| The bacterial community found in the human colon plays a substantive role in human health and nutrition. Examples of these beneficial roles include the production of short-chain fatty acids or vitamins and exclusion of pathogenic bacteria. This community has also been linked to various intestinal diseases. It is believed that modification of these communities through ingestion of live bacteria (probiotics) or exogenous substrates (prebiotics) can promote host health. A widely promoted probiotic genus is Bifidobacterium . Supplementation of the diet using these bacteria is claimed to prevent the colonization of pathogenic bacteria and/or stimulation of the immune system. However, this genus is acid-sensitive and viability is significantly reduced following storage in acidic foods and transit through the acid barrier the stomach. Here, we have investigated the impact of acidic conditions on the survival of five species of bifidobacteria and examined the acid tolerance response (ATR). Results indicate that while species vary in acid-sensitivity, viability can be improved through the provision of a fermentable substrate (e.g., glucose) and the use of stationary-phase cells. The ATR of B. longum and B. breve were further investigated. An ATR can improve cell viability under acidic conditions by preconditioning cells under mild acidic stress. Proteomic analysis of acid treated exponential-phase cells of B. longum found significantly fewer proteins, including the absence of proteins known to be used by bifidobacteria during acid stress, suggesting that exposure to even mild acid inhibits the production of new proteins.;Fructans are prebiotics, claimed to stimulate the growth of colonic bifidobacteria, thereby eliminating the need to introduce viable cells into the community. We have investigated the impact of diet supplementation on the dominant human fecal community using comparative sequence analysis of l6S ribosomal RNA genes. Three diets supplemented with fructans, soy protein and a combination of both were fed for four-weeks each to three subjects. Overall, each subject had a unique fecal community which responded differently to each of the three diet supplements. In only two subjects did diet cause significant changes within the community (p ≤ 0.05). There was little commonality among the phylotypes which appeared in response to a given substrate or among the three subjects on the same diet. Also, no consistent change in total bacterial numbers, community richness or increased abundance of bifidobacteria was observed. In only one subject did diet affect community structure, and this only occurred following single supplemented diets. Results from this study raise questions concerning the bifidogenic properties of fructans and suggest that individuals harbour unique fecal communities which respond differently to the same dietary substrate. |
