Faecal steroids and colorectal cancer: the effect of lactulose on faecal bacterial metabolism in a continuous culture model of the large intestine

Abstract

The effect of lactulose on intestinal bacterial metabolism in two identical single-stage chemostats has been studied. The study was designed as a single stage crossover. Both cultures were inoculated and treated in an identical manner except that whilst one fermenter was subject to pH control the other was not and vice versa. Complex bacterial populations were maintained for 23 days and many of the metabolic functions carried out by the micro-organisms in vivo were demonstrated in vitro. The predominant organisms belonged to the genera Bacteroides, Bifidobacterium and Clostridium with abundant levels of anaerobic Gram-positive rods. The redox potential in each fermenter showed considerable variation with maximum values of below -300 eV being attained. An indication of the stability of the bacterial communities was the production of short-chain volatile fatty acids; during steady-state conditions the mean ratio of the major acids acetic, propionic and butyric being 3.90:0.69:1.00 and 3.65:0.76:1.00 in each fermenter, respectively. During steady-state conditions, 7 alpha-dehydroxylation of the primary bile acid chenodeoxycholic acid was maintained at a constant rate with lithocholic acid representing over 85% of total steroid. Addition of the soluble fibre lactulose to the cultures had a profound effect on many of the parameters tested. Without pH control the culture pH dropped to below 5.0 and this coincided with a substantial decrease in total anaerobes, especially Bacteroides spp., an increase in Lactobacillus spp. (concomitant with an increase in lactic acid), a decrease in the concentration of short-chain volatile fatty acids and a substantial decrease in 7 alpha-dehydroxylation of chenodeoxycholic acid. These results show that it is possible to maintain viable intestinal bacteria in vitro using a continuous culture chemostat and that the cultures respond to changing conditions as evinced by the inhibition of 7 alpha-dehydroxylation of chenodeoxycholic acid on addition of lactulose. This indicates that the model may be of further use in studying the modulation of secondary bile acid formation, a possible risk factor in colorectal carcinogenesis.