Serotype, Shiga toxin (Stx) type, and antimicrobial resistance of Stx-producing Escherichia coli isolated from humans in Shizuoka Prefecture, Japan (2003-2007).

The serotype, Shiga toxin (Stx) type, and antimicrobial resistance patterns of 138 Stx-producing Escherichia coli (STEC) strains isolated from humans between 2003 and 2007 in Shizuoka Prefecture, Japan were characterized. The predominant O serogroups of the STEC isolates were O157, O26, and O111. Antimicrobial susceptibility testing of the STEC isolates showed that 31 of the 138 isolates (22.5%) were resistant to antibiotics. Compared to the results reported in the previous studies, a higher rate of STEC O157 isolates were susceptible to all the antimicrobial agents used in this study. However, antimicrobial susceptibility data from this study showed that antimicrobial resistance patterns have increased by 6 compared to the survey performed by Masuda et al. between 1987 and 2002 (Jpn. J. Food Microbiol., 21, 44-51, 2004). This indicates that STEC isolates have evolved to show a variety of antimicrobial resistance patterns. It is important to consider the population of isolates showing decreased susceptibility to clinically relevant drugs such as ciprofloxacin (CPFX) and fosfomycin (FOM). All the 3 STEC isolates resistant to nalidixic acid showed low susceptibility to CPFX (MIC, 0.25-0.5 μg/ml). In addition, a decreased susceptibility to FOM was clearly observed in the E. coli O26 isolates. Our findings also showed that 1 STEC O26 strain could possibly be a chromosomal AmpC β-lactamase hyperproducer. These results suggest that antimicrobial therapy may be less effective in patients with non-O157 STEC infections than in those with STEC O157 infections.