Manipulation of the human gut microflora.

Manipulation of the human gut microflora is being attempted as a means of introducing into the digestive tract new micro-organisms which are beneficial to the host or which are able to bring about advantageous changes to the equilibrium populations and metabolic activities of the indigenous flora. In theory, this can be achieved by ingestion of nutrients or growth substances specifically favouring certain species, or by administration of exogenous selected micro-organisms. These micro-organisms, through their own complement of enzymes, may be able to compensate for some deficient intestinal enzyme activities of the host. However, the administration of the exogenous flora is primarily aimed at improving the digestive flora acquired under abnormal conditions, such as (a) inadequate transfer of micro-organisms from the mother to the infant in a highly hygienic environment. (b) consumption of sterile or nearly sterile food, (c) frequent antibiotic treatment (Fuller, 1991). Improvement in the resistance of the host to intestinal pathogens, whether by a ‘barrier’ effect of the flora or by stimulation of the immune system, and alteration of the profile of the colonic flora and its enzymic capacity are among the most widely investigated putative effects of exogenous micro-organisms. A few studies, which have dealt with their potential extra-gastrointestinal actions will not be reviewed here (see Marteau et al. 1993). Micro-organisms that are likely to be used to achieve these goals have been called probiotics, i.e. ‘live microbial feed supplements which beneficially affect the host animal (or human) by improving its microbiological balance’ (Fuller, 1991). However, some of the putative effects of probiotics are beyond the limits of this definition. An essential determinant in the choice of a probiotic is its ability to survive in the environment where it is intended to act. Unfortunately, most studies on probiotic actions have ignored this basic requirement. Moreover, a micro-organism cannot affect its environment unless its population reaches a certain minimum level which has not been exactly determined, but is probably about 107-108 CFU/g in the large intestine (Ducluzeau & Raibaud, 1989). The probiotic preparations currently available are still mainly the lactic acid bacteria and bifidobacteria, which have been used empirically for as long as fermented milk has been consumed (Table 1).