QUINOLONE-RESISTANCE GENE ON A MULTIDRUG-RESISTANT ESCHERICHIA COLI FROM THE UNITED STATES

The emergence of multidrug resistant (MDR) bacteria poses a significant public health threat, especially since these organisms are particularly difficult to combat. Resistance to quinolones, a major class of antibiotics, has been observed particularly in Enterobacteriaceae. High-level resistance is due to chromosomal mutations in DNA gyrase and topoisomerase IV, quinolone’s main targets. The presence of plasmid-mediated quinolone resistance (PMQR) genes, like qnrS1, lengthen the bacteria’s mutant selection window by encoding for protection of quinolone’s main targets. This allows bacteria to acquire chromosomal mutations hence full resistance. We hypothesized that the environment is a reservoir of qnrS1 genes. The objective of this study is to identify qnrS1 genes isolated from bacteria in the environment and characterize the mobile genetic elements that facilitate their dissemination. As part of a larger environmental study, water was collected from a creek in Laguna Beach, California. Based on disk susceptibility test results, a MDR E. coli was selected for its high number of antibiotic resistant genes (ARGs). The MDR E. coli plasmids were isolated, sequenced and analyzed. Comparative sequence analysis was performed on the isolate and previously sequenced isolates from clinical and environmental settings. To assess plasmid mobility, conjugation with plasmid-encoded ARGs and sodium azide-resistant J53 E. coli as the recipient was performed. The antimicrobial resistance profile of the transconjugant (TC) was determined. The MDR E. coli contained five plasmids: 91 kb, 85 kb, 69 kb, 49 kb and 42 kb. The 85 kb IncF conjugative plasmid contained ARGs conferring resistance to aminoglycoside (aadA17), beta-lactams (blaTEM-1B), lincosamide (lnuF) and phenicol (floR). The 49 kb plasmid was classified as an IncR non-conjugative mobilizable plasmid containing ARGs conferring resistance to five antibiotic classes: aminoglycoside (aadA8b), quinolone (qnrS1), sulfonamide (sul3), trimethoprim (dfrA12) and tetracycline (tetA). The TC contained the 85 kb and 49 kb plasmids and was resistant to aminoglycoside, beta-lactams, lincosamide, quinolone, sulfonamide, trimethoprim, tetracycline, and phenicol, showing that the plasmid carrying qnrS1 is mobilizable. Transposon IS26 flanks qnrS1 upstream, while downstream it is flanked by transposon IS26 that was truncated by ISKpn19. This is consistent with previous studies suggesting that qnrS1 is usually part of an IS26 element. Through comparative analysis of qnrS1-containing plasmids, this transposition unit seems to be conserved. The gene qnrS1 has been formerly identified in Europe, Asia, Africa and North America. This study shows a first record of an environmental isolate carrying qnrS1 in the United States. This highlights the fact that the environment might be a reservoir of qnrS1 causing reduced susceptibility to quinolones.