Salmonella

In 1982 a rapid Salmonella detection procedure based on the HGMF was described (37). The method was subjected to collaborative study in 1984 and was granted official recognition by AOAC in that same year (38). Over time, it became evident that the plating media used in the original HGMF method—namely, Hektoen Enteric Agar and Selective Lysine Agar (SLA)—were not adequately specific to Salmonella. As a result, SLA was redesigned to increase its selective and differential properties. The improved medium, EF-18 Agar, relied on a dual biochemical differential reaction: lysine decarboxylation and sucrose fermentation. Alkaline colonies (ie, lysine positive and sucrose negative) were considered to be presumptive positive Salmonella and were subcultured for confirmation. Acid colonies (ie, lysine negative or sucrose positive organisms) were discarded.

EF-18 Agar was incorporated into the HGMF Salmonella method together with some relatively minor procedural changes. The improved method was subjected to extensive in-house validation against the conventional AOAC Salmonella method in a study comprising nearly 1000 naturally contaminated and inoculated food and feed samples (39). Overall, the HGMF method produced a false negative rate of 2.0% versus 1.9% for the reference method. The presumptive false-positive rate for the HGMF method was 0.3%, as compared with 8% for the reference method.

Following this successful validation, an AOAC-spon-sored collaborative study of the method was organized. Thirty laboratories based in the United States and Canada took part, including both government and industry labs. The AOAC determined that the HGMF Salmonella method performance was statistically equivalent to that of the conventional AOAC Salmonella method, and awarded approval to the HGMF procedure (40).

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