Deletion of a dehydratase important for intracellular growth and cording renders rough Mycobacterium abscessus avirulent

Interplay of Mycobacterium abscessus and Pseudomonas aeruginosa in experimental models of coinfection: Biofilm dynamics and host immune response

The incidence of infection by nontuberculous mycobacteria, mainly Mycobacterium abscessus, is increasing in patients with cystic fibrosis and other chronic pulmonary diseases, leading to an accelerated lung function decline. In most cases, M. abscessus coinfects Pseudomonas aeruginosa, the most common pathogen in these conditions. However, how these two bacterial species interact during infection remains poorly understood. This study explored their behaviour in three relevant pathogenic settings: dual-species biofilm development using a recently developed method to monitor individual species in dual-species biofilms, coinfection in bronchial epithelial cells, and in vivo coinfection in the Galleria mellonella model. The results demonstrated that both species form stable mixed biofilms and reciprocally inhibit single-biofilm progression. Coinfections in bronchial epithelial cells significantly decreased cell viability, whereas in G. mellonella, coinfections induced lower survival rates than individual infections. Analysis of the immune response triggered by each bacterium in bronchial epithelial cell assays and G. mellonella larvae revealed that P. aeruginosa induces the overexpression of proinflammatory and melanization cascade responses, respectively. In contrast, M. abscessus and P. aeruginosa coinfection significantly inhibited the immune response in both models, resulting in worse consequences for the host than those generated by a single P. aeruginosa infection. Overall, this study highlights the novel role of M. abscessus in suppressing immune responses during coinfection with P. aeruginosa, emphasizing the clinical implications for the management of cystic fibrosis and other pulmonary diseases. Understanding these interactions could inform the development of new therapeutic strategies to mitigate the severity of coinfections in vulnerable patients.

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

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

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

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

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

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

Mycobacteroides abscessus ability to interact with the host mucosal cells plays an important role in pathogenesis of the infection

Non-tuberculous mycobacteria (NTM) are opportunistic pathogens ubiquitous in the environment. Mycobacteroides abscessus is a relatively new pathogen associated with underlying lung chronic pathologies, accounting for most of the pulmonary infections linked to the rapidly growing mycobacteria group. This includes chronic obstructive pulmonary disease, bronchiectasis, or cystic fibrosis. Patient outcomes from M. abscessus infections are poor due to complicated treatments and other factors. Intrinsic drug resistance plays an important role. The M. abscessus toolbox of resistance is varied leading to complex strategies for treatment. Mechanisms include waxy cell walls, drug export mechanisms, and acquired resistance. Many studies have also shown the impact of extracellular DNA found in the biofilm matrix during early infection and its possible advantage in pathogenicity. In this review, we discuss the current knowledge of early infection focusing on biofilm formation, an environmental strategy, and which treatments prevent its formation improving current antibiotic treatment outcomes in preliminary studies.

Structural and functional characterization of itaconyl-CoA hydratase and citramalyl-CoA lyase involved in itaconate metabolism of Pseudomonas aeruginosa

Itaconate is a key anti-inflammatory/antibacterial metabolite in pathogen-macrophage interactions that induces adaptive changes in Pseudomonas aeruginosa-exposed airways. However, the impact and mechanisms underlying itaconate metabolism remain unclear. Our study reveals that itaconate significantly upregulates the expression of pyoverdine in P. aeruginosa and enhances its tolerance to tobramycin. Notably, the enzymes responsible for efficient itaconate metabolism, PaIch and PaCcl, play crucial roles in both utilizing itaconate and clearing its toxic metabolic intermediates. By using protein crystallography and molecular dynamics simulations analyses, we have elucidated the unique catalytic center and substrate-binding pocket of PaIch, which contribute to its highly efficient catalysis. Meanwhile, analysis of PaCcl has revealed how interactions between domains regulate the conformational changes of the active sites and binding pockets, influencing the catalytic process. Overall, our research uncovers the significance and mechanisms of PaIch and PaCcl in the efficient metabolism of itaconate by P. aeruginosa.

Modeling nontuberculous mycobacterial infections in zebrafish

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

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

BACKGROUND

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

OBJECTIVES

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

SOURCES

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

CONTENT

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

IMPLICATIONS

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

Alternative therapies against Mycobacterium abscessus infections

BACKGROUND

Mycobacterium abscessus (Mab) is considered as the most pathogenic rapid-growing mycobacteria in humans, causing pulmonary and extra-pulmonary diseases, especially in patients with cystic fibrosis. Mab shows intrinsic and acquired resistance to many drugs, leaving limited treatment options that lead to a generally poor prognosis. The standard therapeutic regimen last for more than 6 months and consists of a drug cocktail that ideally includes a macrolide and amikacin. Yet, toxicity and efficacy are suboptimal due also to the high toxicity. There is a need to introduce innovative and out-of-the-box approaches to improve treatments.

OBJECTIVES

In this narrative review, we summarize the recent research on the alternative strategies proposed and discuss the importance of using appropriate experimental assays to assess their activity.

SOURCES

Included articles were identified by searching PubMed and MEDLINE until June 2023. The search terms were 'Mycobacterium abscessus', 'antimicrobial', and 'alternative therapies'. Additional relevant references were obtained from articles retrieved from the primary search.

CONTENT

Therapies against Mab including host directed therapies, repurposed drugs, phage therapy, anti-virulence strategies, essential oils, and inhalation therapies.

IMPLICATIONS

Alternative treatments may represent a valid tool to cope the burden of antimicrobial resistance in Mab-caused diseases.

Structural and functional insights of itaconyl-CoA hydratase from Pseudomonas aeruginosa highlight a novel N-terminal hotdog fold

Itaconyl-CoA hydratase in Pseudomonas aeruginosa (PaIch) converts itaconyl-CoA to (S)-citramalyl-CoA upon addition of a water molecule, a part of an itaconate catabolic pathway in virulent organisms required for their survival in humans host cells. Crystal structure analysis of PaIch showed that a unique N-terminal hotdog fold containing a 4-residue short helical segment α3-, named as an "eaten sausage", followed by a flexible loop region slipped away from the conserved β-sheet scaffold, whereas the C-terminal hotdog fold is similar to all MaoC. A conserved hydratase motif with catalytic residues provides mechanistic insights into catalysis, and existence of a longer substrate binding tunnel may suggest the binding of longer CoA derivatives.

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

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

Mechanopathology of biofilm-like Mycobacterium tuberculosis cords

Mycobacterium tuberculosis (Mtb) cultured axenically without detergent forms biofilm-like cords, a clinical identifier of virulence. In lung-on-chip (LoC) and mouse models, cords in alveolar cells contribute to suppression of innate immune signaling via nuclear compression. Thereafter, extracellular cords cause contact-dependent phagocyte death but grow intercellularly between epithelial cells. The absence of these mechanopathological mechanisms explains the greater proportion of alveolar lesions with increased immune infiltration and dissemination defects in cording-deficient Mtb infections. Compression of Mtb lipid monolayers induces a phase transition that enables mechanical energy storage. Agent-based simulations demonstrate that the increased energy storage capacity is sufficient for the formation of cords that maintain structural integrity despite mechanical perturbation. Bacteria in cords remain translationally active despite antibiotic exposure and regrow rapidly upon cessation of treatment. This study provides a conceptual framework for the biophysics and function in tuberculosis infection and therapy of cord architectures independent of mechanisms ascribed to single bacteria.

Delineation of transcriptional regulators involve in biofilm formation cycle of Mycobacterium abscessus

Mycobacterium abscessus is an intrinsically and acquired multidrug resistant (MDR) intracellular pathogen with biofilm formation capability and limited option for treatment. Biofilm is the major characteristic that leads to failure and prolong treatment, intensifies treatment cost and increases mortality/morbidity rate. However, the biofilm formation regulations of M. abscessus remain largely unexplored. In this study, we identify the putative/hypothetical transcriptional regulator (TR) of M. abscessus that are involved in biofilm formation. This study includes fifty TRs belonging to thirteen different families viz., AraC, ArsR, AsnC, CarD, CdaR, GntR, IclR, LysR, MarR, PadR, PrrA, TetR and WhiB, including TRs of unknown family. The promoter of these putative TRs were fused individually with GFP and analyzed their expression using CLSM in planktonic phase and early, mid and mature stages of biofilm formation phase, which overall termed as biofilm formation cycle. Further, qRT-PCR was carried out for selected TRs to analyze their differential expressions. This study found thirteen numbers of TR belonging to TetR family, five TRs belonging to MarR family, four TRs of unannotated TR family, two AraC TRs, two LysR, two GntR, two AsnC, one each of ArsR family, CarD family, IclR family, PadR family, PrrA family and WhiB family selected for this study are involved in biofilm formation cycle. Our study characterized the TRs with respect to their role in biofilm formation for the first time in M. abscessus and also found that their biofilm formation is regulated by diverse TR families.

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.

Targeted Chromosomal Barcoding Establishes Direct Genotype-Phenotype Associations for Antibiotic Resistance in Mycobacterium abscessus

A bedaquiline-resistant Mycobacterium abscessus isolate was sequenced, and a candidate mutation in the atpE gene was identified as responsible for the antibiotic resistance phenotype. To establish a direct genotype-phenotype relationship of this mutation which results in a Asp-to-Ala change at position 29 (D29A), we developed a recombineering-based method consisting of the specific replacement of the desired mutation in the bacterial chromosome. As surrogate bacteria, we used two M. abscessus bedaquiline-susceptible strains: ATCC 19977 and the SL541 clinical isolate. The allelic exchange substrates used in recombineering carried either the sole D29A mutation or a genetic barcode of silent mutations in codons flanking the D29A mutation. After selection of bedaquiline-resistant M. abscessus colonies transformed with both substrates, we obtained equivalent numbers of recombinants. These resistant colonies were analyzed by allele-specific PCR and Sanger sequencing, and we demonstrated that the presence of the genetic barcode was linked to the targeted incorporation of the desired mutation in its chromosomal location. All recombinants displayed the same MIC to bedaquiline as the original isolate, from which the D29A mutation was identified. Finally, to demonstrate the broad applicability of this method, we confirmed the association of bedaquiline resistance with the atpE A64P mutation in analysis performed in independent M. abscessus strains and by independent researchers. IMPORTANCE Antimicrobial resistance (AMR) threatens the effective prevention and treatment of an ever-increasing range of infections caused by microorganisms. On the other hand, infections caused by Mycobacterium abscessus affect people with chronic lung diseases, and their incidence has grown alarmingly in recent years. Further, these bacteria are known to easily develop AMR to the few therapeutic options available, making their treatment long-lasting and challenging. The recent introduction of new antibiotics against M. abscessus, such as bedaquiline, makes us anticipate a future when a plethora of antibiotic-resistant strains will be isolated and sequenced. However, in the era of whole-genome sequencing, one of the challenges is to unequivocally assign a biological function to each identified polymorphism. Thus, in this study, we developed a fast, robust, and reliable method to assign genotype-phenotype associations for putative antibiotic-resistant polymorphisms in M. abscessus.

DS86760016, a Leucyl-tRNA Synthetase Inhibitor, Is Active against Mycobacterium abscessus

Benzoxaboroles are a new class of leucyl-tRNA synthetase inhibitors. Epetraborole, a benzoxaborole, is a clinical candidate developed for Gram-negative infections and has been confirmed to exhibit favorable activity against a well known pulmonary pathogen, Mycobacterium abscessus. However, according to ClinicalTrials.gov, in 2017, a clinical phase II study on the use of epetraborole to treat complicated urinary tract and intra-abdominal infections was terminated due to the rapid emergence of drug resistance during treatment. Nevertheless, epetraborole is in clinical development for nontuberculous mycobacteria (NTM) disease especially for Mycobacterium avium complex-related pulmonary disease (MAC-PD). DS86760016, an epetraborole analog, was further demonstrated to have an improved pharmacokinetic profile, lower plasma clearance, longer plasma half-life, and higher renal excretion than epetraborole in animal models. In this study, DS86760016 was found to be similarly active against M. abscessus in vitro, intracellularly, and in zebrafish infection models with a low mutation frequency. These results expand the diversity of druggable compounds as new benzoxaborole-based candidates for treating M. abscessus diseases.

The cell envelope of Mycobacterium abscessus and its role in pathogenesis

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

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

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

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.

Mycobacterium abscessus: It's Complex

Mycobacterium abscessus (M. abscessus) is an opportunistic pathogen usually colonizing abnormal lung airways and is often seen in patients with cystic fibrosis. Currently, there is no vaccine available for M. abscessus in clinical development. The treatment of M. abscessus-related pulmonary diseases is peculiar due to intrinsic resistance to several commonly used antibiotics. The development of either prophylactic or therapeutic interventions for M. abscessus pulmonary infections is hindered by the absence of an adequate experimental animal model. In this review, we outline the critical elements related to M. abscessus virulence mechanisms, host-pathogen interactions, and treatment challenges associated with M. abscessus pulmonary infections. The challenges of effectively combating this pathogen include developing appropriate preclinical animal models of infection, developing proper diagnostics, and designing novel strategies for treating drug-resistant M. abscessus.

Calcium carbonate mineralization is essential for biofilm formation and lung colonization

Biofilms are differentiated microbial communities held together by an extracellular matrix. μCT X-ray revealed structured mineralized areas within biofilms of lung pathogens belonging to two distant phyla - the proteobacteria Pseudomonas aeruginosa and the actinobacteria Mycobacterium abscessus. Furthermore, calcium chelation inhibited the assembly of complex bacterial structures for both organisms with little to no effect on cell growth. The molecular mechanisms promoting calcite scaffold formation were surprisingly conserved between the two pathogens as biofilm development was similarly impaired by genetic and biochemical inhibition of calcium uptake and carbonate accumulation. Moreover, chemical inhibition and mutations targeting mineralization significantly reduced the attachment of P. aeruginosa to the lung, as well as the subsequent damage inflicted by biofilms to lung tissues, and restored their sensitivity to antibiotics. This work offers underexplored druggable targets for antibiotics to combat otherwise untreatable biofilm infections.

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

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

Rough and smooth variants of Mycobacterium abscessus are differentially controlled by host immunity during chronic infection of adult zebrafish

Prevalence of Mycobacterium abscessus infections is increasing in patients with respiratory comorbidities. After initial colonisation, M. abscessus smooth colony (S) variants can undergo an irreversible genetic switch into highly inflammatory, rough colony (R) variants, often associated with a decline in pulmonary function. Here, we use an adult zebrafish model of chronic infection with R and S variants to study M. abscessus pathogenesis in the context of fully functioning host immunity. We show that infection with an R variant causes an inflammatory immune response that drives necrotic granuloma formation through host TNF signalling, mediated by the tnfa, tnfr1 and tnfr2 gene products. T cell-dependent immunity is stronger against the R variant early in infection, and regulatory T cells associate with R variant granulomas and limit bacterial growth. In comparison, an S variant proliferates to high burdens but appears to be controlled by TNF-dependent innate immunity early during infection, resulting in delayed granuloma formation. Thus, our work demonstrates the applicability of adult zebrafish to model persistent M. abscessus infection, and illustrates differences in the immunopathogenesis induced by R and S variants during granulomatous infection.

The pathogen Mycobacterium abscessus can switch from a smooth colony form (S) into a highly inflammatory, rough colony form (R) during infection. Here, Kam et al. use an adult zebrafish model of M. abscessus chronic infection to illustrate differences in the immunopathogenesis induced by R and S variants.

Mycobacterial Adhesion: From Hydrophobic to Receptor-Ligand Interactions

Adhesion is crucial for the infective lifestyles of bacterial pathogens. Adhesion to non-living surfaces, other microbial cells, and components of the biofilm extracellular matrix are crucial for biofilm formation and integrity, plus adherence to host factors constitutes a first step leading to an infection. Adhesion is, therefore, at the core of pathogens’ ability to contaminate, transmit, establish residency within a host, and cause an infection. Several mycobacterial species cause diseases in humans and animals with diverse clinical manifestations. Mycobacterium tuberculosis, which enters through the respiratory tract, first adheres to alveolar macrophages and epithelial cells leading up to transmigration across the alveolar epithelium and containment within granulomas. Later, when dissemination occurs, the bacilli need to adhere to extracellular matrix components to infect extrapulmonary sites. Mycobacteria causing zoonotic infections and emerging nontuberculous mycobacterial pathogens follow divergent routes of infection that probably require adapted adhesion mechanisms. New evidence also points to the occurrence of mycobacterial biofilms during infection, emphasizing a need to better understand the adhesive factors required for their formation. Herein, we review the literature on tuberculous and nontuberculous mycobacterial adhesion to living and non-living surfaces, to themselves, to host cells, and to components of the extracellular matrix.

Host-Pathogen Interactions Operative during Mycobacteroides abscessus Infection

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

Progress of the Art of Macrophage Polarization and Different Subtypes in Mycobacterial Infection

Mycobacteriosis, mostly resulting from Mycobacterium tuberculosis (MTb), nontuberculous mycobacteria (NTM), and Mycobacterium leprae (M. leprae), is the long-standing granulomatous disease that ravages several organs including skin, lung, and peripheral nerves, and it has a spectrum of clinical-pathologic features based on the interaction of bacilli and host immune response. Histiocytes in infectious granulomas mainly consist of infected and uninfected macrophages (Mφs), multinucleated giant cells (MGCs), epithelioid cells (ECs), and foam cells (FCs), which are commonly discovered in lesions in patients with mycobacteriosis. Granuloma Mφ polarization or reprogramming is the crucial appearance of the host immune response to pathogen aggression, which gets a command of endocellular microbe persistence. Herein, we recapitulate the current gaps and challenges during Mφ polarization and the different subpopulations of mycobacteriosis.

Mycobacterium abscessus biofilms produce an extracellular matrix and have a distinct mycolic acid profile

A non-tuberculous mycobacterium, Mycobacterium abscessus is an emerging opportunistic pathogen associated with difficult to treat pulmonary infections, particularly in patients suffering from cystic fibrosis. It is capable of forming biofilms in vitro that result in an increase of already high levels of antibiotic resistance in this bacterium. Evidence that M. abscessus forms biofilm-like microcolonies in patient lungs and on medical devices further implicated the need to investigate this biofilm in detail. Therefore, in this study we characterized the M. abscessus pellicular biofilm, formed on a liquid-air interface, by studying its molecular composition, and its transcriptional profile in comparison to planktonic cells. Using scanning electron micrographs and fluorescence microscopy, we showed that M. abscessus biofilms produce an extracellular matrix composed of lipids, proteins, carbohydrates and extracellular DNA. Transcriptomic analysis of biofilms revealed an upregulation of pathways involved in the glyoxylate shunt, redox metabolism and mycolic acid biosynthesis. Genes involved in elongation and desaturation of mycolic acids were highly upregulated in biofilms and, mirroring those findings, biochemical analysis of mycolates revealed molecular changes and an increase in mycolic acid chain length. Together these results give us an insight into the complex structure of M. abscessus biofilms, the understanding of which may be adapted for clinical use in treatment of biofilm infections, including strategies for dispersing the extracellular matrix, allowing antibiotics to gain access to bacteria within the biofilm.

Mycobacterium abscessus Strain Morphotype Determines Phage Susceptibility, the Repertoire of Therapeutically Useful Phages, and Phage Resistance

Mycobacterium abscessus infections in cystic fibrosis patients are challenging to treat due to widespread antibiotic resistance. The therapeutic use of lytic bacteriophages presents a new potential strategy, but the great variation among clinical M. abscessus isolates demands determination of phage susceptibility prior to therapy.

Mycobacterium abscessus is an opportunistic pathogen whose treatment is confounded by widespread multidrug resistance. The therapeutic use of bacteriophages against Mycobacterium abscessus infections offers a potential alternative approach, although the spectrum of phage susceptibilities among M. abscessus isolates is not known. We determined the phage infection profiles of 82 M. abscessus recent clinical isolates and find that colony morphotype—rough or smooth—is a key indicator of phage susceptibility. None of the smooth strains are efficiently killed by any phages, whereas 80% of rough strains are infected and efficiently killed by at least one phage. The repertoire of phages available for potential therapy of rough morphotype infections includes those with relatively broad host ranges, host range mutants of Mycobacterium smegmatis phages, and lytically propagated viruses derived from integrated prophages. The rough colony morphotype results from indels in the glycopeptidolipid synthesis genes mps1 and mps2, negating reversion to smooth as a common route to phage resistance. Resistance is thus rare, and although mutations in polyketide synthesis, uvrD2, and rpoZ can confer resistance, these likely also impair survival in vivo. The expanded therapeutic repertoire and the resistance profiles show that small cocktails or single phages could be suitable for controlling infections with rough strains.

Mab_3083c Is a Homologue of RNase J and Plays a Role in Colony Morphotype, Aggregation, and Sliding Motility of Mycobacterium abscessus

Mycobacterium abscessus is an opportunistic pathogen causing human diseases, especially in immunocompromised patients. M. abscessus strains with a rough morphotype are more virulent than those with a smooth morphotype. Morphotype switch may occur during a clinical infection. To investigate the genes involved in colony morphotype switching, we performed transposon mutagenesis in a rough clinical strain of M. abscessus. A morphotype switching mutant (smooth) named mab_3083c::Tn was obtained. This mutant was found to have a lower aggregative ability and a higher sliding motility than the wild type strain. However, its glycopeptidolipid (GPL) content remained the same as those of the wild type. Complementation of the mutant with a functional mab_3083c gene reverted its morphotype back to rough, indicating that mab_3083c is associated with colony morphology of M. abscessus. Bioinformatic analyses showed that mab_3083c has a 75.4% identity in amino acid sequence with the well-characterized ribonuclease J (RNase J) of M. smegmatis (RNase J_Msmeg). Complementation of the mutant with the RNase J gene of M. smegmatis also switched its colony morphology from smooth back to rough. These results suggest that Mab_3083c is a homologue of RNase J and involved in regulating M. abscessus colony morphotype switching.

Glycopeptidolipid Genotype Correlates With the Severity of Mycobacterium abscessus Lung Disease

BACKGROUND

Smooth and rough colony morphotypes of Mycobacterium abscessus are associated with virulence, but some isolates form both smooth and rough colonies, impeding successful morphotype identification. Reportedly, smooth/rough morphotypes are also related to the glycopeptidolipid (GPL) genotype. However, the accuracy of GPL genotyping to discriminate morphotypes and the relationship between GPL genotype and clinical characteristics of M abscessus lung disease have not been verified.

METHODS

A retrospective analysis of colony morphology, GPL genotype, and clinical data from 182 patients with M abscessus lung disease was conducted.

RESULTS

Of 194 clinical isolates, 126 (65.0%), 15 (7.7%), and 53 (27.3%) exhibited rough, smooth, and mixed colony morphotypes, respectively. Glycopeptidolipid genotyping indicated that 86.7% (13 of 15) of smooth isolates belonged to the GPL-wild type (WT) group, whereas 98.4% (124 of 126) of rough isolates belonged to the GPL-mutant type (MUT) group. Therefore, GPL genotyping accurately distinguished between smooth and rough morphotypes. Mixed colony morphotypes were also divided into GPL-WT (18.9%) and GPL-MUT (81.1%) groups. Further analysis revealed that patients infected with the GPL-MUT group presented with significantly worse baseline clinical characteristics and exacerbated episodes of lung disease.

CONCLUSIONS

Glycopeptidolipid genotyping accurately distinguishes smooth and rough colony morphotypes. Patients infected with the GPL-MUT genotype exhibit worse clinical characteristics and are at a higher risk of exacerbated lung disease.

Functional Characterization of the N-Acetylmuramyl-l-Alanine Amidase, Ami1, from Mycobacterium abscessus

Peptidoglycan (PG) is made of a polymer of disaccharides organized as a three-dimensional mesh-like network connected together by peptidic cross-links. PG is a dynamic structure that is essential for resistance to environmental stressors. Remodeling of PG occurs throughout the bacterial life cycle, particularly during bacterial division and separation into daughter cells. Numerous autolysins with various substrate specificities participate in PG remodeling. Expression of these enzymes must be tightly regulated, as an excess of hydrolytic activity can be detrimental for the bacteria. In non-tuberculous mycobacteria such as Mycobacterium abscessus, the function of PG-modifying enzymes has been poorly investigated. In this study, we characterized the function of the PG amidase, Ami1 from M. abscessus. An ami1 deletion mutant was generated and the phenotypes of the mutant were evaluated with respect to susceptibility to antibiotics and virulence in human macrophages and zebrafish. The capacity of purified Ami1 to hydrolyze muramyl-dipeptide was demonstrated in vitro. In addition, the screening of a 9200 compounds library led to the selection of three compounds inhibiting Ami1 in vitro. We also report the structural characterization of Ami1 which, combined with in silico docking studies, allows us to propose a mode of action for these inhibitors.

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.

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.

Non-tuberculous mycobacteria and the rise of Mycobacterium abscessus

Infections caused by non-tuberculous mycobacteria (NTM) are increasing globally and are notoriously difficult to treat due to intrinsic resistance of these bacteria to many common antibiotics. NTM are diverse and ubiquitous in the environment, with only a few species causing serious and often opportunistic infections in humans, including Mycobacterium abscessus. This rapidly growing mycobacterium is one of the most commonly identified NTM species responsible for severe respiratory, skin and mucosal infections in humans. It is often regarded as one of the most antibiotic-resistant mycobacteria, leaving us with few therapeutic options. In this Review, we cover the proposed infection process of M. abscessus, its virulence factors and host interactions and highlight the commonalities and differences of M. abscessus with other NTM species. Finally, we discuss drug resistance mechanisms and future therapeutic options. Taken together, this knowledge is essential to further our understanding of this overlooked and neglected global threat.

Thiostrepton: A Novel Therapeutic Drug Candidate for Mycobacterium abscessus Infection

Mycobacterium abscessus is a rapid-growing, multidrug-resistant, non-tuberculous mycobacterial species responsible for a variety of human infections, such as cutaneous and pulmonary infections. M. abscessus infections are very difficult to eradicate due to the natural and acquired multidrug resistance profiles of M. abscessus. Thus, there is an urgent need for the development of effective drugs or regimens against M. abscessus infections. Here, we report the activity of a US Food and Drug Administration approved drug, thiostrepton, against M. abscessus. We found that thiostrepton significantly inhibited the growth of M. abscessus wild-type strains, subspecies, clinical isolates, and drug-resistant mutants in vitro and in macrophages. In addition, treatment of macrophages with thiostrepton significantly decreased proinflammatory cytokine production in a dose-dependent manner, suggesting an inhibitory effect of thiostrepton on inflammation induced during M. abscessus infection. We further showed that thiostrepton exhibits antimicrobial effects in vivo using a zebrafish model of M. abscessus infection.

Mycobacterium abscessus, an Emerging and Worrisome Pathogen among Cystic Fibrosis Patients

Nontuberculous mycobacteria (NTM) have recently emerged as important pathogens among cystic fibrosis (CF) patients worldwide. Mycobacterium abscessus is becoming the most worrisome NTM in this cohort of patients and recent findings clarified why this pathogen is so prone to this disease. M. abscessus drug therapy takes up to 2 years and its failure causes an accelerated lung function decline. The M. abscessus colonization of lung alveoli begins with smooth strains producing glycopeptidolipids and biofilm, whilst in the invasive infection, "rough" mutants are responsible for the production of trehalose dimycolate, and consequently, cording formation. Human-to-human M. abscessus transmission was demonstrated among geographically separated CF patients by whole-genome sequencing of clinical isolates worldwide. Using a M. abscessus infected CF zebrafish model, it was demonstrated that CFTR (cystic fibrosis transmembrane conductance regulator) dysfunction seems to have a specific role in the immune control of M. abscessus infections only. This pathogen is also intrinsically resistant to many drugs, thanks to its physiology and to the acquisition of new mechanisms of drug resistance. Few new compounds or drug formulations active against M. abscessus are present in preclinical and clinical development, but recently alternative strategies have been investigated, such as phage therapy and the use of β-lactamase inhibitors.

1H-Benzo[d]Imidazole Derivatives Affect MmpL3 in Mycobacterium tuberculosis

1H-benzo[d]imidazole derivatives exhibit antitubercular activity in vitro at a nanomolar range of concentrations and are not toxic to human cells, but their mode of action remains unknown. Here, we showed that these compounds are active against intracellular Mycobacterium tuberculosis To identify their target, we selected drug-resistant M. tuberculosis mutants and then used whole-genome sequencing to unravel mutations in the essential mmpL3 gene, which encodes the integral membrane protein that catalyzes the export of trehalose monomycolate, a precursor of the mycobacterial outer membrane component trehalose dimycolate (TDM), as well as mycolic acids bound to arabinogalactan. The drug-resistant phenotype was also observed in the parental strain overexpressing the mmpL3 alleles carrying the mutations identified in the resistors. However, no cross-resistance was observed between 1H-benzo[d]imidazole derivatives and SQ109, another MmpL3 inhibitor, or other first-line antitubercular drugs. Metabolic labeling and quantitative thin-layer chromatography (TLC) analysis of radiolabeled lipids from M. tuberculosis cultures treated with the benzoimidazoles indicated an inhibition of trehalose dimycolate (TDM) synthesis, as well as reduced levels of mycolylated arabinogalactan, in agreement with the inhibition of MmpL3 activity. Overall, this study emphasizes the pronounced activity of 1H-benzo[d]imidazole derivatives in interfering with mycolic acid metabolism and their potential for therapeutic application in the fight against tuberculosis.

Aggregation of Nontuberculous Mycobacteria Is Regulated by Carbon-Nitrogen Balance

Free-living bacteria can assemble into multicellular structures called biofilms. Biofilms help bacteria tolerate multiple stresses, including antibiotics and the host immune system. Nontuberculous mycobacteria are a group of emerging opportunistic pathogens that utilize biofilms to adhere to household plumbing and showerheads and to avoid phagocytosis by host immune cells. Typically, bacteria regulate biofilm formation by controlling expression of adhesive structures to attach to surfaces and other bacterial cells. Mycobacteria harbor a unique cell wall built chiefly of long-chain mycolic acids that confers hydrophobicity and has been thought to cause constitutive aggregation in liquid media. Here we show that aggregation is instead a regulated process dictated by the balance of available carbon and nitrogen. Understanding that mycobacteria utilize metabolic cues to regulate the transition between planktonic and aggregated cells reveals an inroad to controlling biofilm formation through targeted therapeutics.

Nontuberculous mycobacteria (NTM) are emerging opportunistic pathogens that colonize household water systems and cause chronic lung infections in susceptible patients. The ability of NTM to form surface-attached biofilms in the nonhost environment and corded aggregates in vivo is important to their ability to persist in both contexts. Underlying the development of these multicellular structures is the capacity of mycobacterial cells to adhere to one another. Unlike most other bacteria, NTM spontaneously and constitutively aggregate in vitro, hindering our ability to understand the transition between planktonic and aggregated cells. While culturing a model NTM, Mycobacterium smegmatis, in rich medium, we fortuitously discovered that planktonic cells accumulate after ∼3 days of growth. By providing selective pressure for bacteria that disperse earlier, we isolated a strain with two mutations in the oligopeptide permease operon (opp). A mutant lacking the opp operon (Δopp) disperses earlier than wild type (WT) due to a defect in nutrient uptake. Experiments with WT M. smegmatis revealed that growth as aggregates is favored when carbon is replete, but under conditions of low available carbon relative to available nitrogen, M. smegmatis grows as planktonic cells. By adjusting carbon and nitrogen sources in defined medium, we tuned the cellular C/N ratio such that M. smegmatis grows either as aggregates or as planktonic cells. C/N-mediated aggregation regulation is widespread among NTM with the possible exception of rough-colony Mycobacterium abscessus isolates. Altogether, we show that NTM aggregation is a controlled process that is governed by the relative availability of carbon and nitrogen for metabolism.

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).

Antibiotics versus biofilm: an emerging battleground in microbial communities

Biofilm is a complex structure of microbiome having different bacterial colonies or single type of cells in a group; adhere to the surface. These cells are embedded in extracellular polymeric substances, a matrix which is generally composed of eDNA, proteins and polysaccharides, showed high resistance to antibiotics. It is one of the major causes of infection persistence especially in nosocomial settings through indwelling devices. Quorum sensing plays an important role in regulating the biofilm formation. There are many approaches being used to control infections by suppressing its formation but CRISPR-CAS (gene editing technique) and photo dynamic therapy (PDT) are proposed to be used as therapeutic approaches to subside bacterial biofim infections, especially caused by deadly drug resistant bad bugs.

Lsr2 Is an Important Determinant of Intracellular Growth and Virulence in Mycobacterium abscessus

Mycobacterium abscessus, a pathogen responsible for severe lung infections in cystic fibrosis patients, exhibits either smooth (S) or rough (R) morphotypes. The S-to-R transition correlates with inhibition of the synthesis and/or transport of glycopeptidolipids (GPLs) and is associated with an increase of pathogenicity in animal and human hosts. Lsr2 is a small nucleoid-associated protein highly conserved in mycobacteria, including M. abscessus, and is a functional homolog of the heat-stable nucleoid-structuring protein (H-NS). It is essential in Mycobacterium tuberculosis but not in the non-pathogenic model organism Mycobacterium smegmatis. It acts as a master transcriptional regulator of multiple genes involved in virulence and immunogenicity through binding to AT-rich genomic regions. Previous transcriptomic studies, confirmed here by quantitative PCR, showed increased expression of lsr2 (MAB_0545) in R morphotypes when compared to their S counterparts, suggesting a possible role of this protein in the virulence of the R form. This was addressed by generating lsr2 knock-out mutants in both S (Δlsr2-S) and R (Δlsr2-R) variants, demonstrating that this gene is dispensable for M. abscessus growth. We show that the wild-type S variant, Δlsr2-S and Δlsr2-R strains were more sensitive to H₂O₂ as compared to the wild-type R variant of M. abscessus. Importantly, virulence of the Lsr2 mutants was considerably diminished in cellular models (macrophage and amoeba) as well as in infected animals (mouse and zebrafish). Collectively, these results emphasize the importance of Lsr2 in M. abscessus virulence.

Phagosome Escape of Rough Mycobacterium abscessus Strains in Murine Macrophage via Phagosomal Rupture Can Lead to Type I Interferon Production and Their Cell-To-Cell Spread

Mycobacterium abscessus complex (MAB) is a rapidly growing mycobacterium(RGM) whose clinical significance as an emerging human pathogen has been increasing worldwide. It has two types of colony morphology, a smooth (S) type, producing high glycopeptidolipid (GPL) content, and a rough (R) type, which produces low levels of GPLs and is associated with increased virulence. However, the mechanism responsible for their difference in virulence is poorly known. By ultrastructural examination of murine macrophages infected, we found that MAB-R strains could replicate more actively in the macrophage phagosome than the S variants and that they could escape into cytosol via phagosomal rupture. The cytosolic access of MAB-R strains via phagosomal rupture led to enhanced Type I interferon (IFN) production and cell death, which resulted in their cell-to-cell spreading. This behavior can provide an additional niche for the survival of MAB-R strains. In addition, we found that their enhancement of cell death mediated cell spreading are dependent on Type I IFN signaling via comparison of wild-type and IFNAR1 knockout mice. In conclusion, our data indicated that a transition of MAB-S strains into MAB-R variants increased their virulence via enhanced Type I IFN production, which led to enhanced survival in infected macrophage via cell death mediated cell-to-cell spreading. This result provides not only a novel insight into the difference in virulence between MAB-R and -S variants but also hints to their treatment strategy.

Mycobacterium abscessus: Shapeshifter of the Mycobacterial World

In this review we will focus on unique aspects of Mycobacterium abscessus (MABS) which we feel earn it the designation of "shapeshifter of the mycobacterial world." We will review its emergence as a distinct species, the recognition and description of MABS subspecies which are only now being clearly defined in terms of pathogenicity, its ability to exist in different forms favoring a saprophytic lifestyle or one more suitable to invasion of mammalian hosts, as well as current challenges in terms of antimicrobial therapy and future directions for research. One can see in the various phases of MABS, a species transitioning from a free living saprophyte to a host-adapted pathogen.

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.

Mycobacterium tuberculosis cording in alveolar macrophages of patients with pulmonary tuberculosis is likely associated with increased mycobacterial virulence

Mycobacterium tuberculosis (Mtb) is an infectious agent that causes tuberculosis (TB) in humans. A study of the volume of Mtb population and the detection of Mtb virulence in the lungs of patients with pulmonary TB are of great importance for understanding the infectious process and the outcome of the disease. We analyzed the functional state of Mtb and their number in alveolar macrophages obtained from the resected lungs of patients with TB in ex vivo culture and determined that the number of Mtb, referred mainly to the Beijing genotype family (A0 and B0/W148 clusters), were significantly different in cells between different patients. Only single Mtb were found in alveolar macrophages of some patients, while Mtb were actively replicated in colonies in alveolar macrophages of other patients, including cord morphology of Mtb growth (the indicator of Mtb virulence). Our data demonstrated association between the formation of Mtb cording in alveolar macrophages of patients and increased virulence of Mtb from the lungs of these patients in guinea pig TB model. The find of cording formation by replicating Mtb in human alveolar macrophages may be used for preliminary quick estimation of increased Mtb virulence in individual patients with pulmonary TB.

Glycopeptidolipids, a Double-Edged Sword of the Mycobacterium abscessus Complex

Mycobacterium abscessus is a rapidly-growing species causing a diverse panel of clinical manifestations, ranging from cutaneous infections to severe respiratory disease. Its unique cell wall, contributing largely to drug resistance and to pathogenicity, comprises a vast panoply of complex lipids, among which the glycopeptidolipids (GPLs) have been the focus of intense research. These lipids fulfill various important functions, from sliding motility or biofilm formation to interaction with host cells and intramacrophage trafficking. Being highly immunogenic, the induction of a strong humoral response is likely to select for rough low-GPL producers. These, in contrast to the smooth high-GPL producers, display aggregative properties, which strongly impacts upon intracellular survival. A propensity to grow as extracellular cords allows these low-GPL producing bacilli to escape the innate immune defenses. Transitioning from high-GPL to low-GPL producers implicates mutations within genes involved in biosynthesis or transport of GPL. This leads to induction of an intense pro-inflammatory response and robust and lethal infections in animal models, explaining the presence of rough isolates in patients with decreased pulmonary functions. Herein, we will discuss how, thanks to the generation of defined GPL mutants and the development of appropriate cellular and animal models to study pathogenesis, GPL contribute to M. abscessus biology and physiopathology.

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.

Zebrafish Infection: From Pathogenesis to Cell Biology

The study of host-pathogen interactions has illuminated fundamental research avenues in both infection and cell biology. Zebrafish (Danio rerio) larvae are genetically tractable, optically accessible, and present a fully functional innate immune system with macrophages and neutrophils that mimic their mammalian counterparts. A wide variety of pathogenic bacteria have been investigated using zebrafish models, providing unprecedented resolution of the cellular response to infection in vivo. In this review, we illustrate how zebrafish models have contributed to our understanding of cellular microbiology by providing an in vivo platform to study host-pathogen interactions from the single cell to whole animal level. We also highlight discoveries made from zebrafish infection that hold great promise for translation into novel therapies for humans.

Identification of genes required for Mycobacterium abscessus growth in vivo with a prominent role of the ESX-4 locus

Mycobacterium abscessus, a rapidly growing mycobacterium (RGM) and an opportunistic human pathogen, is responsible for a wide spectrum of clinical manifestations ranging from pulmonary to skin and soft tissue infections. This intracellular organism can resist the bactericidal defense mechanisms of amoebae and macrophages, an ability that has not been observed in other RGM. M. abscessus can up-regulate several virulence factors during transient infection of amoebae, thereby becoming more virulent in subsequent respiratory infections in mice. Here, we sought to identify the M. abscessus genes required for replication within amoebae. To this end, we constructed and screened a transposon (Tn) insertion library of an M. abscessus subspecies massiliense clinical isolate for attenuated clones. This approach identified five genes within the ESX-4 locus, which in M. abscessus encodes an ESX-4 type VII secretion system that exceptionally also includes the ESX conserved EccE component. To confirm the screening results and to get further insight into the contribution of ESX-4 to M. abscessus growth and survival in amoebae and macrophages, we generated a deletion mutant of eccB4 that encodes a core structural element of ESX-4. This mutant was less efficient at blocking phagosomal acidification than its parental strain. Importantly, and in contrast to the wild-type strain, it also failed to damage phagosomes and showed reduced signs of phagosome-to-cytosol contact, as demonstrated by a combination of cellular and immunological assays. This study attributes an unexpected and genuine biological role to the underexplored mycobacterial ESX-4 system and its substrates.