MAB_3551c encodes the primary triacylglycerol synthase involved in lipid accumulation in Mycobacterium abscessus

Restriction of mitochondrial oxidation of glutamine or fatty acids enhances intracellular growth of Mycobacterium abscessus in macrophages

Mycobacterium abscessus (Mab), a nontuberculous mycobacterium, is increasing in prevalence worldwide and causes treatment-refractory pulmonary diseases. However, how Mab rewires macrophage energy metabolism to facilitate its survival is poorly understood. We compared the metabolic profiles of murine bone marrow-derived macrophages (BMDMs) infected with smooth (S)- and rough (R)-type Mab using extracellular flux technology. Mab infection shifted BMDMs towards a more energetic phenotype, marked by increased oxidative phosphorylation (OXPHOS) and glycolysis, with a significantly greater enhancement in OXPHOS. This metabolic adaptation was characterized by enhanced ATP production rates, particularly in cells infected with S-type Mab, highlighting OXPHOS as a key energy source. Notably, Mab infection also modulated mitochondrial substrate preferences, increasing fatty acid oxidation capabilities while revealing significant changes in glutamine dependency and flexibility. R-type Mab infections exhibited a marked decrease in glutamine reliance but enhanced metabolic flexibility and capacity. Furthermore, targeting metabolic pathways related to glutamine and fatty acid oxidation exacerbated Mab growth within macrophages, suggesting these pathways play a protective role against infection. These insights advance our understanding of Mab's impact on host cell metabolism and propose a novel avenue for therapeutic intervention. By manipulating host mitochondrial metabolism, we identify a potential host-directed therapeutic strategy against Mab, offering a promising alternative to conventional treatments beleaguered by drug resistance. This study underscores the importance of exploring metabolic interventions to combat Mab infection, paving the way for innovative approaches in the fight against this formidable pathogen.

Rv3371, a triacylglycerol synthase promotes survival of Mycobacterium tuberculosis in the host through its contributions to redox homeostasis and propionate detoxification

Triacylglycerol (TAG) is the major storage lipid of mycobacteria. Mycobacterium tuberculosis (Mtb) genome encodes 15 triacylglycerol synthases (Tgs), which are speculated to differ in substrate preference, suggesting specific physiological roles. In this study, we investigated the role of a Tgs, Rv3371, in the context of infection. Rv3371 knock-out (KO) Mtb was attenuated in mice, with corresponding poor fitness inside macrophages. The KO was more sensitive to free long-chain fatty acids, but was more tolerant to oxidative and nitrosative stresses. Enzyme kinetics of Rv3371 showed its preference for propionyl-CoA. Excess propionate in growth medium retarded the growth of the KO more significantly than the wild type and complemented mutant. This suggests an additional role of Rv3371 in reducing toxic levels of propionate in Mtb by synthesising propionyl TAG. Moreover, chemical inhibition of methylcitrate cycle caused a decrease in methyl-branched lipids in the KO. Overall, the results suggest a role of Rv3371 in Mtb survival in the host through its roles beyond TAG storage.

Mycobacterium abscessus resides within lipid droplets and acquires a dormancy-like phenotype in adipocytes

Mycobacterium abscessus (M. abscessus) is an emerging, rapidly growing mycobacterium that causes chronic lung infection, particularly in patients with cystic fibrosis, as well as skin and soft tissue infections. Adipose tissue is recognized as an important niche that supports M. tuberculosis persistence. However, the dormancy, latency, and persistence mechanisms of M. abscessus in the host remain poorly understood. This study investigated how adipose tissue serves as a niche for M. abscessus using both a human adipose tissue ex vivo infection model and a murine adipose tissue in vivo infection model. M. abscessus infected not only the cytosol of adipocytes but also entered host lipid droplets, where it formed intracytoplasmic lipid inclusions in the bacterial cell. To our knowledge, this unique localization has never been reported for M. abscessus or any other mycobacterium. Within host lipid droplets, M. abscessus lost acid-fastness and gained Nile Red positivity. These results suggest that M. abscessus acquires a dormancy-like phenotype within host lipid droplets of adipocytes, potentially contributing to its persistence, virulence, and resistance to treatment. These findings provide valuable insights into mycobacterial persistence mechanisms and could offer a promising foundation for developing novel therapeutic approaches.

A proteomic and functional view of intrabacterial lipid inclusion biogenesis in mycobacteria

During infection and granuloma formation, pathogenic mycobacteria store triacylglycerol as intrabacterial lipid inclusions (ILIs). This accumulation of nutrients provides a carbon source for bacterial persistence and slows down intracellular metabolism. Mycobacterium abscessus (Mab), a rapidly growing non-tuberculous actinobacterium, produces ILI throughout its infection cycle. Here, Mab was used as a model organism to identify proteins associated with ILI accumulation on a global scale. By using the APEX2 proximity labeling method in an in vitro model for ILI accumulation, we identified 228 proteins possibly implicated in ILI biosynthesis. Fluorescence microscopy of strains overexpressing eight ILI-associated proteins (IAP) candidates fused to superfolder green fluorescent protein showed co-localization with ILI. Genetic inactivation of these potential IAP-encoding genes and subsequent lipid analysis emphasized the importance of MAB_3486 and MAB_4532c as key enzymes influencing triacylglycerol storage. This study underscores the dynamic process of ILI biogenesis and advances our understanding of lipid metabolism in pathogenic mycobacteria. Identifying major IAP in lipid accumulation offers new therapeutic perspectives to control the growth and persistence of pathogenic mycobacteria.

IMPORTANCE

This study sheds light into the complex process of intracellular lipid accumulation and storage in the survival and persistence of pathogenic mycobacteria, which is of clinical relevance. By identifying the proteins involved in the formation of intrabacterial lipid inclusions and revealing their impact on lipid metabolism, our data may lead to the development of new therapeutic strategies to target and control pathogenic mycobacteria, potentially improving outcomes for patients with mycobacterial infections.

Toward better cures for Mycobacterium abscessus lung disease

SUMMARYThe opportunistic pathogen Mycobacterium abscessus (Mab) causes fatal lung infections that bear similarities-and notable differences-with tuberculosis (TB) pulmonary disease. In contrast to TB, no antibiotic is formally approved to treat Mab disease, there is no reliable cure, and the discovery and development pipeline is incredibly thin. Here, we discuss the factors behind the unsatisfactory cure rates of Mab disease, namely intrinsic resistance and persistence of the pathogen, and the use of underperforming, often parenteral and toxic, repurposed drugs. We propose preclinical strategies to build injectable-free sterilizing and safe regimens: (i) prioritize oral bactericidal antibiotic classes, with an initial focus on approved agents or advanced clinical candidates to provide immediate options for desperate patients, (ii) test drug combinations early, (iii) optimize novel leads specifically for M. abscessus, and (iv) consider pharmacokinetic-pharmacodynamic targets at the site of disease, the lung lesions in which drug tolerant bacterial populations reside. Knowledge and tool gaps in the preclinical drug discovery process are identified, including validated mouse models and computational platforms to enable in vitro mouse-human translation. We briefly discuss recent advances in clinical development, the need for readouts and biomarkers that correlate with cure, and clinical trial concepts adapted to the uniqueness of Mab patient populations for new regimen development. In an era when most pharmaceutical firms have withdrawn from antimicrobial drug discovery, the breakthroughs needed to fill the regimen development pipeline will likely come from partnerships between academia, biotech, pharma, non-profit organizations, and governments, with incentives that reward cooperation.

Biofilm formation and polar lipid biosynthesis in Mycobacterium abscessus are inhibited by naphthylmethylpiperazine

Mycobacterium abscessus is a biofilm-forming, non-tuberculous mycobacterium that is highly resistant to antibiotics. Bacterial efflux pumps contribute to biofilm formation, export of biofilm-associated lipids and antibiotic tolerance. The Resistance Nodulation Cell Division (RND) and ATP-Binding Cassette (ABC) families of efflux pumps export lipids to the mycobacterial cell surface. 1-(1-naphthyl methyl)-piperazine (NMP) is a chemosensitizer that causes membrane destabilization and is an inhibitor of RND efflux pumps. The effects of NMP on biofilm formation and lipid metabolism in M. abscessus biofilms have not been investigated. Plumbagin (PLU) is an inhibitor of ABC efflux pumps that has not been studied for its effects on antibiotic tolerance in M. abscessus biofilms. In this study, we report that the efflux pump inhibitors NMP and PLU inhibit biofilm formation by 50% at sub-MIC levels. We show that NMP inhibits the incorporation of the radiolabeled long-chain fatty acid 14C-palmitate into glycopeptidolipids in cell surface lipids of log-phase M. abscessus. NMP also inhibits the utilization of the radiolabel in the biosynthesis of phosphatidylethanolamine in the cell surface and cellular lipids of M. abscessus cells in log-phase and in biofilms. Incorporation of the radiolabel into cardiolipin in the cellular lipids of M. abscessus biofilms was inhibited by NMP. The incorporation of 14C-acetate into cell surface phosphatidylethanolamine in log-phase and biofilm cells was also inhibited by NMP. Triacylglycerol biosynthesis using 14C-palmitate and 14C-acetate in cellular lipids of log-phase and biofilm cells was increased several folds by NMP. Efflux pump activity in M. abscessus cells was inhibited by 97% and 68% by NMP and PLU respectively. NMP and PLU caused 5-fold decreases in the minimum inhibitory concentrations of ciprofloxacin and clarithromycin against M. abscessus. Our results demonstrate that NMP and PLU affect important physiological processes in M. abscessus associated with its pathogenesis.

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.

An anti-mycobacterial conjugated oligoelectrolyte effective against Mycobacterium abscessus

Infections caused by nontuberculous mycobacteria have increased more than 50% in the past two decades and more than doubled in the elderly population. Mycobacterium abscessus (Mab), one of the most prevalent of these rapidly growing species, is intrinsically resistant to numerous antibiotics. Current standard-of-care treatments are not satisfactory, with high failure rate and notable adverse effects. We report here a potent anti-Mab compound from the flexible molecular framework afforded by conjugated oligoelectrolytes (COEs). A screen of structurally diverse, noncytotoxic COEs identified a lead compound, COE-PNH2, which was bactericidal against replicating, nonreplicating persisters and intracellular Mab.COE-PNH2 had low propensity for resistance development, with a frequency of resistance below 1.25 × 10-9 and showed no detectable resistance upon serial passaging. Mechanism of action studies were in line with COE-PNH2 affecting the physical and functional integrity of the bacterial envelope and disrupting the mycomembrane and associated essential bioenergetic pathways. Moreover, COE-PNH2 was well-tolerated and efficacious in a mouse model of Mab lung infection. This study highlights desirable in vitro and in vivo potency and safety index of this COE structure, which represents a promising anti-mycobacterial to tackle an unmet medical need.

Intrabacterial lipid inclusion-associated proteins: a core machinery conserved from saprophyte Actinobacteria to the human pathogen Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb), the aetiologic agent of tuberculosis (TB), stores triacylglycerol (TAG) in the form of intrabacterial lipid inclusions (ILI) to survive and chronically persist within its host. These highly energetic molecules represent a major source of carbon to support bacterial persistence and reactivation, thus playing a leading role in TB pathogenesis. However, despite its physiological and clinical relevance, ILI metabolism in Mtb remains poorly understood. Recent discoveries have suggested that several ILI-associated proteins might be widely conserved across TAG-producing prokaryotes, but still very little is known regarding the nature and the biological functions of these proteins. Herein, we performed an in silico analysis of three independent ILI-associated proteomes previously reported to computationally define a potential core ILI-associated proteome, referred to as ILIome. Our investigation revealed the presence of 70 orthologous proteins that were strictly conserved, thereby defining a minimal ILIome core. We further narrowed our analysis to proteins involved in lipid metabolism and discuss here their putative biological functions, along with their molecular interactions and dynamics at the surface of these bacterial organelles. We also highlight the experimental limitations of the original proteomic investigations and of the present bioinformatic analysis, while describing new technological approaches and presenting biological perspectives in the field. The in silico investigation presented here aims at providing useful datasets that could constitute a scientific resource of broad interest for the mycobacterial community, with the ultimate goal of enlightening ILI metabolism in prokaryotes with a special emphasis on Mtb pathogenesis.

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.

Mycobacterium abscessus DosRS two-component system controls a species-specific regulon required for adaptation to hypoxia

Mycobacterium abscessus (Mab), an emerging opportunistic pathogen, predominantly infects individuals with underlying pulmonary diseases such as cystic fibrosis (CF). Current treatment outcomes for Mab infections are poor due to Mab's inherent antibiotic resistance and unique host interactions that promote phenotypic tolerance and hinder drug access. The hypoxic, mucus-laden airways in the CF lung and antimicrobial phagosome within macrophages represent hostile niches Mab must overcome via alterations in gene expression for survival. Regulatory mechanisms important for the adaptation and long-term persistence of Mab within the host are poorly understood, warranting further genetic and transcriptomics study of this emerging pathogen. DosRS Mab , a two-component signaling system (TCS), is one proposed mechanism utilized to subvert host defenses and counteract environmental stress such as hypoxia. The homologous TCS of Mycobacterium tuberculosis (Mtb), DosRS Mtb , is known to induce a ~50 gene regulon in response to hypoxia, carbon monoxide (CO) and nitric oxide (NO) in vitro and in vivo. Previously, a small DosR Mab regulon was predicted using bioinformatics based on DosR Mtb motifs however, the role and regulon of DosRS Mab in Mab pathogenesis have yet to be characterized in depth. To address this knowledge gap, our lab generated a Mab dosRS knockout strain (MabΔdosRS) to investigate differential gene expression, and phenotype in an in vitro hypoxia model of dormancy. qRT-PCR and lux reporter assays demonstrate Mab_dosR and 6 predicted downstream genes are induced in hypoxia. In addition, RNAseq revealed induction of a much larger hypoxia response comprised of >1000 genes, including 127 differentially expressed genes in a dosRS mutant strain. Deletion of DosRS Mab led to attenuated growth under low oxygen conditions, a shift in morphotype from smooth to rough, and down-regulation of 216 genes. This study provides the first look at the global transcriptomic response of Mab to low oxygen conditions encountered in the airways of CF patients and within macrophage phagosomes. Our data also demonstrate the importance of DosRS Mab for adaptation of Mab to hypoxia, highlighting a distinct regulon (compared to Mtb) that is significantly larger than previously described, including both genes conserved across mycobacteria as well as Mab-specific genes.

Mycobacterium abscessus alkyl hydroperoxide reductase C promotes cell invasion by binding to tetraspanin CD81

Mycobacterium abscessus (Mab) is an increasingly recognized pulmonary pathogen. How Mab is internalized by macrophages and establishes infection remains unknown. Here, we show that Mab uptake is significantly reduced in macrophages pre-incubated with neutralizing anti-CD81 antibodies or in cells in which the large extracellular loop (LEL) of CD81 has been deleted. Saturation of Mab with either soluble GST-CD81-LEL or CD81-LEL-derived peptides also diminished internalization of the bacilli. The mycobacterial alkyl hydroperoxide reductase C (AhpC) was unveiled as a major interactant of CD81-LEL. Pre-exposure of macrophages with soluble AhpC inhibited mycobacterial uptake whereas overexpression of AhpC in Mab enhanced its internalization. Importantly, pre-incubation of macrophages with anti-CD81-LEL antibodies inhibited phagocytosis of AhpC-coated beads, indicating that AhpC is a direct interactant of CD81-LEL. Conditional depletion of AhpC in Mab correlated with decreased internalization of Mab. These compelling data unravel a previously unexplored role for CD81/AhpC to promote uptake of pathogenic mycobacteria by host cells.

Isolation of Staphylococcus aureus, Uropathogenic Escherichia coli, and Nontuberculous Mycobacteria Strains from Pasteurized Cheeses and Unpasteurized Cream Sold at Traditional Open Markets in Mexico City

ABSTRACT

Fresh cheeses and cream are important garnishes of traditional Mexican food, often purchased at street or itinerant open markets or tianguis. However, there is scarce information regarding the microbiological quality of cheeses and cream sold in tianguis. For 2 years, three dairy stalls from three tianguis in Mexico City were visited once each season, trading practices were registered, and 96 dairy products were purchased. In total 72 fresh pasteurized cheeses that were hand-cut to order (24 Panela, 24 Canasto, and 24 Doble Crema) and 24 unpasteurized Crema de Rancho samples were collected. All dairy products remained without refrigeration for 8 h. Based on the National Guidelines limits, 87.5% of cheeses and 8% of Crema de Rancho samples were of low microbiological quality, and 1 sample of each type of cheese and 3 samples of Crema de Rancho exceeded the guidelines limits for Staphylococcus aureus. All dairy products were negative for Salmonella, Listeria monocytogenes, and all diarrheagenic Escherichia coli pathotypes, including Shiga toxin-producing E. coli. Among the 96 dairy samples, the prevalence of uropathogenic E. coli (UPEC) and of mycobacteria strains were determined because food items contaminated with these strains have been associated with urinary tract infections and mycobacteriosis, respectively. UPEC strains were isolated from 43% of cut-to-order cheeses and 29% of Crema de Rancho samples. Nontuberculous mycobacteria (NTM) strains were identified in 12.5% of Doble Crema cheese samples and 21% of Crema de Rancho samples. From the eight NTM-positive samples, 10 strains were identified (3 strains of Mycolicibacterium fortuitum, 2 of Mycobacteroides abscessus, 2 of Mycobacteroides chelonae, 2 of Mycolicibacterium porcinum, and 1 of Mycolicibacterium rhodesiae). All produced biofilms, and 70% had sliding motility (both virulence traits). Trading practices of cut-to-order pasteurized cheeses and unpasteurized Crema de Rancho in tianguis increase the risk of microbiological contamination of these products, including with human pathogens, and their consumption may cause human illness.

HIGHLIGHTS

The High Plasticity of Nonpathogenic Mycobacterium brumae Induces Rapid Changes in Its Lipid Profile during Pellicle Maturation: The Potential of This Bacterium as a Versatile Cell Factory for Lipid Compounds of Therapeutic Interest

The immunomodulatory potential of mycobacteria to be used for therapeutic purposes varies by species and culture conditions and is closely related to mycobacterial lipid composition. Although the lipids present in the mycobacterial cell wall are relevant, lipids are mainly stored in intracellular lipid inclusions (ILIs), which have emerged as a crucial structure in understanding mycobacteria-host interaction. Little is known about ILI ultrastructure, production, and composition in nonpathogenic species. In this study, we compared the lipid profiles of the nonpathogenic immunomodulatory agent Mycobacterium brumae during pellicle maturation under different culture conditions with qualitative and quantitative approaches by using high-resolution imaging and biochemical and composition analyses to understand ILI dynamics. The results showed wax esters, mainly in early stages of development, and acylglycerols in mature ILI composition, revealing changes in dynamics, amount, and morphometry, depending on pellicle maturation and the culture media used. Low-glycerol cultures induced ILIs with lower molecular weights which were smaller in size in comparison with the ILIs produced in glycerol-enriched media. The data also indicate the simple metabolic plasticity of lipid synthesis in M. brumae, as well as its high versatility in generating different lipid profiles. These findings provide an interesting way to enhance the production of key lipid structures via the simple modulation of cell culture conditions.

The role of triacylglycerols and repurposing DGAT1 inhibitors for the treatment of Mycobacterium tuberculosis

Latent tuberculosis poses a significant threat to global health through the incubation of undiagnosed infections within the community, and through its tolerance to antibiotics. This Special Features article explores the mechanisms by which the dormant Mycobacterium tuberculosis pathogen can store energy in the form of lipid inclusion bodies and triacylglycerols, which may be key in the development of novel therapeutics to treat TB.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Glycopeptidolipid glycosylation controls surface properties and pathogenicity in Mycobacterium abscessus

Mycobacterium abscessus is an emerging and difficult-to-manage mycobacterial species that exhibits smooth (S) or rough (R) morphotypes. Disruption of glycopeptidolipid (GPL) production results in transition from S to R and severe lung disease. A structure-activity relationship study was undertaken to decipher the role of GPL glycosylation in morphotype transition and pathogenesis. Deletion of gtf3 uncovered the prominent role of the extra rhamnose in enhancing mannose receptor-mediated internalization of M. abscessus by macrophages. In contrast, the absence of the 6-deoxy-talose and the first rhamnose in mutants lacking gtf1 and gtf2, respectively, affected M abscessus phagocytosis but also resulted in the S-to-R transition. Strikingly, gtf1 and gtf2 mutants displayed a strong propensity to form cords and abscesses in zebrafish, leading to robust and lethal infection. Together, these results underscore the importance and differential contribution of GPL monosaccharides in promoting virulence and infection outcomes.

Exploring the value of Mycobacterium tuberculosis modified lipoprotein as a potential biomarker for TB detection in children

Background

Current TB diagnostic methods available have been developed for adults and development efforts have neglected the differences in disease and sampling that occur between adults and children. Diagnostic challenges are even greater in HIV co-infected children and infants.

Methods and results

We established a sandwich ELISA assay to detect Mycobacterium tuberculosis modified lipoprotein (TLP) ex vivo in plasma. The study population contains plasma samples from 21 patients with active TB and 24 control samples with no TB, collected in the International Maternal Pediatric Adolescent AIDS Clinical Trails (IMPAACT) P1041 study. Retrospective analysis was performed and the results demonstrate that the median plasma levels of TLP in control subjects are 2.7 fold higher than the median plasma values in active TB subjects (p < 0.001).

Conclusions

Plasma levels of TLP are elevated with active TB disease in HIV positive subjects and deserves further exploration as an indicator for TB detection in children.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-022-07140-9.

Characterization of key enzymes involved in triacylglycerol biosynthesis in mycobacteria

Phosphatidic acid phosphatase (PAP) catalyzes the dephosphorylation of phosphatidic acid (PA) yielding diacylglycerol (DAG), the lipid precursor for triacylglycerol (TAG) biosynthesis. PAP activity has a key role in the regulation of PA flux towards TAG or glycerophospholipid synthesis. In this work we have characterized two Mycobacterium smegmatis genes encoding for functional PAP proteins. Disruption of both genes provoked a sharp reduction in de novo TAG biosynthesis in early growth phase cultures under stress conditions. In vivo labeling experiments demonstrated that TAG biosynthesis was restored in the ∆PAP mutant when bacteria reached exponential growth phase, with a concomitant reduction of phospholipid synthesis. In addition, comparative lipidomic analysis showed that the ∆PAP strain had increased levels of odd chain fatty acids esterified into TAGs, suggesting that the absence of PAP activity triggered other rearrangements of lipid metabolism, like phospholipid recycling, in order to maintain the wild type levels of TAG. Finally, the lipid changes observed in the ∆PAP mutant led to defective biofilm formation. Understanding the interaction between TAG synthesis and the lipid composition of mycobacterial cell envelope is a key step to better understand how lipid homeostasis is regulated during Mycobacterium tuberculosis infection.

Intrabacterial lipid inclusions in mycobacteria: unexpected key players in survival and pathogenesis?

Mycobacterial species, including Mycobacterium tuberculosis, rely on lipids to survive and chronically persist within their hosts. Upon infection, opportunistic and strict pathogenic mycobacteria exploit metabolic pathways to import and process host-derived free fatty acids, subsequently stored as triacylglycerols under the form of intrabacterial lipid inclusions (ILI). Under nutrient-limiting conditions, ILI constitute a critical source of energy that fuels the carbon requirements and maintain redox homeostasis, promoting bacterial survival for extensive periods of time. In addition to their basic metabolic functions, these organelles display multiple other biological properties, emphasizing their central role in the mycobacterial lifecycle. However, despite of their importance, the dynamics of ILI metabolism and their contribution to mycobacterial adaptation/survival in the context of infection has not been thoroughly documented. Herein, we provide an overview of the historical ILI discoveries, their characterization, and current knowledge regarding the micro-environmental stimuli conveying ILI formation, storage and degradation. We also review new biological systems to monitor the dynamics of ILI metabolism in extra- and intracellular mycobacteria and describe major molecular actors in triacylglycerol biosynthesis, maintenance and breakdown. Finally, emerging concepts regarding to the role of ILI in mycobacterial survival, persistence, reactivation, antibiotic susceptibility and inter-individual transmission are also discuss.

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

Background

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

Methods

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

Results

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

Conclusions

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

Lipolytic enzymes inhibitors: A new way for antibacterial drugs discovery

Tuberculosis (TB) caused by Mycobacterium tuberculosis (M. tb) still remains the deadliest infectious disease worldwide with 1.5 million deaths in 2018, of which about 15% are attributed to resistant strains. Another significant example is Mycobacterium abscessus (M. abscessus), a nontuberculous mycobacteria (NTM) responsible for cutaneous and pulmonary infections, representing up to 95% of NTM infections in cystic fibrosis (CF) patients. M. abscessus is a new clinically relevant pathogen and is considered one of the most drug-resistant mycobacteria for which standardized chemotherapeutic regimens are still lacking. Together the emergence of M. tb and M. abscessus multi-drug resistant strains with ineffective and expensive therapeutics, have paved the way to the development of new classes of anti-mycobacterial agents offering additional therapeutic options. In this context, specific inhibitors of mycobacterial lipolytic enzymes represent novel and promising antibacterial molecules to address this challenging issue. The results highlighted here include a complete overview of the antibacterial activities, either in broth medium or inside infected macrophages, of two families of promising and potent anti-mycobacterial multi-target agents, i.e. oxadiazolone-core compounds (OX) and Cyclophostin & Cyclipostins analogs (CyC); the identification and biochemical validation of their effective targets (e.g., the antigen 85 complex and TesA playing key roles in mycolic acid metabolism) together with their respective crystal structures. To our knowledge, these are the first families of compounds able to target and impair replicating as well as intracellular bacteria. We are still impelled in deciphering their mode of action and finding new potential therapeutic targets against mycobacterial-related diseases.

O-Methylation of the Glycopeptidolipid Acyl Chain Defines Surface Hydrophobicity of Mycobacterium abscessus and Macrophage Invasion

Mycobacterium abscessus, an emerging pathogen responsible for severe lung infections in cystic fibrosis patients, displays either smooth (S) or rough (R) morphotypes. The S-to-R transition is associated with reduced levels of glycopeptidolipid (GPL) production and is correlated with increased pathogenicity in animal and human hosts. While the structure of GPL is well established, its biosynthetic pathway is incomplete. In addition, the biological functions of the distinct structural parts of this complex lipid remain elusive. Herein, the fmt gene encoding a putative O-methyltransferase was deleted in the M. abscessus S variant. Subsequent biochemical and structural analyses demonstrated that methoxylation of the fatty acyl chain of GPL was abrogated in the Δfmt mutant, and this defect was rescued upon complementation with a functional fmt gene. In contrast, the introduction of fmt derivatives mutated at residues essential for methyltransferase activity failed to complement GPL defects, indicating that fmt encodes an O-methyltransferase. Unexpectedly, phenotypic analyses showed that Δfmt was more hydrophilic than its parental progenitor, as demonstrated by hexadecane-aqueous buffer partitioning and atomic force microscopy experiments with hydrophobic probes. Importantly, the invasion rate of THP-1 macrophages by Δfmt was reduced by 50% when compared to the wild-type strain. Together, these results indicate that Fmt O-methylates the lipid moiety of GPL and plays a substantial role in conditioning the surface hydrophobicity of M. abscessus as well as in the early steps of the interaction between the bacilli and macrophages.

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

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

Nitrogen deprivation induces triacylglycerol accumulation, drug tolerance and hypervirulence in mycobacteria

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.

Triacylglycerols: Fuelling the Hibernating Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) has the remarkable ability to persist with a modified metabolic status and phenotypic drug tolerance for long periods in the host without producing symptoms of active tuberculosis. These persisters may reactivate to cause active disease when the immune system becomes disrupted or compromised. Thus, the infected hosts with the persisters serve as natural reservoir of the deadly pathogen. Understanding the host and bacterial factors contributing to Mtb persistence is important to devise strategies to tackle the Mtb persisters. Host lipids act as the major source of carbon and energy for Mtb. Fatty acids derived from the host cells are converted to triacylglycerols (triglycerides or TAG) and stored in the bacterial cytoplasm. TAG serves as a dependable, long-term energy source of lesser molecular mass than other storage molecules like glycogen. TAG are found in substantial amounts in the mycobacterial cell wall. This review discusses the production, accumulation and possible roles of TAG in mycobacteria, pointing out the aspects that remain to be explored. Finally, the essentiality of TAG synthesis for Mtb is discussed with implications for identification of intervention strategies.

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

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

Delineating the Physiological Roles of the PE and Catalytic Domains of LipY in Lipid Consumption in Mycobacterium-Infected Foamy Macrophages

Within tuberculous granulomas, a subpopulation of Mycobacterium tuberculosis resides inside foamy macrophages (FM) that contain abundant cytoplasmic lipid bodies (LB) filled with triacylglycerol (TAG). Upon fusion of LB with M. tuberculosis-containing phagosomes, TAG is hydrolyzed and reprocessed by the bacteria into their own lipids, which accumulate as intracytosolic lipid inclusions (ILI). This phenomenon is driven by many mycobacterial lipases, among which LipY participates in the hydrolysis of host and bacterial TAG. However, the functional contribution of LipY's PE domain to TAG hydrolysis remains unclear. Here, enzymatic studies were performed to compare the lipolytic activities of recombinant LipY and its truncated variant lacking the N-terminal PE domain, LipY(ΔPE). Complementarily, an FM model was used where bone marrow-derived mouse macrophages were infected with M. bovis BCG strains either overexpressing LipY or LipY(ΔPE) or carrying a lipY deletion mutation prior to being exposed to TAG-rich very-low-density lipoprotein (VLDL). Results indicate that truncation of the PE domain correlates with increased TAG hydrolase activity. Quantitative electron microscopy analyses showed that (i) in the presence of lipase inhibitors, large ILI (ILI⁺³) were not formed because of an absence of LB due to inhibition of VLDL-TAG hydrolysis or inhibition of LB-neutral lipid hydrolysis by mycobacterial lipases, (ii) ILI⁺³ profiles in the strain overexpressing LipY(ΔPE) were reduced, and (iii) the number of ILI⁺³ profiles in the ΔlipY mutant was reduced by 50%. Overall, these results delineate the role of LipY and its PE domain in host and mycobacterial lipid consumption and show that additional mycobacterial lipases take part in these processes.

Mechanistic and Structural Insights Into the Unique TetR-Dependent Regulation of a Drug Efflux Pump in Mycobacterium abscessus

Mycobacterium abscessus is an emerging human pathogen causing severe pulmonary infections and is refractory to standard antibiotherapy, yet few drug resistance mechanisms have been reported in this organism. Recently, mutations in MAB_4384 leading to up-regulation of the MmpS5/MmpL5 efflux pump were linked to increased resistance to thiacetazone derivatives. Herein, the DNA-binding activity of MAB_4384 was investigated by electrophoretic mobility shift assays using the palindromic sequence IR_S5/L5 located upstream of mmpS5/mmpL5. Introduction of point mutations within IR_S5/L5 identified the sequence requirements for optimal binding of the regulator. Moreover, formation of the protein/IR_S5/L5 complex was severely impaired for MAB_4384 harboring D14N or F57L substitutions. IR_S5/L5/lacZ reporter fusions in M. abscessus demonstrated increased β-galactosidase activity either in strains lacking a functional MAB_4384 or in cultures treated with the TAC analogs. In addition, X-ray crystallography confirmed a typical TetR homodimeric structure of MAB_4384 and unraveled a putative ligand binding site in which the analogs could be docked. Overall, these results support drug recognition of the MAB_4384 TetR regulator, alleviating its binding to IR_S5/L5 and steering up-regulation of MmpS5/MmpL5. This study provides new mechanistic and structural details of TetR-dependent regulatory mechanisms of efflux pumps and drug resistance in mycobacteria.

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.

Elucidation of Mycobacterium abscessus aminoglycoside and capreomycin resistance by targeted deletion of three putative resistance genes

Objectives

Mycobacterium abscessus is innately resistant to a variety of drugs thereby limiting therapeutic options. Bacterial resistance to aminoglycosides (AGs) is conferred mainly by AG-modifying enzymes, which often have overlapping activities. Several putative AG-modifying enzymes are encoded in the genome of M. abscessus . The aim of this study was to investigate the molecular basis underlying AG resistance in M. abscessus .

Methods

M. abscessus deletion mutants deficient in one of three genes potentially involved in AG resistance, aac(2 ' ) , eis1 and eis2 , were generated by targeted gene inactivation, as were combinatorial double and triple deletion mutants. MICs were determined to study susceptibility to a variety of AG drugs and to capreomycin.

Results

Deletion of aac(2 ' ) increased susceptibility of M. abscessus to kanamycin B, tobramycin, dibekacin and gentamicin C. Deletion of eis2 increased susceptibility to capreomycin, hygromycin B, amikacin and kanamycin B. Deletion of eis1 did not affect drug susceptibility. Equally low MICs of apramycin, arbekacin, isepamicin and kanamycin A for WT and mutant strains indicate that these drugs are not inactivated by either AAC(2 ' ) or Eis enzymes.

Conclusions

M. abscessus expresses two distinct AG resistance determinants, AAC(2 ' ) and Eis2, which confer clinically relevant drug resistance.

Cyclipostins and cyclophostin analogs inhibit the antigen 85C from Mycobacterium tuberculosis both in vitro and in vivo

An increasing prevalence of cases of drug-resistant tuberculosis requires the development of more efficacious chemotherapies. We previously reported the discovery of a new class of cyclipostins and cyclophostin (CyC) analogs exhibiting potent activity against Mycobacterium tuberculosis both in vitro and in infected macrophages. Competitive labeling/enrichment assays combined with MS have identified several serine or cysteine enzymes in lipid and cell wall metabolism as putative targets of these CyC compounds. These targets included members of the antigen 85 (Ag85) complex (i.e. Ag85A, Ag85B, and Ag85C), responsible for biosynthesis of trehalose dimycolate and mycolylation of arabinogalactan. Herein, we used biochemical and structural approaches to validate the Ag85 complex as a pharmacological target of the CyC analogs. We found that CyC_7β, CyC_8β, and CyC₁₇ bind covalently to the catalytic Ser¹²⁴ residue in Ag85C; inhibit mycolyltransferase activity (i.e. the transfer of a fatty acid molecule onto trehalose); and reduce triacylglycerol synthase activity, a property previously attributed to Ag85A. Supporting these results, an X-ray structure of Ag85C in complex with CyC_8β disclosed that this inhibitor occupies Ag85C's substrate-binding pocket. Importantly, metabolic labeling of M. tuberculosis cultures revealed that the CyC compounds impair both trehalose dimycolate synthesis and mycolylation of arabinogalactan. Overall, our study provides compelling evidence that CyC analogs can inhibit the activity of the Ag85 complex in vitro and in mycobacteria, opening the door to a new strategy for inhibiting Ag85. The high-resolution crystal structure obtained will further guide the rational optimization of new CyC scaffolds with greater specificity and potency against M. tuberculosis.

Controlling Extra- and Intramacrophagic Mycobacterium abscessus by Targeting Mycolic Acid Transport

Mycobacterium abscessus is a rapidly growing mycobacterium (RGM) causing serious infections especially among cystic fibrosis patients. Extremely limited therapeutic options against M. abscessus and a rise in infections with this mycobacterium require novel chemotherapies and a better understanding of how the bacterium causes infection. Different from most RGM, M. abscessus can survive inside macrophages and persist for long durations in infected tissues. We recently delineated differences in the infective programs followed by smooth (S) and rough (R) variants of M. abscessus. Unexpectedly, we found that the S variant behaves like pathogenic slow growing mycobacteria, through maintaining a block on the phagosome maturation process and by inducing phagosome-cytosol communications. On the other hand, R variant infection triggers autophagy and apoptosis, reminiscent of the way that macrophages control RGM. However, the R variant has an exquisite capacity to form extracellular cords, allowing these bacteria to rapidly divide and evade phagocytosis. Therefore, new chemotherapeutic interventions against M. abscessus need to efficiently deal with both the reservoir of intracellular bacilli and the extracellular cords. In this context, we recently identified two chemical entities that were very effective against both M. abscessus populations. Although being structurally unrelated these two chemotypes inhibit the activity of the essential mycolic acid transporter, MmpL3. In this Perspective, we aimed to highlight recent insights into how M. abscessus interacts with phagocytic cells and how the inhibition of mycolic acid transport in this pathogenic RGM could be an efficient means to control both intracellular and extracellular populations of the bacterium.

The Diverse Cellular and Animal Models to Decipher the Physiopathological Traits of Mycobacterium abscessus Infection

Mycobacterium abscessus represents an important respiratory pathogen among the rapidly-growing non-tuberculous mycobacteria. Infections caused by M. abscessus are increasingly found in cystic fibrosis (CF) patients and are often refractory to antibiotic therapy. The underlying immunopathological mechanisms of pathogenesis remain largely unknown. A major reason for the poor advances in M. abscessus research has been a lack of adequate models to study the acute and chronic stages of the disease leading to delayed progress of evaluation of therapeutic efficacy of potentially active antibiotics. However, the recent development of cellular models led to new insights in the interplay between M. abscessus with host macrophages as well as with amoebae, proposed to represent the environmental host and reservoir for non-tuberculous mycobacteria. The zebrafish embryo has also appeared as a useful alternative to more traditional models as it recapitulates the vertebrate immune system and, due to its optical transparency, allows a spatio-temporal visualization of the infection process in a living animal. More sophisticated immunocompromised mice have also been exploited recently to dissect the immune and inflammatory responses to M. abscessus. Herein, we will discuss the limitations, advantages and potential offered by these various models to study the pathophysiology of M. abscessus infection and to assess the preclinical efficacy of compounds active against this emerging human pathogen.

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

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

Experimental Models of Foamy Macrophages and Approaches for Dissecting the Mechanisms of Lipid Accumulation and Consumption during Dormancy and Reactivation of Tuberculosis

Despite a slight decline since 2014, tuberculosis (TB) remains the major deadly infectious disease worldwide with about 1.5 million deaths each year and with about one-third of the population being latently infected with Mycobacterium tuberculosis, the etiologic agent of TB. During primo-infection, the recruitment of immune cells leads to the formation of highly organized granulomas. Among the different cells, one outstanding subpopulation is the foamy macrophage (FM), characterized by the abundance of triacylglycerol-rich lipid bodies (LB). M. tuberculosis can reside in FM, where it acquires, from host LB, the neutral lipids which are subsequently processed and stored by the bacilli in the form of intracytosolic lipid inclusions (ILI). Although host LB can be viewed as a reservoir of nutrients for the pathogen during latency, the molecular mechanisms whereby intraphagosomal mycobacteria interact with LB and assimilate the LB-derived lipids are only beginning to be understood. Past studies have emphasized that these physiological processes are critical to the M. tuberculosis infectious-life cycle, for propagation of the infection, establishment of the dormancy state and reactivation of the disease. In recent years, several animal and cellular models have been developed with the aim of dissecting these complex processes and of determining the nature and contribution of their key players. Herein, we review some of the in vitro and in vivo models which allowed to gain significant insight into lipid accumulation and consumption in M. tuberculosis, two important events that are directly linked to pathogenicity, granuloma formation/maintenance and survival of the tubercle bacillus under non-replicative conditions. We also discuss the advantages and limitations of each model, hoping that this will serve as a guide for future investigations dedicated to persistence and innovative therapeutic approaches against TB.