Stress induced virulence of Staphylococcus aureus and Listeria monocytogenes

Sammanfattning: Control of the dissemination of pathogens in the food production chain is an on-going process with new challenges emerging as pathogenic bacteria evolve and express enhanced virulence characteristics. Staphylococcus aureus and Listeria monocytogenes are two notorious pathogens responsible for staphylococcal food poisoning (SFP) and listeriosis: a foodborne intoxication and an infectious disease respectively. In this work, S. aureus was investigated regarding the regulatory mechanism of the prophage-encoded enterotoxin A (SEA), and the impact of the food environment on the SEA produced. Moreover, L. monocytogenes’ capacity to survive and grow was evaluated under conditions of osmotic and acid stress. The expression of stress and virulence-related genes under these conditions was evaluated, and related to acquired resistance towards further stress. In the S. aureus studies, a strain-dependent SEA production was demonstrated, and linked to the different sea gene alleles (sea1, sea2). The investigated strains were grouped as high and low SEA-producers, carrying the sea1 and sea2 allele respectively. Furthermore, it was shown how the life cycle of the sea-carrying phages, and specifically prophage induction, increases the levels of SEA produced in some of the high SEA-producers by increasing both sea gene expression and the number of copies of the gene in the induced cells. A phage-activated transcript, alongside the transcript from the endogenous P1 sea promoter, characterized these strains, which were further categorized as inducible high SEA-producers. Prophage induction was found to be linked to the SOS response, through the activation of the RecA protein. A study of S. aureus on pork sausages led to the observation that SEA production and growth rates should not be considered as coupled events, and that food components may lead to prophage induction and increased SEA formation in the food product. A study of the impact of NaCl and sorbic acid on prophage induction and SEA production showed that increased NaCl activates the phage and increases the potential risk for SFP, however, the presence of sorbic acid in pH conditions critical for S. aureus growth lowers this risk. L. monocytogenes was found to be more susceptible to sequential stress applications, where acid and osmotic shifts combined with low temperature delayed the initiation of growth. Strain dependence characterized the survival and growth potential of the pathogen under adverse conditions, and adaptive responses were triggered only under mild stress habituation that allowed growth. However, stress adaptation could potentially increase virulence, even in cases where the phenotypic responses indicate a low risk of pathogen survival. In summary, the genetic complexity of bacterial phenotypic responses when exposed to stress in the food processing environment was demonstrated in this work. The obtained knowledge could be incorporated into predictive modeling for more accurate hazard evaluation and effective control of pathogenic bacteria in food.