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Animal feed may serve as a carrier for a wide variety of microorganisms. The primary mode of inoculation of feed materials is the transference of soil by wind, rain, mechanical agitation, or insects to standing crops. Some of the microorganisms are adapted to the desiccated and relatively nutrient-poor conditions in soil and survive in similar niches on growing crops. Gastrointestinal pathogens can also introduced into the food chain by animals defecating in the farm environment or by fertilization of crops with manures. Other microorganisms are introduced during storage. In general, the amount of available water in the feed matrix determines whether a microorganism will grow or survive. Some microorganisms, primarily moulds, are adapted to the low amount of available moisture and grow actively within stored seeds and grains. Others will produce spores or enter survival state until the moisture is high enough for bacterial action. There are numerous ways contaminating microorganisms can affect feed quality negatively including reducing dry matter and nutrients, causing musty or sour odours, causing caking of the feed and producing toxins. Finally, feed can act as a carrier for animal and human pathogens. The type of feed, processing treatments and storage conditions can all be factors that influence the population levels and types of microorganisms present. The incidence and variation in the microflora found in animal feed and feed materials are reviewed. A select number of important human and animal pathogens are discussed. Finally there is a brief overview over the detection, surveillance and management strategies of microbial contamination in feed and feed materials.

The effect of varying concentrations of different protein sources on the survival of a Salmonella typhimurium marker strain resistant to both novobiocin and nalidixic acid in nonsterile poultry mash diets was determined. A dry inoculum containing a primary poultry isolate of S. typhimurium was used to evaluate survival in mash containing one of two protein sources, soybean meal or meat and bone meal, at five concentrations (100, 150, 200, 250 and 300 g crude protein kg ˇ1 mash) and stored at room temperature (2118C) for 56 days. In general, aerobic enumerations recovered from diets containing meat and bone meal were greater than those from diets containing soybean meal excluding day 0 and 56 (P

Buffered propionic acid (BPA) was evaluated as a potential treatment for the elimination of Salmonella spp. in poultry mash. A primary poultry isolate marker strain of Salmonella typhimurium was added as either a broth or in a dry chalk carrier form to poultry mash containing soybean meal as a protein supplement. The mash was supplemented with buffered propionic acid at 2, 4, 6, 8, 10, 20, 30, 50 and 100 g kg ˇ1 diet and samples were enumerated for indigenous aerobic bacteria, fungi and the S. typhimurium marker strain. Total indigenous aerobic bacteria and fungal populations were generally decreased by addition of more than 20 g BPA kg ˇ1 , but an addition of 100 g BPA kg ˇ1 mash was usually required to achieve reductions of approximately 90% of indigenous aerobic bacteria and 99% of indigenous fungi. After 7 days of storage, 8 g BPA kg ˇ1 mash also reduced S. typhimurium populations by more than 90% in mash inoculated via chalk, while at least 50 g BPA kg ˇ1 mash was required to provide the same level of reduction in mash inoculated with a liquid culture of S. typhimurium. Although BPA does not appear to be an overly effective antimicrobial agent with respect to indigenous aerobic bacterial populations in animal feed, higher concentrations may have the potential for reducing fungal and Salmonella spp. contamination in poultry mash. # 1998 Elsevier Science B.V.