Correlation between penicillin-binding protein 2 mutations and carbapenem resistance in Escherichia coli

It is well known that carbapenem-resistant mutations in penicillin-binding proteins (PBPs) are not observed in most Gram-negative bacteria under either clinical or experimental conditions. To understand the mechanisms involved in carbapenem resistance, this study constructed a mutS- and tolC-deficient Escherichia coli strain, which was expected to have elevated mutation frequencies and to lack drug efflux. Using this mutant, carbapenem-resistant strains with target mutations were successfully and efficiently isolated. The mutations T547I/A, M574I and G601D were identified in the PBP2 gene. Meropenem (MEPM)-resistant strains with the PBP2 T547I mutation showed fourfold increased resistance to 1-β-methyl-substituted carbapenems, such as doripenem, MEPM and biapenem, but not to non-substituted carbapenems such as imipenem and panipenem and other β-lactams. In addition, resistance resulting from the G601D mutation was limited to MEPM, whilst the M574I mutation conferred resistance to MEPM, imipenem and panipenem. This is the first report, to the best of our knowledge, that E. coli also has a carbapenem-resistance mechanism as a result of PBP2 mutations, and it provides insight into the resistance profiles of PBP2 mutations to carbapenems with and without the 1-β-methyl group.