Cefoxitin resistance by a chromosomal cephalosporinase in Escherichia coli

Cefoxitin resistance, an unique property found in clinical isolates of Escherichia coli was investigated. Cefoxitin resistant strains, 255 and GN206, produced cephalosporinase constitutively. The cephalosporinase was located in the periplasm, and its production was considered to be mediated by chromosomal gene(s). Cephalosporinase-less mutants from both strains were susceptible to cefoxitin as well as other beta-lactam antibiotics, suggesting that the cephalosporinase was responsible for the resistance to beta-lactam antibiotics including cefoxitin. The cephalosporinases from the E. coli strains were partially purified and their enzymological properties were compared with those of cephalosporinases of Citrobacter freundii and Proteus morganii. Although the cephalosporinases of E. coli, as well as other cephalosporinases, showed little activity for cefoxitin-hydrolysis, the E. coli cephalosporinases exhibited a significantly higher affinity for cefoxitin than other cephalosporinases. It was assumed that the E. coli enzyme located around the targets of cefoxitin protected the targets from the antibiotic by its high affinity for the antibiotic.