Back mutations to the TEM-1 beta-lactamase from TRC-1 lead to restored sensitivity to clavulanic acid.

Back mutations from the TRC-1 beta-lactamase to the TEM-1 enzyme were selected in vitro. The revertant beta-lactamase was obtained from Escherichia coli strain J62.2 carrying plasmid pUK901 which encodes the TRC-1 beta-lactamase. The revertant was obtained after repeated subculture of E. coli J62.2 (pUK901) in amoxycillin 512 mg/L for 5 days. The revertant beta-lactamase had the same pI as TEM-1 (5.4) and had restored inhibition by clavulanic acid (ID50 reduced from 4.2 microM to 0.15 microM). The prevalence of these beta-lactamases in the clinical population may be the result of a two-way flux, with mutations in both forward and backward directions.