Adaptation of the colonic flora to lactose may contribute to lactose digestion in lactose maldigesters, and supplementation with Lactobacillus acidophilus may modify colonic fermentation of lactose and short-chain fatty acid production. We evaluated the capability of colonic bacteria to ferment lactose and the ability of L. acidophilus to modify lactose fermentation by the colonic microflora in vitro. An anaerobic continuous culture was established and inoculated with fresh samples of human feces. Lactose infusion was maintained at 25 g/d and pH at 6.7. L. acidophilus strain LA-1 (1.5 x 10(10) cells) was introduced into the fermenter on d 0 or added daily on d 0 through 4. The control was the continuous culture without the addition of lactobacilli. Rapid adaptation of colonic bacteria to lactose occurred within 1-2 d, with a significant decrease in lactose concentration and increase in beta-galactosidase activity, and lactose concentrations fell below 3 mmol/L by d 7. Supplementation with strain LA-1 resulted in a significantly greater decrease in lactose concentration and greater increase in acetate and propionate production within the first day compared with the control group. However, there was no significant difference between the fermentation treated with L. acidophilus daily and the control after the first day. These data suggest that the colonic bacteria adapt quickly to lactose, causing efficient utilization of lactose. L. acidophilus supplementation may enhance lactose fermentation during early periods when the adaptation is not established in this model.