Subinhibitory concentrations of nalidixic acid alter bacterial physiology and induce anthropogenic resistance in a commensal strain of Escherichia coli in vitro

The human gut houses a complex group of bacterial genera, including both opportunistic pathogens and commensal micro-organisms. These are regularly exposed to antibiotics, and their subinhibitory concentrations play a pivotal role in shaping the microbial responses. This study was aimed to investigate the effects exerted by sub-MICs of nalidixic acid (NA) on the growth rate, bacterial motility, biofilm formation and expression of outer membrane proteins (OMPs) in a commensal strain of E. coli. The NA-sensitive strain was sequentially passaged under sub-MICs of NA. E-test was used to determine the MIC values of NA. Results indicated significant changes in the growth profile of commensal E. coli upon exposure to NA at sub-MICs. Differential expression of OMPs was observed in cells treated with sub-MICs of NA. Bacterial motility was reduced under 1/2 MIC of NA. Interestingly, successive passaging under 1/2 MIC of NA led to the emergence of resistant E. coli with an increased MIC value of 64 µg ml⁻¹ in just 24 days. The NA-resistant variant was confirmed by comparing its 16S rRNA sequence to that of the sensitive commensal strain. Mutations in the Quinolone Resistance-Determining Regions (QRDRs) of chromosomal gyrA, and Topoisomerase IV-encoding parC genes were detected in NA-resistant E. coli. Our results demonstrate how antibiotics play an important role as signalling molecules or elicitors in driving the pathogenicity of commensal bacteria in vitro.