Salmonella presents a particular problem in food testing regimes since other non-pathogenic micro-organisms commonly found in food, such as Citrobacter, Proteus and Morganella species, may present with similar morphology on standard diagnostic culture media. Such presumptive positive colonies must then be further tested to eliminate or confirm the presence of Salmonella. Traditionally, this relies on expensive biochemical and serological tests and can take up to 24 hours to obtain a result.
In the meantime, until a negative result is obtained, the release of the product is delayed, production may be suspended and often the plant may be cleaned down as a precaution, resulting in considerable cost and concern for the manufacturer.
In the majority of cases it is sufficient to confirm that suspect colonies are NOT Salmonella, without identifying the species. It was in recognising this potential that Thermo Scientific Limited developed OBIS Salmonella (OBIS. - Thermo Scientific Biochemical Identification System).
OBIS Salmonella incorporates two simple biochemical reactions on a single disposable test card. The first, PYR, detects PYRase activity in Citrobacter species and the second, NPA, demonstrates deaminase activity in Proteus and Morganella species (see illustration). Positive activity is demonstrated by a vivid colour reaction (PYR - purple, NPA - orange), indicating the presence of non-Salmonella species. Colonies that test negative for both reactions are confirmed as presumptive Salmonella.
By confirming that suspect colonies are not Salmonella in just 5 minutes, OBIS Salmonella allows batches to be released and production to continue up to 24 hours earlier than by traditional methods. Furthermore, fewer API tests (to identify Salmonella) are required, saving time and resources in the laboratory.
In trials involving 1000 samples grown on a variety of selective culture media, 470 produced presumptive positive Salmonella colonies. Of these OBIS Salmonella screened out 406 false positives, reducing the number of presumptive positive plates requiring further testing to 64. All positive Salmonella samples were detected (ref1).
1. Colem an, D.J., Barnwell, A.P. and Raggett, J. Birmingham PHLS West Midlands, Poster Presentation, Thermo Scientific Limited.