Effects on poultry and livestock of feed contamination with bacteria and fungi
Introduction
The chemical and nutritional constituents of animal feeds are important for livestock nutrition and growth, but are only part of the animal feed matrix. From an ecological standpoint, harvested grains are not only ingredients for livestock diets, but can act as substrate and transmission vectors for simple unicellular prokaryotic and eukaryotic organisms. Feeds may contain diverse microflora that is acquired from multiple environmental sources, including dust, soil, water, and insects. Feed materials may be inoculated at any time during growing, harvesting, processing, storage and dispersal of the feed. The microflora found in feed materials come from a variety of ecological niches, such as soil and gastrointestinal tracts, and have to adapt to the conditions found in animal feed and feed components in order to survive and/or grow. The microbial diversity found in different feeds is dependent on the water activity, oxygen tension, pH and nutrient composition of the feed matrix. Microfloral growth is dependent on the moisture content of the feed material. Some microorganisms, primarily moulds, have adapted to conditions without free water and can actively grow in stored grains. However, the majority of microorganisms must exercise various strategies to survive until there is sufficient water to support microbial activity. Microflora can decrease grain value through nutritional changes, physical damage, or the production of toxins deleterious to animal health.
Section snippets
Origins of bacteria found in feed
Grains and oilseed crops possess a diverse microflora, with populations ranging from 5 × 103 to 1.6 × 108 CFU/g, that are highly resistant to low moisture conditions (Richard-Molard, 1988, Multon, 1988). Plant material is primarily inoculated by dust generated when soil is disturbed during mechanical harvesting, strong wind or rain. The soil environment is a collection of microhabitats comprised of clay particles, organic matter and aqueous domains that vary in pH, redox potential, ionic strength,
Important bacterial pathogens found in feed
An overview of clinically important foodborne pathogens found in feed can be found in Table 1.
Origins of feed mycoflora
Mycoflora (moulds) can also be present in feed and present a potential threat to feed quality and seed survival. Moulds may cause a decrease in seed germination, musty or sour odours, dry matter and nutrient loss, caking, mycotoxin formation and, ultimately, a reduction in feed monetary value (Beuchat, 1978, Sauer et al., 1992).
Different populations of moulds may be found in growing versus stored grain and can be divided into two large groups: field fungi and storage fungi. Field fungi may
Presence of indicator organisms
Given the difficulty of monitoring the myriad of microorganisms which may contaminate animal feeds, tracking a variety of indicator organisms have been proposed as a means to monitor overall hygiene and fecal contamination. Toranzos and McFeters (1997) reviewed several indicators of fecal contamination, including total coliforms (Gram-negative, nonsporeforming rods that ferment lactose and produce acid and gas within 48 h at 35 °C), fecal coliforms (coliforms that produce gas and acid at 44.5 °C),
Control of bacterial contamination
An overview of several methods used to control bacterial contamination of feeds may be found in Table 3. Several strategies that have been tried to overcome feed degradation including: shortening storage time to prevent browning and caking of the feed, and supplementation with soybean oil to overcome fat losses (Bartov et al., 1982). To prevent overgrowth by storage microorganisms, rapid drying has been widely used to preserve grain (ICMSF, 1998). Zinc bacitracin can be added to feed to control
Summary
Feed may serve as a substrate for a wide variety of microorganisms. Some of the microflora are adapted to the desiccated conditions in soil and are transferred by insects, dust, and wind to similar niches in feed. Some bacteria are adapted to a niche where they are capable of degrading organic matter and/or exist in a survival state until the moisture is high enough for bacterial action. While other microorganisms, primarily moulds, actively grow within stored seeds and use seed nutrients and
Acknowledgements
This research was supported by the Texas Higher Education Coordinating Board's Advanced Technology Program (Grant #999902-165) and the Research Enhancement Program grant of the Texas Agricultural Experiment Station of the Texas A&M University System (Grant #2-102). This research was also supported by Hatch grant H8311 administered by the Texas Agricultural Experiment Station. K.G.M. was supported by an Endowed Graduate Fellowship from Pilgrim's Pride, Inc., Pittsburg, TX, and the Heep
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Current address: Department of Agriculture and Natural Resources, 1200 North DuPont Highway, Delaware State University, Dover, DE 19901, United States.
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Current address: Department of Food Science, 2650 N. Young Ave., University of Arkansas, Fayetteville, AR 72704, United States.