Intestinal microbes live in diminished or deprived oxygen conditions. Facultative anaerobic bacteria of the family Enterobacteriaceae ferment glucose in the gut using two main pathways: mixed acid and neutral fermentations. The aim of the present study was to clarify the roles of these fermentations in an in vitro model. Acid-producing Escherichia coli ATCC 25922 and neutral end-products producing Klebsiella mobilis ATCC 13048 were cultured in a Portable Microbe Enrichment Unit (PMEU) which permitted the selection of the gas phase in standardized conditions. In the butanediol production experiments also Klebsiella pneumoniae ssp. pneumoniae IIIa2 E111 strain was also used. Two isomers of 2,3-butanediol were observed in both aerobic and microaerobic Klebsiella cultures. During 7h cultures both E. coli and K. mobilis grew from densities of some million up to some billions per ml. The growth was almost equal in both aerobic and microaerobic conditions. In anaerobic conditions a balance prevailed in mixed E. coli and Klebsiella cultures between the species (final pH was 5.8-6.1), whereas in aerobic conditions the klebsiellas were favoured, and the pH rose up to about 8.5 (in 20h) as in pure Klebsiella cultures. In microaerobic Klebsiella cultures the final pH varied between 6.7 and 7.5. In mixed cultures the growth yields of both bacteria equalled those of the separate pure cultures indicating mutual benefits of the co-existence. Apparently E. coli produced a set of organic acids, which lowered pH. Klebsiellas increased the pH up to 2units probably due to acid conversion in anaerobic conditions to ethanol. We propose that the balance of acid and ethanol as well 2,3-butanediol fermentations contributes to the small bowel pH regulation, which also aids the host in the nutrient uptake in the small intestine.