Acetic acid increases the phage-encoded enterotoxin A expression in Staphylococcus aureus

The sea expression profile was similar from pH 7.0 to 5.5, with the relative expression peaking in the transition between exponential and stationary growth phase and falling during stationary phase. The levels of sea mRNA were below the detection limit at pH 5.0 and 4.5, confirmed by very low SEA levels at these pH values. The level of relative sea expression at pH 6.0 and 5.5 were nine and four times higher, respectively, in the transitional phase than in the exponential growth phase, compared to pH 7.0 and pH 6.5, where only a slight increase in relative expression in the transitional phase was observed. Furthermore, the increase in sea expression levels at pH 6.0 and 5.5 were observed to be linked to increased intracellular sea gene copy numbers and extracellular sea-containing phage copy numbers. The extracellular SEA levels increased over time, with highest levels produced at pH 6.0 in the four growth phases investigated. Using mitomycin C, it was verified that SEA was at least partially produced as a consequence of prophage induction of the sea-phage in the three S. aureus strains tested. Finally, genetic analysis of six S. aureus strains carrying the sea gene showed specific sea phage-groups and two versions of the sea gene that may explain the different sea expression and production levels observed in this study.Our findings suggest that the increased sea expression in S. aureus caused by acetic acid induced the sea-encoding prophage, linking SEA production to the lifecycle of the phage.Staphylococcal enterotoxins (SEs) are extracellular proteins, produced mainly by Staphylococcus aureus, causing food intoxication when ingested. Staphylococcal food poisoning (SFP) was the fourth most common causative agent in food-borne illness within the EU in 2008 [1]. It is associated with food, generally rich in protein, which requires extensive manual handling, often in combination with inadequate heating and/or inappropriate storage of the food [2,3]. To date, 21 staph