Mashed potato made with raw bovine milk was suspected to have been the source of a food poisoning outbreak. Almost 8 x 10(8)Staphylococcus aureus CFU g(-1) were detected in the mashed potato. S. aureus was also found in bulk milk from the farm that had supplied milk for the mashed potato. Isolates from mashed potato and bulk milk were positive for the gene encoding staphylococcal enterotoxin H (seh), and the corresponding protein toxin, SEH, was detected by ELISA in the mashed potato. Production of SEH by S. aureus isolates from mashed potato (n = 4) and bulk milk (n = 4) was also demonstrated by ELISA. Sequencing of seh from one mashed potato isolate and one bulk milk isolate confirmed that the gene was a variant seh, and that the genes in both isolates were identical. Macrorestriction of chromosomal DNA with Sma1 followed by pulsed-field gel electrophoresis of seh-positive S. aureus from mashed potato and bulk milk revealed indistinguishable banding patterns between isolates from both sources. It seems likely that raw bovine milk used in the preparation of mashed potato contained S. aureus that subsequently produced sufficient SEH in the mashed potato to cause food poisoning.

Aim: To find out if testing of up to 10 Staphylococcus aureus isolates from each sample from raw milk and raw milk products for staphylococcal enterotoxin (SE) might increase the chances of identifying potential sources of food intoxication. Methods and Results: Altogether 386 S. aureus isolates were tested for the presence of SE by reversed passive latex agglutination (SET-RPLA), and SE genes (se) by a multiplex polymerase chain reaction (PCR). In 18 of 34 (53%) S. aureus positive samples a mixture of SE and/or se positive and negative isolates were identified. Multiplex PCR increased the number of potential SE producing strains, i.e. isolates that harboured se, with 51% among the product and 48% among the raw bovine milk isolates. Examination by pulsed-field gel electrophoresis mostly confirmed clonal similarity among isolates sharing SE/se profile, but did not further differentiate between them. Conclusions: Isolates of S. aureus collected from one sample may show great diversity in SE production and different plating media seem to suppress or favour different strains of S. aureus. Significance and Impact of the Study: Several isolates of S. aureus from each sample should be tested for enterotoxin production in cases with typical SE intoxication symptoms with methods that are able to reveal new SE/se.

The performance of a multiplex PCR assay that separates the four major serovars of the pathogenic Listeria monocytogenes into four distinct PCR groups was evaluated through a multicenter typing study. Identical panels of 90 Listeria isolates were distributed to five participating laboratories that were blind to the nature of the isolates. Isolates were characterized using the previously standardized protocol. Overall concordance was 96.6 to 100%, sufficient for the assay to be used as an alternative to serotyping and confidently applied in laboratories involved in L. monocytogenes typing.

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