Effects of Temperature Abuse on the Growth and Staphylococcal Enterotoxin A Gene (sea) Expression of Staphylococcus aureus in Milk

Abstract

The aim of this study was to determine the effects of different temperatures and storage time on Staphylococcus aureus growth, sea gene expression, and synthesis of staphylococcal enterotoxin A (SEA) in the pasteurized and UHT-pasteurized milk. Pasteurized and UHT-pasteurized milk were inoculated with 3.98 log(10) CFU/mL of S. aureus (ATCC 13565). Inoculated milk samples were stored at 8 degrees C, 15 degrees C, and 22 degrees C for 24, 48, and 72 h, respectively. SEA synthesis was detected with a fully automated miniVIDAS instrument using the Enzyme-Linked Fluorescent Assay (ELFA) technology. The patterns of gene regulation were detected by quantitative reverse transcriptase PCR. The 2(-Delta Delta CT) method has been used as a relative quantification strategy for gene expression responses data analysis. The results indicated that growth rate, sea gene expression, and SEA synthesis were influenced by type of milk, storage time, and temperature. Incubation of milk at different temperatures (15 degrees C and 22 degrees C) and times was used to simulate inadequate transport and storage conditions. Storage of pasteurized milk at 22 degrees C for 24 h significantly upregulated the expression of sea gene compared with milk stored at 8 degrees C, which coincides with the achieved S. aureus number of 10(5) CFU/mL and detected amount of SEA. In addition, storage of UHT-pasteurized milk at 22 degrees C for 24 h and at 15 degrees C for 48 h significantly upregulated the sea gene expression compared with milk stored at 8 degrees C, which coincides with the detected amount of SEA and the dynamics of S. aureus number change. It can, therefore, be concluded that implementing good hygiene practices to avoid pre- and post-heat treatment milk contamination and maintaining the cold chain at temperature <8 degrees C throughout the entire dairy production chain are of paramount importance to decrease the risk of staphylococcal food poisoning.