Characterization of the chromosomal class A β-lactamase CKO from Citrobacter koseri

Abstract The gene blaCKO encoding the chromosomal class A β-lactamase of Citrobacter koseri was cloned and sequenced. CKO was found to display only 41% identity with SED-1 from Citrobacter sedlakii and 36% with CdiA from Citrobacter amalonaticus (formerly Citrobacter diversus). No transcriptional regulator was found upstream from blaCKO. Silent and missense mutations were detected in four blaCKO genes amplified from different C. koseri clinical isolates, but the CKO variants displayed identical biochemical behaviours. A blaCKO-specific polymerase chain reaction confirmed that blaCKO is present only in C. koseri and therefore represents an interesting tool with which to differentiate C. koseri from the other Citrobacter spp.