In vitro activity of β-lactams and quinolones against AmpC β-lactamase-producing Escherichia coli

We studied the antimicrobial susceptibility of AmpC β-lactamase-producing Escherichia coli isolates collected at ten medical institutions in the Kinki area of Japan during a 6-month period (November 2002 through April 2003). Of 2845 E. coli isolates tested, 29 (1.0%) showed a minimum inhibitory concentration (MIC) for cefazolin of more than 8μg/ml and were three-dimensional extract test positive. In standard inoculum susceptibility tests against these 29 strains, the MIC90s for the four carbapenems tested ranged from 0.06 μg/ml to 0.5 μg/ml, and these compounds were more active than the other β-lactams, with meropenem being the most active. The MIC90s for β- lactams, except carbapenems, ranged from 4 μg/ml to 32 μg/ ml, with cefepime being the most active. In high inoculum susceptibility tests against these strains, the MIC90s for the four carbapenems and cefepime were 8 μg/ml or less, and these compounds were more active than other β-lactams. The MIC90s for β-lactams, except carbapenems and cefepime, were 32 μg/ml or more. The MIC90s for the five quinolones tested ranged from 4 μg/ml to 16 μg/ml, and the order of increasing susceptibility was ciprofloxacin > levofloxacin, gatifloxacin and pazufloxacin > prulifloxacin.