Effects of a SigH mutation on tigecycline resistance and the SigH-RshA interaction in Mycobacteroides abscessus

Abstract Mycobacteroides abscessus is an opportunistic pathogen ubiquitous in the environment owing to its ability to remain viable on nutrient-poor surfaces. It is a frequent cause of infections in the cosmetic industry, with patients being infected through cutaneous invasive procedures such as tattooing, piercings and cosmetic surgeries. In immunocompromised hosts, infections can be severe and difficult to treat as M. abscessus has many intrinsic and acquired resistances to different classes of antibiotics. Tigecycline, a tetracycline derivative introduced two decades ago, is a broad-spectrum antibiotic with activity on bacteria that are resistant to many existing antibiotics. Unfortunately, tigecycline-resistant strains of M. abscessus have been reported in recent years. This study aimed to investigate the resistance mechanism of CL7, a tigecycline-resistant, spontaneous mutant derived from M. abscessus ATCC 19977. CL7 notably had a 7 amino-acid truncation in the SigH protein, a sigma factor (transcriptional activator) responsible for mycobacterial responses to environmental stress. RNA sequencing showed that sigH and other genes were up-regulated in CL7 (as compared to ATCC 19977). The gene set enrichment analysis demonstrated that the SigH regulon was significantly over-represented among these genes up-regulated in CL7. A bacterial-2-hybrid assay was performed to investigate the effect of the mutation on the SigH interaction with RshA, the anti-sigma factor that inhibits SigH intracellularly. The results, supported by RNA sequencing, showed that the interaction between RshA and the mutant SigH was impeded. This reduced interaction could lead to a decreased inhibition of SigH by RshA, causing the up-regulation of the sigH gene. Coupled with the RNA polymerase, SigH would then up-regulate genes under its regulation, leading to tigecycline resistance. In general, this study enhances our understanding of tigecycline resistance mechanisms in M. abscessus, and contributes to the development of novel antibacterial therapies and diagnostic tools for managing M. abscessus infections.