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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.Non-tuberculous mycobacteria, such as Mycobacterium abscessus, are an increasing global health burden, in part due their extensive drug resistance. In this Review, Johansen, Herrmann and Kremer discuss the infection process, host interactions, mechanisms of drug resistance and drug development.

Mycobacterium abscessus is the most common rapidly growing non-tuberculous mycobacteria to cause pulmonary disease in patients with impaired lung function such as cystic fibrosis. M . abscessus displays high intrinsic resistance to common antibiotics and inducible resistance to macrolides like clarithromycin. As such, M . abscessus is clinically resistant to the entire regimen of front-line M . tuberculosis drugs, and treatment with antibiotics that do inhibit M . abscessus in the lab results in cure rates of 50% or less. Here, we identified epetraborole (EPT) from the MMV pandemic response box as an inhibitor against the essential protein leucyl-tRNA synthetase (LeuRS) in M . abscessus . EPT protected zebrafish from lethal M . abscessus infection and did not induce self-resistance nor against clarithromycin. Contrary to most antimycobacterials, the whole-cell activity of EPT was greater against M . abscessus than M . tuberculosis , but crystallographic and equilibrium binding data showed that EPT binds LeuRS Mabs and LeuRS Mtb with similar residues and dissociation constants. Since EPT-resistant M . abscessus mutants lost LeuRS editing activity, these mutants became susceptible to misaminoacylation with leucine mimics like the non-proteinogenic amino acid norvaline. Proteomic analysis revealed that when M . abscessus LeuRS mutants were fed norvaline, leucine residues in proteins were replaced by norvaline, inducing the unfolded protein response with temporal changes in expression of GroEL chaperonins and Clp proteases. This supports our in vitro data that supplementation of media with norvaline reduced the emergence of EPT mutants in both M . abscessus and M . tuberculosis . Furthermore, the combination of EPT and norvaline had improved in vivo efficacy compared to EPT in a murine model of M . abscessus infection. Our results emphasize the effectiveness of EPT against the clinically relevant cystic fibrosis pathogen M . abscessus , and these findings also suggest norvaline adjunct therapy with EPT could be beneficial for M . abscessus and other mycobacterial infections like tuberculosis.

Mycobacterium abscessus is the most frequently isolated rapidly growing mycobacterium in human disease and recently has emerged as responsible for severe pulmonary infections in cystic fibrosis patients. However, little is known about the virulence mechanisms of this human pathogen. We adapted the zebrafish embryo as a tractable infection model to study, at a spatiotemporal level, the physiopathology of M. abscessus infection. We describe the high propensity of virulent rough variant M. abscessus to produce serpentine cords in vivo , which are not observed with the less virulent smooth variant. We demonstrate that extracellular cording allows the bacterium to withstand phagocytosis, leading to uncontrolled growth and establishment of an acute and lethal infection, thus constituting a determinant of virulence. Mycobacterium abscessus is a rapidly growing Mycobacterium causing a wide spectrum of clinical syndromes. It now is recognized as a pulmonary pathogen to which cystic fibrosis patients have a particular susceptibility. The M. abscessus rough (R) variant, devoid of cell-surface glycopeptidolipids (GPLs), causes more severe clinical disease than the smooth (S) variant, but the underlying mechanisms of R-variant virulence remain obscure. Exploiting the optical transparency of zebrafish embryos, we observed that the increased virulence of the M. abscessus R variant compared with the S variant correlated with the loss of GPL production. The virulence of the R variant involved the massive production of serpentine cords, absent during S-variant infection, and the cords initiated abscess formation leading to rapid larval death. Cording occurred within the vasculature and was highly pronounced in the central nervous system (CNS). It appears that M. abscessus is transported to the CNS within macrophages. The release of M. abscessus from apoptotic macrophages initiated the formation of cords that grew too large to be phagocytized by macrophages or neutrophils. This study is a description of the crucial role of cording in the in vivo physiopathology of M. abscessus infection and emphasizes cording as a mechanism of immune evasion.

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.

Treatments of Mycobacterium marinum , a common non-tuberculous mycobacterium associated with cutaneous infections are very challenging, emphasizing the development of new therapeutic approaches. Here we report the functionalization of mesoporous silica nanoparticles (MSN) with a series of triphenylphosphonium (TPP) substituents, which endowed them with affinity towards the surface of M. marinum in vitro, as well as within infected THP-1 cells. The presence of these nanoparticles at the bacterial surface prevents their uptake by human macrophages and dendritic cells. When loaded with doxycycline, the nanosystem exerts a potent anti-bacterial effect in planktonic cultures, biofilms, and in M. marinum -infected macrophages. Strikingly, in the M. marinum/ zebrafish infection model, the doxycycline-loaded nanoparticles are associated with a pronounced decrease in the bacterial burden and a high embryo survival rate. These results disclose the proposed MSN nanosystems as a promising alternative for the treatment of M. marinum infection and, presumably, against a broader range of mycobacterial infections. Current treatment options for Mycobacterium marinum infections are extensive and challenging. In this work, authors explore the potential of functionalized mesoporous silica nanoparticles as a carrier for delivery of drugs against M. marinum .

Trehalose polyphleates (TPPs) are complex lipids associated with the mycobacterial cell surface and were identified 50 years ago. While the TPP biosynthetic pathway has been described recently, the role of these lipids in the biology of mycobacteria remains yet to be established. The wide distribution of TPPs across mycobacterial species suggests that they may exhibit important functions in these actinobacteria. Here, we demonstrate that Mycobacterium abscessus, an emerging multidrug-resistant pathogen that causes severe lung diseases in cystic fibrosis patients, requires TPPs for survival in macrophages and virulence in a zebrafish model of infection. These findings support the importance of this underexplored family of lipids in mycobacterial pathogenesis.

The complex comprises several closely related species, causing pulmonary and extra-pulmonary infections. However, there is very limited knowledge about the disease pathogenesis involved in infections, particularly due to the lack of suitable animal models. Using the zebrafish model, we show that embryos are susceptible to infection in a dose-dependent manner. Furthermore, zebrafish embryos form granulomas from as early as 2 days post-infection, recapitulating critical aspects of mycobacterial pathogenesis observed in other pathogenic species. The formation of extracellular cords in infected embryos highlights a previously unknown pathogenic feature of . The formation of large corded structures occurs also during growth, suggesting that this is not a host-adapted stress mechanism deployed during infection. Moreover, transient macrophage depletion led to rapid embryo death with increased extracellular cords, indicating that macrophages are essential determinants of infection control. Importantly, morpholino depletion of the cystic fibrosis transmembrane conductance regulator ( ) significantly increased embryo death, bacterial burden, bacterial cords and abscesses. There was a noticeable decrease in the number of -deficient infected embryos with granulomas as compared to infected controls, suggesting that loss of CFTR leads to impaired host immune responses and confers hypersusceptiblity to infection. Overall, these findings highlight the application of the zebrafish embryo to study and emphasizes previously unexplored aspects of disease pathogenesis of this significant mycobacterial species.

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 (Δ MAB_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. Δ MAB_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. Leclercq et al report the structure of a glycosylated lipooctapeptide (GL8P) produced by Mycobacterium abscessus . GL8P is shown to induce a strong humoral response in infected patients and to contribute to intramacrophage survival and virulence in mice.

Mycobacteria share with other actinomycetes the ability to produce large quantities of triacylglycerol (TAG), which accumulate as intracytoplasmic lipid inclusions (ILI) also known as lipid droplets (LD). Mycobacterium tuberculosis ( M. tb ), the etiologic agent of tuberculosis, acquires fatty acids from the human host which are utilized to synthesize TAG, subsequently stored in the form of ILI to meet the carbon and nutrient requirements of the bacterium during long periods of persistence. However, environmental factors governing mycobacterial ILI formation and degradation remain poorly understood. Herein, we demonstrated that in the absence of host cells, carbon excess and nitrogen starvation promote TAG accumulation in the form of ILI in M. smegmatis and M. abscessus , used as surrogate species of M. tb . Based on these findings, we developed a simple and reversible in vitro model to regulate ILI biosynthesis and hydrolysis in mycobacteria. We also showed that TAG formation is tgs1 dependent and that lipolytic enzymes mediate TAG breakdown. Moreover, we confirmed that the nitrogen-deprived and ILI-rich phenotype was associated with an increased tolerance towards several drugs used for treating mycobacterial infections. Importantly, we showed that the presence of ILI substantially enhanced the bacterial burden and granuloma abundance in zebrafish embryos infected with lipid-rich M. abscessus as compared to embryos infected with lipid-poor M. abscessus , suggesting that ILI are actively contributing to mycobacterial virulence and pathogenesis.

Infection by multidrug-resistant Mycobacterium abscessus is increasingly prevalent in cystic fibrosis (CF) patients, leaving clinicians with few therapeutic options. A compassionate study showed the clinical improvement of a CF patient with a disseminated M. abscessus (GD01) infection, following injection of a phage cocktail, including phage Muddy. Broadening the use of phage therapy in patients as a potential antibacterial alternative necessitates the development of biological models to improve the reliability and successful prediction of phage therapy in the clinic. Herein, we demonstrate that Muddy very efficiently lyses GD01 in vitro, an effect substantially increased with standard drugs. Remarkably, this cooperative activity was retained in an M. abscessus model of infection in CFTR-depleted zebrafish, associated with a striking increase in larval survival and reduction in pathological signs. The activity of Muddy was lost in macrophage-ablated larvae, suggesting that successful phage therapy relies on functional innate immunity. CFTR-depleted zebrafish represent a practical model to rapidly assess phage treatment efficacy against M. abscessus isolates, allowing the identification of drug combinations accompanying phage therapy and treatment prediction in patients. This article has an associated First Person interview with the first author of the paper.