OXA-type carbapenemases

In recent years, the number of class D beta-lactamases with carbapenem-hydrolysing properties has increased substantially. Based on amino acid sequence identities, these class D or OXA-type carbapenemases are divided into eight distantly related groups, and they are only remotely related to other class D beta-lactamases. A putative ancestor to one of the plasmid-encoded OXA-type carbapenemases has been found. OXA-type carbapenemases are not integrated into integrons as gene cassettes like many class D oxacillinases, but most of the OXA-type carbapenemases are instead encoded by chromosomal genes. Some of these OXA-type carbapenemases are widely dispersed in Pseudomonas aeruginosa and especially in Acinetobacter baumannii. Although most of the OXA-type carbapenemases show only weak carbapenemase activity, carbapenem resistance may result from a combined action an OXA-type carbapenemase and a secondary resistance mechanism such as porin deficiencies or overexpressed efflux pumps. This article reviews the phylogeny and the genetic environments of the encoding genes and kinetic properties of the OXA-type carbapenemases.