Identification and measurement of staphylococcal enterotoxin M from Staphylococcus aureus isolate associated with staphylococcal food poisoning.

Staphylococcus aureus produces a wide variety of staphylococcal enterotoxins (SEs, SEA to SEX), which are responsible for staphylococcal food poisoning. This study is aimed to establish a system to detect staphylococcal enterotoxin M (SEM) protein in food matrixes. In the present study, sem gene was characterized in a S. aureus isolate H4 associated with food poisoning. The amino acid sequence of the deduced SEM protein was same as that of previously identified SEM from S. aureus 04-02981. Subsequently, mature SEM protein was expressed in Escherichia coli BL21 (DE3) cells and purified with a Ni-NTA spin column. The polyclonal and monoclonal antibody against it were prepared. Using these antibodies, a highly sensitive, specific sandwich enzyme-linked immunosorbent assay (ELISA) system was developed capable of detecting SEM in milk, meat and rice. Cross-reactivity with SEB, SEI and SEK in this method was insignificant. Quantification of SEM secretion in vitro using this novel capture ELISA revealed that SEM was mainly secreted during the transition from the exponential to the stationary phase. Furthermore, sem gene and SEM protein production were screened by PCR and the developed ELISA system. The results indicated that there were two SEM+ strains of 19 S. aureus isolates originating in cold dishes and humans suffering from food poisoning. The investigations make it possible to assess SEM in food hygiene supervision in near future. Staphylococcal enterotoxins (SE) are the main causative agents of staphylococcal food poisoning. Unlike classical SEs (SEA to SEE), the relationship between newly identified SEs (SEG to SEX) and staphylococcal food poisoning has not been clearly elucidated. Recently, mild emetic potential of staphylococcal enterotoxin M (SEM) has been demonstrated, which indicated that SEM might be associated with food poisoning. However, there is currently no commercial enzyme-linked immunosorbent assay (ELISA) kit available for immunological detection of it. Therefore, we developed a highly sensitive, specific sandwich ELISA system and assayed SEM in food matrixes. This assay facilitates the study of SEM expression in vitro and in vivo. In addition, the investigations would be helpful in addressing the relative incidence of SEM+ strains in near future.