Abstract
The susceptibility of 416 clinical isolates of Escherichia coli to ampicillin, cephaloridine and cefoxitin was determined, and 39 isolates were found to be resistance to 25 μg/ml or higher concentrations of cephaloridine. Five of the cephaloridine-resistant strains further exhibited resistance to cefoxitin which is a novel cephamycin antibiotic stable to many types of β-lactamases. The cephaloridine-resistant strains except the strains resistant to cefoxitin produced a large amount of type I penicillinase mediated by R plasmid, while all the cefoxitin-resistant strains produced a cephalosporinase (2 strains) or both of the cephalosporinase and the type I penicillinase (3 strains). Although the cephalosporinase failed to hydrolyze cefoxitin effectively in vitro, the close relationship between the cephalosporinase production and the cephaloridine and cefoxitin resistances in the E. coli strains was suggested. The conjugal transfer of the cefoxitin resistance to an E. coli strain was unsuccessful.
