The metabolism of lactulose by intestinal bacteria

Abstract

About 60 strains of intestinal bacteria were cultured in lactulose-containing media to quantitate both sugar fermentation and non-gaseous end-products. Certain species of Clostridia (especially C. perfringens), lactobacilli and bacteroides utilised the disaccharide extensively, while other organisms were unable to metabolise this substrate, beta-Galactosidase activity did not parallel growth on lactulose in all cases. The major fermentation products were acetic, lactic and butyric acids. Hydrogen and carbon dioxide were the only gases qualitatively detected. Clostridial strains exhibited a butyric type fermentation, and most lactobacilli were homofermentative. Fermentation products were estimated for selected species throughout their growth cycles. The products of lactulose fermentation by mixed bacterial cultures varied with incubation conditions such as pH, but correlated well with those produced by pure cultures. Studies on lactulose transport by C. perfringens indicated 14 methodological limitations in assaying (C) lactose uptake and in the use of NADH-based procedures. o-Nitrophenyl O-D-galactopyranoside uptake by lactulose grown whole cells and an absence of phospho-beta-D-galactosidase suggested an active transport of the disaccharide. The inducible beta-galactosidase was partially purified and characterised fructose 1,6-bisphosphate inhibited enzyme activity by 26%, and lactulose or lactose hydrolysis required K ions. Galactokinase was inducible in galactose, lactulose or lactose grown cells. Fructose 1-phosphate kinase (FIPK) and fructose 6-phosphate kinase were detected in fructose grown cell-free extracts; FIPK was partially purified five-fold by affinity chromatography. The glucose effect was observed in C. perfringens grown on lactulose, and could not be eleviated by external cyclic AMP or dibutyryl cAMP. Assays for the cyclic nucleotide in lactulose grown cells and extracellular fluid were in the negative. This inhibition was not observed during growth on a mixture of lactulose with fructose, and co-utilisation of lactulose with galactose and lactose respectively was apparent.