The cell envelope of Mycobacterium abscessus and its role in pathogenesis

Pathogenicity and virulence of Mycobacterium abscessus

Non-tuberculous mycobacteria (NTM), such as Mycobacterium abscessus (Mab) are an increasing cause of human disease. While the majority of immunocompetent hosts control Mab infections, the robust survival of Mab within the environment has shaped survival in human cells to help drive persistence and cause inflammatory damage in susceptible individuals. With high intrinsic resistance to antibiotics, there is an important need to fully understand how Mab causes infection, define protective host pathways that control disease, and develop new strategies to treat those at high risk. This review will examine the existing literature related to host-Mab interactions with a focus on virulence, the host response, and therapy development. The goal is to highlight key gaps in our understanding and describe novel approaches to encourage new research avenues that better define the pathogenesis and host response against this increasingly important human pathogen.

Mycobacteroides abscessus subspecies: a comparative genomic analysis reveals unique metabolic activities and drug resistance patterns

BACKGROUND

Mycobacteroides abscessus poses a considerable and growing threat to public health due to its resistance against most antibiotics and low cure rate. For a comprehensive understanding of the genomic characteristics and drug resistance mechanisms of M. abscessus, clinical isolates from diverse sources were collected and analyzed.

RESULTS

The clinical M. abscessus complex analyzed herein primarily comprised two subspecies: Mycobacteroides abscessus subsp. abscessus and Mycobacteroides abscessus subsp. massiliense. Furthermore, comparative genomic and single nucleotide polymorphism analyses revealed distinct metabolic activities among subspecies. Subsequent examination of core hub gene mutations confirmed the presence of distinct metabolic and biosynthetic pathways between M. abscessus subspecies, which may have contributed to their differential drug resistance and may aid in providing targeted interventions. Understanding this subtle genomic variation is crucial for improving treatment strategies and patient outcomes. Additional analyses identified potential novel amikacin and moxifloxacin resistance genes, offering a promising avenue for investigating M. abscessus drug resistance.

CONCLUSIONS

Through comparative genomic analysis, we revealed the unique metabolic activities of M. abscessus subsp. abscessus and M. abscessus subsp. massiliense, providing a scientific basis for future diagnostic and personalized management strategies. Identifying possible novel amikacin and moxifloxacin resistance genes within these subspecies offers insights for future drug development efforts and enhances our understanding of the mechanisms underlying M. abscessus drug resistance.

Insights on the Pathogenesis of Mycobacterium abscessus Infection in Patients with Cystic Fibrosis

People with CF (pwCF) have a significant risk for pulmonary infections with non-tuberculous mycobacteria (NTM), particularly Mycobacterium abscessus (Mab). Mab is an emerging pathogen, which causes pulmonary infections in patients with chronic lung diseases, particularly CF; Mab pulmonary disease leads to progressive pulmonary dysfunction and increased morbidity and mortality. Despite advances in CF care, including CFTR modulators (CFTRm), Mab continues to pose a therapeutic challenge, with significant long-term medical burden. This review provides insights into the complex host-pathogen interplay of Mab infections in pwCF. It provides a detailed overview of Mab bacterial virulence factors, including biofilm formation, secretion systems, the virulence-associated rough morphotype, and antibiotic resistance mechanisms. This review also summarizes features conferring susceptibility of the CF host to Mab infections, alongside the contribution of the CF-host environment to the pathogenesis of Mab infection, such as antibiotic-derived microbial selection, within-host mycobacterial evolution, and interactions with co-pathogens such as Pseudomonas aeruginosa (PA). Finally, the therapeutic implications and novel treatments for Mab are discussed, considering the complex host-pathogen interplay.

Sphingosine kills Mycobacteria and suppresses mycobacterial lung infections

Tuberculous mycobacterial infections pose a substantial global health burden because of their prevalence and multi-drug resistance. The current approach to tackling these infections primarily involves developing new antibiotics or combining existing ones, an approach that often proves ineffective in the specific targeting of mycobacteria. We investigated the effect of sphingosine on tuberculous Mycobacteria in vitro and mycobacterial infections in vivo to test whether sphingosine could potentially be used as a novel drug against tuberculosis. Sphingosine inhibited mycobacterial growth and eradicated mycobacteria in vitro. Mechanistically, sphingosine increased bacterial membrane permeability and induced marked changes on the bacterial plasma membrane evidenced by electron microscopy studies. Administration of sphingosine in a mouse model of pulmonary infection with Bacillus Calmette-Guérin (BCG) greatly reduced the number of bacteria in the lung and prevented pulmonary inflammation. Furthermore, infection of ex vivo human lung tissue samples with BCG and treatment with sphingosine showed that sphingosine also kills BCG in human bronchi. Our findings suggest that sphingosine may be a potential therapeutic intervention against mycobacterial infections. KEY MESSAGES: Sphingosine inhibits mycobacterial growth in vitro. Sphingosine disrupts bacterial membrane integrity. Sphingosine reduces bacterial load in mouse pulmonary infection model. Sphingosine eradicates mycobacteria in human bronchi ex vivo.

Cell Death Mechanisms in Mycobacterium abscessus Infection: A Double-Edged Sword

Infections caused by non-tuberculous mycobacteria (NTM), such as Mycobacterium abscessus, elicit diverse cell death mechanisms including apoptosis, necrosis, and pyroptosis, which play key roles in immunopathogenesis. NTM can manipulate these cell death pathways to evade host immune responses, ensuring their intracellular survival and persistence. Apoptosis may aid in antigen presentation and immune activation, while necrosis and pyroptosis trigger excessive inflammation, leading to tissue damage. Autophagy, a crucial cellular defense mechanism, is often induced in response to NTM infection; however, M. abscessus has evolved mechanisms to inhibit autophagic processes, enhancing its ability to survive within host cells. This manipulation of cell death pathways, particularly the dysregulation of autophagy and ferroptosis, contributes to chronic infection, immune evasion, and tissue damage, complicating disease management. Understanding these mechanisms offers potential therapeutic targets for improving treatment strategies against M. abscessus infections.

Infection model of THP-1 cells, growth dynamics, and antimicrobial susceptibility of clinical Mycobacterium abscessus isolates from cystic fibrosis patients: Results from a multicentre study

Mycobacterium abscessus (MABS) is an emerging pathogen causing severe infections, particularly in cystic fibrosis (CF) patients. A prospective multicentre study included CF patients from four hospitals in Madrid between January 2022 and January 2024. Respiratory samples were collected, and MABS isolates were analysed to determine their antibiotic resistance profiles, growth dynamics, infection kinetics, intracellular behaviour, and pathogenicity. Intracellular bacterial growth and macrophage viability were evaluated through THP-1 cell infection experiments, with and without amikacin. Phenotypic susceptibility testing and genotypic susceptibility testing were also conducted. Among 148 patients, 28 MABS isolates were detected from 16 patients (10.8%), and the first isolate from each patient was analysed. Isolation was more prevalent in younger individuals (median age 24.4 vs. 28.4 years, p = 0.049), and most isolates (81.25%) were identified as M. abscessus subsp. abscessus (MABSa). MABS isolates exhibited high resistance rates (>85%) to doxycycline, tobramycin, ciprofloxacin, moxifloxacin (75%) and cotrimoxazole (56.3%). Amikacin resistance (18.8%) was higher than expected, and inducible (10/16 isolates) or acquired (1/16 isolate) macrolide resistance was found in 68.8% of strains. Phenotypic and genotypic testing results were fully concordant. Tigecycline demonstrated strong in vitro activity, and resistance to imipenem, linezolid, and cefoxitin remained low. Rough strains displayed lower optical density values in later growth stages, probably due to their increased aggregation. In THP-1 cell infection experiments, rough strains showed higher intracellular bacterial loads with statistically significant differences observed at 2 hours (both with and without amikacin) and at 72 hours (with amikacin) post infection. Notably, rough strains also exhibited a higher internalisation index and greater impact on THP-1 cell viability, especially in the absence of amikacin.

Alleviation of mycobacterial infection by impairing motility and biofilm formation via natural and synthetic molecules

Mycobacterium species show distinctive characteristics with significant medical implications. Mycobacteria, including Mycobacterium tuberculosis and non-tuberculous mycobacteria, can form biofilms that facilitate their survival in hostile environments and contribute to development of antibiotic resistance and responses by the host immune system. Mycobacterial biofilm development is a complex process involving multiple genetic determinants, notably mmpL genes, which regulate lipid transport and support cell wall integrity, and the groEL gene, which is essential for biofilm maturation. Sliding motility, a passive form of surface movement observed across various mycobacterial species, is closely associated with biofilm formation and colony morphology. The unique sliding motility and biofilm-forming capabilities of Mycobacterium spp. are pivotal for their pathogenicity and persistence in diverse environments. A comprehensive understanding of the regulatory mechanisms governing these processes is crucial for the development of novel therapeutic strategies against mycobacterial infections. This review provides a detailed examination of our current knowledge regarding mycobacterial biofilm formation and motility, with a focus on regulation of these processes, their impact on pathogenicity, and potential avenues for therapeutic intervention. To this end, the potential of natural and synthetic compounds, including nanomaterials, in combating mycobacterial biofilms and inhibiting sliding motility are discussed as well. These compounds offer new avenues for the treatment of drug-resistant mycobacterial infections.

AgNP-Containing Niosomes Functionalized with Fucoidan Potentiated the Intracellular Killing of Mycobacterium abscessus in Macrophages

Intracellular pathogens represent a challenge for therapy because the antibiotics used need to diffuse into the cytoplasm to target the pathogens. The situation is more complicated in the mycobacteria family because members of this family infect and multiply within macrophages, the cells responsible for clearing microorganisms in the body. In addition, mycobacteria members are enclosed inside pathogen-containing vesicles or phagosomes. The treatments of these pathogens are aggravated when these pathogens acquire resistance to antibiotic molecules. As a result, new antimicrobial alternatives are needed. Niosomes are vesicles composed of cholesterol and nonionic surfactants that can be used for antibiotic encapsulation and delivery. The current study developed a systematic formulation of niosomes to determine the best option for niosome functionalizing for precise delivery to the intracellular pathogen Mycobacterium abscessus. Silver nanoparticles (AgNPs) were synthesized using gallic acid as an antibacterial agent. Then, niosomes were prepared and characterized, following the encapsulation of AgNPs functionalized with a single-chain antibody screened against the cell wall glycopeptidolipid of Mycobacterium abscessus. For a precise delivery of the cargo into macrophages, the niosomes were also functionalized with the polysaccharide fucoidan, taken specifically by the scavenger receptor class A expressed on the surface of macrophages. Results of the study showed a steady decrease in the intracellular pathogen load after 48 h post-infection. In conclusion, this system could be developed into a platform to target other types of intracellular pathogens and as an option for antimicrobial therapy.

Harnessing hypoxia: bacterial adaptation and chronic infection in cystic fibrosis

The exquisite ability of bacteria to adapt to their environment is essential for their capacity to colonize hostile niches. In the cystic fibrosis (CF) lung, hypoxia is among several environmental stresses that opportunistic pathogens must overcome to persist and chronically colonize. Although the role of hypoxia in the host has been widely reviewed, the impact of hypoxia on bacterial pathogens has not yet been studied extensively. This review considers the bacterial oxygen-sensing mechanisms in three species that effectively colonize the lungs of people with CF, namely Pseudomonas aeruginosa, Burkholderia cepacia complex, and Mycobacterium abscessus and draws parallels between their three proposed oxygen-sensing two-component systems: BfiSR, FixLJ, and DosRS, respectively. Moreover, each species expresses regulons that respond to hypoxia: Anr, Lxa, and DosR, and encode multiple proteins that share similar homologies and function. Many adaptations that these pathogens undergo during chronic infection, including antibiotic resistance, protease expression, or changes in motility, have parallels in the responses of the respective species to hypoxia. It is likely that exposure to hypoxia in their environmental habitats predispose these pathogens to colonization of hypoxic niches, arming them with mechanisms than enable their evasion of the immune system and establish chronic infections. Overcoming hypoxia presents a new target for therapeutic options against chronic lung infections.

Mechanisms of Virulence of Mycobacterium abscessus and Interaction with the Host Immune System

Mycobacterium abscessus is a non-tuberculosis fast-growing mycobacterium that has recently become a serious concern due to its rapidly increasing prevalence worldwide, mainly in individuals with a high susceptibility to pulmonary infections, for example, patients with cystic fibrosis, bronchiectasis, chronic obstructive pulmonary disease, and previous tuberculosis infection. According to present estimations, at least 20% of patients with cystic fibrosis are infected with M. abscessus. This bacterium is extremely resistant to most drugs, leading to a severe and difficult-to-treat infection. That is why M. abscessus, previously classified as a low-virulent opportunistic pathogen, is now reconsidered as a true pathogenic bacterium. There are no effective drugs for successful M. abscessus infection therapy, as well as no vaccines to prevent its spread. This review focuses on the molecular mechanisms ensuring M. abscessus resistance to immune response and its ability to survive in the aggressive intracellular environment of human immune cells, and describes virulence factors that can serve as potential targets for the development of innovative therapeutic approaches to combat the spread of infections caused by M. abscessus.

Polymicrobial infection in cystic fibrosis and future perspectives for improving Mycobacterium abscessus drug discovery

Polymicrobial communities inhabit the cystic fibrosis (CF) airway, whereby microbial interactions can occur. One prominent CF pathogen is Mycobacterium abscessus, whose treatment is largely unsuccessful. This creates a need to discover novel antimicrobial agents to treat M. abscessus, however the methods used within antibiotic discovery are typically monomicrobial. This review will discuss this pathogen whilst considering the CF polymicrobial environment, to highlight future perspectives to improve M. abscessus drug discovery.

Mycobacterial biofilms: Understanding the genetic factors playing significant role in pathogenesis, resistance and diagnosis

Even though the genus Mycobacterium is a diverse group consisting of a majority of environmental bacteria known as non-tuberculous mycobacteria (NTM), it also contains some of the deadliest pathogens (Mycobacterium tuberculosis) in history associated with chronic disease called tuberculosis (TB). Formation of biofilm is one of the unique strategies employed by mycobacteria to enhance their ability to survive in hostile conditions. Biofilm formation by Mycobacterium species is an emerging area of research with significant implications for understanding its pathogenesis and treatment of related infections, specifically TB. This review provides an overview of the biofilm-forming abilities of different species of Mycobacterium and the genetic factors influencing biofilm formation with a detailed focus on M. tuberculosis. Biofilm-mediated resistance is a significant challenge as it can limit antibiotic penetration and promote the survival of dormant mycobacterial cells. Key genetic factors promoting biofilm formation have been explored such as the mmpL genes involved in lipid transport and cell wall integrity as well as the groEL gene essential for mature biofilm formation. Additionally, biofilm-mediated antibiotic resistance and pathogenesis highlighting the specific niches, sites of infection along with the possible mechanisms of biofilm dissemination have been discussed. Furthermore, drug targets within mycobacterial biofilm and their role as potential biomarkers in the development of rapid diagnostic tools have been highlighted. The review summarises the current understanding of the complex nature of Mycobacterium biofilm and its clinical implications, paving the way for advancements in the field of disease diagnosis, management and treatment against its multi-drug resistant species.

The diversity of clinical Mycobacterium abscessus isolates in morphology, glycopeptidolipids and infection rates in a macrophage model

Introduction. Mycobacterium abscessus (MABS) is a pathogenic bacterium that can cause severe lung infections, particularly in individuals with cystic fibrosis. MABS colonies can exhibit either a smooth (S) or rough (R) morphotype, influenced by the presence or absence of glycopeptidolipids (GPLs) on their surface, respectively. Despite the clinical significance of these morphotypes, the relationship between GPL levels, morphotype and the pathogenesis of MABS infections remains poorly understood.Gap statement. The mechanisms and implications of GPL production and morphotypes in clinical MABS infections are unclear. There is a gap in understanding their correlation with infectivity and pathogenicity, particularly in patients with underlying lung disease.Aim. This study aimed to investigate the correlation between MABS morphology, GPL and infectivity by analysing strains from cystic fibrosis patients' sputum samples.Methodology. MABS was isolated from patient sputum samples and categorized by morphotype, GPL profile and replication rate in macrophages. A high-content ex vivo infection model using THP-1 cells assessed the infectivity of both clinical and laboratory strains.Results. Our findings revealed that around 50 % of isolates displayed mixed morphologies. GPL analysis confirmed a consistent relationship between GPL content and morphotype that was only found in smooth isolates. Across morphotype groups, no differences were observed in vitro, yet clinical R strains were observed to replicate at higher levels in the THP-1 infection model. Moreover, the proportion of infected macrophages was notably higher among clinical R strains compared to their S counterparts at 72 h post-infection. Clinical variants also infected THP-1 cells at significantly higher rates compared to laboratory strains, highlighting the limited translatability of lab strain infection data to clinical contexts.Conclusion. Our study confirmed the general correlation between morphotype and GPL levels in smooth strains yet unveiled more variability within morphotype groups than previously recognized, particularly during intracellular infection. As the R morphotype is the highest clinical concern, these findings contribute to the expanding knowledge base surrounding MABS infections, offering insights that can steer diagnostic methodologies and treatment approaches.

Transfer learning predicts species-specific drug interactions in emerging pathogens

Machine learning (ML) algorithms are necessary to efficiently identify potent drug combinations within a large candidate space to combat drug resistance. However, existing ML approaches cannot be applied to emerging and under-studied pathogens with limited training data. To address this, we developed a transfer learning and crowdsourcing framework (TACTIC) to train ML models on data from multiple bacteria. TACTIC was built using 2,965 drug interactions from 12 bacterial strains and outperformed traditional ML models in predicting drug interaction outcomes for species that lack training data. Top TACTIC model features revealed genetic and metabolic factors that influence cross-species and species-specific drug interaction outcomes. Upon analyzing ~600,000 predicted drug interactions across 9 metabolic environments and 18 bacterial strains, we identified a small set of drug interactions that are selectively synergistic against Gram-negative (e.g., A. baumannii) and non-tuberculous mycobacteria (NTM) pathogens. We experimentally validated synergistic drug combinations containing clarithromycin, ampicillin, and mecillinam against M. abscessus, an emerging pathogen with growing levels of antibiotic resistance. Lastly, we leveraged TACTIC to propose selectively synergistic drug combinations to treat bacterial eye infections (endophthalmitis).

Construction of Composite Correlation Index Matrix and Analysis of Cultural Properties of Representatives of Mycobacterium abscessus Complex Isolated from Patients with Cystic Fibrosis

BACKGROUND

Microbiological diagnosis of mycobacteriosis is often difficult, as it is necessary to differentiate between transient colonization and active infection.

METHODS

We studied the cultural properties of Mycobacterium abscessus complex (MABSc) strains obtained from cystic fibrosis patients, and also analyzed composite correlation index (CCI) values in patients with repeated MABSc inoculation and their correlation with the presence of clinical and radiological manifestations of mycobacteriosis.

RESULTS

As a result, MABSc more often grew in S-form colonies in patients without clinical manifestations of chronic infection, while R-form colonies were characteristic of patients with chronic infection and clinical symptoms. At the same time, in patients examined once, no growth of colonies in the R-form was recorded, and all strains produced growth in the form of either S-colonies or in the S- and R-forms simultaneously. Statistically significant results were obtained for the relationship of the CCI with the clinical and radiological picture. In addition, a heterogeneous MABSc population with low CCI score values correlated with the development of mycobacteriosis in patients. In patients with high CCI score values (homogeneity of isolated strains), on the contrary, there were no radiological or clinical signs of the disease.

CONCLUSION

These data make it possible to build a strategy for monitoring patients depending on changes in CCI score values. The use of CCI matrix to evaluate microorganisms' identification results is a potentially new method that expands the use of matrix-assisted laser desorption ionization time-of-flight mass spectrometry.

Druggable redox pathways against Mycobacterium abscessus in cystic fibrosis patient-derived airway organoids

Mycobacterium abscessus (Mabs) drives life-shortening mortality in cystic fibrosis (CF) patients, primarily because of its resistance to chemotherapeutic agents. To date, our knowledge on the host and bacterial determinants driving Mabs pathology in CF patient lung remains rudimentary. Here, we used human airway organoids (AOs) microinjected with smooth (S) or rough (R-)Mabs to evaluate bacteria fitness, host responses to infection, and new treatment efficacy. We show that S Mabs formed biofilm, and R Mabs formed cord serpentines and displayed a higher virulence. While Mabs infection triggers enhanced oxidative stress, pharmacological activation of antioxidant pathways resulted in better control of Mabs growth and reduced virulence. Genetic and pharmacological inhibition of the CFTR is associated with better growth and higher virulence of S and R Mabs. Finally, pharmacological activation of antioxidant pathways inhibited Mabs growth, at least in part through the quinone oxidoreductase NQO1, and improved efficacy in combination with cefoxitin, a first line antibiotic. In conclusion, we have established AOs as a suitable human system to decipher mechanisms of CF-driven respiratory infection by Mabs and propose boosting of the NRF2-NQO1 axis as a potential host-directed strategy to improve Mabs infection control.