Animal Waste, Water Quality and Human Health

134Animal Waste, Water Quality and Human Health(2003) reported that a mixture of probiotic E. coli strains isolated from adult cattlereduced faecal shedding of E. coli O157:H7 and enterohaemorrhagic E. coliserotype O111:NM from young cattle within 8–30 days and 6–12 daysfollowing treatment, respectively. Sargeant et al. (2007) reported that, based ona systematic review of several subsequent studies in the literature, feedingprobiotic bacteria was the only one of the many pre-harvest treatments whichthey investigated that was efficacious in reducing the faecal shedding of E. coliO157:H7 by cattle.Coupling the use of competitive exclusion and prebiotics, in a process known assynbiotics, could yield a synergistic effect in the reduction of foodborne pathogenicbacterial populations in animals for consumption.Oligosaccharides and other organic compounds which enhance growth of thenormal gastrointestinal flora are termed prebiotics: they promote the growth ofprobiotic species. Probiotic bacteria, such as lactic acid bacteria, are thought tocreate an antagonistic environment for pathogens through a number of possiblemechanisms that parallel those thought to play a role in competitive exclusion.These include production of organic acids, competition for colonization sites,competition for nutrients and enhancement of the host immune system (Doyle &Erickson 2006). It has been shown that as populations of Lactobacillus andBifidobacterium spp increase there is a reduction in Salmonella prevalence in birds(Fernandez et al. 2002, Xu et al. 2003). Prebiotics in feeds such as β-glucans(Lowry et al. 2005) and fructo-oligosaccharides appear to reduce Salmonellacolonization in chickens (Donalson et al. 2007, Babu & Raybourne 2008).Bacteriocins are bactericidal toxins produced by certain bacteria whicheliminate other competing bacterial species including subtypes within their ownspecies. Bacteriocin-producing bacteria are being studied for the control of E.coli O157:H7 faecal shedding in cattle (Duncan et al. 1999, Schamberger &Diez-Gonzalez 2002, Zhao et al. 2003) and faecal shedding of Salmonella inpoultry (Wooley et al. 1999).4.5.2 BacteriophagesBacteriophages are viruses that form an important part of the normal “microflora”of the gastrointestinal tract of animals. Bacteriophages can be either lytic ortemperate. Lytic bacteriophages enter the bacterial host, take over cell processessuch as DNA and protein synthesis, and then burst the cell to release new phageprogeny. In contrast, temperate bacteriophages exist in a latent state integratedinto the genomes of bacteria. Stressors on the bacterial cell cause the latentbacteriophage DNA to be excised from the bacterial genome and trigger thelytic cycle. Lytic bacteriophages were first investigated for their role in