Rapid Cultural Methods for Detection of Salmonella in Feeds and Feed Ingredients

Abstract

Efficacy of standard, 6-h standard and direct enrichment methods for detection of Salmonella in naturally contaminated feeds and feed ingredients was compared. Analysis by the standard method involved preenrichment of feed slurries in nutrient broth, selective enrichment in tetrathionate brilliant green (43°C) and selenite cystine (35°C), and isolation of presumptive isolates on bismuth sulfite and brilliant green sulfa agar media. Sample analysis by the 6-h standard method was identical to the above except that incubation of enrichment broths was reduced to 6 h; for direct enrichment. preenrichment in nutrient broth was omitted. Of 287 samples tested, 75 were found to contain salmonellae by the three methods combined. Ability of the standard and 6-h standard methods to identify the same 58 contaminated samples underlines the reliability of the 6-h standard method for the more rapid detection of Salmonella in animal feeds. Identification of 68 positive samples by direct enrichment presumably resulted from equilibration (3 to 4 h) of feed slurries at reduced water activity before analysis. Addition of novobiocin (40 μg/ml) to selective enrichment broths did not facilitate isolation of Salmonella through repression of competitive flora. Productivity of the six enrichment-plating combinations used in this study was comparable, and no single medium played a determinant role in recovery.

Organic acids for control of Salmonellain different feed materials

“…Salmonella counts in naturally contaminated feed or feed ingredients are usually low [22,23]. With the observed reduction in Salmonella counts in compound feed the levels after acid treatment would probably be less than 1 CFU/kg.…”

Accuracy and Sensitivity of Commercial PCR-Based Methods for Detection of Salmonella enterica in Feed

“…Efficient isolation methods and sampling plans are important tools in tracing scenarios, control programs, and decision-making processes by the industry or authorities. The low levels of Salmonella in compound feed and feed raw ingredients, the dehydration of the Salmonella present, the uneven distribution of contamination in feed commodities, and the high levels of competing aerobic bacteria combine to put great demands on the isolation methods (15,23,39).…”

“…Molecular techniques, such as PCR, for Salmonella detection are capable of obtaining positive or negative results in 2 days but still require preenrichment to obtain the 10 2 to 10 4 Salmonella CFU/ml required by the method for detection (unpublished studies). While the addition of the preenrichment step increases detection time, it reduces the concern that PCR methods will detect DNA from dead cells since feed is usually contaminated with a very low number of Salmonella cells (15,40).…”

Rapid and Specific Detection ofSalmonellaspp. in Animal Feed Samples by PCR after Culture Enrichment

“…Animal feed is a recognized source of Salmonella enterica for farm livestock and might also act as an indirect cause of infection of people consuming foods of animal origin. Salmonellae can survive for several years in feed (14), and low concentrations have been found to be infective in, for instance, chickens (18). Transmission of salmonellae from feed to animals such as cattle and chickens is well documented (18,22).…”

“…In several countries, salmonellae have been isolated from animal feed ingredients as well as the final product, of both vegetable and animal origin, with a prevalence of between 0 and 6% (17,30). The levels of salmonellae in feed are low; D'Aoust and Sewell (14) quantified salmonellae in several kinds of feed ingredients and compound feeds and found the levels to be less than 1 bacterium per g of feed.…”