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

The gene bla CKO 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 bla CKO. Silent and missense mutations were detected in four bla CKO genes amplified from different C. koseri clinical isolates, but the CKO variants displayed identical biochemical behaviours. A bla CKO-specific polymerase chain reaction confirmed that bla CKO is present only in C. koseri and therefore represents an interesting tool with which to differentiate C. koseri from the other Citrobacter spp.