Microfluidic analysis of salt-stress-mediated antibiotic tolerance in Mycobacterium smegmatis

Understanding how bacteria respond to complex environmental stresses is essential for addressing antibiotic resistance. In the natural environment, bacteria can experience salt stress, for instance, due to spontaneous water evaporation. Here, we present a microfluidic platform that enables long-term culture of Mycobacterium smegmatis, a fast-growing, non-pathogenic model for mycobacteria. Using a microfluidic gradient generator, we established stable salt and antibiotic concentration profiles across growth chambers and monitored bacterial proliferation over multiple generations. When exposed to antibiotics in conjunction with elevated salt concentrations, M. smegmatis exhibited a significant increase in the minimum inhibitory concentration, indicating a salt-induced drug resistance. Salt stress also led to slower growth, shorter cell length, and reduced division asymmetry. While efflux pump inhibitors partially restored antibiotic sensitivity, gene expression profiles and dye-based efflux assays showed minimal early activation of known efflux genes, but upregulation of ribosomal biosynthesis and stress adaptation. In general, these findings demonstrate how abiotic stress promotes phenotypic drug tolerance and reshapes antibiotic susceptibility prior to developing genetic resistance, thus providing valuable insights into managing the global threat of antibiotic resistance.