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Salmonella Typhimurium pathogenesis relies mainly on the expression of genes of two pathogenicity islands, Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2). Each island has its own pattern of expression and regulation. Success in suppression of the responsible key activator of each island would be an effective way of controlling Salmonella, especially with the emerging problem of antibiotic-resistant strains. Probiotics have been shown to inhibit several foodborne pathogens, and their mode of action may partly involve down-regulation of virulence genes. To investigate whether probiotics played a role in the regulation of the pathogenicity islands SPI1 and SPI2 in Salmonella, two reporter strains were constructed in which the general regulator of SPI1, hilA, and the response regulator of SPI2, ssrB, were fused with luxCDABE genes. These constructs were used to screen the effect of probiotics on the expression of each gene. Molecules secreted by Bifidobacterium bifidum were able to down-regulate both genes.

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