Molecular Analysis of Intestinal Microbiota Composition to Evaluate the Effect of PEG and Lactulose Laxatives in Humans

Methods using bacteriological cultures allow only 30–50% of colonic bacterial populations to be detected. Our aim was to assess the effect of osmotic laxative administration on fecal bacteria during chronic idiopathic constipation, by a culture-independent molecular method. Forty-eight constipated patients were included in a randomized blinded trial and received either polyethylene glycol (PEG, n =23) or lactulose ( n= 25) for 28 days. Non-fermentable laxative (PEG) should not affect the bacterial groups, whereas fermentable (lactulose) could modulate them. The treatment dosage was 20 g per day for a 7-day period, then adjusted according to ef? cacy for 7 days and constant from 10 to 30 g per day for 14 days. Stools were collected at d0, d21 and d28, and analyzed by quantitative dot-blot hybridization using 16S ribosomal RNA-targeted oligonucleotide probes. We used a new Bifidobacterium group specific probe and five other probes to monitor the dominant groups of the intestinal bacteria: Bacteroides -Prevotella-Porphyromonas , Lactobacillus -Enterococcus- Streptococcus , enterobacteria: Clostridium coccoides and Clostridium leptum . Lactulose and PEG administration were associated with an increase of bifidobacterial rRNA index in 72% (18/25) and 17% (4/23) of cases, respectively. In the lactulose group, bifidobacterial rRNA index increased from [mean (SEM)] 1.1% (0.2%) at d0 (a), to 4.3% (1.2%) at d21 (b) and 4.5% (1.1%) at d28 (b), (a ≠ b, p=0.011). Lactulose treatment when administered at doses usually prescribed for constipation selectively stimulated bifidobacteria within the intestinal microbiota. The quantitative rRNA dot-blot hybridization method is an efficient tool to quantify intestinal bacteria, without the limitations of classical culture methods. Keywords: constipation, lactulose, quantitative dot-blot hybridization, Bifidobacterium probe, 16S rRNA.