Identification of Mycobacterium abscessus using the peaks of ribosomal protein L29, L30 and hemophore-related protein by MALDI-MS proteotyping

Mycobacteroides (Mycobacterium) abscessus, which causes a variety of infectious diseases in humans, is becoming detected more frequently in clinical specimens as cases are spreading worldwide. Taxonomically, M. abscessus is composed of three subspecies of M. abscessus subsp. abscessus, M. abscessus subsp. bolletii, and M. abscessus subsp. massiliense, with different susceptibilities to macrolides. In order to identify rapidly these three subspecies, we determined useful biomarker proteins, including ribosomal protein L29, L30, and hemophore-related protein, for distinguishing the subspecies of M. abscessus using the matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) profiles. Thirty-three clinical strains of M. abscessus were correctly identified at the subspecies-level by the three biomarker protein peaks. This study ultimately demonstrates the potential of routine MALDI-MS-based laboratory methods for early identification and treatment for M. abscessus infections.

Itaconic acid inhibits nontuberculous mycobacterial growth in pH dependent manner while 4-octyl-itaconic acid enhances THP-1 clearance of nontuberculous mycobacteria in vitro

Increasingly prevalent, nontuberculous mycobacteria (NTM) infections affect approximately 20% of people with cystic fibrosis (CF). Previous studies of CF sputum identified lower levels of the host metabolite itaconate in those infected with NTM. Itaconate can inhibit the growth of M. tuberculosis (MTB) in vitro via the inhibition of the glyoxylate cycle enzyme (ICL), but its impact on NTM is unclear. To test itaconic acid's (IA) effect on NTM growth, laboratory and CF clinical strains of Mycobacterium abscessus and Mycobacterium avium were cultured in 7H9 minimal media supplemented with 1-10 mM of IA and short-chain fatty acids (SCFA). M. avium and M. abscessus grew when supplemented with SCFAs, whereas the addition of IA (≥ 10 mM) completely inhibited NTM growth. NTM supplemented with acetate or propionate and 5 mM IA displayed slower growth than NTM cultured with SCFA and ≤ 1 mM of IA. However, IA's inhibition of NTM was pH dependent; as similar and higher quantities (100 mM) of pH adjusted IA (pH 7) did not inhibit growth in vitro, while in an acidic minimal media (pH 6.1), 1 to 5 mM of non-pH adjusted IA inhibited growth. None of the examined isolates displayed the ability to utilize IA as a carbon source, and IA added to M. abscessus isocitrate lyase (ICL) decreased enzymatic activity. Lastly, the addition of cell-permeable 4-octyl itaconate (4-OI) to THP-1 cells enhanced NTM clearance, demonstrating a potential role for IA/itaconate in host defense against NTM infections.

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

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

IMPORTANCE

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

Mycobacterium abscessus extracellular vesicles increase mycobacterial resistance to clarithromycin in vitro

Nontuberculous Mycobacteria (NTM) are a group of emerging bacterial pathogens that have been identified in cystic fibrosis (CF) patients with microbial lung infections. The treatment of NTM infection in CF patients is challenging due to the natural resistance of NTM species to many antibiotics. Mycobacterium abscessus is one of the most common NTM species found in the airways of CF patients. In this study, we characterized the extracellular vesicles (EVs) released by drug-sensitive M. abscessus untreated or treated with clarithromycin (CLR), one of the frontline anti-NTM drugs. Our data show that exposure to CLR increases mycobacterial protein trafficking into EVs as well as the secretion of EVs in culture. Additionally, EVs released by CLR-treated M. abscessus increase M. abscessus resistance to CLR when compared to EVs from untreated M. abscessus. Proteomic analysis further indicates that EVs released by CLR-treated M. abscessus carry an increased level of 50S ribosomal subunits, the target of CLR. Taken together, our results suggest that EVs play an important role in M. abscessus resistance to CLR treatment.

Pharmacological validation of dihydrofolate reductase as a drug target in Mycobacterium abscessus

The Mycobacterium abscessus drug development pipeline is poorly populated, with particularly few validated target-lead couples to initiate de novo drug discovery. Trimethoprim, an inhibitor of dihydrofolate reductase (DHFR) used for the treatment of a range of bacterial infections, is not active against M. abscessus. Thus, evidence that M. abscessus DHFR is vulnerable to pharmacological intervention with a small molecule inhibitor is lacking. Here, we show that the pyrrolo-quinazoline PQD-1, previously identified as a DHFR inhibitor active against Mycobacterium tuberculosis, exerts whole cell activity against M. abscessus. Enzyme inhibition studies showed that PQD-1, in contrast to trimethoprim, is a potent inhibitor of M. abscessus DHFR and over-expression of DHFR causes resistance to PQD-1, providing biochemical and genetic evidence that DHFR is a vulnerable target and mediates PQD-1's growth inhibitory activity in M. abscessus. As observed in M. tuberculosis, PQD-1 resistant mutations mapped to the folate pathway enzyme thymidylate synthase (TYMS) ThyA. Like trimethoprim in other bacteria, PQD-1 synergizes with the dihydropteroate synthase (DHPS) inhibitor sulfamethoxazole (SMX), offering an opportunity to exploit the successful dual inhibition of the folate pathway and develop similarly potent combinations against M. abscessus. PQD-1 is active against subspecies of M. abscessus and a panel of clinical isolates, providing epidemiological validation of the target-lead couple. Leveraging a series of PQD-1 analogs, we have demonstrated a dynamic structure-activity relationship (SAR). Collectively, the results identify M. abscessus DHFR as an attractive target and PQD-1 as a chemical starting point for the discovery of novel drugs and drug combinations that target the folate pathway in M. abscessus.

Pathological granuloma fibrosis induced by agar-embedded Mycobacterium abscessus in C57BL/6JNarl mice

Introduction

Pulmonary granuloma diseases caused by Mycobacterium abscessus (M. abscessus) have increased in past decades, and drug-resistance in this pathogen is a growing public health concern. Therefore, an animal model of chronic granuloma disease is urgently needed.

Methods

In this study, M. abscessus embedded within agar beads (agar-AB) was used to develop such a model in C57BL/6JNarl mice.

Results

Chronic infection was sustained for at least 3 months after agar-AB infection, visible granulomas spread in the lungs, and giant cells and foamy cells appeared in the granulomas. More importantly, pulmonary fibrosis progressed for 3 months, and collagen fibers were detected by Masson trichrome staining. Further, inducible nitric oxide synthase (iNOS) was highly expressed within the alveolar space, and the fibrosis-mediator transforming growth factor beta (TGF-β) began to be expressed at 1 month. Hypoxia-inducible factor (HIF-1α) expression also increased, which aided in normalizing oxygen partial pressure.

Discussion

Although the transient fibrosis persisted for only 3 months, and the pulmonary structure resolved when the pathogen was cleard, this pulmonary fibrosis model for M. abscessus infection will provide a novel test platform for development of new drugs, regimens, and therapies.

Hierarchy and interconnected networks in the WhiB7 mediated transcriptional response to antibiotic stress in Mycobacterium abscessus

Mycobacterium abscessus is intrinsically resistant to antibiotics effective against other pathogenic mycobacteria largely due to the drug-induced expression of genes that confer resistance. WhiB7 is a major hub controlling the induction of resistance to ribosome-targeting antibiotics. It activates the expression of >100 genes, 7 of which are known determinants of drug resistance; the function of most genes within the regulon is however unknown, but some conceivably encode additional mechanisms of resistance. Furthermore, the hierarchy of gene expression within the regulon, if any, is poorly understood. In the present work we have identified 56 WhiB7 binding sites using chromatin immunoprecipitation sequencing (CHIP-Seq) which accounts for the WhiB7-dependent upregulation of 72 genes, and find that M. abscessus WhiB7 functions exclusively as a transcriptional activator at promoters recognized by σA/σB. We have investigated the role of 18 WhiB7 regulated genes in drug resistance. Our results suggest that while some genes within the regulon (eg. erm41, hflX, eis2 and the ABCFs) play a major role in resistance, others make smaller contributions (eg. MAB_4324c and MAB_1409c) and the observed hypersensitivity ΔMabwhiB7 is a cumulative effect of these individual contributions. Moreover, our CHIP-Seq data implicate additional roles of WhiB7 induced genes beyond antibiotic resistance. Finally, we identify a σH-dependent network in aminoglycoside and tigecycline resistance which is induced upon drug exposure and is further activated by WhiB7 demonstrating the existence of a crosstalk between components of the WhiB7-dependent and -independent circuits.

Impaired ESX-3 Induces Bedaquiline Persistence in Mycobacterium abscessus Growing Under Iron-Limited Conditions

ESX-3 is a secretion pathway which is essential for mycobactin-mediated iron acquisition under iron-limited conditions. Although present in all Mycobacterium sp., ESX-3 remains to be elucidated in Mycobacterium abscessus. In the study reported here, impaired ESX-3 seriously restricts the growth of M. abscesses under iron-limited conditions; growth is salvaged by functional ESX-3 or iron supplementation. Notably, impaired ESX-3 does not kill M. abscesses when environmental iron is insufficient but induces persistence to bedaquiline, a diarylquinoline class antibiotic used to treat multidrug-resistant mycobacteria. One potential mechanism contributing to persistence is the iron deficiency due to impaired ESX-3 suppressing succinate dehydrogenase activity, which dysregulates the tricarboxylic acid cycle and inactivates bedaquiline. Experiments conducted here also demonstrate that the regulator, MtrA, can bind ESX-3 and promote the survival of M. abscessus. As such, this study suggests that a novel pathway involving MtrA, ESX-3, iron metabolism, and the TCA cycle contributes to bedaquiline persistence in M. abscesses growing under iron-limited conditions.

New therapeutic strategies for Mycobacterium abscessus pulmonary diseases - untapping the mycolic acid pathway

INTRODUCTION

Treatment options against Mycobacterium abscessus infections are very limited. New compounds are needed to cure M. abscessus pulmonary diseases. While the mycolic acid biosynthetic pathway has been largely exploited for the treatment of tuberculosis, this metabolic process has been overlooked in M. abscessus, although it offers many potential drug targets for the treatment of this opportunistic pathogen.

AREAS COVERED

Herein, the authors review the role of the MmpL3 membrane protein and the enoyl-ACP reductase InhA involved in the transport and synthesis of mycolic acids, respectively. They discuss their importance as two major vulnerable drug targets in M. abscessus and report the activity of MmpL3 and InhA inhibitors. In particular, they focus on NITD-916, a direct InhA inhibitor against M. abscessus, particularly warranted in the context of multidrug resistance.

EXPERT OPINION

There is an increasing body of evidence validating the mycolic acid pathway as an attractive drug target to be further exploited for M. abscessus lung disease treatments. The NITD-916 studies provide a proof-of-concept that direct inhibitors of InhA are efficient in vitro, in macrophages and in zebrafish. Future work is now required to improve the activity and pharmacological properties of these inhibitors and their evaluation in pre-clinical models.

Crystal structure of MAB_4123, a putative flavin-dependent monooxygenase from Mycobacterium abscessus

Numerous bacteria from different phylae can perform desulfurization reactions of organosulfur compounds. In these degradation or detoxification pathways, two-component flavin-dependent monooxygenases that use flavins (FMN or FAD) as a cofactor play important roles as they catalyse the first steps of these metabolic routes. The TdsC or DszC and MsuC proteins belong to this class of enzymes as they process dibenzothiophene (DBT) and methanesulfinate. Elucidation of their X-ray structures in apo, ligand-bound and cofactor-bound forms has provided important molecular insights into their catalytic reaction. Mycobacterial species have also been shown to possess a DBT degradation pathway, but no structural information is available on these two-component flavin-dependent monooxygenases. In this study, the crystal structure of the uncharacterized MAB_4123 protein from the human pathogen Mycobacterium abscessus is presented. The structure solved at high resolution displays high similarity to homologs from Rhodococcus, Paenibacillus and Pseudomonas species. In silico docking approaches suggest that MAB_4123 binds FMN and may use it as a cofactor. Structural analysis strongly suggests that MAB_4123 is a two-component flavin-dependent monooxygenase that could act as a detoxifying enzyme of organosulfur compounds in mycobacteria.

Unveiling the Biosynthetic Pathway for Short Mycolic Acids in Nontuberculous Mycobacteria: Mycobacterium smegmatis MSMEG_4301 and Its Ortholog Mycobacterium abscessus MAB_1915 Are Essential for the Synthesis of α'-Mycolic Acids

Mycolic acids, a hallmark of the genus Mycobacterium, are unique branched long-chain fatty acids produced by a complex biosynthetic pathway. Due to their essentiality and involvement in various aspects of mycobacterial pathogenesis, the synthesis of mycolic acids-and the identification of the enzymes involved-is a valuable target for drug development. Although most of the core pathway is comparable between species, subtle structure differences lead to different structures delineating the mycolic acid repertoire of tuberculous and some nontuberculous mycobacteria. We here report the characterization of an α'-mycolic acid-deficient Mycobacterium smegmatis mutant obtained by chemical mutagenesis. Whole-genome sequencing and bioinformatic analysis identified a premature stop codon in MSMEG_4301, encoding an acyl-CoA synthetase. Orthologs of MSMEG_4301 are present in all mycobacterial species containing α'-mycolic acids. Deletion of the Mycobacterium abscessus ortholog MAB_1915 abrogated synthesis of α'-mycolic acids; likewise, deletion of MSMEG_4301 in an otherwise wild-type M. smegmatis background also caused loss of these short mycolates. IMPORTANCE Mycobacterium abscessus is a nontuberculous mycobacterium responsible for an increasing number of hard-to-treat infections due to the impervious nature of its cell envelope, a natural barrier to several antibiotics. Mycolic acids are key components of that envelope; thus, their synthesis is a valuable target for drug development. Our results identify the first enzyme involved in α'-mycolic acids, a short-chain member of mycolic acids, loss of which greatly affects growth of this opportunistic pathogen.

Type I Interferons Are Involved in the Intracellular Growth Control of Mycobacterium abscessus by Mediating NOD2-Induced Production of Nitric Oxide in Macrophages

Mycobacterium abscessus (MAB) is one of the rapidly growing, multidrug-resistant non-tuberculous mycobacteria (NTM) causing various diseases including pulmonary disorder. Although it has been known that type I interferons (IFNs) contribute to host defense against bacterial infections, the role of type I IFNs against MAB infection is still unclear. In the present study, we show that rIFN-β treatment reduced the intracellular growth of MAB in macrophages. Deficiency of IFN-α/β receptor (IFNAR) led to the reduction of nitric oxide (NO) production in MAB-infected macrophages. Consistently, rIFN-β treatment enhanced the expression of iNOS gene and protein, and NO production in response to MAB. We also found that NO is essential for the intracellular growth control of MAB within macrophages in an inhibitor assay using iNOS-deficient cells. In addition, pretreatment of rIFN-β before MAB infection in mice increased production of NO in the lungs at day 1 after infection and promoted the bacterial clearance at day 5. However, when alveolar macrophages were depleted by treatment of clodronate liposome, rIFN-β did not promote the bacterial clearance in the lungs. Moreover, we found that a cytosolic receptor nucleotide-binding oligomerization domain 2 (NOD2) is required for MAB-induced TANK binding kinase 1 (TBK1) phosphorylation and IFN-β gene expression in macrophages. Finally, increase in the bacterial loads caused by reduction of NO levels was reversed by rIFN-β treatment in the lungs of NOD2-deficient mice. Collectively, our findings suggest that type I IFNs act as an intermediator of NOD2-induced NO production in macrophages and thus contribute to host defense against MAB infection.

Developing Tadpole Xenopus laevis as a Comparative Animal Model to Study Mycobacterium abscessus Pathogenicity

Mycobacterium abscessus (Mab) is an emerging, nontuberculosis mycobacterium (NTM) that infects humans. Mab has two morphotypes, smooth (S) and rough (R), related to the production of glycopeptidolipid (GPL), that differ in pathogenesis. To further understand the pathogenicity of these morphotypes in vivo, the amphibian Xenopus laevis was used as an alternative animal model. Mab infections have been previously modeled in zebrafish embryos and mice, but Mab are cleared early from immunocompetent mice, preventing the study of chronic infection, and the zebrafish model cannot be used to model a pulmonary infection and T cell involvement. Here, we show that X. laevis tadpoles, which have lungs and T cells, can be used as a complementary model for persistent Mab infection and pathogenesis. Intraperitoneal (IP) inoculation of S and R Mab morphotypes disseminated to tadpole tissues including liver and lungs, persisting for up to 40 days without significant mortality. Furthermore, the R morphotype was more persistent, maintaining a higher bacterial load at 40 days postinoculation. In contrast, the intracardiac (IC) inoculation with S Mab induced significantly greater mortality than inoculation with the R Mab form. These data suggest that X. laevis tadpoles can serve as a useful comparative experimental organism to investigate pathogenesis and host resistance to M. abscessus.

Fast chemical force microscopy demonstrates that glycopeptidolipids define nanodomains of varying hydrophobicity on mycobacteria

Mycobacterium abscessus is an emerging multidrug-resistant bacterial pathogen causing severe lung infections in cystic fibrosis patients. A remarkable trait of this mycobacterial species is its ability to form morphologically smooth (S) and rough (R) colonies. The S-to-R transition is caused by the loss of glycopeptidolipids (GPLs) in the outer layer of the cell envelope and correlates with an increase in cording and virulence. Despite the physiological and medical importance of this morphological transition, whether it involves changes in cell surface properties remains unknown. Herein, we combine recently developed quantitative imaging (QI) atomic force microscopy (AFM) with hydrophobic tips to quantitatively map the surface structure and hydrophobicity of M. abscessus at high spatiotemporal resolution, and to assess how these properties are modulated by the S-to-R transition and by treatment with an inhibitor of the mycolic acid transporter MmpL3. We discover that loss of GPLs leads to major modifications in surface hydrophobicity, without any apparent change in cell surface ultrastructure. While R bacilli are homogeneously hydrophobic, S bacilli feature unusual variations of nanoscale hydrophobic properties. These previously undescribed cell surface nanodomains are likely to play critical roles in bacterial adhesion, aggregation, phenotypic heterogeneity and transmission, and in turn in virulence and pathogenicity. Our study also suggests that MmpL3 inhibitors show promise in nanomedicine as chemotherapeutic agents to interfere with the highly hydrophobic nature of the mycobacterial cell wall. The advantages of QI-AFM with hydrophobic tips are the ability to map chemical and structural properties simultaneously and at high resolution, applicable to a wide range of biosystems.

Anti-inflammatory effects of α-MSH through p-CREB expression in sarcoidosis like granuloma model

Lung inflammation due to sarcoidosis is characterized by a complex cascade of immunopathologic events, including leukocyte recruitment and granuloma formation. α-melanocyte stimulating hormone (α-MSH) is a melanocortin signaling peptide with anti-inflammatory properties. We aimed to evaluate the effects of α-MSH in a novel in vitro sarcoidosis model. An in vitro sarcoidosis-like granuloma model was developed by challenging peripheral blood mononuclear cells (PBMCs) derived from patients with confirmed treatment-naïve sarcoidosis with microparticles generated from Mycobacterium abscessus cell walls. Unchallenged PBMCsand developed granulomas were treated daily with 10 μM α-MSH or saline as control. Cytokine concentrations in supernatants of culture and in cell extracts were measured using Illumina multiplex Elisa and western blot, respectively. Gene expression was analyzed using RNA-Seq and RT-PCR. Protein secretion and gene expression of IL-7, IL-7R, IFN-γ, MC1R, NF-κB, phosphorylated NF-κB (p-NF-κB), MARCO, and p-CREB were measured with western blot and RNAseq. A significant increase in IL-7, IL-7R, and IFN-γ protein expression was found in developed granulomas comparing to microparticle unchallenged PBMCs. IL-7, IL-7R, and IFN-γ protein expression was significantly reduced in developed granulomas after exposure to α-MSH compared with saline treated granulomas. Compared with microparticle unchallenged PBMCs, total NF-κB and p-NF-κB were significantly increased in developed granulomas, while expression of p-CREB was not changed. Treatment with α-MSH promoted a significantly higher concentration of p-CREB in granulomas. The anti-inflammatory effects of α-MSH were blocked by specific p-CREB inhibition. α-MSH has anti-inflammatory properties in this in vitro granuloma model, which is an effect mediated by induction of phosphorylation of CREB.

Structure-guided fragment-based drug discovery at the synchrotron: screening binding sites and correlations with hotspot mapping

Structure-guided drug discovery emerged in the 1970s and 1980s, stimulated by the three-dimensional structures of protein targets that became available, mainly through X-ray crystal structure analysis, assisted by the development of synchrotron radiation sources. Structures of known drugs or inhibitors were used to guide the development of leads. The growth of high-throughput screening during the late 1980s and the early 1990s in the pharmaceutical industry of chemical libraries of hundreds of thousands of compounds of molecular weight of approximately 500 Da was impressive but still explored only a tiny fraction of the chemical space of the predicted 10⁴⁰ drug-like compounds. The use of fragments with molecular weights less than 300 Da in drug discovery not only decreased the chemical space needing exploration but also increased promiscuity in binding targets. Here we discuss advances in X-ray fragment screening and the challenge of identifying sites where fragments not only bind but can be chemically elaborated while retaining their positions and binding modes. We first describe the analysis of fragment binding using conventional X-ray difference Fourier techniques, with Mycobacterium abscessus SAICAR synthetase (PurC) as an example. We observe that all fragments occupy positions predicted by computational hotspot mapping. We compare this with fragment screening at Diamond Synchrotron Light Source XChem facility using PanDDA software, which identifies many more fragment hits, only some of which bind to the predicted hotspots. Many low occupancy sites identified may not support elaboration to give adequate ligand affinity, although they will likely be useful in drug discovery as 'warm spots' for guiding elaboration of fragments bound at hotspots. We discuss implications of these observations for fragment screening at the synchrotron sources. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Mycobacterial genomics and structural bioinformatics: opportunities and challenges in drug discovery

Of the more than 190 distinct species of Mycobacterium genus, many are economically and clinically important pathogens of humans or animals. Among those mycobacteria that infect humans, three species namely Mycobacterium tuberculosis (causative agent of tuberculosis), Mycobacterium leprae (causative agent of leprosy) and Mycobacterium abscessus (causative agent of chronic pulmonary infections) pose concern to global public health. Although antibiotics have been successfully developed to combat each of these, the emergence of drug-resistant strains is an increasing challenge for treatment and drug discovery. Here we describe the impact of the rapid expansion of genome sequencing and genome/pathway annotations that have greatly improved the progress of structure-guided drug discovery. We focus on the applications of comparative genomics, metabolomics, evolutionary bioinformatics and structural proteomics to identify potential drug targets. The opportunities and challenges for the design of drugs for M. tuberculosis, M. leprae and M. abscessus to combat resistance are discussed.

Antimicrobial and Efflux Inhibitor Activity of Usnic Acid Against Mycobacterium abscessus

New drugs are needed to treat infections with antimicrobial-resistant Mycobacterium abscessus; therefore, we evaluated usnic acid as an antimicrobial agent and efflux inhibitor (EI) against M. abscessus. Usnic acid showed antimicrobial activity, and synergistically, the EI verapamil increased this activity. In addition, when we evaluated the interaction of antimicrobials with usnic acid, the increase of their activity was observed. Finally, usnic acid showed an efflux inhibitory effect between the classical EIs verapamil and carbonyl cyanide m-chlorophenylhydrazine. In conclusion, usnic acid showed both antimicrobial and EI activity, indicating that this natural compound may be a promising scaffold for new drugs against this difficult-to-treat microorganism.

MmpL8MAB controls Mycobacterium abscessus virulence and production of a previously unknown glycolipid family

Mycobacterium abscessus is a peculiar rapid-growing Mycobacterium (RGM) capable of surviving within eukaryotic cells thanks to an arsenal of virulence genes also found in slow-growing mycobacteria (SGM), such as Mycobacterium tuberculosis A screen based on the intracellular survival in amoebae and macrophages (MΦ) of an M. abscessus transposon mutant library revealed the important role of MAB_0855, a yet uncharacterized Mycobacterial membrane protein Large (MmpL). Large-scale comparisons with SGM and RGM genomes uncovered MmpL12 proteins as putative orthologs of MAB_0855 and a locus-scale synteny between the MAB_0855 and Mycobacterium chelonae mmpL8 loci. A KO mutant of the MAB_0855 gene, designated herein as mmpL8MAB , had impaired adhesion to MΦ and displayed a decreased intracellular viability. Despite retaining the ability to block phagosomal acidification, like the WT strain, the mmpL8MAB mutant was delayed in damaging the phagosomal membrane and in making contact with the cytosol. Virulence attenuation of the mutant was confirmed in vivo by impaired zebrafish killing and a diminished propensity to induce granuloma formation. The previously shown role of MmpL in lipid transport prompted us to investigate the potential lipid substrates of MmpL8MAB Systematic lipid analysis revealed that MmpL8MAB was required for the proper expression of a glycolipid entity, a glycosyl diacylated nonadecyl diol (GDND) alcohol comprising different combinations of oleic and stearic acids. This study shows the importance of MmpL8MAB in modifying interactions between the bacteria and phagocytic cells and in the production of a previously unknown glycolipid family.