Enhanced Recovery and Isolation of Salmonella and Listeria Using a Novel Culture-Transfer-Inoculation Device

Abstract

A novel culture-transfer-inoculation device (CTID), comprised of a fiber-foam matrix suspended in the enrichment medium and thereafter used in subsequent transfers and inoculations, was used in the analysis of foods for Salmonella using the Food and Drug Administration (FDA) method and for Listeria using the FDA and United States Department of Agriculture (USDA) procedures. In these studies, the food sample was introduced into the enrichment medium including the CTID. Following incubation, transfer from the primary enrichment culture and subsequent transfers were made by both conventional procedures and by the CTID. Four-hundred-and-twenty-five samples were analyzed for Salmonella of which 115 were positive. Using conventional transfer and inoculation techniques, 87 positive results were observed; whereas using the CTID-modified technique, 112 positive results were obtained. The false-negative rate for the conventional transfer method was 21.7%. A 2.6% false-negative rate was observed with the CTID. The difference between conventional and CTID results were statistically significant (p<0.0160). One-hundred samples were analyzed for Listeria using the FDA method with 32 positive results. Using the CTID 42 positives were detected from the FDA enrichments. One-hundred-and-ninety-eight samples were analyzed using the USDA method with 77 positive results. With the CTID, the same samples yielded 86 positives. No false-negatives resulted through the use of the CTID while a 10.5% to 23.8% rate was observed with the conventional transfer method. The difference between conventional USDA and CTID-modified USDA results were statistically significant (p<0.0086). These data indicate that the fiber-foam matrix of the CTID provides a micro-environment enhancing the sensitivity of the culture methods for the detection of Salmonella and Listeria.