Probiotics for chickens

Abstract

10.1 INTRODUCTION As with other animals, the development of probiotics for poultry has developed out of our increasing understanding of the microflora of the alimentary tract. Although Pasteur's observations suggested that the host and its intestinal micro flora were interdependent, the intellectual origins of probiosis started with Metchnikoff (1907). He thought that, on balance, the intestinal microflora was detrimental to the host because of absorption of toxic bacterial metabolites. By correlating the longevity of some of the rural inhabitants of the Caucasus with their ingestion of fermented milk throughout their life, he suggested that health status and longevity could be improved by consumption of milks fermented by lactobacilli. This description of the intestinal micro flora in adversarial terms was perpetuated by Dubas et a1. (1965) who divided the indigenous micro flora into the autochthonous organisms, such as lactobacilli and Bacteroides, which had developed an evolutionary symbiotic relation-ship with the host and allochthonous organisms such as Escherichia and Clostridium which were potential pathogens. These, together with non-enteric organisms acquired from the environment, comprised the normal intestinal flora. These descriptions are far too simplistic and must be seen as early models attempting to describe several highly complex ecosystems. For example, microbial opportunism and true commensalism are largely ignored. Regarding the flora as a climax community in which every niche is occupied is also patently inaccurate. Our inadequate under-standing of microbial taxa at that time presumably led to regarding Escherichia coli as a potential pathogen although many strains may be beneficial to the host and can certainly be used in that way (Linton et aI., 1978; Duval-Iflah et aI., 1983). However, these hypotheses provided an important stimulus to studying the microecology of the alimentary R. Fuller, Probiotics © Chapman & Hall 1992 226 Probiotics for chickens tract. These early models had a profound effect on the development of probiotics. Many preparations currently used for poultry and other animals are based on the assumption that the early hypotheses are correct with the result that the approach to probiosis is often too simplistic. 10.2 THE NORMAL INTESTINAL FLORA OF POULTRY The gut micro flora of poultry is complex and the interactions between different types of organisms must be very complicated. Those interac-tions that are best understood are probably the most simple. It is clear that the basis upon which many probiotic preparations have been selected and used is often also too simplistic. Thus, in reviewing work on the use of probiotics for poultry the scientific basis for their selection and use must be analysed. This cannot be done without first reviewing our current knowledge of gut microorganisms and what is known of their interactions both with the host and with each other. Our knowledge of the components of the gut flora of chickens is continually increasing. Information for other poultry species is much less detailed. The flora and its activities must be studied in relation to the host anatomy and physiology and to the conditions existing in individual sections of the alimentary tract. Early studies on the normal flora were hampered by the use of highly selective media with aerobic incubation producing a very inaccurate picture of the organisms present. Anaerobic jars have now been replaced by methods which prevent any oxygen from coming into contact with samples or cultures. These techniques, together with complex non-selective media, allow a more detailed examination ofthe highly oxygen-sensitive organisms present in the caeca. However, even now some organisms, such as the budding bacteria, cannot be cultured and others such as methanogens have not been found even though there is chemical and metabolic evidence for their existence. This must presumably indicate nutritional inadequacies in the techniques and media used. Despite the fact that the flora can be subdivided most conveniently according to the area of the alimentary tract involved it must be remembered that the flora is almost continuous throughout the length of the gut. Microorganisms from the crop which survive the low pH of the gizzard generally multiply in the small intestine. Organisms from this organ may be taken into the caeca. The microbial content of the cloaca and faeces depends on whether they contain material from the small intestine or from the caeca. Caecal droppings are discharged two to four times every day.