Balance studies in germ-free (GF) and conventional (CV) rats have shown that in-vivo breakdown of amylomaize starch depends on endogenous and bacterial enzymes. In GF rats, one-third of amylomaize starch is resistant to endogeneous digestion, whereas in CV rats starch is totally degraded. In order to study the contribution of endogenous and bacterial enzymes, the in-vivo breakdown of amylomaize starch was compared in GF and in gnotobiotic rats associated with bacterial strains isolated from the conventional rat microflora. These bacteria were (i) a non-amylolytic Peptostreptococcus (GNP rats), (ii) an amylolytic strain of Eubacterium able to hydrolyse resistant starch (GNE rats), and (iii) an association of Eubacterium with three other bacterial strains (Veillonella, Enterococcus and Peptostreptococcus) able to ferment the products of starch hydrolysis (GNT rats). Various studies were performed to assess the starch breakdown in the caecal contents: balance studies, light and electron microscopy and analyses of bacterial metabolites. The results show that in GNP as in GF rats, around 65% of amylomaize starch was resistant to digestion. Digestive utilization of starch was significantly improved in GNE rats (from 68 to 92%) and was further enhanced to 97% in GNT rats. The increase in starch degradation in the caecum of rats was related to a higher amylase activity and led to an increase in production of bacterial metabolites, and to a change in the proportions of short-chain fatty acids, lactic acid and succinic acid. Ultrastructure study of the starch in the caecal contents showed a progressive granule hydrolysis in the order GF, GNE and GNT rats: (i) an endocorrosion of the central part of the starch granules in GF rats, (ii) a peripheral digestion with bacteria penetrating the granules in GNE rats, and (iii) a more pronounced hydrolysis in the caecum of GNT rats where the outer part of the granules were almost entirely hydrolysed. These results suggest that starch breakdown in the hindgut is not only due to amylolytic bacteria but also to the contribution of other glycolytic strains able to utilize the products of starch hydrolysis.
