Animal Waste, Water Quality and Human Health

Zoonotic waterborne pathogens in livestock 133pathogens. Diet-dependent, digestive processes also create physical and chemicalstress for the organisms. Therefore it is not surprising that certain zoonoticpathogen control efforts have focused on using these obstacles to tip theecological balance in the gastrointestinal tract in favour of the beneficialmicroflora and against the survival of pathogenic species.4.5.1 Prebiotics, probiotics and competitive exclusionDecades ago it was discovered that feeding the caecal contents of adult birds toyoung chicks makes them resistant to colonization with Salmonella (Rantala &Nurmi 1973), and, in contrast to the use of antimicrobials in feeds, inhibitoryeffects on the shedding of Salmonella persisted long after the treatment wasstopped. This method of combating Salmonella in poultry, called “competitiveexclusion,” became widely used in Finland in their Salmonella controlprogramme. Hirn et al. (1992) reported that less than 5% of the Finnish poultryflocks were Salmonella-positive and that 70–80% of salmonellosis cases inhumans in Finland were acquired abroad. Stern et al. (2001) in the USA alsoused anaerobic cultures derived from mucosal scraping of the intestines of adultchickens in competitive exclusion experiments and demonstrated that theircultures reduce Salmonella in the caeca of chickens as well as faecal sheddingof Campylobacter. However, attempts to control Campylobacter in poultryusing competitive exclusion have been less consistent.The biological basis of competitive exclusion is poorly understood; however, itmay be related to enhancing the immune function of the host, nutrient competitionamong bacteria, or the elaboration of toxic substances such as volatile fatty acids,antimicrobial peptides, or bacteriocins (Joerger 2003). Studies on bacterialpopulations also suggest that certain bacterial hormones allow “communication”among different bacterial species and may play a role in controlling populationlevels of specific bacterial species (a process called quorum sensing) forexample a recent report suggests that a common anaerobe in the humanintestine, Bacteroides thetaiotaomicron, can repress expression of Shiga toxinby E. coli O157:H7 (de Sablet et al. 2009).Cultures of micro-organisms that limit pathogenic bacterial populations in thegut have been termed probiotics. Other micro-organisms, such as lactobacilli andBifidobacterium, and complex carbohydrates that promote the growth ofpopulations of these members of the normal gut flora have been extensivelystudied as agents to control faecal shedding of enteric bacterial pathogens suchas Salmonella in poultry (Stavric 1992, Gusils et al. 1999, Fernandez et al.2002). The strategy of using probiotic bacteria is also being explored forcontrol of other bacterial pathogens in other animal species. Tkalcic et al.