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Bacteria can be infected by bacterial viruses (bacteriophages, or simply phages). By nature, they are very specific such that an individual phage may only infect and subsequently kill a specific bacterial strain within a species. This high degree of specificity allows phages to be used against targeted microorganisms in a mixed population such as the ruminant gut without perturbing the microbial ecosystem. Bacteriophages are common natural members of the gastrointestinal microbial ecosystem of food animals, including ruminants (Adams et al., 1966; Klieve and Bauchop, 1988).
Virulent bacteriophages attach to specific receptors on the surface of bacteria, inject their DNA, then express genes that “hijack” a bacterium; directing a transition from host to phage metabolism that leads to the synthesis of new phage DNA and new phage particles, ending with programmed cell lysis and the release of dozens or hundreds of new phage particles. This exponential increase in the number of phages continues as long as the targeted bacteria are present, thus allowing phages to persist in the gut rather than simply degrade over time as do antibiotics. Because phages are “infectious” agents, they can be passed between hosts, colonizing other animals and infecting the target bacteria in this new host; however, phages they are also self-limiting, if the target bacterium (prey) is removed from the environment, then the phage population, like any predator, will diminish. Ever since their discovery in 1917 by D’herelle they have been applied in both human and animal therapy in many parts of the world, often as a direct replacement for antibiotics.
Hide as a source of E. coli O157:H7
For years the gastrointestinal tract was thought to be the proximate source of E. coli O157:H7 contamination of cattle carcasses in the slaughter plant. Recent research has implicated the hide of cattle as contributing a greater proportion of the contamination of carcasses at slaughter. Several intervention strategies have been proposed to reduce the carriage of pathogens on the hide entering the slaughter plant, including chemical dehairing and steam vacuum treatments. These treatments share a common drawback, they must be administered immediately prior to slaughter.
Bacteriophage offer a unique opportunity to control E. coli O157:H7 on the hides of cattle, that of a biological control method that can be used prior to shipment. If phage can be used to reduce pathogens on the hide before other intervention strategies are utilized at the plant, then the effectiveness of these already beneficial treatments can be enhanced.
The stated objectives for this work were:
To anaerobically isolate several E. coli O157:H7 infecting bacteriophage from feedlot manure and soils. To determine if previously isolated phage and these new phage could be used to reduce E. coli O157:H7 on the hides of cattle prior to their entry to the food chain.
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