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An atypical Enterobacteriaceae strain with a β-lactam susceptibility pattern of inducible cephalosporinase was isolated in Tenon Hospital (Paris, France) from a patient’s skull wound infection. Identifications by the API-50CHE biochemical system and 16S rRNA gene sequencing concluded that it was a member of the Buttiauxella genus. The bla gene was cloned and sequenced. The deduced translated product was a 383-amino acid protein (BUT-1) with 75–78% identity with the chromosomal AmpC β-lactamases of Citrobacter freundii , Enterobacter aerogenes , Enterobacter cloacae and Escherichia coli . The isoelectric point of 9.0 and the kinetic constants of BUT-1 were comparable with results described for other Ambler class C enzymes. bla BUT-1 and the associated ampR transcriptional regulator gene were divergently transcribed from a common intercistronic region, a genetic organization already described for other inducible class C β-lactamases. The deduced amino acid sequence of AmpR shared 85% and 81% identity with AmpR from E. cloacae and C. freundii respectively.

The amplification and sequence of ampC genes in Enterobacter asburiae, Enterobacter cancerogenus, Enterobacter dissolvens, Enterobacter hormaechei and Enterobacter intermedius bring the number of known cephalosporinase sequences from the genus Enterobacter to seven. Expression in Escherichia coli of the ampC genes from E. asburiae, E. hormaechei and E. intermedius established the functional nature of these genes. ampC from E. asburiae shows 96.5% identity to bla(ACT-1) encoding a plasmid-borne cephalosporinase previously believed to derive from Enterobacter cloacae. The reassignment of ACT-1 ancestry to E. asburiae is confirmed by the 95.5% identity between ampR upstream of bla(ACT-1) and ampR from E. asburiae.