Dual β-lactams for the treatment of Mycobacterium abscessus: a review of the evidence and a call to act against an antibiotic nightmare

Mycobacterium abscessus complex is a group of rapidly growing non-tuberculous mycobacteria (NTM), increasingly emerging as opportunistic pathogens. Current treatment options for these microorganisms are limited and associated with a high rate of treatment failure, toxicity and recurrence. In search of new therapeutic strategies, interest has grown in dual β-lactam (DBL) therapy, as research recently discovered that M. abscessus cell wall synthesis is mainly regulated by two types of enzymes (d,d-transpeptidases and l,d-transpeptidases) differently susceptible to inhibition by distinct β-lactams. In vitro studies testing several DBL combinations have shown synergy in extracellular broth cultures as well as in the intracellular setting: cefoxitin/imipenem, ceftaroline/imipenem, ceftazidime/ceftaroline and ceftazidime/imipenem. The addition of specific β-lactamase inhibitors (BLIs) targeting M. abscessus β-lactamase did not significantly enhance the activity of DBL combinations. However, in vivo data are lacking. We reviewed the literature on DBL/DBL-BLI-based therapies for M. abscessus infections to raise greater attention on this promising yet overlooked treatment option and to guide future preclinical and clinical studies.

Emerging and alternative strategies for the treatment of nontuberculous mycobacterial infections

INTRODUCTION

Nontuberculous mycobacteria (NTM) infections have emerged as a significant clinical challenge due to their intrinsic multidrug resistance and the limited efficacy of existing treatments. These infections are becoming increasingly prevalent, with a need for new and effective therapeutic strategies.

AREAS COVERED

This review addresses several key aspects of NTM infections: i) pathogenesis and epidemiology; ii) the limitations and challenges of current treatment options; iii) emerging and alternative therapeutic strategies; iv) advanced drug delivery systems such as nanoparticles and efflux pump inhibitors; v) innovative antibacterial alternatives like antimicrobial peptides, bacteriophage therapy, and phytochemicals; and vi) other potential treatment modalities such as inhaled nitric oxide, small molecules, surgical debridement, phototherapy, and immunomodulatory therapy.

EXPERT OPINION

Personalized medicine, advanced drug delivery systems, and alternative therapies hold promise for the future of NTM treatment. Early and accurate identification of NTM species, enabled by improved diagnostic methods, is critical for tailoring treatment regimens. Emerging therapies show promise against drug-resistant NTM strains, but overcoming barriers like clinical trials, regulatory hurdles, and high production costs is crucial. Continued research and innovation are essential to improve treatment efficacy and patient outcomes.

[A pharmacologic approach to treatment of Mycobacterium abscessus pulmonary disease]

Mycobacterium abscessus is a fast-growing non-tuberculous mycobacteria complex causing pulmonary infections, comprising the subspecies abscessus, massiliense and bolletii. Differences are based predominantly on natural inducible macrolide resistance, active in most Mycobacterium abscessus spp abscessus species and in Mycobacterium abscessus spp bolletii but inactive in Mycobacterium abscessus spp massiliense. Therapy consists in long-term treatment, combining multiple antibiotics. Prognosis is poor, as only 40% of patients experience cure. Pharmacodynamic and pharmacokinetic data on M. abscessus have recently been published, showing that therapy ineffectiveness might be explained by intrinsic bacterial resistance (macrolides…) and by the unfavorable pharmacokinetics of the recommended antibiotics. Other molecules and inhaled antibiotics are promising.

Clofazimine as a comparator for preclinical efficacy evaluations of experimental therapeutics against pulmonary M. abscessus infection in mice

Mycobacteroides abscessus (Mab, also known as Mycobacterium abscessus) can cause chronic pulmonary disease in the setting of structural lung conditions. Current treatment recommendations require at least one year of daily therapy with repurposed antibiotics. Yet these therapies are often ineffective and associated with significant adverse events. To address this challenge, research efforts are underway to develop new antibiotics and regimens. During the preclinical phase of treatment development, experimental agents require testing and comparison alongside positive controls that are known agents with clinical history. As there are no FDA approved treatments for this indication, here, we have considered repurposed antibiotics currently included in the recommendation for treating Mab disease as candidates for selection of an ideal standard comparator that can serve as a positive control in preclinical studies. Clofazimine meets the criteria for an ideal positive control as it can be administered via the least invasive route, requires only once-daily dosing, is well tolerated, and is widely available in high purity from independent sources. Using a mouse model of pulmonary Mab disease, we assessed for ideal dosages of clofazimine in C3HeB/FeJ and BALB/c mice in a six-week treatment window. Clofazimine, 25 mg/kg, once daily, produced desired reduction in Mab burden in the lungs of C3HeB/FeJ and BALB/c mice. Based on these findings, we conclude that clofazimine meets the criteria for a positive control comparator in mice for use in preclinical efficacy assessments of agents for treatment of Mab pulmonary disease. Although not included in the current standard-of-care for treating Mab disease, rifabutin, 20 mg/kg, also produced desired reduction in Mab lung burden in C3HeB/FeJ mice but not in BALB/c mice. IMPORTANCE: Mycobacteroides abscessus can cause life-threatening infections in patients with chronic lung conditions. New treatments are needed as cure rate using existing drugs is low. During pre-clinical phase of treatment development, it is important to compare the efficacy of the experimental drug against existing ones with known history. Here, we demonstrate that clofazimine, one of the antibiotics repurposed for treating Mab disease, can serve as a positive control comparator for efficacy assessments of experimental drugs and regimens to treat M. abscessus disease in mice.

Liposomal amikacin and Mycobacterium abscessus: intimate interactions inside eukaryotic cells

BACKGROUND

Mycobacterium abscessus (Mabs), a rapidly growing Mycobacterium species, is considered an MDR organism. Among the standard antimicrobial multi-drug regimens against Mabs, amikacin is considered as one of the most effective. Parenteral amikacin, as a consequence of its inability to penetrate inside the cells, is only active against extracellular mycobacteria. The use of inhaled liposomal amikacin may yield improved intracellular efficacy by targeting Mabs inside the cells, while reducing its systemic toxicity.

OBJECTIVES

To evaluate the colocalization of an amikacin liposomal inhalation suspension (ALIS) with intracellular Mabs, and then to measure its intracellular anti-Mabs activity.

METHODS

We evaluated the colocalization of ALIS with Mabs in eukaryotic cells such as macrophages (THP-1 and J774.2) or pulmonary epithelial cells (BCi-NS1.1 and MucilAir), using a fluorescent ALIS and GFP-expressing Mabs, to test whether ALIS reaches intracellular Mabs. We then evaluated the intracellular anti-Mabs activity of ALIS inside macrophages using cfu and/or luminescence.

RESULTS

Using confocal microscopy, we demonstrated fluorescent ALIS and GFP-Mabs colocalization in macrophages and epithelial cells. We also showed that ALIS was active against intracellular Mabs at a concentration of 32 to 64 mg/L, at 3 and 5 days post-infection. Finally, ALIS intracellular activity was confirmed when tested against 53 clinical Mabs isolates, showing intracellular growth reduction for nearly 80% of the isolates.

CONCLUSIONS

Our experiments demonstrate the intracellular localization and intracellular contact between Mabs and ALIS, and antibacterial activity against intracellular Mabs, showing promise for its future use for Mabs pulmonary infections.

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.

Efficacy and Mode of Action of a Direct Inhibitor of Mycobacterium abscessus InhA

There is an unmet medical need for effective treatments against Mycobacterium abscessus pulmonary infections, to which cystic fibrosis (CF) patients are particularly vulnerable. Recent studies showed that the antitubercular drug isoniazid is inactive against M. abscessus due to the incapacity of the catalase-peroxidase to convert the pro-drug into a reactive metabolite that inhibits the enoyl-ACP reductase InhA. To validate InhA_MAB as a druggable target in M. abscessus, we assayed the activity of NITD-916, a 4-hydroxy-2-pyridone lead candidate initially described as a direct inhibitor of InhA that bypasses KatG bioactivation in Mycobacterium tuberculosis. The compound displayed low MIC values against rough and smooth clinical isolates in vitro and significantly reduced the bacterial burden inside human macrophages. Moreover, treatment with NITD-916 reduced the number and size of intracellular mycobacterial cords, regarded as markers of the severity of the infection. Importantly, NITD-916 significantly lowered the M. abscessus burden in CF-derived lung airway organoids. From a mechanistic perspective, NITD-916 abrogated de novo synthesis of mycolic acids and NITD-916-resistant spontaneous mutants harbored point mutations in InhA_MAB at residue 96. That NITD-916 targets InhA_MAB directly without activation requirements was confirmed genetically and by resolving the crystal structure of the protein in complex with NADH and NITD-916. These findings collectively indicate that InhA_MAB is an attractive target to be exploited for future chemotherapeutic developments against this difficult-to-treat mycobacterium and highlight the potential of NITD-916 derivatives for further evaluation in preclinical settings.

In Vitro Bedaquiline and Clofazimine Susceptibility Testing in Mycobacterium abscessus

Bedaquiline and clofazimine are increasingly used to treat infections with Mycobacterium abscessus. We determined distributions of MICs by broth microdilution for bedaquiline and clofazimine for 61 M. abscessus clinical isolates using different media and incubation times. We show that incubation time and growth media critically influence the MIC. Our data will aid in defining future clinical breakpoints for in vitro susceptibility testing for bedaquiline and clofazimine in M. abscessus.

Omadacycline Potentiates Clarithromycin Activity Against Mycobacterium abscessus

Mycobacterium abscessus is a difficult respiratory pathogen to treat, when compared to other nontuberculus mycobacteria (NTM), due to its drug resistance. In this study, we aimed to find a new clarithromycin partner that potentiated strong, positive, synergy against M. abscessus among current anti-M. abscessus drugs, including omadacycline, amikacin, rifabutin, bedaquiline, and cefoxitine. First, we determined the minimum inhibitory concentrations required of all the drugs tested for M. abscessus subsp. abscessus CIP104536^T treatment using a resazurin microplate assay. Next, the best synergistic partner for clarithromycin against M. abscessus was determined using an in vitro checkerboard combination assay. Among the drug combinations evaluated, omadacycline showed the best synergistic effect with clarithromycin, with a fractional inhibitory concentration index of 0.4. This positive effect was also observed against M. abscessus clinical isolates and anti-M. abscessus drug resistant strains. Lastly, this combination was further validated using a M. abscessus infected zebrafish model. In this model, the clarithromycin-omadacyline regimen was found to inhibit the dissemination of M. abscessus, and it significantly extended the lifespan of the M. abscessus infected zebrafish. In summation, the synergy between two anti-M. abscessus compounds, clarithromycin and omadacycline, provides an attractive foundation for a new M. abscessus treatment regimen.

In Vitro Activity and Clinical Outcomes of Clofazimine for Nontuberculous Mycobacteria Pulmonary Disease

Limited data are available regarding the in vitro activity of clofazimine against nontuberculous mycobacteria (NTM) or on outcomes of clofazimine-containing regimens in NTM-pulmonary disease (PD). Therefore, we evaluated the in vitro activity of clofazimine and the clinical outcomes of clofazimine-containing regimens. We evaluated clofazimine in vitro activity for 303 NTM isolates from NTM-PD patients. Fifty-seven clarithromycin-resistant and 35 amikacin-resistant isolates were also analyzed. Culture conversion after a 12-month treatment regimen containing clofazimine was evaluated in 58 NTM-PD patients, including 20 patients with drug-resistant isolates. Most of the 303 isolates (238/303) had minimum inhibitory concentrations (MICs) ≤ 0.25 µg/mL for clofazimine (57/63 Mycobacterium avium, 53/57 M. intracellulare, 49/52 M. kansasii, 22/64 M. abscessus, and 57/67 M. massiliense). For the 57 clarithromycin-resistant and 35 amikacin-resistant isolates, most had MICs ≤ 0.25 µg/mL (47/57 and 32/35, respectively). Among the 38 NTM-PD patients without resistance to clarithromycin or amikacin, 47% achieved culture conversion (8/27 M. abscessus, 9/9 M. massiliense, 0/1 M. avium, and 1/1 M. intracellulare). The conversion rate was higher in the MIC ≤ 0.25 µg/mL group than in the MIC = 0.5 µg/mL group (13/18 vs. 5/20, p = 0.004), and an MIC ≤ 0.25 µg/mL remained a significant factor in multivariable analysis. Culture conversion was achieved in 20% of 20 patients with clarithromycin- or amikacin-resistant isolates. However, a clofazimine MIC ≤ 0.25 µg/mL was not significant for culture conversion in the 58 NTM-PD patients, regardless of the drug resistance pattern. Clofazimine was effective in vitro against NTM species. Some patients on clofazimine-containing regimens achieved culture conversion.

Current Molecular Therapeutic Agents and Drug Candidates for Mycobacterium abscessus

Mycobacterium abscessus has been recognised as a dreadful respiratory pathogen among the non-tuberculous mycobacteria (NTM) because of misdiagnosis, prolonged therapy with poor treatment outcomes and a high cost. This pathogen also shows extremely high antimicrobial resistance against current antibiotics, including the anti-tuberculosis agents. Therefore, current chemotherapies require a long curative period and the clinical outcomes are not satisfactory. Thus, there is an urgent need for discovering and developing novel, more effective anti-M. abscessus drugs. In this review, we sum the effectiveness of the current anti-M. abscessus drugs and drug candidates. Furthermore, we describe the shortcomings and difficulties associated with M. abscessus drug discovery and development.

Clofazimine susceptibility testing of Mycobacterium avium complex and Mycobacterium abscessus: a meta-analysis study

OBJECTIVES

The incidence of infections due to Mycobacterium avium complex (MAC) and Mycobacterium abscessus (MABS) is increasing worldwide. Current antimycobacterial agents are not sufficiently effective against nontuberculous mycobacteria (NTM) and there is a need for new drugs. This study aimed to estimate the overall in vitro activity of clofazimine (CFZ) against MAC and MABS clinical isolates.

METHODS

We systematically searched four databases up to 1 March 2020 to identify relevant studies. Studies were included if they used the Clinical and Laboratory Standards Institute (CLSI) criteria for drug susceptibility testing (DST). We assessed the pooled in vitro CFZ resistance rate in MAC and MABS clinical isolates using a random- effects model. Sources of heterogeneity were evaluated using Cochran's Q and the I² statistic. Potential for publication bias was explored using Begg's and Egger's tests. All analyses were conducted using Stata 14.0.

RESULTS

A total of 20 publications (11 reports for MAC and 15 for MABS) were included. The pooled rates of in vitro resistance to CFZ in clinical isolates of MAC and MABS were 9.0% [95% confidence interval (CI) 3.0-17.0%] and 16.0% (95% CI 4.0-34.0%), respectively. There was no evidence of publication bias.

CONCLUSION

This study reports the frequency of CFZ resistance in clinical isolates of MAC and MABS. According to the results, establishing accurate DST methods for detecting CFZ resistance, performing DST for all NTM isolates to provide effective treatment, and continuous monitoring of drug resistance are suggested for the prevention and control of CFZ-resistant NTM.

Rifabutin Is Bactericidal against Intracellular and Extracellular Forms of Mycobacterium abscessus

Mycobacterium abscessus is increasingly recognized as an emerging opportunistic pathogen causing severe lung diseases. As it is intrinsically resistant to most conventional antibiotics, there is an unmet medical need for effective treatments. Repurposing of clinically validated pharmaceuticals represents an attractive option for the development of chemotherapeutic alternatives against M. abscessus infections. In this context, rifabutin (RFB) has been shown to be active against M. abscessus and has raised renewed interest in using rifamycins for the treatment of M. abscessus pulmonary diseases. Here, we compared the in vitro and in vivo activity of RFB against the smooth and rough variants of M. abscessus, differing in their susceptibility profiles to several drugs and physiopathologial characteristics. While the activity of RFB is greater against rough strains than in smooth strains in vitro, suggesting a role of the glycopeptidolipid layer in susceptibility to RFB, both variants were equally susceptible to RFB inside human macrophages. RFB treatment also led to a reduction in the number and size of intracellular and extracellular mycobacterial cords. Furthermore, RFB was highly effective in a zebrafish model of infection and protected the infected larvae from M. abscessus-induced killing. This was corroborated by a significant reduction in the overall bacterial burden, as well as decreased numbers of abscesses and cords, two major pathophysiological traits in infected zebrafish. This study indicates that RFB is active against M. abscessus both in vitro and in vivo, further supporting its potential usefulness as part of combination regimens targeting this difficult-to-treat mycobacterium.

Dissecting the antibacterial activity of oxadiazolone-core derivatives against Mycobacterium abscessus

Mycobacterium abscessus (M. abscessus), a rapidly growing mycobacterium, is an emergent opportunistic pathogen responsible for chronic bronchopulmonary infections in individuals with respiratory diseases such as cystic fibrosis. Most treatments of M. abscessus pulmonary infections are poorly effective due to the intrinsic resistance of this bacteria against a broad range of antibiotics including anti-tuberculosis agents. Consequently, the number of drugs that are efficient against M. abscessus remains limited. In this context, 19 oxadiazolone (OX) derivatives have been investigated for their antibacterial activity against both the rough (R) and smooth (S) variants of M. abscessus. Several OXs impair extracellular M. abscessus growth with moderated minimal inhibitory concentrations (MIC), or act intracellularly by inhibiting M. abscessus growth inside infected macrophages with MIC values similar to those of imipenem. Such promising results prompted us to identify the potential target enzymes of the sole extra and intracellular inhibitor of M. abscessus growth, i.e., compound iBpPPOX, via activity-based protein profiling combined with mass spectrometry. This approach led to the identification of 21 potential protein candidates being mostly involved in M. abscessus lipid metabolism and/or in cell wall biosynthesis. Among them, the Ag85C protein has been confirmed as a vulnerable target of iBpPPOX. This study clearly emphasizes the potential of the OX derivatives to inhibit the extracellular and/or intracellular growth of M. abscessus by targeting various enzymes potentially involved in many physiological processes of this most drug-resistant mycobacterial species.

Alternative and Experimental Therapies of Mycobacterium abscessus Infections

Mycobacterium abscessus is a non-tuberculous mycobacterium notoriously known for causing severe, chronic infections. Treatment of these infections is challenging due to either intrinsic or acquired resistance of M. abscessus to multiple antibiotics. Despite prolonged poly-antimicrobial therapy, treatment of M. abscessus infections often fails, leading to progressive morbidity and eventual mortality. Great research efforts are invested in finding new therapeutic options for M. abscessus. Clofazimine and rifabutin are known anti-mycobacterial antibiotics, repurposed for use against M. abscessus. Novel antimicrobials active against M. abscessus include delamanid, pretomanid and PIPD1 and the recently approved beta-lactamase inhibitors avibactam, relebactam and vaborbactam. Previously unused antimicrobial combinations, e.g. vancomycin–clarithromycin and dual beta-lactam therapy, have been shown to have synergistic effect against M. abscessus in experimental models, suggesting their possible use in multiple-drug regimens. Finally, engineered phage therapy has been reported to be clinically successful in a severe case of disseminated M. abscessus infection. While many of these experimental therapeutics have shown activity against M. abscessus in vitro, as well as in intracellular and/or animal models, most have little if any evidence of effect in human infections. Clinical studies of M. abscesssus treatments are needed to reliably determine the value of their incorporation in therapeutic regimens.

Clofazimine: an old drug for never-ending diseases

Clofazimine (CFZ), an old hydrophobic riminophenazine, has a wide range of antimycobacterial activity ranging from leprosy to nontuberculous mycobacterial diseases. CFZ has several advantages such as a favorable pharmacokinetic profile, dose-dependent side effects as well as low price. In this narrative review, we have assessed the clinical development of CFZ, starting from the potential in vitro mechanism of actions, to the spectrum of side effects and potential drug-drug interactions, highlighting its current place in therapy and future possible use in leprosy, nontuberculous mycobacterial diseases and drug-resistant tuberculosis.

Synergistic Interactions of Indole-2-Carboxamides and β-Lactam Antibiotics against Mycobacterium abscessus

New drugs or therapeutic combinations are urgently needed against Mycobacterium abscessus Previously, we demonstrated the potent activity of indole-2-carboxamides 6 and 12 against M. abscessus We show here that these compounds act synergistically with imipenem and cefoxitin in vitro and increase the bactericidal activity of the β-lactams against M. abscessus In addition, compound 12 also displays synergism with imipenem and cefoxitin within infected macrophages. The clinical potential of these new drug combinations requires further evaluation.

State-of-the-art treatment strategies for nontuberculous mycobacteria infections

INTRODUCTION

Non-tuberculous Mycobacteria (NTM) are a group of organisms whose importance in medicine seems to be increasing in recent times. The increasing number of patients susceptible to these diseases make it necessary to expand our knowledge of therapeutic options and to explore future possibilities for the development of a therapeutic arsenal.

AREAS COVERED

In this review, the authors provide a brief introduction about the present importance of NTM and describe the present recommendations of the available guidelines for their treatment. They include a description of the future options for the management of these patients, especially focusing on new antibiotics. The authors also look at possibilities for future therapeutic options, such as antibiofilm strategies.

EXPERT OPINION

No actual changes have been made to the current recommendations for the management of most NTM infections (except perhaps the availability of nebulized amikacin). However, it is also true that we have increased the number of available antibiotic treatment options with good in vitro activity against NTM. The use of these drugs in selected cases could increase the therapeutic possibilities. However, some problems are still present, such as the knowledge of the actual meaning of a NTM isolate, and will probably be a key part of future research.

Active Benzimidazole Derivatives Targeting the MmpL3 Transporter in Mycobacterium abscessus

The prevalence of pulmonary infections due to nontuberculous mycobacteria such as Mycobacterium abscessus has been increasing and surpassing tuberculosis (TB) in some industrialized countries. Because of intrinsic resistance to most antibiotics that drastically limits conventional chemotherapeutic treatment options, new anti-M. abscessus therapeutics are urgently needed against this emerging pathogen. Extensive screening of a library of benzimidazole derivatives that were previously shown to be active against Mycobacterium tuberculosis led to the identification of a lead compound exhibiting very potent in vitro activity against a wide panel of M. abscessus clinical strains. Designated EJMCh-6, this compound, a 2-(2-cyclohexylethyl)-5,6-dimethyl-1H-benzo[d]imidazole), also exerted very strong activity against intramacrophage-residing M. abscessus. Moreover, the treatment of infected zebrafish embryos with EJMCh-6 was correlated with significantly increased embryo survival and a decrease in the bacterial burden as compared to those for untreated fish. Insights into the mechanism of action were inferred from the generation of spontaneous benzimidazole-resistant strains and the identification of a large set of missense mutations in MmpL3, the mycolic acid transporter in mycobacteria. Overexpression of the mutated mmpL3 alleles in a susceptible M. abscessus strain was associated with high resistance levels to EJMCh-6 and to other known MmpL3 inhibitors. Mapping the mutations conferring resistance on an MmpL3 three-dimensional homology model defined a potential EJMCh-6-binding cavity. These data emphasize a yet unexploited chemical structure class against M. abscessus with promising translational development for the treatment of M. abscessus lung diseases.

Naturally-Occurring Polymorphisms in QcrB Are Responsible for Resistance to Telacebec in Mycobacterium abscessus

Mycobacterium abscessus (M. abscessus) is a rapidly growing nontuberculous mycobacteria that is quickly emerging as a global health concern. M. abscessus pulmonary infections are frequently intractable due to the high intrinsic resistance to most antibiotics. Therefore, there is an urgent need to discover effective pharmacological options for M. abscessus infections. In this study, the potency of the antituberculosis drug Telacebec (Q203) was evaluated against M. abscessus. Q203 is a clinical-stage drug candidate targeting the subunit QcrB of the cytochrome bc₁:aa₃ terminal oxidase. We demonstrated that the presence of four naturally-occurring polymorphisms in the M. abscessus QcrB is responsible for the high resistance of the bacterium to Q203. Genetics reversion of the four polymorphisms sensitized M. abscessus to Q203. While this study highlights the limitation of a direct drug repurposing approach of Q203 and related drugs for M. abscessus infections, it reveals that the M. abscessus cytochrome bc₁:aa₃ respiratory branch is sensitive to chemical inhibition.

The TetR Family Transcription Factor MAB_2299c Regulates the Expression of Two Distinct MmpS-MmpL Efflux Pumps Involved in Cross-Resistance to Clofazimine and Bedaquiline in Mycobacterium abscessus

Mycobacterium abscessus is a human pathogen responsible for severe respiratory infections, particularly in patients with underlying lung disorders. Notorious for being highly resistant to most antimicrobials, new therapeutic approaches are needed to successfully treat M. abscessus-infected patients. Clofazimine (CFZ) and bedaquiline (BDQ) are two antibiotics used for the treatment of multidrug-resistant tuberculosis and are considered alternatives for the treatment of M. abscessus pulmonary disease. To get insights into their mechanisms of resistance in M. abscessus, we previously characterized the TetR transcriptional regulator MAB_2299c, which controls expression of the MAB_2300-MAB_2301 genes, encoding an MmpS-MmpL efflux pump. Here, in silico studies identified a second mmpS-mmpL (MAB_1135c-MAB_1134c) target of MAB_2299c. A palindromic DNA sequence upstream of MAB_1135c, sharing strong homology with the one located upstream of MAB_2300, was found to form a complex with the MAB_2299c regulator in electrophoretic mobility shift assays. Deletion of MAB_1135c-1134c in a wild-type strain led to increased susceptibility to both CFZ and BDQ. In addition, deletion of these genes in a CFZ/BDQ-susceptible mutant lacking MAB_2299c as well as MAB_2300-MAB_2301 further exacerbated the sensitivity of this strain to both drugs in vitro and inside macrophages. Overall, these results indicate that MAB_1135c-1134c encodes a new MmpS-MmpL efflux pump system involved in the intrinsic resistance to CFZ and BDQ. They also support the view that MAB_2299c controls the expression of two separate MmpS-MmpL efflux pumps, substantiating the importance of MAB_2299c as a marker of resistance to be considered when assessing drug susceptibility in clinical isolates.

The Contribution of Efflux Pumps in Mycobacterium abscessus Complex Resistance to Clarithromycin

The basis of drug resistance in Mycobacterium abscessus is still poorly understood. Nevertheless, as seen in other microorganisms, the efflux of antimicrobials may also play a role in M. abscessus drug resistance. Here, we investigated the role of efflux pumps in clarithromycin resistance using nine clinical isolates of M. abscessus complex belonging to the T28 erm(41) sequevar responsible for the inducible resistance to clarithromycin. The strains were characterized by drug susceptibility testing in the presence/absence of the efflux inhibitor verapamil and by genetic analysis of drug-resistance-associated genes. Efflux activity was quantified by real-time fluorometry. Efflux pump gene expression was studied by RT-qPCR upon exposure to clarithromycin. Verapamil increased the susceptibility to clarithromycin from 4- to ≥64-fold. The efflux pump genes MAB_3142 and MAB_1409 were found consistently overexpressed. The results obtained demonstrate that the T28 erm(41) polymorphism is not the sole cause of the inducible clarithromycin resistance in M.abscessus subsp. abscessus or bolletii with efflux activity providing a strong contribution to clarithromycin resistance. These data highlight the need for further studies on M. abscessus efflux response to antimicrobial stress in order to implement more effective therapeutic regimens and guidance in the development of new drugs against these bacteria.

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).

Mutations in the MAB_2299c TetR Regulator Confer Cross-Resistance to Clofazimine and Bedaquiline in Mycobacterium abscessus

New therapeutic approaches are needed against Mycobacterium abscessus, a respiratory mycobacterial pathogen that evades efforts to successfully treat infected patients. Clofazimine and bedaquiline, two drugs used for the treatment of multidrug-resistant tuberculosis, are being considered as alternatives for the treatment of lung diseases caused by M. abscessus With the aim to understand the mechanism of action of these agents in M. abscessus, we sought herein to determine the means by which M. abscessus can develop resistance. Spontaneous resistant strains selected on clofazimine, followed by whole-genome sequencing, identified mutations in MAB_2299c, encoding a putative TetR transcriptional regulator. Unexpectedly, mutants with these mutations were also cross-resistant to bedaquiline. MAB_2299c was found to bind to its target DNA, located upstream of the divergently oriented MAB_2300-MAB_2301 gene cluster, encoding MmpS/MmpL membrane proteins. Point mutations or deletion of MAB_2299c was associated with the concomitant upregulation of the mmpS and mmpL transcripts and accounted for this cross-resistance. Strikingly, deletion of MAB_2300 and MAB_2301 in the MAB_2299c mutant strain restored susceptibility to bedaquiline and clofazimine. Overall, these results expand our knowledge with respect to the regulatory mechanisms of the MmpL family of proteins and a novel mechanism of drug resistance in this difficult-to-treat respiratory mycobacterial pathogen. Therefore, MAB_2299c may represent an important marker of resistance to be considered in the treatment of M. abscessus diseases with clofazimine and bedaquiline in clinical settings.

Mycobacterium abscessus and β-Lactams: Emerging Insights and Potential Opportunities

β-lactams, the most widely used class of antibiotics, are well-tolerated, and their molecular mechanisms of action against many bacteria are well-documented. Mycobacterium abscessus (Mab) is a highly drug-resistant rapidly-growing nontuberculous mycobacteria (NTM). Only in recent years have we started to gain insight into the unique relationship between β-lactams and their targets in Mab. In this mini-review, we summarize recent findings that have begun to unravel the molecular basis for overall efficacy of β-lactams against Mab and discuss emerging evidence that indicates that we have yet to harness the full potential of this antibiotic class to treat Mab infections.

Clofazimine: A useful antibiotic for drug-resistant tuberculosis

Drug resistance is still the major threat to global tuberculosis (TB) control, and drug-resistant (DR) Mycobacterium tuberculosis (M. tuberculosis) strains have become the main challenge worldwide. Currently used antibiotics for treatment of DR-TB are often poorly tolerated and not sufficiently effective. Since the therapeutic options are still limited, the main strategy for treatment of DR-TB is to repurpose existing anti-mycobacterial agents. Clofazimine (CFZ) is one such drug that has recently attracted interest against DR-TB. CFZ is a hydrophobic riminophenazine that was initially synthesized as an anti-TB antibiotic. Although the mechanisms of action of CFZ are not yet entirely understood, it has been suggested that outer membrane is its primary action site, and the respiratory chain and ion transporters are the putative targets. In this review, we will discuss the anti-mycobacterial properties of CFZ, and provide new insights into the clinical use of this drug.

Novel Mutations Associated with Clofazimine Resistance in Mycobacterium abscessus

Mycobacterium abscessus is a major nontuberculous mycobacterial (NTM) pathogen and is responsible for about 80% of all pulmonary infections caused by rapidly growing mycobacteria. Clofazimine is an effective drug active against M. abscessus, but the mechanism of resistance to clofazimine in M. abscessus is unknown. To investigate the molecular basis of clofazimine resistance in M. abscessus, we isolated 29 M. abscessus mutants resistant to clofazimine and subjected them to whole-genome sequencing to identify possible mutations associated with clofazimine resistance. We found that mutations in the MAB_2299c gene (which encodes a possible transcriptional regulatory protein), MAB_1483, and MAB_0540 are most commonly associated with clofazimine resistance. In addition, mutations in MAB_0416c, MAB_4099c, MAB_2613, MAB_0409, and MAB_1426 were also associated with clofazimine resistance but less frequently. Two identical mutations which are likely to be polymorphisms unrelated to clofazimine resistance were found in MAB_4605c and MAB_4323 in 13 mutants. We conclude that mutations in MAB_2299c, MAB_1483, and MAB_0540 are the major mechanisms of clofazimine resistance in M. abscessus Future studies are needed to address the role of the identified mutations in clofazimine resistance in M. abscessus Our findings have implications for understanding mechanisms of resistance to clofazimine and for rapid detection of clofazimine resistance in this organism.

Teicoplanin - Tigecycline Combination Shows Synergy Against Mycobacterium abscessus

Lung disease caused by non-tuberculous mycobacteria (NTM), relatives of Mycobacterium tuberculosis, is increasing. M. abscessus is the most prevalent rapid growing NTM. This environmental pathogen is intrinsically resistant to most commonly used antibiotics, including anti-tuberculosis drugs. Current therapies take years to achieve cure, if cure if achieved. Thus, there is an urgent medical need to identify new, more efficacious treatments. Here, we explore the possibility of repurposing antibiotics developed for other indications. We asked whether novel two-drug combinations of clinically used antibiotics can be identified that show synergistic activity against this mycobacterium. An in vitro checkerboard titration assay was employed to test 180 dual combinations of 41 drugs against the clinical isolate M. abscessus Bamboo. The most attractive novel combination was further profiled against reference strains representing three sub-species (M. abscessus subsp. abscessus, massiliense and bolletii) and a collection of clinical isolates. This resulted in the identification of a novel synergistic antibiotic pair active against the M. abscessus complex: the glycopeptide teicoplanin with the glycylcycline tigecycline showed inhibitory activity at 2–3 μM (teicoplanin) and 1–2 μM (tigecycline). This novel combination can now be tested in M. abscessus animal models of infection and/or patients.

NTM drug discovery: status, gaps and the way forward

Incidence of pulmonary diseases caused by non-tuberculous mycobacteria (NTM), relatives of Mycobacterium tuberculosis, is increasing at an alarming rate, surpassing tuberculosis in many countries. Current chemotherapies require long treatment times and the clinical outcomes are often disappointing. There is an urgent medical need to discover and develop new, more-efficacious anti-NTM drugs. In this review, we summarize the current status of NTM drug development, and highlight knowledge gaps and scientific obstacles in NTM drug discovery. We propose strategies to reduce biological uncertainties and to begin to populate a NTM drug pipeline with attractive leads and drug candidates.

A Simple and Rapid Gene Disruption Strategy in Mycobacterium abscessus: On the Design and Application of Glycopeptidolipid Mutants

Little is known about the disease-causing genetic determinants that are used by Mycobacterium abscessus, increasingly acknowledged as an important emerging pathogen, notably in cystic fibrosis. The presence or absence of surface exposed glycopeptidolipids (GPL) conditions the smooth (S) or rough (R) M. abscessus subsp. abscessus (M. abscessus) variants, respectively, which are characterized by distinct infective programs. However, only a handful of successful gene knock-out and conditional mutants have been reported in M. abscessus, testifying that genetic manipulation of this mycobacterium is difficult. To facilitate gene disruption and generation of conditional mutants in M. abscessus, we have designed a one-step single cross-over system that allows the rapid and simple generation of such mutants. Cloning of as small as 300 bp of the target gene allows for efficient homologous recombination to occur without additional exogenous recombination-promoting factors. The presence of tdTomato on the plasmids allows easily sifting out the large background of mutants spontaneously resistant to antibiotics. Using this strategy in the S genetic background and the target gene mmpL4a, necessary for GPL synthesis and transport, nearly 100% of red fluorescent clones exhibited a rough morphotype and lost GPL on the surface, suggesting that most red fluorescent colonies obtained after transformation incorporated the plasmid through homologous recombination into the chromosome. This system was further exploited to generate another strain with reduced GPL levels to explore how the presence of these cell wall-associated glycolipids influences M. abscessus hydrophobicity as well as virulence in the zebrafish model of infection. This mutant exhibited a more pronounced killing phenotype in zebrafish embryos compared to its S progenitor and this effect correlated with the production of abscesses in the central nervous system. Overall, these results suggest that the near-complete absence of GPL on the bacterial surface is a necessary condition for optimal pathogenesis of this mycobacterium. They also suggest that GPL content affects hydrophobicity of M. abscessus, potentially altering the aerosol transmission, which is of particular importance from an epidemiological and clinical perspective.

Vancomycin and Clarithromycin Show Synergy against Mycobacterium abscessus In Vitro

Lung disease caused by Mycobacterium abscessus is increasing, and current clarithromycin-based treatment regimens are only moderately effective. Here, we determined the effect of clarithromycin-vancomycin combination against M. abscessus complex isolates in vitro. Synergy was found with a fractional inhibitory concentration index (FICI) score of ≤0.5 and a 4- to 10-fold decrease in MIC.

Bedaquiline Inhibits the ATP Synthase in Mycobacterium abscessus and Is Effective in Infected Zebrafish

Pulmonary infections caused by Mycobacterium abscessus are emerging as a global threat, especially in cystic fibrosis patients. Further intensifying the concern of M. abscessus infection is the recent evidence of human-to-human transmission of the infection. M. abscessus is a naturally multidrug-resistant fast-growing pathogen for which pharmacological options are limited. Repurposing antitubercular drugs represents an attractive option for the development of chemotherapeutic alternatives against M. abscessus infections. Bedaquiline (BDQ), an ATP synthase inhibitor, has recently been approved for the treatment of multidrug-resistant tuberculosis. Herein, we show that BDQ has a very low MIC against a vast panel of clinical isolates. Despite being bacteriostatic in vitro, BDQ was highly efficacious in a zebrafish model of M. abscessus infection. Remarkably, a very short period of treatment was sufficient to protect the infected larvae from M. abscessus-induced killing. This was corroborated with reduced numbers of abscesses and cords, considered to be major pathophysiological signs in infected zebrafish. Mode-of-action studies revealed that BDQ triggered a rapid depletion of ATP in M. abscessusin vitro, consistent with the drug targeting the F_oF₁ ATP synthase. Importantly, despite a failure to select in vitro for spontaneous mutants that are highly resistant to BDQ, the transfer of single nucleotide polymorphisms leading to D29V or A64P substitutions in atpE conferred high resistance, thus resolving the target of BDQ in M. abscessus Overall, this study indicates that BDQ is active against M. abscessusin vitro and in vivo and should be considered for clinical use against the difficult-to-manage M. abscessus pulmonary infections.

Mycobacterium abscessus Complex Infections in Children: A Review

PURPOSE OF REVIEW

Infections in children with Mycobacterium abscessus complex represent a particular challenge for clinicians. Increasing incidence of these infections worldwide has necessitated focused attention to improve both diagnostic as well as treatment modalities. Published medical literature was reviewed, with emphasis on material published in the past 5 years.

RECENT FINDINGS

Increasing availability of new diagnostic tools, such as matrix-assisted laser desorption ionization-time of flight mass spectrometry and custom PCRs, has provided unique insights into the subspecies within the complex and improved diagnostic certainty. Microbiological review of all recent isolates at the University of Minnesota Medical Center was also conducted, with description of the antimicrobial sensitivity patterns encountered in our center, and compared with those published from other centers in the recent literature. A discussion of conventional antimicrobial treatment regimens, alongside detailed description of the relevant antimicrobials, is derived from recent publications. Antimicrobial therapy, combined with surgical intervention in some cases, remains the mainstay of pediatric care. Ongoing questions remain regarding the transmission mechanics, immunologic vulnerabilities exploited by these organisms in the host, and the optimal antimicrobial regimens necessary to enable a reliable cure. Updated treatment guidelines based on focused clinical studies in children and accounting especially for the immunocompromised children at greatest risk are very much needed.

In vitro activity between linezolid and other antimicrobial agents against Mycobacterium abscessus complex

Linezolid (LZD) serves as an effective option in the treatment of Mycobacterium abscessus complex (MABC) infection. Unfortunately, the combined activities of LZD with other antimicrobial agents against MABC have not been evaluated systemically. In this study, we randomly selected 32 Mycobacterium abscessus and 32 Mycobacterium massiliense isolates for the determination of in vitro synergistic effect between LZD and other antimicrobial agents, including amikacin (AMK), moxifloxacin (MOX), cefoxitin (CFX) and tigecycline (TGC). Out of 64 MABC isolates tested, only one (1.6%, 1/64) and two (3.2%, 2/64) exhibited resistance to AMK and LZD, respectively. Statistical analysis revealed that the percentage of TGC-resistant isolates was significantly lower among M. massiliense (9.4%, 3/32) than that among M. abscessus (25.0%, 8/32, P<0.001). In addition, LZD and AMK showed synergy for 29 MABC isolates (45.3%), whereas no antagonism was noted for this combination. The second mostly frequent synergistic effect was found in LZD plus TGC combination, and 26.6% (17/64) of the strains tested exhibited synergy. In contrast, LZD-CFX and LZD-MOX combinations appeared antagonistic for half of the isolates (48.4%, 31/64 for CFX and 51.6%, 33/64), and almost no synergistic effect was reported in any of the strains for these two combinations. In conclusion, our data reveal that LZD and AMK show the most potent activity against MABC. The frequent synergism is observed in LZD-AMK and LZD-TGC combinations, while LZD rarely exhibits in vitro synergy with MOX and CFX when tested against MABC.

Clofazimine in Nontuberculous Mycobacterial Infections: A Growing Niche

Infection secondary to rapidly growing mycobacteria (RGM) is associated with significant morbidity and mortality, especially in individuals with underlying structural lung disease or immune compromise. Such infections, particularly those caused by the Mycobacterium abscessus group, are challenging to treat due to high virulence, antibiotic resistance, and the lack of effective and tolerable therapies. Although novel antimycobacterials are under development, clofazimine-a drug historically administered as part of multidrug therapy regimens for Mycobacterium leprae-holds promise as a chemotherapeutic for the treatment of RGM. The history, pharmacologic properties of clofazimine, as well as in vitro and in vivo studies against RGM are described here and highlight a potential new niche for an old drug.

Clofazimine-Containing Regimen for the Treatment of Mycobacterium abscessus Lung Disease

Patients with lung disease caused by Mycobacterium abscessus subsp. abscessus (here M. abscessus) typically have poor treatment outcomes. Although clofazimine (CFZ) has been increasingly used in the treatment of M. abscessus lung disease in clinical practice, there are no reported data on its effectiveness for this disease. This study sought to evaluate the clinical efficacy of a CFZ-containing regimen for the treatment of M. abscessus lung disease. We performed a retrospective review of the medical records of 42 patients with M. abscessus lung disease who were treated with CFZ-containing regimens between November 2013 and January 2015. CFZ was administered in combination with other antibiotics as an initial antibiotic regimen in 15 (36%) patients (initial treatment group), and it was added to an existing antibiotic regimen for refractory M. abscessus lung disease in 27 (64%) patients (salvage treatment group). Overall, there was an 81% treatment response rate based on symptoms and a 31% response rate based on radiographic findings. Conversion to culture-negative sputum samples was achieved in 10 (24%) patients after CFZ-containing antibiotic treatment, and during treatment, there were significant decreases in the positivity of semiquantitative sputum cultures for acid-fast bacilli in both the initial (P = 0.018) and salvage (P = 0.001) treatment groups. Our study suggests that CFZ-containing regimens may improve treatment outcomes in patients with M. abscessus lung disease and that a prospective evaluation of CFZ in M. abscessus lung disease is warranted.

Resistance to Thiacetazone Derivatives Active against Mycobacterium abscessus Involves Mutations in the MmpL5 Transcriptional Repressor MAB_4384

Available chemotherapeutic options are very limited against Mycobacterium abscessus, which imparts a particular challenge in the treatment of cystic fibrosis (CF) patients infected with this rapidly growing mycobacterium. New drugs are urgently needed against this emerging pathogen, but the discovery of active chemotypes has not been performed intensively. Interestingly, however, the repurposing of thiacetazone (TAC), a drug once used to treat tuberculosis, has increased following the deciphering of its mechanism of action and the detection of significantly more potent analogues. We therefore report studies performed on a library of 38 TAC-related derivatives previously evaluated for their antitubercular activity. Several compounds, including D6, D15, and D17, were found to exhibit potent activity in vitro against M. abscessus, Mycobacterium massiliense, and Mycobacterium bolletii clinical isolates from CF and non-CF patients. Similar to TAC in Mycobacterium tuberculosis, the three analogues act as prodrugs in M. abscessus, requiring bioactivation by the EthA enzyme, MAB_0985. Importantly, mutations in the transcriptional TetR repressor MAB_4384, with concomitant upregulation of the divergently oriented adjacent genes encoding an MmpS5/MmpL5 efflux pump system, accounted for high cross-resistance levels among all three compounds. Overall, this study uncovered a new mechanism of drug resistance in M. abscessus and demonstrated that simple structural optimization of the TAC scaffold can lead to the development of new drug candidates against M. abscessus infections.

Nontuberculous Mycobacteria in Cystic Fibrosis

Nontuberculous mycobacteria (NTM) are found in approximately 10 % of cystic fibrosis (CF) patients, but only a portion will develop NTM disease. The management of CF lung disease should be optimized, including antibiotic therapy targeted to the individual's usual airway bacteria, prior to considering treatment for NTM lung disease. Those who meet criteria for NTM lung disease may not necessarily require treatment and could be monitored expectantly if symptoms and radiographic findings are minimal. However, the presence of Mycobacterium abscessus complex (MABSC), severe lung disease, and/or anticipated lung transplant should prompt NTM therapy initiation. For CF patients with Mycobacterium avium complex (MAC), recommended treatment includes triple antibiotic therapy with a macrolide, rifampin, and ethambutol. Azithromycin is generally our preferred macrolide in CF as it is better tolerated and has fewer drug-drug interactions. MABSC treatment is more complex and requires an induction phase (oral macrolide and two IV agents including amikacin) as well as a maintenance phase (nebulized amikacin and two to three oral antibiotics including a macrolide). The induction phase may range from one to three months (depending on infection severity, treatment response, and medication tolerability). For both MAC and MABSC, treatment duration is extended 1-year post-culture conversion. However, in patients who do not achieve culture negative status but tolerate therapy, we consider ongoing treatment for mycobacterial suppression and prevention of disease progression.

Mechanisms of action and therapeutic efficacies of the lipophilic antimycobacterial agents clofazimine and bedaquiline

Drug-resistant (DR)-TB is the major challenge confronting the global TB control programme, necessitating treatment with second-line anti-TB drugs, often with limited therapeutic efficacy. This scenario has resulted in the inclusion of Group 5 antibiotics in various therapeutic regimens, two of which promise to impact significantly on the outcome of the therapy of DR-TB. These are the 're-purposed' riminophenazine, clofazimine, and the recently approved diarylquinoline, bedaquiline. Although they differ structurally, both of these lipophilic agents possess cationic amphiphilic properties that enable them to target and inactivate essential ion transporters in the outer membrane of Mycobacterium tuberculosis. In the case of bedaquiline, the primary target is the key respiratory chain enzyme F₁/F₀-ATPase, whereas clofazimine is less selective, apparently inhibiting several targets, which may underpin the extremely low level of resistance to this agent. This review is focused on similarities and differences between clofazimine and bedaquiline, specifically in respect of molecular mechanisms of antimycobacterial action, targeting of quiescent and metabolically active organisms, therapeutic efficacy in the clinical setting of DR-TB, resistance mechanisms, pharmacodynamics, pharmacokinetics and adverse events.

Infections caused by Mycobacterium abscessus: epidemiology, diagnostic tools and treatment

INTRODUCTION

Mycobacterium abscessus is an emerging mycobacteria that is responsible for lung diseases and healthcare-associated extrapulmonary infections. Recent findings support its taxonomic status as a single species comprising 3 subspecies designated abscessus, bolletii and massiliense. We performed a review of English-language publications investigating all three of these subspecies. Areas covered: Worldwide, human infections are often attributable to environmental contamination, although the isolation of M. abscessus in this reservoir is very rare. Basic research has demonstrated an association between virulence and cell wall components and cording, and genome analysis has identified gene transfer from other bacteria. The bacteriological diagnosis of M. abscessus is based on innovative tools combining molecular biology and mass spectrometry. Genotypic and phenotypic susceptibility testing are required to predict the success of macrolide (clarithromycin or azithromycin)-based therapeutic regimens. Genotyping methods are helpful to assess relapse and cross-transmission and to search for a common source. Treatment is not standardised, and outcomes are often unsatisfactory. Expert commentary: M. abscessus is still an open field in terms of clinical and bacteriological research. Further knowledge of its ecology and transmission routes, as well as host-pathogen interactions, is required. Because the number of human cases is increasing, it is also necessary to identify more active treatments and perform clinical trials to assess standard effective regimens.

Tigecycline Potentiates Clarithromycin Activity against Mycobacterium avium In Vitro

Thein vitroactivities of clarithromycin and tigecycline alone and in combination againstMycobacterium aviumwere assessed. The activity of clarithromycin was time dependent, highly variable, and often resulted in clarithromycin resistance. Tigecycline showed concentration-dependent activity, and mycobacterial killing could only be achieved at high concentrations. Tigecycline enhanced clarithromycin activity againstM. aviumand prevented clarithromycin resistance. Whether there is clinical usefulness of tigecycline in the treatment ofM. aviuminfections needs further study.

Update on pulmonary disease due to non-tuberculous mycobacteria

Non-tuberculous mycobacteria (NTM) are emerging worldwide as significant causes of chronic pulmonary infection, posing a number of challenges for both clinicians and researchers. While a number of studies worldwide have described an increasing prevalence of NTM pulmonary disease over time, population-based data are relatively sparse and subject to ascertainment bias. Furthermore, the disease is geographically heterogeneous. While some species are commonly implicated worldwide (Mycobacterium avium complex, Mycobacterium abscessus), others (e.g., Mycobacterium malmoense, Mycobacterium xenopi) are regionally important. Thoracic computed tomography, microbiological testing with identification to the species level, and local epidemiology must all be taken into account to accurately diagnose NTM pulmonary disease. A diagnosis of NTM pulmonary disease does not necessarily imply that treatment is required; a patient-centered approach is essential. When treatment is required, multidrug therapy based on appropriate susceptibility testing for the species in question should be used. New diagnostic and therapeutic modalities are needed to optimize the management of these complicated infections.

Systematic synergy modeling: understanding drug synergy from a systems biology perspective

Owing to drug synergy effects, drug combinations have become a new trend in combating complex diseases like cancer, HIV and cardiovascular diseases. However, conventional synergy quantification methods often depend on experimental dose–response data which are quite resource-demanding. In addition, these methods are unable to interpret the explicit synergy mechanism. In this review, we give representative examples of how systems biology modeling offers strategies toward better understanding of drug synergy, including the protein-protein interaction (PPI) network-based methods, pathway dynamic simulations, synergy network motif recognitions, integrative drug feature calculations, and “omic”-supported analyses. Although partially successful in drug synergy exploration and interpretation, more efforts should be put on a holistic understanding of drug-disease interactions, considering integrative pharmacology and toxicology factors. With a comprehensive and deep insight into the mechanism of drug synergy, systems biology opens a novel avenue for rational design of effective drug combinations.