Autoinducer-2 Signaling Molecules Produced by Pediococcus Suppress Growth and Virulence Gene Regulation in Salmonella

Introduction: Probiotics provide numerous benefits to the host including protection from pathogens by competing for nutrients, space and producing antimicrobial substances. Recently, a novel mechanism of protection is discovered by which Pediococcus suppressed the growth of Salmonella and interfered with gene regulation in Salmonella. These findings were discovered by using sterile spent medium produced by Pediococcus in which Salmonella was cultured. We hypothesized that the quorum sensing signal Autoinducer-2 (AI-2), present in the sterile medium, was responsible for this gene interference and further experiments confirmed this hypothesis. 

Purpose: Further investigate the mechanism by which Pediococcus retards the growth of and interferes with the gene regulation in Salmonella.

Methods: Pediococcus was cultured in MRS broth for 24 h and this medium was filter sterilized to produce the sterile spent medium. Growth curves were conducted by culturing Salmonella in the sterile spent medium and measuring optical density every hour for 24 h. To investigate virulence gene regulation interference, total RNA was extracted from Salmonella cultured in the sterile spent medium and Real-Time PCR (RT-PCR) was used to evaluate gene expression. Finally, Salmonella was cultured in the medium containing 100 µM or 200 µM of AI-2 and gene expression analysis was repeated. 

Results: Salmonella growth was inhibited and the expression of the virulence gene hilA was down regulated when Salmonella was cultured in the sterile spent medium produced by Pediococcus. A more targeted investigation confirmed that AI-2, a chemical molecule present in the medium, was responsible for suppression of hilA gene expression.

Significance: From these experiments, we found a new mechanism by which probiotics Pediococcus may protect the host against pathogen infection; by suppressing growth or interfering with gene regulation. Further studies are needed to refine our knowledge of the interaction between AI-2 and other virulence genes in Salmonella and the relationship between quorum sensing and virulence.