2026 年 13 巻 5 号 p. 215-222
The spread of antibiotic-resistant bacteria in natural environments is a serious threat to public health. We isolated four levofloxacin-resistant Escherichia coli strains (TLR101, TLR104, TLR105, and TLR108) from the Tone River in Japan. To understand the prevalence of levofloxacin-resistant bacteria and develop countermeasures, we investigated the genetic mechanisms underlying levofloxacin resistance and the characteristics of isolated E. coli strains. Antimicrobial susceptibility was determined using the agar dilution method. Quinolone resistance determining region (QRDR) analysis and detection of plasmid-mediated quinolone resistance (PMQR) genes were performed using PCR and sequence analysis. Conjugation was performed using E. coli ML4905. Biofilm formation was assessed using crystal violet staining. Amino acid mutations in the QRDR were identified in all four resistant strains. Strain TLR101 was a multiple mutant with five amino acid changes in GyrA (Ser83→Leu, Asp87→Asn), ParC (Ser80→Ile, Glu84→Val), and ParE (Val450→Asp), and harbored the PMQR gene aac-(6')-Ib-cr. In contrast, strains TLR104, 105, and 108 showed no QRDR mutations but carried the PMQR gene, qnrS1. In these three strains, qnrS1 was transferable and increased the minimum inhibitory concentration of levofloxacin by 16-fold. In strain TLR104, biofilm formation-related genes, along with tetracycline and chloramphenicol resistance genes, were co-transferred with qnrS1. These results demonstrate that qnrS1 plays a crucial role in the spread of multidrug resistance and enhances pathogenicity. To our knowledge, this is the first report of transferable plasmids containing multidrug resistance and virulence factors in the Tone River.
