Effect of overproduction of PBP3 on the susceptibility of L. monocytogenes to β-lactams. (A) Susceptibility of L. monocytogenes pAKB (Lm pAKB) and L. monocytogenes pAKB-lmo1438 (Lm pAKB-lmo1438) to ampicillin measured using the E-test. The extent of the zone of partial autolysis of L. monocytogenes pAKB-lmo1438 is indicated by an arrow. (B) Survival of L. monocytogenes pAKB (○) and L. monocytogenes pAKB-lmo1438 (•) in the presence of a lethal dose of penicillin G (0.6 μg/ml). Following nisin induction, penicillin G was added (at the time indicated by an arrow) to the cultures in BHI broth and incubation at 37°C was continued. Survival was measured by performing viable cell counts. Error bars represent standard deviations from the means of three independent experiments, each performed in triplicate.