Clinically relevant mutations in the PhoR sensor kinase of host-adapted Mycobacterium abscessus isolates impact response to acidic pH and virulence

Cell envelope polysaccharide modifications alter the surface properties and interactions of Mycobacterium abscessus with innate immune cells in a morphotype-dependent manner

Mycobacterium abscessus is one of the leading causes of pulmonary infections caused by non-tuberculous mycobacteria. The ability of M. abscessus to establish a chronic infection in the lung relies on a series of adaptive mutations impacting, in part, global regulators and cell envelope biosynthetic enzymes. One of the genes under strong evolutionary pressure during host adaptation is ubiA, which participates in the elaboration of the arabinan domains of two major cell envelope polysaccharides: arabinogalactan (AG) and lipoarabinomannan (LAM). We here show that patient-derived UbiA mutations not only cause alterations in the AG, LAM, and mycolic acid contents of M. abscessus but also tend to render the bacterium more prone to forming biofilms while evading uptake by innate immune cells and enhancing their pro-inflammatory properties. The fact that the effects of UbiA mutations on the physiology and pathogenicity of M. abscessus were impacted by the rough or smooth morphotype of the strain suggests that the timing of their selection relative to morphotype switching may be key to their ability to promote chronic persistence in the host.IMPORTANCEMultidrug-resistant pulmonary infections caused by Mycobacterium abscessus and subspecies are increasing in the U.S.A. and globally. Little is known of the mechanisms of pathogenicity of these microorganisms. We have identified single-nucleotide polymorphisms (SNPs) in a gene involved in the biosynthesis of two major cell envelope polysaccharides, arabinogalactan and lipoarabinomannan, in lung-adapted isolates from 13 patients. Introduction of these individual SNPs in a reference M. abscessus strain allowed us to study their impact on the physiology of the bacterium and its interactions with immune cells. The significance of our work is in identifying some of the mechanisms used by M. abscessus to colonize and persist in the human lung, which will facilitate the early detection of potentially more virulent clinical isolates and lead to new therapeutic strategies. Our findings may further have broader biomedical impacts, as the ubiA gene is conserved in other tuberculous and non-tuberculous mycobacterial pathogens.

Exposure of Mycobacteriodes abscessus clones to mucin affects bacterial phenotype

In the past 20 years infections caused by Mycobacterioides abscessus have become increasingly common in patients with chronic lung conditions. The microorganisms are also resistant to a number of antibiotic classes, making treatment challenging. To begin understanding how the bacterium adapts to the lung environment, pure colonies of M. abscessus strain 19,977 were grown in 7H9 broth with or without mucin over 30-day intervals for 6 months and analyzed for colony morphology, 7 day-growth curves, the ability to form biofilms after 14 days, and the susceptibility to antibiotics determined using previous published methods. In presence of mucin by month 3 the non-replicating stage of growth occurred by day 3, compared to earlier months. Similar characteristics were seen in colonies grown in absence of mucin by month 5. During biofilm formation, the amount of protein in the matrix started to decrease at month 3 between day 7 and day 14, with progressive overall biomass decreased in month 6. Mucin exposed clones had less of this decrease between day 7 and day 14 compared to clones naïve to mucin. The number of bacteria in the biofilms were similar in all 6 months in 7H9 medium and 7H9 with mucin. Some of the strains increased the amount of carbohydrates in the biofilm matrix overtime while others exported more DNA, the matrix containing large amounts of it. The presence of mucin was associated with increased antibiotic resistance to amikacin, 5-fold increase of MIC in 7 out of 8 strains evaluated. Some of the colonies transitioned from smooth to rough morphotypes, again indicating the influence of different environments. Overall, M. abscessus phenotype changes overtime influenced by mucin, an important component in the host lung environment. Bacteria clones arrested growth, produced different biofilms compositions, increased the resistance to antibiotics, and some changed the cell wall surfaces. These observations have direct implications in virulence and the response of the pathogen to treatment.

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

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