CTX-M Expression and Selection of Ertapenem Resistance in Klebsiella pneumoniae and Escherichia coli

In vitro selection of mutants with decreased susceptibility to ertapenem was performed using Escherichia coli and Klebsiella pneumoniae clinical strains producing either the bla(CTX-M-2), bla(CTX-M-3), bla(CTX-M-9), or bla(CTX-M-15) gene. Frequencies of mutants with decreased susceptibilities to ertapenem were similar for all beta-lactamases expressed.