Enterotoxin genes of Staphylococci isolated from bovine milk

Abstract

Staphylococcal food poisoning is an important foodborne problem worldwide. It is caused by the contamination of enterotoxins produced by staphylococci. The aims of this study were 1) to investigate classical staphylococcal enterotoxin genes, including sea, seb, sec, sed and see, among Staphylococcus aureus and Coagulase-negative Staphylococci (CNS) isolated from milk samples associated with bovine mastitis, and 2) to determine the effect of temperature on the expression of classical staphylococcal enterotoxin genes of staphylococci inoculated in milk. For the first objective, S. aureus (n=51) and CNS (n=47) were analyzed for their presenting of enterotoxin genes using PCR assays. For the second objective, the expressions of classical enterotoxin genes including sea, seb, sec, sed, and see were determined during the growth of staphylococci in UHT milk at two different temperature conditions including; 1) 8 °C as a representative of refrigerated temperature and 2) Room temperature. Patterns of gene expressions were analyzed by quantitative real-time PCR. Among S. aureus isolates, 18 isolates (35.3%) carried at least one enterotoxin gene. The most frequent gene profile detected was sec (15/51; 29.4%) followed by sea + sec (3/51; 5.7%). Among CNS isolates, only 6 isolates (6/47; 12.7%) carried enterotoxin genes with sec being the most prevalent detected gene (3/47; 6.4%), followed by see (2/47, 4.3%), and seb (1/47, 2%), respectively. After the inoculation of a strain of either S. aureus or CNS carrying sea, seb, sec, or see genes into the UHT milk, the expressions of sea and sec genes were significantly higher at room temperature compared to those at 8 °C after 16 hours of incubation (p<0.05) However, the expressions of seb and see genes at both temperatures were not significantly different (p>0.05). In conclusion, the temperature has an effect on the expressions of sea and sec genes of staphylococci in milk. Milk contaminated with staphylococci carrying these genes should be kept at the refrigerated temperature as soon as possible. Long period of storing milk contaminated with staphylococci at room temperature may increase the risk of high contamination of enterotoxin in milk, consequently increase the risk of foodborne poisoning from consuming contaminated milk.