Identification of a ferritin-like protein of Listeria monocytogenes as a mediator of β-lactam tolerance and innate resistance to cephalosporins

Background: The food-borne pathogen Listeria monocytogenes is the causative agent of listeriosis. The beta-lactam antibiotics penicillin G and ampicillin are the current drugs of choice for the treatment of listerial infections. While isolates of L. monocytogenes are susceptible to these antibiotics, their action is only bacteriostatic and consequently, this bacterium is regarded as tolerant to beta-lactams. In addition, L. monocytogenes has a high level of innate resistance to the cephalosporin family of beta-lactams frequently used to treat sepsis of unknown etiology. Given the high mortality rate of listeriosis despite rational antibiotic therapy, it is important to identify genes that play a role in the susceptibility and tolerance of L. monocytogenes to beta-lactams. Results: The hly-based promoter trap system was applied to identify penicillin G-inducible genes of L. monocytogenes. The results of reporter system studies, verified by transcriptional analysis, identified ten penicillin G-inducible genes. The contribution of three of these genes, encoding a ferritin-like protein (fri), a two-component phosphate-response regulator (phoP) and an AraC/XylS family transcriptional regulator (axyR), to the susceptibility and tolerance of L. monocytogenes to beta-lactams was examined by analysis of nonpolar deletion mutants. The absence of PhoP or AxyR resulted in more rapid growth of the strains in the presence of sublethal concentration of beta-lactams, but had no effect on the MIC values or the ability to survive a lethal dose of these antibiotics. However, the Delta fri strain showed impaired growth in the presence of sublethal concentrations of penicillin G and ampicillin and a significantly reduced ability to survive lethal concentrations of these beta-lactams. A lack of Fri also caused a 2-fold increase in the sensitivity of L. monocytogenes to cefalotin and cephradine. Conclusions: The present study has identified Fri as an important mediator of beta-lactam tolerance and innate resistance to cephalosporins in L. monocytogenes. PhoP and AxyR are probably involved in transmitting signals to adjust the rate of growth of L. monocytogenes under beta-lactam pressure, but these regulators do not play a significant role in susceptibility and tolerance to this class of antibiotics.