The survival and enterotoxin gene expression of Staphylococcus aureus planktonic and biofilm cells in quick-frozen food

Food safety issues caused by food-borne pathogenic microorganisms during low-temperature food storage have attracted broad attention. In the current study, Staphylococcus aureus cells in planktonic and different stages of biofilm formation were inoculated into four types of quick-frozen rice and flour products. The changes in culturable cell number of S. aureus in the artificially contaminated food samples stored at 4 °C and −20 °C were recorded for 60 days. Next, the PMA-qPCR method was used to explore the viable cell number and determine whether low temperature induced S. aureus enter into the viable but non-culturable (VBNC) state. Finally, the expression changes of enterotoxin gene seb in S. aureus were determined by RT-qPCR during food sample storage. Results showed that the culturable cell number of S. aureus was not significantly changed during 60 days except for the decreased culturable cell number in the 8 h biofilm cells in MF stored at −20 °C. In addition, S. aureus cells did not enter into the VBNC state during low temperature storage. Moreover, the seb gene was continuously expressed in most samples although with difference in expression level. The findings provided an alert for the risk of S. aureus contamination in quick-frozen rice and flour products.