Multiplex PCR and RPLA identification of Staphylococcus aureus enterotoxigenic strains from bulk tank milk

Enterotoxigenic Staphylococcus aureus in raw milk poses a potential health hazard to consumers, and the identification of such strains should be used as part of a risk analysis of milk and milk products. The primary purpose of this study was to investigate the occurrence of enterotoxigenic S. aureus strains in raw milk supplied for dairy processing in the Czech Republic. A further aim was to compare the production of staphylococcal enterotoxins (SEs) with the presence of the corresponding genes. This was undertaken using multiplex polymerase chain reaction (PCR) and reversed passive latex agglutination (RPLA). Out of 440 bulk tank milk samples from 298 dairy herds, 70 proved positive for S. aureus (15.9%). Staphylococcal enterotoxin genes (ses) were detected in 39 (55.7%) isolates. The genes most commonly detected were sei (38.6%), seg (31.4%) and sea (27.1%). Genes seb, seh, sed, sej and sec were observed in 10%, 4.3%, 2.9%, 2.9% and 1.4% of strains respectively. Genes see and sel did not occur. The most frequently detected genotypes were seg, sei at 11.4%; sea at 10.0%; and sea, seg, sei at 8.6%. Toxin production was observed in nine (12.9%) S. aureus isolates. Seven strains were detected as SEB- (10%) and two as SED- (2.9%) producing. A relatively high number (32%) of discrepancies between the results with multiplex PCR and RPLA assays was obtained, particularly on account of SEA. Nineteen strains were sea positive by PCR but SEA negative by RPLA, and one strain was sec positive and SEC negative. The results of both methods were identical concerning SEB and SED. It was concluded that detection of ses by PCR was a useful additional tool to support identification of enterotoxigenic strains.