Influence of dietary lactose on the gut flora of chicks.

1. The gut microflora of chicks fed on a purified diet containing 300 g lactose plus 300 g starch/kg was compared with that of control birds receiving a diet containing 600 g starch/kg. 2. In 14-d-old conventional chicks, lactose in the diet decreased the incidence of lactobacilli and clostridia in the caecal contents, although when presenting lactose-fed chicks the counts of lactobacilli exceeded those of control chicks. 3. High counts of Proteus sp. were present in the caeca of control birds but they were completely suppressed in conventional birds fed on the lactose diet. In vitro tests showed that this inhibition was partially due to Escherichia coli and Streptococcus faecalis. 4. The growth of Lactobacillus acidophilus was inhibited by lactose when gnotobiotic chicks were monoassociated but not when polyassociated. The protective effect was shown in vitro to be due to L. salivarius. 5. The pH was markedly lowered in the caecum of conventional and polyassociated chicks receiving dietary lactose. Of the strains used in gnotobiotic experiments E. coli, S. faecalis and L. salivarius produced the lowest pH values in the caeca.