Azithromycin can induce SOS response and horizontal gene transfer of SXT element in Vibrio cholerae

The emergence and spread of drug resistance in Vibrio cholerae are mainly attributed to horizontal gene transfer of mobile genetic elements, especially the SXT (sulfamethoxazole and trimethoprim) element, an integrative conjugative element carrying multiple drug resistance genes. SOS (save our souls) bacterial stress response in Vibrio cholerae plays a pivotal role in inducing the SXT element transfer and induction of the CTX prophage, carrying the important virulence factor cholera toxin encoded by the ctxAB gene.

This study investigated whether the subinhibitory concentration of antibiotics like ciprofloxacin, tetracycline, and azithromycin induce SOS response by detecting the expression of recA and lexA, the key genes of SOS response by reverse transcriptase real time PCR (RT-qPCR). We also studied the transfer of SXT element in response to these three antibiotics by bacterial conjugation. Transfer of SXT elements was confirmed by detecting the SXT element-specific conserved genes.

The results of the real-time PCR showed that all three antibiotics induced SOS response with more robust induction by tetracycline and azithromycin relative to ciprofloxacin. We observed a higher frequency of transfer of SXT elements in cultures exposed to these antibiotics and the control mitomycin C compared to unexposed cultures.

Our study indicates that antibiotics including azithromycin, ciprofloxacin, and tetracycline activate SOS response in Vibrio cholerae and demonstrates a robust mechanism for wide dissemination of drug resistance.