ABSTRACTStaphylococcus aureusstrains producing the bacteriophage-encoded staphylococcal enterotoxin A (SEA) were divided into two groups, high- and low-SEA-producing strains, based on the amount of SEA produced. After growth under favorable conditions in batch cultures, 10 of the 21 strains tested produced more than 1,000 ng/ml SEA, and 9 strains produced less than 10 ng/ml SEA; two enterotoxigenic strains, MRSA252 and Newman, produced intermediate levels of SEA (around 450 ng/ml). The differences in the production of SEA were found to be associated with the expression level ofseaand whether the strains hosted thesea1orsea2version. Furthermore, differences in nucleotide sequence in theSiphoviridaephage region showed two clonal lineages of the high-SEA-producing strains. One of these lines was correlated with the capacity for a massive increase in SEA levels by prophage induction as demonstrated using mitomycin C (MC). This was also confirmed by the occurrence of additionalseaexpression, presumed to be initiated by a latent phage promoter located upstream of the endogenousseapromoter. Remarkably, the SEA level was increased up to 10-fold in some strains due to prophage induction. The low-SEA-producing group and the high-SEA-producing subgroup lacking phage-activatedseatranscription showed no increase in SEA formation after the addition of MC. This study demonstrates thatseaexpression in enterotoxigenic strains is correlated with the clonal lineage ofsea-carrying phages. The high-SEA-producing group, in particular the prophage-induciblesea1group, may be more relevant to staphylococcal food poisoning than the low-SEA-producing group, harboring mainlysea2.

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Prevention, prediction, control, and handling of bacterial foodborne diseases – an ongoing, serious, and costly concern worldwide – are continually facing a wide array of difficulties. Not the least due to that food matrices, highly variable and complex, can impact virulence expression in diverse and unpredictable ways. This review aims to present a comprehensive overview of challenges related to the presence of enterotoxigenic Staphylococcus aureus in the food production chain. It focuses on characteristics, expression, and regulation of the highly stable staphylococcal enterotoxins and in particular staphylococcal enterotoxin A (SEA). Together with the robustness of the pathogen under diverse environmental conditions and the range of possible entry routes into the food chain, this poses some of the biggest challenges in the control of SFP. Furthermore, the emergence of new enterotoxins, found to be connected with SFP, brings new questions around their regulatory mechanisms and expression in different food environments. The appearance of increasing amounts of antibiotic resistant strains found in food is also highlighted. Finally, potentials and limitations of implementing existing risk assessment models are discussed. Various quantitative microbial risk assessment approaches have attempted to quantify the growth of the bacterium and production of disease causing levels of toxin under various food chain and domestic food handling scenarios. This requires employment of predictive modeling tools, quantifying the spatiotemporal population dynamics of S. aureus in response to intrinsic and extrinsic food properties. In this context, the armory of predictive modeling employs both kinetic and probabilistic models to estimate the levels that potentiate toxin production, the time needed to reach that levels, and overall, the likelihood of toxin production. Following risk assessment, the main challenge to mitigate the risk of S. aureus intoxication is first to prevent growth of the organism and then to hamper the production of enterotoxins, or at least prevent the accumulation of high levels (e.g., >10–20 ng) in food. The necessity for continued studies indeed becomes apparent based on the challenges to understand, control, and predict enterotoxin production in relation to the food environment. Different types of food, preservatives, processing, and packaging conditions; regulatory networks; and different staphylococcal enterotoxin-producing S. aureus strains need to be further explored to obtain more complete knowledge about the virulence of this intriguing pathogen.

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The prophage-encoded staphylococcal enterotoxin A (SEA) is recognized as the main cause of staphylococcal food poisoning (SFP), a common foodborne intoxication disease, caused by Staphylococcus aureus. Studies on the production of SEA suggest that activation of the SOS response and subsequent prophage induction affect the regulation of the sea gene and the SEA produced, increasing the risk for SFP. The present study aims to evaluate the effect of NaCl and sorbic acid, in concentrations relevant to food production, on SOS response activation, prophage induction and SEA production. The impact of stress was initially evaluated on steady state cells for a homogenous cell response. NaCl 2% was found to activate the SOS response, i.e., recA expression, and trigger prophage induction, in a similar way as the phage-inducer mitomycin C. In contrast, sorbic acid decreased the pH of the culture to a level where prophage induction was probably suppressed, even when combined with NaCl stress. The impact of previous physiological state of the bacteria was also addressed on cells pre-exposed to NaCl, and was found to potentially affect cell response upon exposure to further stress. The results obtained highlight the possible SFP-related risks arising from the use of preservatives during food processing.