Detection of Salmonella in the presence of competitive microorganisms with the use of reference samples. BCR-FOOD Trial 2

Abstract

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The detection of Salmonella in the presence of competitive microorganisms was tested with the use of reference samples. Thirtyeight laboratories investigated 25 samples of which 5 were negative for salmonella. The Rappaport-Vassiliadis broth was used as reference (RV- method) and compared with a selective enrichment broth chosen by the laboratory itself (OWN-method). The Salmonella samples had a contamination level of 5.48 salmonellae/capsule, the competitive microorganisms of about 2x10 cfu/capsule. The strains used for competitors were: Enterobacter cloacae, Escherichia coli, Pseudomonas aeruginosa and Proteus mirabilis. Significant differences were found between laboratories and between the RV and OWN method. With the RV method 42% of the labs found 19 or 20 positive isolations ; the percentage positive isolations (averaged over all labs) was 81.5%. With the OWN method only 19% of the labs found 19 or 20 positives, the percentage positive isolations was 62.3%. Eight laboratories found one or more of the Salmonella negative samples positive for Salmonella. The competitive microorganisms reference samples is mutable to test the isolation procedure for Salmonella under more realistic circumstances.