Transferable class C -lactamases in Escherichia coli strains isolated in Greek hospitals and characterization of two enzyme variants (LAT-3 and LAT-4) closely related to Citrobacter freundii AmpC -lactamase

Escherichia coli is one of the leading causes of nosocomial infections. The most frequent mechanism of resistance to penicillins is the production of the common plasmidmediated TEM-1 -lactamase. Extended-spectrum lactamases of the TEM and SHV families, conferring resistance to newer -lactams such as oxyimino-cephalosporins, have also spread among E. coli clinical isolates. These enzymes are unable to hydrolyse cephamycins with an -methoxy substituent in the C7 position, such as cefoxitin and cefotetan. E. coli resistance to cefoxitin is considered uncommon and is usually attributed to overexpression of the species-specific chromosomal cephalosporinase. The emergence of plasmid-mediated -lactamases with chromosomal cephalosporinase characteristics has been reported mostly in Klebsiella pneumoniae and, to a lesser extent, in E. coli. On the basis of sequence homologies, it appears that LAT-1, LAT-2 CMY-2 and BIL-1 -lactamases comprising the ‘Mediterranean – Middle East’ cluster have been derived from the species-specific chromosomal cephalosporinase of Citrobacter freundii. The LAT-1, LAT-2 and CMY-2 enzymes were found in K. pneumoniae strains isolated in hospitals in Athens; BIL-1 was from an isolate of E. coli possibly acquired in Pakistan. An invariable phenotypic trait of E. coli strains producing these -lactamases is resistance to cefoxitin. Recent records of the Greek WHO-Net Study Group indicate that the current incidence of cefoxitin resistance among E. coli isolates in Athens hospitals is 3%. In this study we show that the predominant mechanism of resistance to cefoxitin in these isolates is the production of transferable C. freundii-derived cephalosporinases.