Investigation of the transcriptional and metabolic response of Staphylococcus aureus following growth on an autoclaved chicken breast model

The capacity of food to support microbial growth is an ongoing concern at every stage of production, from agricultural harvesting to the final preparation for consumption. The Gram-positive pathogen, Staphylococcus aureus, easily grows in foods of animal origin and causes foodborne intoxication by secreting staphylococcal enterotoxins (SE) during growth. To date, most of the experimental evidence concerning the ability of S. aureus to produce SEs has been collected from laboratory media, and little is known about its metabolism in specific food matrices. The objective of this study was to develop a cooked chicken breast model to evaluate methods for the analysis of S. aureus metabolic alterations. Multiple S. aureus lab strains and epidemiologically distinct clinical methicillin-resistant S. aureus were inoculated onto autoclaved chicken breast (ACB), and growth was monitored at 25°C to mimic a meat product undergoing temperature abuse, a factor often associated with staphylococcal food poisoning. The ACB model supports growth of unrelated S. aureus strains equivalent to that observed on rich nutrient agar media. The transcriptome of S. aureus SH1000 on ACB at 18 h and 48 h postinoculation indicates that rearrangement of nitrogen compounds is important for survival; genes required for arginine, biotin, and urea biosynthesis, as well as amino acid transporters, were dramatically up-regulated in the ACB cultures. The transcription of genes for virulence factors and several regulators were expressed at a lower level on ACB. An exception to the typical S. aureus growth pattern observed on ACB was displayed by PS47S, a proline-prototrophic variant of SH1000, which exhibits reduced growth on ACB even in the presence of exogenous arginine and biotin. Analysis of PS47S by whole genome 454 sequencing revealed that 39 unique gene loci contained polymorphisms in 100% of the contigs compared to parent strain NCTC8325. Altered loci include genes for groEL, ccpA, rimM, infC , acpP, dcuC, and ezrA , indicating extensive changes in global metabolism. Bacteria respond to growth under atypical conditions, such as those encountered in foods, by utilizing stress response systems and regulating metabolic pathways to match available nutrients. The ACB model provides an effective matrix for further study of these processes.