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Development of Unique Bacterial Strains for Use as Positive Controls in the Food Microbiology Testing Laboratory

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Nalidixic acid–resistant (NalR) mutants of Salmonella enterica serovar Berta and Escherichia coli O157:H7 were derived from wild-type laboratory cultures to serve as distinguishable control strains for routine use in food microbiology testing programs. The prevalence of the NalR phenotype among different bacteria was verified using panels of related and unrelated strains with the ability to grow vigorously on plating media containing nalidixic acid, being restricted to the NalR mutants. The NalR phenotype was stable in both mutant strains over several generations in the absence of selective pressure and enabled their differentiation from wild-type bacteria on the basis of their ability to grow on plating media containing nalidixic acid. A similar approach for the development of a distinguishable Listeria monocytogenes control strain was not possible due to the inherent resistance of this organism to nalidixic acid. Instead, an L. monocytogenes isolate with rare genotypic and serologic features was identified as a possible candidate to serve as a unique and distinguishable positive control strain.

Document Type: Research Article

Affiliations: 1: Ottawa Laboratory (Carling), Canadian Food Inspection Agency, Building 22, C. E. F., Ottawa, Ontario, Canada K1A 0C6 2: Bureau of Microbial Hazards, Health Canada, Frederick G. Banting Building, Tunney's Pasture, Ottawa, Ontario, Canada K1A 0K9

Publication date: November 1, 2008

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