SummaryScanning electron micrographs are presented of the ileal epithelium of mice aged 5, 15, 20 and 25 days. During this period the villous pattern develops to full maturity. By the twentieth day of life a segmented filamentous microorganism colonizes the ileal epithelium and is firmly attached via a small segment. During the first days of colonization the segmented filamentous micro-organisms themselves are subcolonized by small rod-shaped bacteria, presumably lactobacilli. At the age of 25 days this subcolonization was no longer observed.

The effects of the following changes throughout the association of germ-free mice with increasing numbers of anaerobic bacteria were studied: (i) elution patterns obtained by gel-filtration chromatography of caecal diffusates; (ii) concentration of beta-aspartylglycine in caecal and faecal contents; (iii) polypeptide patterns obtained by sodium dodecyl sulphate-polyacrylamide gel electrophoresis of caecal supernatants; (iv) free amino acid content of caecal supernatants; (v) faecal bile acids, analysed by gas-liquid chromatography; (vi) colonization-resistance. The results indicate that monitoring the normalization (association) process can be accomplished in several ways, but the level of colonization-resistance is most easily measured by high-voltage paper electrophoresis of faecal supernatants to determine the concentration of beta-aspartylglycine. During association, the concentration of beta-aspartylglycine decreased and became undetectable after association with 40 to 50 different strains of bacteria. There was a good negative correlation between the level of colonization-resistance and the concentration of beta-aspartylglycine.

SUMMARY

After oral administration of 5-aminosalicylic acid a substantial proportion of the acetylated form is excreted in the faeces. We have studied the role of the faecal microflora in acetylating 5-aminosalicylic acid. In faeces incubated under both aerobic and anaerobic conditions there was acetylation of 5-aminosalicylic acid. Of the anaerobic bacteria isolated from a 10(8) faecal dilution 44% were able to acetylate 5-aminosalicylic acid. We conclude that the normal faecal microflora contribute to the acetylation of 5-aminosalicylic acid.

Germfree (GF) mice were inoculated with a cultured flora from 10-1, 10-3, 10-5, and 10-7 dilutions of caecal contents from a 'normal' mouse. GF mice associated with a flora of a 'normal' mouse served as controls. The following intestinal parameters were determined: Colonization resistance (CR), Relative caecal weight (RCW), villus:crypt ratio (jejunum and ileum), IgA-producing cells (jejunum and ileum), ß-aspartyl glycine (faeces), volatile and non-volatile fatty acids (caecum) and bile acids (faeces). Only the 10-1 culture was able to induce similar changes in the GF mice to a 'normal' flora. The GF + 10-5 and GF + 10-7 groups deviated markedly from the controls while the GF + 10-3 group showed in general intermediate values between GF + SPF and GF + 10-1 on the one hand and GF + 10-5 and GF + 10-7 on the other hand. ß-aspartyl glycine was present only in the GF + 10-7 group. Scanning electron microscopy (SEM) of ileal contents revealed segmented filamentous organisms in the ileum of controls and the GF + 10-1 group. The faecal flora consisted mainly of fusiform organisms. In the faeces of the 10-5 and 10-7 groups increasing amounts of non-bacterial matter were found, while in the faeces of the other groups virtually only bacteria were seen.