Modulation of PrfA activity in Listeria monocytogenes upon growth in different culture media.

PrfA is the major transcriptional activator of most virulence genes of Listeria monocytogenes. Its activity is modulated by a variety of culture conditions. Here, we studied the PrfA activity in the L. monocytogenes wild-type strain EGD and an isogenic prfA deletion mutant (EGDDeltaprfA) carrying multiple copies of the wild-type prfA or the mutant prfA* gene (strains EGDDeltaprfApPrfA and EGDDeltaprfApPrfA*) in response to growth in brain heart infusion (BHI), Luria-Bertani broth (LB) or a defined minimal medium (MM) supplemented with one of the three phosphotransferase system (PTS) carbohydrates, glucose, mannose and cellobiose, or the non-PTS carbon source glycerol. Low PrfA activity was observed in the wild-type strain in BHI and LB with all of these carbon sources, while PrfA activity was high in minimal medium in the presence of glycerol. EGDDeltaprfApPrfA*, expressing a large amount of PrfA* protein, showed high PrfA activity under all growth conditions. In contrast, strain EGDDeltaprfApPrfA, expressing an equally high amount of PrfA protein, showed high PrfA activity only when cultured in BHI, and not in LB or MM (in the presence of any of the carbon sources). A ptsH mutant (lacking a functional HPr) was able to grow in BHI but not in LB or MM, regardless of which of the four carbon sources was added, suggesting that in LB and MM the uptake of the used PTS carbohydrates and the catabolism of glycerol are fully dependent on the functional common PTS pathway. The BHI culture medium, in contrast, apparently contains carbon sources (supporting listerial growth) which are taken up and metabolized by L. monocytogenes independently of the common PTS pathway. The growth rates of L. monocytogenes were strongly reduced in the presence of large amounts of PrfA (or PrfA*) protein when growing in MM, but were less reduced in LB and only slightly reduced in BHI. The expression of the genes encoding the PTS permeases of L. monocytogenes was determined in the listerial strains under the applied growth conditions. The data obtained further support the hypothesis that PrfA activity correlates with the expression level and the phosphorylation state of specific PTS permeases.