Mycobacterium abscessus morphotype comparison in a murine model

Preclinical murine models for the testing of antimicrobials against Mycobacterium abscessus pulmonary infections: Current practices and recommendations

Mycobacterium abscessus, a rapidly growing nontuberculous mycobacterium, is increasingly recognized as an important pathogen of the human lung, disproportionally affecting people with cystic fibrosis (CF) and other susceptible individuals with non-CF bronchiectasis and compromised immune functions. M. abscessus infections are extremely difficult to treat due to intrinsic resistance to many antibiotics, including most anti-tuberculous drugs. Current standard-of-care chemotherapy is long, includes multiple oral and parenteral repurposed drugs, and is associated with significant toxicity. The development of more effective oral antibiotics to treat M. abscessus infections has thus emerged as a high priority. While murine models have proven instrumental in predicting the efficacy of therapeutic treatments for M. tuberculosis infections, the preclinical evaluation of drugs against M. abscessus infections has proven more challenging due to the difficulty of establishing a progressive, sustained, pulmonary infection with this pathogen in mice. To address this issue, a series of three workshops were hosted in 2023 by the Cystic Fibrosis Foundation (CFF) and the National Institute of Allergy and Infectious Diseases (NIAID) to review the current murine models of M. abscessus infections, discuss current challenges and identify priorities toward establishing validated and globally harmonized preclinical models. This paper summarizes the key points from these workshops. The hope is that the recommendations that emerged from this exercise will facilitate the implementation of informative murine models of therapeutic efficacy testing across laboratories, improve reproducibility from lab-to-lab and accelerate preclinical-to-clinical translation.

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.

The roles of neutrophils in non-tuberculous mycobacterial pulmonary disease

Non-tuberculous Mycobacterial Pulmonary Disease (NTM-PD) is an increasingly recognised global health issue. Studies have suggested that neutrophils may play an important role in controlling NTM infection and contribute to protective immune responses within the early phase of infection. However, these cells are also adversely associated with disease progression and exacerbation and can contribute to pathology, for example in the development of bronchiectasis. In this review, we discuss the key findings and latest evidence regarding the diverse functions of neutrophils in NTM infection. First, we focus on studies that implicate neutrophils in the early response to NTM infection and the evidence reporting neutrophils' capability to kill NTM. Next, we present an overview of the positive and negative effects that characterise the bidirectional relationship between neutrophils and adaptive immunity. We consider the pathological role of neutrophils in driving the clinical phenotype of NTM-PD including bronchiectasis. Finally, we highlight the current promising treatments in development targeting neutrophils in airways diseases. Clearly, more insights on the roles of neutrophils in NTM-PD are needed in order to inform both preventative strategies and host-directed therapy for these important infections.

Mycobacterium abscessus infection results in decrease of oxidative metabolism of lung airways cells and relaxation of the epithelial mucosal tight junctions

Mycobacterium abscessus complex is a group of environmental pathogens that recently have been isolated more from patients with underlying lung diseases, such and COPD, bronchiectasis, and cystic fibrosis. The mechanisms involved in the pathogenesis of these diseases have only recently been investigated. Infection is associated with biofilm formation on the airway mucosa, invasion of the mucosal epithelial cells and a time-dependent impairment of the integrity of the monolayer. Using electron microscopy, it was shown that Mycobacterium abscessus induced lesions of the cell surface structures. Tight junction proteins claudin-1 and occludin-1 have increased transcription in cells exposed to Mycobacterium abscessus, in contrast to cells exposed to Mycobacterium avium. Infection of A549 alveolar epithelial cells by Mycobacterium abscessus reduced the oxidative metabolism of the cell, without inducing necrosis. A transposon library screen identified mutants that do not alter the metabolism of the A549 cells.Once the bacterium crosses the epithelial barrier, it may encounter sub-epithelial macrophages. Select mutants were used for infection assays to determine their effects on membrane integrity. Translocated select mutants were attenuated in macrophages compared to wild type Mycobacterium abscessus. In summary, the dynamics of Mycobacterium abscessus infection appears to be different from other non-tuberculous mycobacteria (NTMs). Future studies will attempt to address the mechanism involved in airway membrane lesions.

Preclinical murine models to study lung infection with Mycobacterium abscessus complex

Mycobacterium abscessus is a non-tuberculous mycobacterium (NTM) able to cause invasive pulmonary infections, named NTM pulmonary disease. The therapeutic approaches are limited, and infections are difficult to treat due to antibiotic resistance conferred by an impermeable cell wall, drug efflux pumps, or drug-modifying enzymes. The development of new therapeutics, intended as antimicrobials or drug limiting immunopathology, is urgently necessary. In this context, the preclinical murine models of M. abscessus represent a useful tool to validate and translate in vitro-proofed concepts. These in vivo models are essential for developing new targets and drugs, ameliorating our knowledge in combinatorial regimens of current existing antibiotic treatments, and repurposing existing drugs for new therapeutic options against M. abscessus infection. Thus, this review aims at providing an overview of the current state of the art of preclinical murine models to study M. abscessus lung infection and its exploitation for new therapeutic approaches. This review discusses the murine models available focusing on the different bacterial challenges (aerosol, intranasal, intratracheal, and intravenous administrations), murine genetic background, and additional bacterial related factors. Then, we discuss the successful preclinical models for M. abscessus respiratory infection exploited to study the efficacy and safety of new antimicrobials or to determine the best dosage and route of administration of existing drugs. Finally, we present the current murine models exploited to develop new therapeutic approaches to modulate the host immune response and limit immunopathological damage during M. abscessus lung disease. In conclusion, our review article provides an overview of current and available murine models to characterize acute or chronic infections and to study the outcome of new therapeutic strategies against M. abscessus lung infection.

Mice with lung airway ciliopathy develop persistent Mycobacterium abscessus lung infection and have a proinflammatory lung phenotype associated with decreased T regulatory cells

Introduction

Human pulmonary infection with non-tuberculous mycobacteria (NTM) such as Mycobacterium abscessus (Mabs) occurs in seemingly immunocompetent patients with underlying structural lung disease such as bronchiectasis in which normal ciliary function is perturbed. In addition to alterations in mucociliary clearance, the local immunologic milieu may be altered in patients with structural lung disease, but the nature of these changes and how they relate to NTM persistence remain unclear.

Methods

We used a mouse strain containing a conditional floxed allele of the gene IFT88, which encodes for the protein Polaris. Deletion of this gene in adult mice reportedly leads to loss of cilia on lung airway epithelium and to the development of bronchiectasis. In a series of experiments, IFT88 control mice and IFT88 KO mice received different preparations of Mabs lung inocula with lung CFU assessed out to approximately 8 weeks post-infection. In addition, cytokine levels in bronchoalveolar lavage (BAL) fluid, lung T cell subset analysis, and lung histopathology and morphometry were performed at various time points.

Results

Mabs embedded in agarose beads persisted in the lungs of IFT88 KO mice out to approximately 8 weeks (54 days), while Mabs agarose beads in the lungs of IFT88 control mice was cleared from the lungs of all mice at this time point. T cells subset analysis showed a decrease in the percentage of CD4⁺FoxP3⁺ T cells in the total lymphocyte population in the lungs of IFT88 KO mice relative to IFT88 control mice. Proinflammatory cytokines were elevated in the BAL fluid from infected IFT88 KO mice compared to infected IFT88 control mice, and histopathology showed an increased inflammatory response and greater numbers of granulomas in the lungs of infected IFT88 KO mice compared to the lungs of infected IFT88 control mice. Scanning lung morphometry did not show a significant difference comparing lung airway area and lung airway perimeter between IFT88 KO mice and IFT88 control mice.

Discussion

Persistent lung infection in our model was established using Mabs embedded in agarose beads. The utility of using IFT88 mice is that a significant difference in Mabs lung CFU is observed comparing IFT88 KO mice to IFT88 control mice thus allowing for studies assessing the mechanism(s) of Mabs lung persistence. Our finding of minimal differences in lung airway area and lung airway diameter comparing IFT88 KO mice to IFT88 control mice suggests that the development of a proinflammatory lung phenotype in IFT88 KO mice contributes to Mabs lung persistence independent of bronchiectasis. The contribution of cilia to immune regulation is increasingly recognized, and our results suggest that ciliopathy associated with structural lung disease may play a role in NTM pulmonary infection via alteration of the local immunologic lung milieu.

In vitro effects of diazabicyclooctane β-lactamase inhibitors relebactam and nacubactam against three subspecies of Mycobacterium abscessus complex

BACKGROUND

Mycobacterium abscessus complex (MABC) pulmonary disease is notoriously difficult to treat due to intrinsic resistance to many common antibiotics. MABC is β-lactam-resistant as it produces class A β-lactamases, such as bla_Mab, which are inhibited by diazabicyclooctane (DBO) β-lactamase inhibitors.

OBJECTIVES

To investigate the microbiological effects of the combination of β-lactam and DBO β-lactamase inhibitors (relebactam and nacubactam) against MABC and determine if the effects are associated with the MABC subspecies and colony morphotype.

METHODS

The antimicrobial susceptibility of three type strains and 20 clinical isolates of MABC to the combination of seven β-lactams with relebactam or nacubactam was evaluated using broth microdilution checkerboard assays. For these strains, expression levels of bla_Mab were assessed using quantitative real-time polymerase chain reaction and genotypic diversity was evaluated using 18-locus variable number tandem repeat assay.

RESULTS

Relebactam and nacubactam lowered the minimum inhibitory concentrations of β-lactams, particularly imipenem, meropenem, and tebipenem, against MABC. There was no difference in efficacy of combination treatment between three subspecies, but rough morphotypes tended to be less susceptible than smooth morphotypes. There were no differences in bla_Mab expression levels and genotypic diversity between the morphotypes.

CONCLUSIONS

The combination of β-lactam with relebactam or nacubactam improved the efficacy of β-lactams against all MABC subspecies, but higher concentrations of β-lactams were needed for rough morphotypes.

Alternatives to Antibiotics against Mycobacterium abscessus

Mycobacterium abscessus complex is extremely difficult to treat. Intrinsic and acquired bacterial resistance makes this species one of the most challenging pathogens and treatments last from months to years, associated with potential risky antibiotic toxicity and a high number of failures. Nonantibiotic antimicrobial agents against this microorganism have recently been studied so as to offer an alternative to current drugs. This review summarizes recent research on different strategies such as host modulation using stem cells, photodynamic therapy, antibiofilm therapy, phage therapy, nanoparticles, vaccines and antimicrobial peptides against M. abscessus both in vitro and in vivo.

Early IL-17A production helps establish Mycobacterium intracellulare infection in mice

Nontuberculous mycobacteria (NTM) infection is common in patients with structural lung damage. To address how NTM infection is established and causes lung damage, we established an NTM mouse model by intranasal inoculation of clinical isolates of M. intracellulare. During the 39-week course of infection, the bacteria persistently grew in the lung and caused progressive granulomatous and fibrotic lung damage with mortality exceeding 50%. Lung neutrophils were significantly increased at 1 week postinfection, reduced at 2 weeks postinfection and increased again at 39 weeks postinfection. IL-17A was increased in the lungs at 1–2 weeks of infection and reduced at 3 weeks postinfection. Depletion of neutrophils during early (0–2 weeks) and late (32–34 weeks) infection had no effect on mortality or lung damage in chronically infected mice. However, neutralization of IL-17A during early infection significantly reduced bacterial burden, fibrotic lung damage, and mortality in chronically infected mice. Since it is known that IL-17A regulates matrix metalloproteinases (MMPs) and that MMPs contribute to the pathogenesis of pulmonary fibrosis, we determined the levels of MMPs in the lungs of M. intracellulare-infected mice. Interestingly, MMP-3 was significantly reduced by anti-IL-17A neutralizing antibody. Moreover, in vitro data showed that exogenous IL-17A exaggerated the production of MMP-3 by lung epithelial cells upon M. intracellulare infection. Collectively, our findings suggest that early IL-17A production precedes and promotes organized pulmonary M. intracellulare infection in mice, at least in part through MMP-3 production.

To determine how nontuberculous mycobacteria (NTM) infection is established and how NTM disease progresses, we established a chronic NTM mouse model by intranasal inoculation of M. intracellulare, one of the most frequently isolated strains in NTM patients. The bacteria persistently grew in the lungs and caused fibrotic lung damage with over 50% mortality over 39 weeks. Neutrophils and IL-17A rapidly increased in the lung during early (1–2 weeks) infection, and neutrophils reappeared at 39 weeks postinfection. Depletion of neutrophils during early (0–2 weeks) and chronic (32–34 weeks) infection had no effect on mortality or lung damage in chronically infected mice. Neutralization of IL-17A during early (0–2 weeks) infection significantly reduced mortality, bacterial burden, fibrotic lung damage, and lung matrix metalloproteinase (MMP)-3 at 39 weeks postinfection. Exogenous IL-17A exaggerated the production of MMP-3, but not MMP-9, by lung epithelial cells upon M. intracellulare infection. This study demonstrates that early IL-17A production contributes to established M. intracellulare infection in mice.

Virulence Mechanisms of Mycobacterium abscessus: Current Knowledge and Implications for Vaccine Design

Mycobacterium abscessus is a member of the non-tuberculous mycobacteria (NTM) group, responsible for chronic infections in individuals with cystic fibrosis (CF) or those otherwise immunocompromised. While viewed traditionally as an opportunistic pathogen, increasing research into M. abscessus in recent years has highlighted its continued evolution into a true pathogen. This is demonstrated through an extensive collection of virulence factors (VFs) possessed by this organism which facilitate survival within the host, particularly in the harsh environment of the CF lung. These include VFs resembling those of other Mycobacteria, and non-mycobacterial VFs, both of which make a notable contribution in shaping M. abscessus interaction with the host. Mycobacterium abscessus continued acquisition of VFs is cause for concern and highlights the need for novel vaccination strategies to combat this pathogen. An effective M. abscessus vaccine must be suitably designed for target populations (i.e., individuals with CF) and incorporate current knowledge on immune correlates of protection against M. abscessus infection. Vaccination strategies must also build upon lessons learned from ongoing efforts to develop novel vaccines for other pathogens, particularly Mycobacterium tuberculosis (M. tb); decades of research into M. tb has provided insight into unconventional and innovative vaccine approaches that may be applied to M. abscessus. Continued research into M. abscessus pathogenesis will be critical for the future development of safe and effective vaccines and therapeutics to reduce global incidence of this emerging pathogen.

Host-Pathogen Interactions Operative during Mycobacteroides abscessus Infection

Mycobacteroides abscessus (previously Mycobacterium abscessus; Mabc), one of rapidly growing nontuberculous mycobacteria (NTM), is an important pathogen of NTM pulmonary diseases (NTM-PDs) in both immunocompetent and immunocompromised individuals. Mabc infection is chronic and often challenging to treat due to drug resistance, motivating the development of new therapeutics. Despite this, there is a lack of understanding of the relationship between Mabc and the immune system. This review highlights recent progress in the molecular architecture of Mabc and host interactions. We discuss several microbial components that take advantage of host immune defenses, host defense pathways that can overcome Mabc pathogenesis, and how host-pathogen interactions determine the outcomes of Mabc infection. Understanding the molecular mechanisms underlying host-pathogen interactions during Mabc infection will enable the identification of biomarkers and/or drugs to control immune pathogenesis and protect against NTM infection.

Antimicrobial Activity of Neutrophils Against Mycobacteria

The mycobacterium genus contains a broad range of species, including the human pathogens M. tuberculosis and M. leprae. These bacteria are best known for their residence inside host cells. Neutrophils are frequently observed at sites of mycobacterial infection, but their role in clearance is not well understood. In this review, we discuss how neutrophils attempt to control mycobacterial infections, either through the ingestion of bacteria into intracellular phagosomes, or the release of neutrophil extracellular traps (NETs). Despite their powerful antimicrobial activity, including the production of reactive oxidants such as hypochlorous acid, neutrophils appear ineffective in killing pathogenic mycobacteria. We explore mycobacterial resistance mechanisms, and how thwarting neutrophil action exacerbates disease pathology. A better understanding of how mycobacteria protect themselves from neutrophils will aid the development of novel strategies that facilitate bacterial clearance and limit host tissue damage.

Growth medium and nitric oxide alter Mycobacterium abscessus morphotype and virulence

Mycobacterium abscessus complex (MABC) infections cause significant morbidity and mortality among patients with chronic lung disease, like cystic fibrosis. MABC exists in smooth (S) and rough (R) morphotypes, but triggers of morphotype switching and associated pathogenicity or antimicrobial susceptibility are poorly understood. We demonstrate that M. abscessus subspecies abscessus (Mab), massiliense (Mms), and bolletii (Mbl) cultured in Middlebrook (MB) broth exhibit S morphotype, whereas the bacteria grown in Luria Bertani (LB) broth adopt the R morphotype, characterized by low glycopeptidolipid (GPL) expression. The components of broth that mediate this selection are complex, with albumin supplementation promoting growth of S morphotype, but not sufficient for complete selection. Consistent with the findings of other groups, R forms of Mab, Mms and Mbl selected by LB broth were internalized in RAW 264.7 macrophages with higher efficiency than S. Intracellular survival of broth-selected organisms was variable and was higher for S Mab, but lower for S Mms and Mbl. It is proposed that growth in R morphotype is induced during stress conditions, such as nutrient poor environments or during inflammation. One key component of inflammation is release of nitric oxide. We demonstrated that a nitric oxide donor (DETA-NONOate) appears to induce growth in an R morphotype, as indicated by reduced GPL expression of Mab. Mab treated with DETA-NONOate also enhanced susceptibility to azithromycin at sub-MIC concentrations. In conclusion, morphotype and macrophage intracellular bacterial load of MABC subspecies can be manipulated by growing the bacteria in different culture conditions. Nitric oxide may also drive morphotype selection and enhanced azithromycin activity against Mab and macrophage killing.

Rufomycin Exhibits Dual Effects Against Mycobacterium abscessus Infection by Inducing Host Defense and Antimicrobial Activities

Nontuberculous mycobacterial pulmonary infection is often aggravated due to antibiotic resistance issues. There is a need for development of new drugs inducing both host immune responses and antimicrobial activities. This study shows that the rufomycins 4/5/6/7 (Rufomycin 4–7), which targets ClpC1 as a subunit of caseinolytic protein complex ClpC1/ClpP1/ClpP2 of mycobacteria, exhibits a dual effect in host innate defense and in vivo antimicrobial activities against a rough morphotype of Mycobacterium abscessus (Mabs-R), a clinically severe morphotype that causes hyperinflammation. Rufomycin 4–7 treatment showed antimicrobial effects against Mabs pulmonary infection in vivo and in macrophages. In addition, Rufomycin 4–7 significantly decreased inflammation, but enhanced the autophagy/lysosomal genes through upregulation of the nuclear translocation of transcription factor EB (TFEB). Furthermore, Rufomycin 4–7 treatment effectively inhibited mitochondrial damage and oxidative stresses in macrophages during Mabs-R infection. Collectively, Rufomycin 4–7-mediated dual effects inducing both antimicrobial activities and host immune defense might confer an advantage to treatment against Mabs-R infection.

Sirtuin 3 is essential for host defense against Mycobacterium abscessus infection through regulation of mitochondrial homeostasis

The global incidence of Mycobacterium abscessus (Mabc), a rapidly growing nontuberculous mycobacterial strain that causes treatment-refractory pulmonary diseases, is increasing. Despite this, the host factors that allow for protection against infection are largely unknown. In this study, we found that sirtuin 3 (SIRT3), a mitochondrial protein deacetylase, plays a critical role in host defense against Mabc infection. Mabc decreased SIRT3 and upregulated mitochondrial oxidative stress in macrophages. SIRT3 deficiency led to increased bacterial loads, histopathological, and mitochondrial damage, and pathological inflammation during Mabc infection. Administration of scavengers of mitochondrial reactive oxygen species significantly decreased the in vivo Mabc burden and excessive inflammation, and induced SIRT3 expression in infected lungs. Notably, SIRT3 agonist (resveratrol) significantly decreased Mabc growth and attenuated inflammation in mice and zebrafishes, indicating the key role for SIRT3 in metazoan host defense. Collectively, these data strongly suggest that SIRT3 is a host-directed therapeutic target against Mabc infection by controlling mitochondrial homeostasis.

Ex vivo infection of murine precision-cut lung tissue slices with Mycobacterium abscessus: a model to study antimycobacterial agents

Background

Multidrug-resistant infections due to Mycobacterium abscessus often require complex and prolonged regimens for treatment. Here, we report the evaluation of a new ex vivo antimicrobial susceptibility testing model using organotypic cultures of murine precision-cut lung slices, an experimental model in which metabolic activity, and all the usual cell types of the organ are found while the tissue architecture and the interactions between the different cells are maintained.

Methods

Precision cut lung slices (PCLS) were prepared from the lungs of wild type BALB/c mice using the Krumdieck^® tissue slicer. Lung tissue slices were ex vivo infected with the virulent M. abscessus strain L948. Then, we tested the antimicrobial activity of two drugs: imipenem (4, 16 and 64 μg/mL) and tigecycline (0.25, 1 and 4 μg/mL), at 12, 24 and 48 h. Afterwards, CFUs were determined plating on blood agar to measure the surviving intracellular bacteria. The viability of PCLS was assessed by Alamar Blue assay and corroborated using histopathological analysis.

Results

PCLS were successfully infected with a virulent strain of M. abscessus as demonstrated by CFUs and detailed histopathological analysis. The time-course infection, including tissue damage, parallels in vivo findings reported in genetically modified murine models for M. abscessus infection. Tigecycline showed a bactericidal effect at 48 h that achieved a reduction of > 4log₁₀ CFU/mL against the intracellular mycobacteria, while imipenem showed a bacteriostatic effect.

Conclusions

The use of this new organotypic ex vivo model provides the opportunity to test new drugs against M. abscessus, decreasing the use of costly and tedious animal models.

Gallium Porphyrin and Gallium Nitrate Synergistically Inhibit Mycobacterial Species by Targeting Different Aspects of Iron/Heme Metabolism

There is an urgent need for new effective and safe antibiotics active against pathogenic mycobacterial species. Gallium (Ga) nitrate (Ga(NO₃)₃) and Ga porphyrin (GaPP) have each been shown to inhibit the growth of a variety of mycobacterial species. The Ga(III) ion derived from Ga(NO₃)₃ has the potential to disrupt the mycobacterial Fe(III) uptake mechanisms and utilization, including replacing iron (Fe) in the active site of enzymes, resulting in the disruption of function. Similarly, noniron metalloporphyrins such as heme mimetics, which can be transported across the bacterial membrane via heme-uptake pathways, would potentially block the acquisition of iron-containing heme and bind to heme-utilizing proteins, making them nonfunctional. Given that they likely act on different aspects of mycobacterial Fe metabolism, the efficacy of combining Ga(NO₃)₃ and GaPP was studied in vitro against Mycobacterium avium, Mycobacterium abscessus, and Mycobacterium tuberculosis (M. tb). The combination was then assessed in vivo in a murine pulmonary infection model of M. abscessus. We observed that Ga(NO₃)₃ in combination with GaPP exhibited synergistic inhibitory activity against the growth of M. avium, M. tb, and M. abscessus, being most active against M. abscessus. Activity assays indicated that Ga(NO₃)₃ and GaPP inhibited both catalase and aconitase at high concentrations. However, the combination showed a synergistic effect on the aconitase activity of M. abscessus. The Ga(NO₃)₃/GaPP combination via intranasal administration showed significant antimicrobial activity in mice infected with M. abscessus. M. abscessus CFU from the lungs of the Ga(NO₃)₃/GaPP-treated mice was significantly less compared to that of nontreated or single Ga(III)-treated mice. These findings suggest that combinations of different Ga(III) compounds can synergistically target multiple iron/heme-utilizing mycobacterial enzymes. The results support the potential of combination Ga therapy for development against mycobacterial pathogens.

A New Model of Chronic Mycobacterium abscessus Lung Infection in Immunocompetent Mice

Pulmonary infections caused by Mycobacterium abscessus (MA) have increased over recent decades, affecting individuals with underlying pathologies such as chronic obstructive pulmonary disease, bronchiectasis and, especially, cystic fibrosis. The lack of a representative and standardized model of chronic infection in mice has limited steps forward in the field of MA pulmonary infection. To overcome this challenge, we refined the method of agar beads to establish MA chronic infection in immunocompetent mice. We evaluated bacterial count, lung pathology and markers of inflammation and we performed longitudinal studies with magnetic resonance imaging (MRI) up to three months after MA infection. In this model, MA was able to establish a persistent lung infection for up to two months and with minimal systemic spread. Lung histopathological analysis revealed granulomatous inflammation around bronchi characterized by the presence of lymphocytes, aggregates of vacuolated histiocytes and a few neutrophils, mimicking the damage observed in humans. Furthermore, MA lung lesions were successfully monitored for the first time by MRI. The availability of this murine model and the introduction of the successfully longitudinal monitoring of the murine lung lesions with MRI pave the way for further investigations on the impact of MA pathogenesis and the efficacy of novel treatments.

Cell Surface Remodeling of Mycobacterium abscessus under Cystic Fibrosis Airway Growth Conditions

Understanding the physiological processes underlying the ability of Mycobacterium abscessus to become a chronic pathogen of the cystic fibrosis (CF) lung is important to the development of prophylactic and therapeutic strategies to better control and treat pulmonary infections caused by these bacteria. Gene expression profiling of a diversity of M. abscessus complex isolates points to amino acids being significant sources of carbon and energy for M. abscessus in both CF sputum and synthetic CF medium and to the bacterium undergoing an important metabolic reprogramming in order to adapt to this particular nutritional environment. Cell envelope analyses conducted on the same representative isolates further revealed unexpected structural alterations in major cell surface glycolipids known as the glycopeptidolipids (GPLs). Besides showing an increase in triglycosylated forms of these lipids, CF sputum- and synthetic CF medium-grown isolates presented as yet unknown forms of GPLs representing as much as 10% to 20% of the total GPL content of the cells, in which the classical amino alcohol located at the carboxy terminal of the peptide, alaninol, is replaced with the branched-chain amino alcohol leucinol. Importantly, both these lipid changes were exacerbated by the presence of mucin in the culture medium. Collectively, our results reveal potential new drug targets against M. abscessus in the CF airway and point to mucin as an important host signal modulating the cell surface composition of this pathogen.

Spontaneous sputum conversion and reversion in Mycobacterium abscessus complex lung disease

Background

We aim to investigate the rate of spontaneous sputum conversion and reversion in patients with Mycobacterium abscessus complex (MABC) lung disease.

Methods

Among 241 patients diagnosed with MABC lung disease between July 2012 and December 2018, 126 patients with persistent sputum positivity for ≥ 6 months without treatment were enrolled at a tertiary referral center in South Korea. Patients were subdivided into two groups, depending on whether or not treatment was initiated within 2 years of diagnosis. The rates of spontaneous sputum culture conversion and reversion was investigated in patients who did not receive treatment within 2 years.

Results

The mean age of 126 patients was 62.9 years. During a mean follow-up duration of 3.2 years, 33 (26.2%) patients received treatment within 2 years of diagnosis. Among the remaining 93 patients not receiving treatment within 2 years, spontaneous sputum conversion occurred in 24 (25.8%) patients during a mean follow-up duration of 3.7 years after diagnosis. No significant differences were observed in time to conversion between Mycobacterium abscessus and Mycobacterium massiliense lung diseases. The Cox regression analysis showed that malignancy as a comorbid disease and the lower number of lobes involved were independent predictors of spontaneous sputum conversion. After spontaneous sputum conversion, reversion occurred in 27.8% patients at a median of 18.2 months after conversion.

Conclusions

Among patients with MABC lung disease who did not receive treatment for at least 2 years after diagnosis, approximately one-fourth experienced spontaneous conversion. However, not a few patients experienced reversion after spontaneous conversion.

The Peroxisome Proliferator-Activated Receptor α- Agonist Gemfibrozil Promotes Defense Against Mycobacterium abscessus Infections

Peroxisome proliferator-activated receptor α (PPARα) shows promising potential to enhance host defenses against Mycobacterium tuberculosis infection. Herein we evaluated the protective effect of PPARα against nontuberculous mycobacterial (NTM) infections. Using a rapidly growing NTM species, Mycobacterium abscessus (Mabc), we found that the intracellular bacterial load and histopathological damage were increased in PPARα-null mice in vivo. In addition, PPARα deficiency led to excessive production of proinflammatory cytokines and chemokines after infection of the lung and macrophages. Notably, administration of gemfibrozil (GEM), a PPARα activator, significantly reduced the in vivo Mabc load and inflammatory response in mice. Transcription factor EB was required for the antimicrobial response against Mabc infection. Collectively, these results suggest that manipulation of PPARα activation has promising potential as a therapeutic strategy for NTM disease.

Synergistic Efficacy of β-Lactam Combinations against Mycobacterium abscessus Pulmonary Infection in Mice

Mycobacterium abscessus is an emerging pathogen capable of causing invasive pulmonary infections in patients with chronic lung diseases. These infections are difficult to treat, necessitating prolonged multidrug therapy, which is further complicated by extensive intrinsic and acquired resistance exhibited by clinical M. abscessus isolates. Therefore, development of novel treatment regimens effective against drug-resistant strains is crucial. Prior studies have demonstrated synergistic efficacy of several β-lactams against M. abscessus in vitro; however, these combinations have never been tested in an animal model of M. abscessus pulmonary disease. We utilized a recently developed murine system of sustained M. abscessus lung infection delivered via an aerosol route to test the bactericidal efficacy of four novel dual β-lactam combinations and one β-lactam/β-lactamase inhibitor combination. All five of the novel combinations exhibited synergy and resulted in at least 6-log₁₀ reductions in bacterial burden in the lungs of mice at 4 weeks compared to untreated controls (P = 0.038).

Mycobacterium avium Infection in a C3HeB/FeJ Mouse Model

Infections caused by Mycobacterium avium complex (MAC) species are increasing worldwide, resulting in a serious public health problem. Patients with MAC lung disease face an arduous journey of a prolonged multidrug regimen that is often poorly tolerated and associated with relatively poor outcome. Identification of new animal models that demonstrate a similar pulmonary pathology as humans infected with MAC has the potential to significantly advance our understanding of nontuberculosis mycobacteria (NTM) pathogenesis as well as provide a tractable model for screening candidate compounds for therapy. One new mouse model is the C3HeB/FeJ which is similar to MAC patients in that these mice can form foci of necrosis in granulomas. In this study, we evaluated the ability of C3HeB/FeJ mice exposure to an aerosol infection of a rough strain of MAC 2285 to produce a progressive infection resulting in small necrotic foci during granuloma formation. C3HeB/FeJ mice were infected with MAC and demonstrated a progressive lung infection resulting in an increase in bacterial burden peaking around day 40, developed micronecrosis in granulomas and was associated with increased influx of CD4⁺ Th1, Th17, and Treg lymphocytes into the lungs. However, during chronic infection around day 50, the bacterial burden plateaued and was associated with the reduced influx of CD4⁺ Th1, Th17 cells, and increased numbers of Treg lymphocytes and necrotic foci during granuloma formation. These results suggest the C3HeB/FeJ MAC infection mouse model will be an important model to evaluate immune pathogenesis and compound efficacy.

The complexities and challenges of preventing and treating nontuberculous mycobacterial diseases

Seemingly innocuous nontuberculous mycobacteria (NTM) species, classified by their slow or rapid growth rates, can cause a wide range of illnesses, from skin ulceration to severe pulmonary and disseminated disease. Despite their worldwide prevalence and significant disease burden, NTM do not garner the same financial or research focus as Mycobacterium tuberculosis. In this review, we outline the most abundant of over 170 NTM species and inadequacies of diagnostics and treatments and weigh the advantages and disadvantages of currently available in vivo animal models of NTM. In order to effectively combat this group of mycobacteria, more research focused on appropriate animal models of infection, screening of chemotherapeutic compounds, and development of anti-NTM vaccines and diagnostics is urgently needed.

Mycobacterium abscessus subsp. massiliense mycma_0076 and mycma_0077 Genes Code for Ferritins That Are Modulated by Iron Concentration

Mycobacterium abscessus complex has been characterized in the last decade as part of a cluster of mycobacteria that evolved from an opportunistic to true human pathogen; however, the factors responsible for pathogenicity are still undefined. It appears that the success of mycobacterial infection is intrinsically related with the capacity of the bacteria to regulate intracellular iron levels, mostly using iron storage proteins. This study evaluated two potential M. abscessus subsp. massiliense genes involved in iron storage. Unlike other opportunist or pathogenic mycobacteria studied, M. abscessus complex has two genes similar to ferritins from M. tuberculosis (Rv3841), and in M. abscessus subsp. massiliense, those genes are annotated as mycma_0076 and mycma_0077. Molecular dynamic analysis of the predicted expressed proteins showed that they have a ferroxidase center. The expressions of mycma_0076 and mycma_0077 genes were modulated by the iron levels in both in vitro cultures as well as infected macrophages. Structural studies using size-exclusion chromatography, circular dichroism spectroscopy and dynamic light scattering showed that r0076 protein has a structure similar to those observed in the ferritin family. The r0076 forms oligomers in solution most likely composed of 24 subunits. Functional studies with recombinant proteins, obtained from heterologous expression of mycma_0076 and mycma_0077 genes in Escherichia coli, showed that both proteins were capable of oxidizing Fe2+ into Fe3+, demonstrating that these proteins have a functional ferroxidase center. In conclusion, two ferritins proteins were shown, for the first time, to be involved in iron storage in M. abscessus subsp. massiliense and their expressions were modulated by the iron levels.

Pathogen Box screening for hit identification against Mycobacterium abscessus

Mycobacterium abscessus is a rapidly growing life-threatening mycobacterium with multiple drug-resistance mechanisms. However, there is no official regimen for M. abscessus therapy. In this study, we screened the Pathogen Box, which contains 400 drug-like molecules active against neglected diseases, to identify active molecules targeting Mycobacterium abscessus using resazurin live/dead assays. In this screening assay, the Z-factor was 0.7, as an indicator of the statistical confidence of the assay. A cut-off of 80% growth inhibition in the screening resulted in the identification of four different compounds at a single concentration (20 μM). Dose-response curves identified three different hit candidates, i.e., MMV688508, MMV688844, and MMV688845, which generated good inhibitory curves. All hit candidates were expected to have different molecular targets. Among them, MMV688844 showed the best minimum inhibitory concentration value for not only wild-type M. abscessus but also for nine different R and S morphotype clinical isolates. Thus, we found that MMV688844, identified from the Pathogen Box screen, may be a promising candidate in the M. abscessus drug discovery pipeline.

Neutrophil killing of Mycobacterium abscessus by intra- and extracellular mechanisms

Mycobacterium abscessus, a rapidly growing nontuberculous mycobacterium, are increasingly present in soft tissue infections and chronic lung diseases, including cystic fibrosis, and infections are characterized by growth in neutrophil-rich environments. M. abscessus is observed as two distinct smooth and rough morphotypes. The environmental smooth morphotype initiates infection and has a relatively limited ability to activate neutrophils. The rough morphotype has increased virulence and immunogenicity. However, the neutrophil response to the rough morphotype has not been explored. Killing of the smooth and rough strains, including cystic fibrosis clinical isolates, was equivalent. Neutrophil uptake of M. abscessus was similar between morphotypes. Mechanistically, both rough and smooth morphotypes enhanced neutrophil reactive oxygen species generation but inhibition of NADPH oxidase activity did not affect M. abscessus viability. However, inhibition of phagocytosis and extracellular traps reduced killing of the smooth morphotype with lesser effects against the rough morphotype. Neutrophils treated with M. abscessus released a heat-labile mycobactericidal activity against the rough morphotype, but the activity was heat-tolerant against the smooth morphotype. Overall, M. abscessus stimulates ineffective neutrophil reactive oxygen species generation, and key mechanisms differ in killing of the smooth (phagocytosis-dependent, extracellular traps, and heat-tolerant secreted factor) and rough (extracellular traps and a heat-labile secreted factor) morphotypes. These studies represent an essential advancement in understanding the host response to M. abscessus, and help explain the recalcitrance of infection.

Survival of pathogenic Mycobacterium abscessus subsp. massiliense in Acanthamoeba castellanii

We used an amoeba model to study the intracellular growth and cytotoxicity of clinical strains of Mycobacterium abscessus subsp. massiliense (Mabsm) isolated from 2 patients (one with cystic fibrosis, the other one with idiopathic bronchiectasis) during the early (smooth colonies) and late stage (rough colonies) of chronic pulmonary infection. Acanthamoeba castellanii were infected with Mabsm (MOI 100) and samples collected every 24 h for 72 h. Results showed Mabsm is able to survive in trophozoites and persist in cysts for at least 7 days. Late Mabsm demonstrated higher cytotoxicity toward A. castellanii when compared to early strains. A. castellanii is a useful in vitro host model to study infection of Mabsm clinical isolates.

Activity of LCB01-0371, a Novel Oxazolidinone, against Mycobacterium abscessus

Mycobacterium abscessus is a highly pathogenic drug-resistant rapidly growing mycobacterium. In this study, we evaluated the in vitro, intracellular, and in vivo activities of LCB01-0371, a novel and safe oxazolidinone derivative, for the treatment of M. abscessus infection and compared its resistance to that of other oxazolidinone drugs. LCB01-0371 was effective against several M. abscessus strains in vitro and in a macrophage model of infection. In the murine model, a similar efficacy to linezolid was achieved, especially in the lungs. We induced laboratory-generated resistance to LCB01-0371; sequencing analysis revealed mutations in rplC of T424C and G419A and a nucleotide insertion at the 503 position. Furthermore, LCB01-0371 inhibited the growth of amikacin-, cefoxitin-, and clarithromycin-resistant strains. Collectively, our data indicate that LCB01-0371 might represent a promising new class of oxazolidinones with improved safety, which may replace linezolid for the treatment of M. abscessus.

The role of hydrophobicity in tuberculosis evolution and pathogenicity

The evolution of tubercle bacilli parallels a route from environmental Mycobacterium kansasii, through intermediate "Mycobacterium canettii", to the modern Mycobacterium tuberculosis complex. Cell envelope outer membrane lipids change systematically from hydrophilic lipooligosaccharides and phenolic glycolipids to hydrophobic phthiocerol dimycocerosates, di- and pentaacyl trehaloses and sulfoglycolipids. Such lipid changes point to a hydrophobic phenotype for M. tuberculosis sensu stricto. Using Congo Red staining and hexadecane-aqueous buffer partitioning, the hydrophobicity of rough morphology M. tuberculosis and Mycobacterium bovis strains was greater than smooth "M. canettii" and M. kansasii. Killed mycobacteria maintained differential hydrophobicity but defatted cells were similar, indicating that outer membrane lipids govern overall hydrophobicity. A rough M. tuberculosis H37Rv ΔpapA1 sulfoglycolipid-deficient mutant had significantly diminished Congo Red uptake though hexadecane-aqueous buffer partitioning was similar to H37Rv. An M. kansasii, ΔMKAN27435 partially lipooligosaccharide-deficient mutant absorbed marginally more Congo Red dye than the parent strain but was comparable in partition experiments. In evolving from ancestral mycobacteria, related to "M. canettii" and M. kansasii, modern M. tuberculosis probably became more hydrophobic by increasing the proportion of less polar lipids in the outer membrane. Importantly, such a change would enhance the capability for aerosol transmission, affecting virulence and pathogenicity.

Mycobacterial Characteristics and Treatment Outcomes in Mycobacterium abscessus Lung Disease

Background

Treatment outcomes of patients with Mycobacterium abscessus subspecies abscessus lung disease are poor, and the microbial characteristics associated with treatment outcomes have not been studied systematically. The purpose of this study was to identify associations between microbial characteristics and treatment outcomes in patients with M. abscessus lung disease.

Methods

Sixty-seven consecutive patients with M. abscessus lung disease undergoing antibiotic treatment for ≥12 months between January 2002 and December 2012 were included. Morphotypic and genetic analyses were performed on isolates from 44 patients.

Results

Final sputum conversion to culture negative occurred in 34 (51%) patients. Compared to isolates from 24 patients with persistently positive cultures, pretreatment isolates from 20 patients with final negative conversion were more likely to exhibit smooth colonies (9/20, 45% vs 2/24, 8%; P = .020), susceptibility to clarithromycin (7/20, 35% vs 1/24, 4%; P = .015), and be of the C28 sequevar with regard to the erm(41) gene (6/20, 30% vs 1/24, 4%; P = .035). Mycobacterium abscessus lung disease recurred in 5 (15%) patients after successful completion of antibiotic therapy. Genotypic analysis revealed that most episodes (22/24, 92%) of persistently positive cultures during antibiotic treatment and all cases of microbiologic recurrence after treatment completion were caused by different M. abscessus genotypes within a patient.

Conclusions

Precise identification to the subspecies level and analysis of mycobacterial characteristics could help predict treatment outcomes in patients with M. abscessus lung disease. Treatment failures and recurrences are frequently associated with multiple genotypes, suggesting reinfection.

Clinical Trials Registration

NCT00970801.

Mycobacterium fortuitum induces A20 expression that impairs macrophage inflammatory responses

Mycobacterium fortuitum is a rapidly growing mycobacterium that has been regarded as an etiological agent of a variety of human infections. However, little is known about the host inflammatory responses and the molecular mechanisms by which MF-induced inflammation is regulated in macrophages. In this study, we report that MF infection leads to the induction of an anti-inflammatory molecule, A20 (also known as TNFAIP3), which is essential for the regulation of MF-induced inflammatory responses in murine bone marrow-derived macrophages (BMDMs). MF triggered the expression of tumor necrosis factor-α and interleukin-6 in BMDMs through signaling of the Toll-like receptor 2 (TLR2)-myeloid differentiation primary response gene 88. Additionally, MF rapidly induced the expression of A20, which inhibited proinflammatory cytokine expression and nuclear factor (NF)-κB reporter gene activities in BMDMs. Notably, MF-induced activation of NF-κB signaling was required for A20 expression and proinflammatory responses in BMDMs. Furthermore, the rough morphotype of the MF clinical strain induced a higher level of proinflammatory signaling activation, but less A20 induction in BMDMs, compared to the smooth morphotype. Taken together, these results suggest that MF-induced activation of host proinflammatory responses is negatively regulated through TLR2-dependent A20 expression.

Clonal Diversification and Changes in Lipid Traits and Colony Morphology in Mycobacterium abscessus Clinical Isolates

The smooth-to-rough colony morphology shift in Mycobacterium abscessus has been implicated in loss of glycopeptidolipid (GPL), increased pathogenicity, and clinical decline in cystic fibrosis (CF) patients. However, the evolutionary phenotypic and genetic changes remain obscure. Serial isolates from nine non-CF patients with persistent M. abscessus infection were characterized by colony morphology, lipid profile via thin-layer chromatography and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), sequencing of eight genes in the GPL locus, and expression level of fadD23, a key gene involved in the biosynthesis of complex lipids. All 50 isolates were typed as M. abscessus subspecies abscessus and were clonally related within each patient. Rough isolates, all lacking GPL, predominated at later disease stages, some showing variation within rough morphology. While most (77%) rough isolates harbored detrimental mutations in mps1 and mps2, 13% displayed previously unreported mutations in mmpL4a and mmpS4, the latter yielding a putative GPL precursor. Two isolates showed no deleterious mutations in any of the eight genes sequenced. Mixed populations harboring different GPL locus mutations were detected in 5 patients, demonstrating clonal diversification, which was likely overlooked by conventional acid-fast bacillus (AFB) culture methods. Our work highlights applications of MALDI-TOF MS beyond identification, focusing on mycobacterial lipids relevant in virulence and adaptation. Later isolates displayed accumulation of triacylglycerol and reduced expression of fadD23, sometimes preceding rough colony onset. Our results indicate that clonal diversification and a shift in lipid metabolism, including the loss of GPL, occur during chronic lung infection with M. abscessus. GPL loss alone may not account for all traits associated with rough morphology.