the difficulty of collecting retrospective real-world data via electronic healthcare records on symptoms, side effects, and radiography from patients who obtained a drug through expanded access. Based on the findings of this study, we recommend further research into the potential use of CFZ in patients with M. abscessus pulmonary infections.

Population heterogeneity in Mycobacterium smegmatis and Mycobacterium abscessus

Bacteria use population heterogeneity, the presence of more than one phenotypic variant in a clonal population, to endure diverse environmental challenges - a 'bet-hedging' strategy. Phenotypic variants have been described in many bacteria, but the phenomenon is not well-understood in mycobacteria, including the environmental factors that influence heterogeneity. Here, we describe three reproducible morphological variants in M. smegmatis - smooth, rough, and an intermediate morphotype that predominated under typical laboratory conditions. M. abscessus has two recognized morphotypes, smooth and rough. Interestingly, M. tuberculosis exists in only a rough form. The shift from smooth to rough in both M. smegmatis and M. abscessus was observed over time in extended static culture, however the frequency of the rough morphotype was high in pellicle preparations compared to planktonic culture, suggesting a role for an aggregated microenvironment in the shift to the rough form. Differences in growth rate, biofilm formation, cell wall composition, and drug tolerance were noted among M. smegmatis and M. abscessus variants. Deletion of the global regulator lsr2 shifted the M. smegmatis intermediate morphotype to a smooth form but did not fully phenocopy the naturally generated smooth morphotype, indicating Lsr2 is likely downstream of the initiating regulatory cascade that controls these morphotypes. Rough forms typically correlate with higher invasiveness and worse outcomes during infection and our findings indicate the shift to this rough form is promoted by aggregation. Our findings suggest that mycobacterial population heterogeneity, reflected in colony morphotypes, is a reproducible, programmed phenomenon that plays a role in adaptation to unique environments and this heterogeneity may influence infection progression and response to treatment.

Characterization of Mycobacterium abscessus colony-biofilms based on bi-dimensional images

Mycobacterium abscessus biofilm aggregates have been shown in the lungs of cystic fibrosis patients and are often tolerant to drugs. Herein, we analyzed bi-dimensional images of either fluorescent or Congo red-stained M. abscessus colony-biofilms grown on a membrane to monitor growth and shape of M. abscessus smooth and rough variants. These colony-biofilms responded differently to rifabutin and bedaquiline, thus highlighting the importance of the morphotype to properly address antibiotic treatment in patients with biofilm-related infections.

Mycobacterium abscessus Opsonization Allows an Escape from the Defensin Bactericidal Action in Drosophila

Mycobacterium abscessus, an intracellular nontuberculous mycobacterium, is considered the most pathogenic species among the group of rapidly growing mycobacteria. The resistance of M. abscessus to the host innate response contributes to its pathogenicity in addition to several virulence factors. We have recently shown in Drosophila that antimicrobial peptides (AMPs), whose production is induced by M. abscessus, are unable to control mycobacterial infection. This could be due to their inability to kill mycobacteria and/or the hidden location of the pathogen in phagocytic cells. Here, we demonstrate that the rapid internalization of M. abscessus by Drosophila macrophages allows it to escape the AMP-mediated humoral response. By depleting phagocytes in AMP-deficient flies, we found that several AMPs were required for the control of extracellular M. abscessus. This was confirmed in the Tep4 opsonin-deficient flies, which we show can better control M. abscessus growth and have increased survival through overproduction of some AMPs, including Defensin. Furthermore, Defensin alone was sufficient to kill extracellular M. abscessus both in vitro and in vivo and control its infection. Collectively, our data support that Tep4-mediated opsonization of M. abscessus allows its escape and resistance toward the Defensin bactericidal action in Drosophila. IMPORTANCE Mycobacterium abscessus, an opportunistic pathogen in cystic fibrosis patients, is the most pathogenic species among the fast-growing mycobacteria. How M. abscessus resists the host innate response before establishing an infection remains unclear. Using Drosophila, we have recently demonstrated that M. abscessus resists the host innate response by surviving the cytotoxic lysis of the infected phagocytes and the induced antimicrobial peptides (AMPs), including Defensin. In this work, we demonstrate that M. abscessus resists the latter response by being rapidly internalized by Drosophila phagocytes. Indeed, by combining in vivo and in vitro approaches, we show that Defensin is able to control extracellular M. abscessus infection through a direct bactericidal action. In conclusion, we report that M. abscessus escapes the host AMP-mediated humoral response by taking advantage of its internalization by the phagocytes.

Multiple Mycobacterium abscessus O-acetyltransferases influence glycopeptidolipid structure and colony morphotype

Mycobacterium abscessus causes severe lung infections. Clinical isolates can have either smooth (S) or rough (R) colony morphotypes; of these, S but not R variants have abundant cell wall glycopeptidolipids (GPL) consisting of a peptidolipid core substituted by a 6-deoxy-α-L-talose (6-dTal) and rhamnose residues. Deletion of gtf1, encoding the 6-dTal transferase, results in the S-to-R transition, mycobacterial cord formation, and increased virulence, underscoring the importance of 6-dTal in infection outcomes. However, since 6-dTal is di-O-acetylated, it is unclear whether the gtf1 mutant phenotypes are related to the loss of the 6-dTal or the result of the absence of acetylation. Here, we addressed whether M. abscessus atf1 and atf2, encoding two putative O-acetyltransferases located within the gpl biosynthetic locus, transfer acetyl groups to 6-dTal. We found deletion of atf1 and/or atf2 did not drastically alter the GPL acetylation profile, suggesting there are additional enzymes with redundant functions. We subsequently identified two paralogs of atf1 and atf2, MAB_1725c and MAB_3448. While deletion of MAB_1725c and MAB_3448 had no effect on GPL acetylation, the triple atf1-atf2-MAB_1725c mutant did not synthetize fully acetylated GPL, and the quadruple mutant was totally devoid of acetylated GPL. Moreover, both triple and quadruple mutants accumulated hyper-methylated GPL. Finally, we show deletion of atf genes resulted in subtle changes in colony morphology but had no effect on M. abscessus internalization by macrophages. Overall, these findings reveal the existence of functionally redundant O-acetyltransferases and suggest that O-acetylation influences the glycan moiety of GPL by deflecting biosynthetic flux in M. abscessus.

Druggable redox pathways against Mycobacterium abscessus in cystic fibrosis patient-derived airway organoids

Mycobacterium abscessus (Mabs) drives life-shortening mortality in cystic fibrosis (CF) patients, primarily because of its resistance to chemotherapeutic agents. To date, our knowledge on the host and bacterial determinants driving Mabs pathology in CF patient lung remains rudimentary. Here, we used human airway organoids (AOs) microinjected with smooth (S) or rough (R-)Mabs to evaluate bacteria fitness, host responses to infection, and new treatment efficacy. We show that S Mabs formed biofilm, and R Mabs formed cord serpentines and displayed a higher virulence. While Mabs infection triggers enhanced oxidative stress, pharmacological activation of antioxidant pathways resulted in better control of Mabs growth and reduced virulence. Genetic and pharmacological inhibition of the CFTR is associated with better growth and higher virulence of S and R Mabs. Finally, pharmacological activation of antioxidant pathways inhibited Mabs growth, at least in part through the quinone oxidoreductase NQO1, and improved efficacy in combination with cefoxitin, a first line antibiotic. In conclusion, we have established AOs as a suitable human system to decipher mechanisms of CF-driven respiratory infection by Mabs and propose boosting of the NRF2-NQO1 axis as a potential host-directed strategy to improve Mabs infection control.

Divergent host innate immune response to the smooth-to-rough M. abscessus adaptation to chronic infection

Mycobacterium abscessus is a nontuberculous mycobacterium emerging as a significant pathogen for individuals with chronic lung disease, including cystic fibrosis and chronic obstructive pulmonary disease. Current therapeutics have poor efficacy. New strategies of bacterial control based on host defenses are appealing, but anti-mycobacterial immune mechanisms are poorly understood and are complicated by the appearance of smooth and rough morphotypes with distinct host responses. We explored the role of the complement system in the clearance of M. abscessus morphotypes by neutrophils, an abundant cell in these infections. M. abscessus opsonized with plasma from healthy individuals promoted greater killing by neutrophils compared to opsonization in heat-inactivated plasma. Rough clinical isolates were more resistant to complement but were still efficiently killed. Complement C3 associated strongly with the smooth morphotype while mannose-binding lectin 2 was associated with the rough morphotype. M. abscessus killing was dependent on C3, but not on C1q or Factor B; furthermore, competition of mannose-binding lectin 2 binding with mannan or N-acetyl-glucosamine during opsonization did not inhibit killing. These data suggest that M. abscessus does not canonically activate complement through the classical, alternative, or lectin pathways. Complement-mediated killing was dependent on IgG and IgM for smooth and on IgG for rough M. abscessus. Both morphotypes were recognized by Complement Receptor 3 (CD11b), but not CR1 (CD35), and in a carbohydrate- and calcium-dependent manner. These data suggest the smooth-to-rough adaptation changes complement recognition of M. abscessus and that complement is an important factor for M. abscessus infection.

Understanding the Phage-Host Interaction Mechanism toward Improving the Efficacy of Current Antibiotics in Mycobacterium abscessus

Pulmonary infections caused by Mycobacterium abscessus (MAB) have been increasing in incidence in recent years, leading to chronic and many times fatal infections due to MAB's natural resistance to most available antimicrobials. The use of bacteriophages (phages) in clinics is emerging as a novel treatment strategy to save the lives of patients suffering from drug-resistant, chronic, and disseminated infections. The substantial research indicates that phage-antibiotic combination therapy can display synergy and be clinically more effective than phage therapy alone. However, there is limited knowledge in the understanding of the molecular mechanisms in phage-mycobacteria interaction and the synergism of phage-antibiotic combinations. We generated the lytic mycobacteriophage library and studied phage specificity and the host range in MAB clinical isolates and characterized the phage's ability to lyse the pathogen under various environmental and mammalian host stress conditions. Our results indicate that phage lytic efficiency is altered by environmental conditions, especially in conditions of biofilm and intracellular states of MAB. By utilizing the MAB gene knockout mutants of the MAB_0937c/MmpL10 drug efflux pump and MAB_0939/pks polyketide synthase enzyme, we discovered the surface glycolipid diacyltrehalose/polyacyltrehalose (DAT/PAT) as one of the major primary phage receptors in mycobacteria. We also established a set of phages that alter the MmpL10 multidrug efflux pump function in MAB through an evolutionary trade-off mechanism. The combination of these phages with antibiotics significantly decreases the number of viable bacteria when compared to phage or antibiotic-alone treatments. This study deepens our understanding of phage-mycobacteria interaction mechanisms and identifies therapeutic phages that can lower bacterial fitness by impairing an antibiotic efflux function and attenuating the MAB intrinsic resistance mechanism via targeted therapy.

The cell envelope of Mycobacterium abscessus and its role in pathogenesis

Mycobacterium abscessus is a nontuberculosis mycobacterium (NTM) that has shown an exponential rise in its ability to cause disease. Due to its ubiquitous presence in the environment, M. abscessus is widely implicated in secondary exacerbations of many nosocomial infections and genetic respiratory disorders, such as cystic fibrosis (CF). Contrary to other rapidly growing NTMs, the cell envelope of M. abscessus harbors several prominent features and undergoes modifications that are responsible for its pathogenesis. Compositional changes of the mycobacterial outer membrane (MOM) significantly decrease the presence of glycopeptidolipids (GPLs) and enable the transition from a colonizing, smooth morphotype into a virulent, rough morphotype. The GPLs are transported to the MOM by the Mycobacterial membrane proteins Large (MmpL), which further act as drug efflux pumps and confer antibiotic resistance. Lastly, M. abscessus possesses 2 type VII secretion systems (T7SS): ESX-3 and ESX-4, both of which have recently been implicated in host-pathogen interactions and virulence. This review summarizes the current knowledge of M. abscessus pathogenesis and highlights the clinically relevant association between the structure and functions of its cell envelope.

A Genome-Wide Screen in Macrophages Defines Host Genes Regulating the Uptake of Mycobacterium abscessus

The interactions between a host cell and a pathogen can dictate disease outcomes and are important targets for host-directed therapies. Mycobacterium abscessus (Mab) is a highly antibiotic resistant, rapidly growing nontuberculous mycobacterium that infects patients with chronic lung diseases. Mab can infect host immune cells, such as macrophages, which contribute to its pathogenesis. However, our understanding of initial host-Mab interactions remains unclear. Here, we developed a functional genetic approach to define these host-Mab interactions by coupling a Mab fluorescent reporter with a genome-wide knockout library in murine macrophages. We used this approach to conduct a forward genetic screen to define host genes that contribute to the uptake of Mab by macrophages. We identified known regulators of phagocytosis, such as the integrin ITGB2, and uncovered a key requirement for glycosaminoglycan (sGAG) synthesis for macrophages to efficiently take up Mab. CRISPR-Cas9 targeting of three key sGAG biosynthesis regulators, Ugdh, B3gat3, and B4galt7 resulted in reduced uptake of both smooth and rough Mab variants by macrophages. Mechanistic studies suggest that sGAGs function upstream of pathogen engulfment and are required for the uptake of Mab, but not Escherichia coli or latex beads. Further investigation found that the loss of sGAGs reduced the surface expression, but not the mRNA expression, of key integrins, suggesting an important role for sGAGs in modulating surface receptor availability. Together, these studies globally define and characterize important regulators of macrophage-Mab interactions and are a first step to understanding host genes that contribute to Mab pathogenesis and disease. IMPORTANCE Pathogen interactions with immune cells like macrophages contribute to pathogenesis, yet the mechanisms underlying these interactions remain largely undefined. For emerging respiratory pathogens, like Mycobacterium abscessus, understanding these host-pathogen interactions is important to fully understand disease progression. Given that M. abscessus is broadly recalcitrant to antibiotic treatments, new therapeutic approaches are needed. Here, we leveraged a genome-wide knockout library in murine macrophages to globally define host genes required for M. abscessus uptake. We identified new macrophage uptake regulators during M. abscessus infection, including a subset of integrins and the glycosaminoglycan synthesis (sGAG) pathway. While ionic characteristics of sGAGs are known to drive pathogen-cell interactions, we discovered a previously unrecognized requirement for sGAGs to maintain robust surface expression of key uptake receptors. Thus, we developed a flexible forward-genetic pipeline to define important interactions during M. abscessus infection and more broadly identified a new mechanism by which sGAGs control pathogen uptake.

Use of niosomes for the treatment of intracellular pathogens infecting the lungs

The delivery of drugs in an encapsulated environment is designed to precisely target specific tissues, avoiding a systemic circulation of the drug. Lungs are organs exposed to the environment with multiple defense barriers. However, many pathogens can still colonize and infect the airways bypassing the hostile environment of the lungs. In more complicated situations, some pathogens have developed strategies to multiply and survive within macrophages, one of the first immune cell responses to clearing infections in mammals. Niosomes are artificial vesicles that can be loaded with drugs, offering an alternative strategy to treat intracellular pathogens as nanocarriers. Members of the mycobacteria genus are intracellular pathogens that have evolved to escape the immunological response, specifically in macrophages, the white cells responsible for the clearance of pathogens. This review analyzed the state-of-the-art niosome synthesis aimed at tackling the problem of intracellular pathogen therapy. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Mycobacterium abscessus resists the innate cellular response by surviving cell lysis of infected phagocytes

Mycobacterium abscessus is the most pathogenic species among the predominantly saprophytic fast-growing mycobacteria. This opportunistic human pathogen causes severe infections that are difficult to eradicate. Its ability to survive within the host was described mainly with the rough (R) form of M. abscessus, which is lethal in several animal models. This R form is not present at the very beginning of the disease but appears during the progression and the exacerbation of the mycobacterial infection, by transition from a smooth (S) form. However, we do not know how the S form of M. abscessus colonizes and infects the host to then multiply and cause the disease. In this work, we were able to show the hypersensitivity of fruit flies, Drosophila melanogaster, to intrathoracic infections by the S and R forms of M. abscessus. This allowed us to unravel how the S form resists the innate immune response developed by the fly, both the antimicrobial peptides- and cellular-dependent immune responses. We demonstrate that intracellular M. abscessus was not killed within the infected phagocytic cells, by resisting lysis and caspase-dependent apoptotic cell death of Drosophila infected phagocytes. In mice, in a similar manner, intra-macrophage M. abscessus was not killed when M. abscessus-infected macrophages were lysed by autologous natural killer cells. These results demonstrate the propensity of the S form of M. abscessus to resist the host’s innate responses to colonize and multiply within the host.

Mycobacterium abscessus DosRS two-component system controls a species-specific regulon required for adaptation to hypoxia

Mycobacterium abscessus (Mab), an emerging opportunistic pathogen, predominantly infects individuals with underlying pulmonary diseases such as cystic fibrosis (CF). Current treatment outcomes for Mab infections are poor due to Mab's inherent antibiotic resistance and unique host interactions that promote phenotypic tolerance and hinder drug access. The hypoxic, mucus-laden airways in the CF lung and antimicrobial phagosome within macrophages represent hostile niches Mab must overcome via alterations in gene expression for survival. Regulatory mechanisms important for the adaptation and long-term persistence of Mab within the host are poorly understood, warranting further genetic and transcriptomics study of this emerging pathogen. DosRS Mab , a two-component signaling system (TCS), is one proposed mechanism utilized to subvert host defenses and counteract environmental stress such as hypoxia. The homologous TCS of Mycobacterium tuberculosis (Mtb), DosRS Mtb , is known to induce a ~50 gene regulon in response to hypoxia, carbon monoxide (CO) and nitric oxide (NO) in vitro and in vivo. Previously, a small DosR Mab regulon was predicted using bioinformatics based on DosR Mtb motifs however, the role and regulon of DosRS Mab in Mab pathogenesis have yet to be characterized in depth. To address this knowledge gap, our lab generated a Mab dosRS knockout strain (MabΔdosRS) to investigate differential gene expression, and phenotype in an in vitro hypoxia model of dormancy. qRT-PCR and lux reporter assays demonstrate Mab_dosR and 6 predicted downstream genes are induced in hypoxia. In addition, RNAseq revealed induction of a much larger hypoxia response comprised of >1000 genes, including 127 differentially expressed genes in a dosRS mutant strain. Deletion of DosRS Mab led to attenuated growth under low oxygen conditions, a shift in morphotype from smooth to rough, and down-regulation of 216 genes. This study provides the first look at the global transcriptomic response of Mab to low oxygen conditions encountered in the airways of CF patients and within macrophage phagosomes. Our data also demonstrate the importance of DosRS Mab for adaptation of Mab to hypoxia, highlighting a distinct regulon (compared to Mtb) that is significantly larger than previously described, including both genes conserved across mycobacteria as well as Mab-specific genes.

Vacuoles in the Breast: A Histologic Clue for an Unusual Presentation of an Atypical Organism

Infections with nontuberculous mycobacteria (NTM) are increasing in prevalence worldwide, and this group of organisms is emerging as significant clinical pathogens. We present a case of a 58-year-old female with persistent furuncles of the breast who was found to have an NTM infection. This case is unique for the lack of risk factors for NTM in the patient's history, the location of the infection in the breast, and the close cooperation needed across disciplines to arrive at the diagnosis. This multi-disciplinary discussion considers the classic clinical presentation of NTM, it is a characteristic morphological appearance on histopathology, the differential diagnosis, treatment, and the ultimate outcome of the case. This case report and discussion will assist both clinicians and pathologists in the diagnosis of this important infectious disease.

Biotin-dependent cell envelope remodelling is required for Mycobacterium abscessus survival in lung infection

Mycobacterium abscessus is an emerging pathogen causing lung infection predominantly in patients with underlying structural abnormalities or lung disease and is resistant to most frontline antibiotics. As the pathogenic mechanisms of M. abscessus in the context of the lung are not well-understood, we developed an infection model using air-liquid interface culture and performed a transposon mutagenesis and sequencing screen to identify genes differentially required for bacterial survival in the lung. Biotin cofactor synthesis was required for M. abscessus growth due to increased intracellular biotin demand, while pharmacological inhibition of biotin synthesis prevented bacterial proliferation. Biotin was required for fatty acid remodelling, which increased cell envelope fluidity and promoted M. abscessus survival in the alkaline lung environment. Together, these results indicate that biotin-dependent fatty acid remodelling plays a critical role in pathogenic adaptation to the lung niche, suggesting that biotin synthesis and fatty acid metabolism might provide therapeutic targets for treatment of M. abscessus infection.

Mycobacterium abscessus alkyl hydroperoxide reductase C promotes cell invasion by binding to tetraspanin CD81

Mycobacterium abscessus (Mab) is an increasingly recognized pulmonary pathogen. How Mab is internalized by macrophages and establishes infection remains unknown. Here, we show that Mab uptake is significantly reduced in macrophages pre-incubated with neutralizing anti-CD81 antibodies or in cells in which the large extracellular loop (LEL) of CD81 has been deleted. Saturation of Mab with either soluble GST-CD81-LEL or CD81-LEL-derived peptides also diminished internalization of the bacilli. The mycobacterial alkyl hydroperoxide reductase C (AhpC) was unveiled as a major interactant of CD81-LEL. Pre-exposure of macrophages with soluble AhpC inhibited mycobacterial uptake whereas overexpression of AhpC in Mab enhanced its internalization. Importantly, pre-incubation of macrophages with anti-CD81-LEL antibodies inhibited phagocytosis of AhpC-coated beads, indicating that AhpC is a direct interactant of CD81-LEL. Conditional depletion of AhpC in Mab correlated with decreased internalization of Mab. These compelling data unravel a previously unexplored role for CD81/AhpC to promote uptake of pathogenic mycobacteria by host cells.

Glycopeptidolipid Defects Leading to Rough Morphotypes of Mycobacterium abscessus Do Not Confer Clinical Antibiotic Resistance

Mycobacterium abscessus is an emerging pathogen causing severe pulmonary infections. Within chronically infected patients, M. abscessus isolates undergo molecular changes leading to increased virulence and antibiotic resistance. Specifically, mutations in glycopeptidolipid (GPL) synthesis genes, leading to the rough phenotype, are associated with invasive, nonremitting infections and a severe clinical course. It has been unclear whether GPL defects confer antibiotic resistance independently of other molecular changes. We used transposon technology to isolate a rough (GPL-defective; Tn MABS_4099cZeoR) mutant and compare it to a fully isogenic parent strain (ATCC 19977) bearing wild-type zeocin resistance (WTZeoR). Antibiotic susceptibility profiles of Tn_4099cZeoR and WTZeoR were tested and compared using the Sensititre RAPMYCOI antimicrobial susceptibility test plate. MICs were evaluated within clinically relevant values according to the Clinical and Laboratory Standards Institute (CLSI) standards. We found that M. abscessus with rough colony morphotype (Tn_4009c) had comparable antibiotic susceptibility to its smooth isogenic WT counterpart. Small differences (a 1:2 dilution) in MICs were found for imipenem, cefoxitin, and tigecycline, yet those small differences did not change the clinical susceptibility report for these antibiotics, as they fell within the same CLSI cutoffs for resistance. While small alternations in susceptibility to imipenem, cefoxitin, and tigecycline were noted, we conclude that the GPL mutations in M. abscessus did not confer clinically significant antibiotic resistance. Increased antibiotic resistance in the clinical setting may occur in an unrelated and parallel manner to GPL mutations. IMPORTANCE Mycobacterium abscessus chronically infects patients with preexisting lung diseases, leading to progressive deterioration in pulmonary function. The common perception among clinicians is that the rough phenotype is associated with progressive disease and severe clinical course, manifested as a widespread inflammatory response and resistance to antibacterials. However, as clinical isolates accumulate hundreds of mutations over the prolonged course of infection, it is unclear whether the rough phenotype per se is responsible for the antibiotic resistance seen in late-stage infections, or whether the resistance is related to other genetic changes in the bacteria. Previous studies mostly compared rough and smooth clinical isolates. Here, for the first time, we compared WT smooth bacteria to a specific rough, GPL-associated, otherwise-isogenic mutant. We determined that the rough morphotype had essentially identical antibiotic susceptibilities as the parent strain. The mechanistic basis for the antibiotic resistance observed in rough clinical isolates is therefore most probably related to other genetic determinants.

Evaluation of the Efficiency of Random and Diblock Methacrylate-Based Amphiphilic Cationic Polymers against Major Bacterial Pathogens Associated with Cystic Fibrosis

Cystic fibrosis (CF) is associated with repeated lung bacterial infection, mainly by Pseudomonas aeruginosa, Staphylococcus aureus, and Mycobacterium abscessus, all known to be or becoming resistant to several antibiotics, often leading to therapeutic failure and death. In this context, antimicrobial peptides and antimicrobial polymers active against resistant strains and less prompt to cause resistance, appear as a good alternative to conventional antibiotics. In the present study, methacrylate-based copolymers obtained by radical chemistry were evaluated against CF-associated bacterial strains. Results showed that the type (Random versus Diblock) and the size of the copolymers affected their antibacterial activity and toxicity. Among the different copolymers tested, four (i.e., Random₁₀₂₀₀, Random₁₅₀₀₀, Random₂₃₉₀₀, and Diblock₉₅₀₀) were identified as the most active and the safest molecules and were further investigated. Data showed that they inserted into bacterial lipids, leading to a rapid membranolytic effect and killing of the bacterial. In relation with their fast bactericidal action and conversely to conventional antibiotics, those copolymers did not induce a resistance and remained active against antibiotic-resistant strains. Finally, the selected copolymers possessed a preventive effect on biofilm formation, although not exhibiting disruptive activity. Overall, the present study demonstrates that methacrylate-based copolymers are an interesting alternative to conventional antibiotics in the treatment of CF-associated bacterial infection.

Cysteamine/Cystamine Exert Anti-Mycobacterium abscessus Activity Alone or in Combination with Amikacin

Host-directed therapies are emerging as a promising tool in the curing of difficult-to-treat infections, such as those caused by drug-resistant bacteria. In this study, we aim to test the potential activity of the FDA- and EMA-approved drugs cysteamine and cystamine against Mycobacterium abscessus. In human macrophages (differentiated THP-1 cells), these drugs restricted M. abscessus growth similar to that achieved by amikacin. Here, we use the human ex vivo granuloma-like structures (GLS) model of infection with the M. abscessus rough (MAB-R) and smooth (MAB-S) variants to study the activity of new therapies against M. abscessus. We demonstrate that cysteamine and cystamine show a decrease in the number of total GLSs per well in the MAB-S and MAB-R infected human peripheral blood mononuclear cells (PBMCs). Furthermore, combined administration of cysteamine or cystamine with amikacin resulted in enhanced activity against the two M. abscessus morpho variants compared to treatment with amikacin only. Treatment with cysteamine and cystamine was more effective in reducing GLS size and bacterial load during MAB-S infection compared with MAB-R infection. Moreover, treatment with these two drugs drastically quenched the exuberant proinflammatory response triggered by the MAB-R variant. These findings showing the activity of cysteamine and cystamine against the R and S M. abscessus morphotypes support the use of these drugs as novel host-directed therapies against M. abscessus infections.

Inflammation-mediated tissue damage in pulmonary tuberculosis and host-directed therapeutic strategies

Treatment of tuberculosis (TB) involves the administration of anti-mycobacterial drugs for several months. The emergence of drug-resistant strains of Mycobacterium tuberculosis (Mtb, the causative agent) together with increased disease severity in people with co-morbidities such as diabetes mellitus and HIV have hampered efforts to reduce case fatality. In severe disease, TB pathology is largely attributable to over-exuberant host immune responses targeted at controlling bacterial replication. Non-resolving inflammation driven by host pro-inflammatory mediators in response to high bacterial load leads to pulmonary pathology including cavitation and fibrosis. The need to improve clinical outcomes and reduce treatment times has led to a two-pronged approach involving the development of novel antimicrobials as well as host-directed therapies (HDT) that favourably modulate immune responses to Mtb. HDT strategies incorporate aspects of immune modulation aimed at downregulating non-productive inflammatory responses and augmenting antimicrobial effector mechanisms to minimise pulmonary pathology and accelerate symptom resolution. HDT in combination with existing antimycobacterial agents offers a potentially promising strategy to improve the long-term outcome for TB patients. In this review, we describe components of the host immune response that contribute to inflammation and tissue damage in pulmonary TB, including cytokines, matrix metalloproteinases, lipid mediators, and neutrophil extracellular traps. We then proceed to review HDT directed at these pathways.

Updated Review on the Mechanisms of Pathogenicity in Mycobacterium abscessus, a Rapidly Growing Emerging Pathogen

In recent years, Mycobacterium abscessus has appeared as an emerging pathogen, with an increasing number of disease cases reported worldwide that mainly occur among patients with chronic lung diseases or impaired immune systems. The treatment of this pathogen represents a challenge due to the multi-drug-resistant nature of this species and its ability to evade most therapeutic approaches. However, although predisposing host factors for disease are well known, intrinsic pathogenicity mechanisms of this mycobacterium are still not elucidated. Like other mycobacteria, intracellular invasiveness and survival inside different cell lines are pathogenic factors related to the ability of M. abscessus to establish infection. Some of the molecular factors involved in this process are well-known and are present in the mycobacterial cell wall, such as trehalose-dimycolate and glycopeptidolipids. The ability to form biofilms is another pathogenic factor that is essential for the development of chronic disease and for promoting mycobacterial survival against the host immune system or different antibacterial treatments. This capability also seems to be related to glycopeptidolipids and other lipid molecules, and some studies have shown an intrinsic relationship between both pathogenic mechanisms. Antimicrobial resistance is also considered a mechanism of pathogenicity because it allows the mycobacterium to resist antimicrobial therapies and represents an advantage in polymicrobial biofilms. The recent description of hyperpathogenic strains with the potential interhuman transmission makes it necessary to increase our knowledge of pathogenic mechanisms of this species to design better therapeutic approaches to the management of these infections.