Effect of sub-lethal doses of nisin on Staphylococcus aureus toxin production and biofilm formation.

Staphylococcus aureus is one of the commonest food-borne pathogens that can cause gastroenteritis owing to having several enterotoxins. Also, biofilm formation can complicate infections caused by this microorganism. Nisin is a safe food bio preservative which is usually used as an agent to prevent pathogen growth; however, it is important to identify the exact impact of nisin on the growth of S. aureus and to determine the suitable concentration needed for elimination of this pathogen in food. In this study, after MIC determination of nisin against S. aureus ATCC 29213, this strain was treated with sub-MIC (1/2) of nisin (4 µg/ml) and transcript levels of toxin-encoding (hla, SEA, SEB, and SED) and biofilm-associated (fnb, ebpS, eno, and icaA) genes were determined using Quantitative Real-time PCR at 2, 8, and 24 h post exposure. All toxin genes were down-regulated following exposure to sub-MIC of nisin, whereas biofilm-associated genes were up-regulated. The expression levels of fnb and icaA in S. aureus were highest after 8 h (4.5-fold and 6.8-fold increase, respectively), while the expression levels of eno and ebpS genes were highest after 2 h (3.3 and 4.5-fold increase, respectively). According to these results, although transcriptional levels of toxin genes were reduced, sub-MIC concentrations of nisin could trigger the expression of biofilm-associated genes in S. aureus. This can further lead to bacteriocin tolerance such that even its higher concentrations cannot kill bacterial cells after exposure to sub-lethal doses. Therefore, it is pivotal to add appropriate concentrations of nisin to food products for preservation purposes.