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The primary habitat of the intracellular pathogen Listeria monocytogenes is considered to be soil and decaying vegetation. As an opportunistic pathogen it must be able to recognize its entry into host tissue and, in response, co-ordinately induce the expression of virulence factors. No signature molecule, which facilitates this regulation, has been identified for any human pathogen. Our studies have demonstrated for the first time that the expression of major virulence determinants in L. monocytogenes can be repressed by an environmentally ubiquitous molecule. Transcriptional hlyA and plcA fusions to luxAB were used to monitor virulent gene expression in the presence of various disaccharides. These studies revealed that the expression of listeriolysin O and phosphatidylinositol-specific phospholipase C is repressed specifically by the plant-derived disaccharide, cellobiose.

A promoter probe vector, which utilized the luxAB genes from Vibriofischeri as reporters of gene expression, was constructed for use in Listeriu monocytogenes. Using this system gene expression can be monitored nondestructively and in real-time, simply by measuring cellular bioluminescence. Derivatives of the promoter probe were constructed that contained the cloned promoters from the hlyA and plcA genes of L. monocytogenes. The activity of these promoters was dependent on the transcriptional activator PrfA. Accordingly, in a strain containing an intact copy of the prfA gene, expression from both the hlyA and plcA promoters was 25-45-fold higher than inprfA mutants. Heat shock was identified as an environmental signal which induced expression of hlyA and plcA. Conversely, oxidative stress had no effect upon the expression of the virulence factors. In addition, the composition of the growth media was found to have a dramatic effect upon the expression of @ A and plcA, suggesting the presence of an unidentified signal which may regulate induction of expression of virulence genes in L. monocytogenes.

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