monocytogenes to β-lactams and have demonstrated that two other TCSs, LiaSR and
VirRS, are also linked to this response . The mechanisms of tolerance of L. monocytogenes to cell envelope-acting antimicrobial agents are much more poorly characterized than the mechanisms of innate resistance to cephalosporins. To date, only the alternative sigma factor SigB has been shown to determine the tolerance of L. monocytogenes to β-lactams . It seems reasonable to assume that certain genes that are important GSK2118436 price for the survival and growth of bacteria in the presence of cell envelope-acting antibiotics are induced during treatment with these antimicrobial agents. Several studies have provided evidence to support this assumption in the case of L. monocytogenes. Stack et al.  showed that htrA, encoding an HtrA-like serine protease, is essential for the growth of L. monocytogenes in the presence of penicillin G, and that this gene is more efficiently transcribed when this β-lactam is present. Gottschalk et al.  demonstrated that the transcription of several cell wall-related genes (controlled by the CesRK two-component system) is induced by β-lactam and glycopeptide antibiotics. Three of these genes, lmo1416, lmo2210 and lmo2812, play a significant role in the survival of the bacterium in
the presence of cell wall-acting antibiotics. More recently, Nielsen et al.  showed the same relationship between the induction of expression and significance of lmo2442 and lmo2568 genes in the susceptibility of L. monocytogenes to the β-lactam Bucladesine price antibiotic cefuroxime. Fulvestrant purchase These observations prompted us to attempt
to identify L. monocytogenes genes induced in the presence of penicillin G, in order to learn more about mechanisms of tolerance to this class of antibiotic. For this purpose, a promoter-trap system based on a promoterless plasmid-borne copy of the hly gene encoding listeriolysin O (LLO) was employed. This system has been used previously to identify L. monocytogenes promoters that are either constitutive or specifically induced during in vivo infection . In the course of this 5-FU study, ten penicillin-G inducible genes were identified. The upregulated expression of these genes under penicillin G pressure was verified by transcriptional analysis. Three of the identified genes, namely fri, phoP and axyR, were selected for further investigation. The fri gene encodes a non-heme, iron-binding ferritin-like protein (Fri) that belongs to the Dps (DNA-binding proteins from starved cells) family of proteins, which play important roles in the response to multiple stresses in many bacterial species (reviewed recently in ). Gene phoP encodes a two-component phosphate-response regulator homologous to B. subtilis phoP, which plays a crucial role in controlling the biosynthesis of teichoic acid, a key component of the gram-positive bacterial cell wall .