Expression of promoter-<i>lacZ</i> fusions in <i>L. monocytogenes</i> strains.

aThe expression of promoter-lacZ fusions in response to the addition of 0.09 µg/ml penicillin G (+ PenG) or in the absence of the antibiotic was determined by β-galactosidase assays. Specific β-galactosidase activity was measured for wild-type (EGD) or Δfri mutant cells containing promoter-lacZ fusions, grown in the presence or absence of the antibiotic. The results are the average of three independent experiments, each performed in triplicate ± standard deviation. bGenes that are organized in an operon (according to Toledo-Arana et al. [66]). cSignificant differences following growth in the presence and absence of penicillin G (Student's t-test; P<0.05). dSignificant differences between the studied strains (Student's t-test; P<0.05).