| Staphylococcus aureus (S. aureus) is a major pathogen which could cause bacterial food poisoning. It spreads widely in the natural environment and can contaminate food through various ways. Staphylococcal food poisoning (SFP) is one of the most common food-borne illnesses in the world. S. aureus can produce many virulence determinants. Staphylococcal enterotoxins (SE) are the key virulences to lead Staphylococcal food poisoning and shock. There are11kinds of SE have been determined. They are SEA, SEB, SEC, SED, SEE, SEG, SHE, SEI, SER, SES and SET. However, the isolates used to institute the distribution and expression of SE were got from clinical cases. Therefor, it is not very meaningful for food safety. Besides, the research of enterotoxin expression on food were few. Therefor, this research studied the enterotoxin genes distribution and relative expression on liquid culture and spiced beef to reveal the temporal expression of enterotoxin genes. At the same time, the study can provide certain basis and support for the prevention and control of SE expression.1. Isolation, identification and distribution of Staphylococcus aureus isolates from various foodsThe objective of this study was to investigate the distribution of different serotype enterotoxins gene from food-borne S. aureus. the2514samples (meat products n=486, bean products n=350, dairy products n=384, aquatic products n=446, raw milk n=480, raw meat n=368) were used to detect the S. aureus isolates through traditional routine microbiological analysis protocol and PCR amplification. In this study, we identified51S. aureus isolates from various foods. The meat products presented highest percentage of S. aureus positive samples among the tested foods (3.91%).Duplex PCR was conducted to genotype the enterotoxins in the study, the16sRNA gene was used as the internal control. The results showed that eight kinds of SE genes were detected in the51food-borne Staphylococcus isolates without ses, set and see.22Out of52isolates (43.14%) were enterotoxigenic27.45%of the isolates contained seg and sei genes. The most prevalent traditional SE genes were sed(13.73%). 2. Temporal expression of Staphylococcus aureus enterotoxin genes in the liquid cultureThe objective of this study was to investigate the temporal expression of different enterotoxin gene in liquid culture. Growth of S. aureus in the liquid broth was monitored by detecting cell density at OD600for each strain. RNA extraction were taken at the corresponding absorbance value. The expression of enterotoxin on mRNAs during the cell cycle were assayed through RT-PCR with ftsZ and ropB genes included as the reference genes. The results showed that the patterns of SE mRNA gene expression were similar in all of the isolates during the growth cycle. The enterotoxin genes on the level of mRNA were peaked in the post-exponential growth phase and then rapidly decreased. However, the relative expression level of different enterotoxin genes in the same isolate was different, and the same SE gene’s expression level also varied among strains.3. Expression of classical Staphylococcus aureus enterotoxin genes on beef productsThe objective of this study was to investigate the expression of classical sea-sed on Chinese traditional spiced beef products. The beef products were contaminated with104CFU/g Staphylococcus aureus which contain enterotoxin (sea, seb, sec, sed) separately and incubated at23℃. Samples were taken after incubation1,2,3,5and7d for qRT-PCR, ELISA analysis, and viable count determination. The viable count reached the maximum at first or second day. The mRNA of enterotoxin genes sea-sed were detected in traditional beef during the entire incubation period with a peak at day one of two. What’s more, a second increase of sea-sed in spiced beef was observed during the incubation. The ELISA detection indicated that all target SE were positive throughout the incubation. |
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