ACI-1 from Acidaminococcus fermentans: Characterization of the First β-Lactamase in Anaerobic Cocci

Acidaminococcus fermentans belongs to the group of strictly anaerobic gram-negative cocci. All previously described Acidaminococcus strains are susceptible to β-lactam antibiotics. An A. fermentans strain (RYC-MR95) resistant to penicillin and expanded-spectrum cephalosporin (amoxicillin and cefotaxime MICs, 64 μg/ml) was isolated from a human perianal abscess. A fragment encoding a β-lactamase from genomic DNA was cloned in Escherichia coli K-12 strain HB101, and the recombinant strain expressed resistance to amoxicillin (MIC, 1,024 μg/ml) and cefotaxime (MIC, 4 μg/ml). Clavulanic acid decreased the MICs to 8 and 0.03 μg/ml, respectively. Analysis of the nucleotide sequence revealed a new class A β-lactamase, ACI-1. In accordance with its biochemical properties, we propose to assign ACI-1 to functional group 2be. The ACI-1 enzyme (estimated pI 4.3) had <50% amino acid identity with any other class A β-lactamases, the closest being ROB-1 from Haemophilus influenzae (44%). ACI-1 was closer to class A β-lactamases from some gram-positive organisms (41 to 44% amino acid identity with Bacillus β-lactamases) than to most class A enzymes from gram-negative organisms (TEM-1, 24.6%). The aci1 gene had a G+C content of 42.1%, in contrast with 56% G+C content for genomic DNA from A. fermentans, thus suggesting that aci1 may have been obtained by horizontal gene transfer.