Manipulation of the gut microflora: experimental approach in animals

Abstract

The microbiologist and the nutritionist sometimes, with different objectives, attempt to manipulate the animals' digestive flora in order to optimize the beneficial effects of the microflora and to diminish its harmful effects, to study its main functions or to determine the role of a given microbial species in the digestive ecosystem or that of microbial associations of varying degrees of complexity. These manipulations are delicate and the results do not always live up to expectations, often being difficult to interpret, because throughout evolution the different microbial species have formed complex relationships to create great stability in the digestive ecosystems. Each species has a specific, limited function but the species are so numerous that their functions may overlap and if necessary be substituted one for another. The equilibrium of the microbial populations is the result of many events that we are unable to control. Attempts to improve performance in farm animals by optimizing the functions of the digestive flora have largely relied on an empirical approach consisting of experimenting with diet composition or using feed additives. In these animals, the main aim is to influence microbial metabolism or to modify microbial enzyme activity. Analytical study of the functions of the flora has been performed mostly in laboratory animals (rats, mice) but also in a few instances in small ruminants. In this case the animal serves as a biological tool and different experimental animal models, inoculated with a more or less defined flora have been developed. There have been many studies dealing with manipulation of the flora and several reviews have already been published (Ducluzeau & Raibaud, 1979; Durand, 1982; Ducluzeau, 1988; Van Nevel & Demeyer, 1988). In the present paper we shall describe and discuss a few examples of manipulation of the digestive flora by the two means currently in use: inoculation of animals with selected microbial species alone or in association; modification of the flora or of its activity by abiotic factors.