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Effect of substrate composition on intestinal flora

Despite extensive previous studies that show little change in flora composition with changes in diet, Peltonen et al. (1) concluded that the changes in the cellular constituent composition of intestinal contents in response to uncooked vegan diets represented changes in the types of bacteria present. Although intestinal species depend heavily upon host mucin for their survival, they do attack lumen substrates. Presumably, the vegan diet modified the available substrate. Apparently, the investigators are unaware of the extreme sensitivity that many bacteria exhibit to changes in the available substrate. Cell composition is sensitive not only to incubation temperature, cell age, and the degree of anaerobiosis (which should not change much in the intestinal tract) but also (and most especially) to the medium composition. A single example of this sensitivity by using Eubacterium lentum (an intestinal species) should suffice. In the laboratory, this species grows best in prereduced arginine medium, but since the growth of many gram-positive anaerobes is stimulated by Tween 80, washed cells from prereduced peptone-yeast extract-glucose-Tween (PYG-T) medium are often used to determine cell composition or to identify anaerobic species. In these two media, the predominant fatty acid constituents of E. lentum are shown in Table 1. Similar changes in the fatty acid constituents of bacterial cells can be shown with many (but not all) bacterial species if brain heart infusion broth, Trypticase soy broth, or other media are used. The effect of substrate composition is of crucial importance both for the interpretation of the data of Peltonen et al. and for the identification of bacteria by their cellular fatty acid compositions. It is well established that reliable identification requires culture media and conditions that are identical to those used to develop the reference data base. In the absence of analytical data to show changes in the ratios of species in the intestinal tract, the data presented by Peltonen et al. almost certainly reflect only changes in the cellular composition rather than significant changes in the composition of the flora.