In vitro activities and detection performances of cefmetazole and flomoxef for extended-spectrum β-lactamase and plasmid-mediated AmpC β-lactamase–producing Enterobacteriaceae

• Molecular detection of resistance genes found ESBL in 69% and pAmpC in 11%.
• Cefmetazole and flomoxef had low MICs against ESBL-positive isolates.
• Flomoxef had a high screening performance for pAmpC-positive isolates.

To investigate the in vitro activities of cephamycins (cefmetazole and flomoxef) for extended-spectrum β-lactamase (ESBL)– and plasmid-mediated AmpC β-lactamase (pAmpC)–producing Enterobacteriaceae, a total of 574 third-generation cephalosporin-resistant clinical isolates were collected at a Japanese multicenter study. PCR and sequencing identified 394 isolates with only ESBL genes, 63 isolates with only pAmpC genes, and 6 isolates with both ESBL and pAmpC genes. blaCTX-M types predominated 95.5% of the ESBL genes, and blaCMY-2 predominated 91.3% of the pAmpC genes. The MIC50/90 values of cefmetazole and flomoxef were ≤1/4 and ≤1/≤1 μg/mL for isolates with only ESBL genes, respectively, and 16/>16 and 8/16 μg/mL for isolates with only pAmpC genes, respectively. Flomoxef ≥4 μg/mL had the best screening performance for the detection of isolates with pAmpC genes. Flomoxef had better in vitro activities against ESBL-producing Enterobacteriaceae and provided a clearer distinction between ESBL and pAmpC-producing Enterobacteriaceae compared to cefmetazole.