Fluoroquinolone (FQ) Contamination Does Not Correlate with Occurrence of FQ-Resistant Bacteria in Aquatic Environments of Vietnam and Thailand

Fluoroquinolone antibiotics (FQs) have been used worldwide for chemotherapy, animal husbandry, and aquaculture, and the occurrence of FQ-resistant (FQs(r)) bacteria in natural environments has been reported. Plasmid-mediated transferable quinolone resistance (PMQR) genes are suspected to originate from the chromosomes of water-dwelling bacteria. However, the occurrence of and the potential reservoir of FQs(r) bacteria and PMQR genes in aquatic environments have not been elucidated. In this study, we detected FQs(r) bacteria and PMQR genes in aquatic environments in Thailand and Vietnam, and measured FQ contamination. Levels of contamination were greater Thailand (avg. 5130, max 46100 ng L-1) than in Vietnam (avg. 235, max 1130 ng L-1); however, the occurrence of FQs(r) bacteria was higher in Vietnam (similar to 15%) than in Thailand (similar to 7.0%), suggesting that contamination by FQs is not directly linked to the development of FQs(r) bacteria. Diverse taxonomic groups of FQs(r)-bacteria were identified, and one of the PMQR genes, qnrB, was detected from bacteria of environmental origin, not enteric bacteria. This suggests that the environmental bacteria are a potential reservoir of antibiotic resistance determinants even at un-contaminated sites.