Trehalose metabolism and its impact on PrfA activity in Listeria monocytogenes

Listeria monocytogenes can grow as a saprophyte on decaying plant material and switch to a pathogenic lifestyle. This switch is mediated by the virulence regulator PrfA, which activates the expression of most virulence genes. PrfA activity is tightly regulated by several mechanisms to ensure that virulence genes are only expressed within the host. One of these regulatory mechanisms is the sugar-dependent inhibition. In the presence of readily metabolizable sugars, which are imported via phosphotransferase systems (PTS), such as cellobiose, PrfA is inhibited; however, the precise mechanism is still unknown. Using a sugar screen, trehalose was identified as the first PTS-dependent sugar that supports the growth of L. monocytogenes, but does not exert a strong inhibitory effect on PrfA. We demonstrated that the PTS permease TreB is the sole trehalose importer. After import, trehalose-6-phosphate is cleaved by the phosphotrehalase TreA; however, loss of TreA does not fully abolish growth on trehalose, suggesting that L. monocytogenes encodes an additional phosphotrehalase. ¹³C-Labeling experiments revealed that trehalose metabolism is repressed in the presence of glucose, while it can be metabolized in the presence of glycerol. Additionally, these experiments provided evidence that trehalose and cellobiose are metabolized via identical pathways, including glycolysis and the incomplete tricarboxylic acid (TCA) cycle, although trehalose has a slower uptake and/or metabolization rate. We therefore hypothesize that sugar-dependent PrfA inhibition correlates with sugar transport and/or consumption rates.

Virulence factors determine the pathogenic potential of bacteria; however, constant expression of these factors is an energetic burden. In Listeria monocytogenes, virulence gene expression is induced by the virulence regulator PrfA, whose activity is repressed in the presence of readily metabolizable sugars. This study reveals that trehalose, unlike other phosphotransferase system (PTS)-imported sugars, supports listerial growth while barely affecting PrfA activity, highlighting an exception to the established model of sugar-dependent PrfA inhibition. Based on ¹³C-labeling experiments, we hypothesize that the metabolic flux rather than the sugar type alone influences PrfA activity. The proposed link between phosphoenolpyruvate availability and PrfA activity offers new insight into how nutrient sensing is linked to pathogenicity.