Molecular characterization of the gene encoding a new AmpC beta-lactamase in Acinetobacter baylyi

The main objective of the present study was to demonstrate the presence of a beta-lactamase ampC gene in the chromosome of the non-pathogenic bacterium Acinetobacter baylyi ADP1.

beta-Lactam MICs were determined by Etest. The ampC gene was amplified by PCR, with specific oligonucleotides, then cloned into pBGS18 and pAT-RA plasmids and transformed into Escherichia coli TG1 and parental A. baylyi as hosts. The gene was sequenced and analysed. The AmpC protein was expressed, purified by affinity chromatography and the kinetic parameters determined.

An ampC gene was amplified from the ADP1 genome. Sequencing of the gene showed typical SVSK and KTG domains and the typical YXN Class C motif. The amplified gene showed significant identity (48.5% to 49.3%) with the AmpC enzymes of Acinetobacter baumannii and AG3 strains, which have recently been renamed ADC-1 to ADC-7. MIC analysis revealed a cephalosporinase profile for the E. coli TG1 clone as well as for the parental A. baylyi strain that overexpressed the ampC gene cloned under the control of an external promoter. Analysis of kinetic parameters of the purified enzyme showed higher catalytic efficiency for cefalotin than for ampicillin.

This study represents the first report of an AmpC beta-lactamase in A. baylyi, which was shown by biochemical and microbiological experiments to have a typical cephalosporinase profile. The presence of the respective gene in the chromosome of A. baylyi ADP1 suggested that this ampC gene is the naturally occurring cephalosporinase in this species, as previously reported for other Acinetobacter spp. We tentatively named the enzyme ADC-8.