Efficiency of various intestinal bacteria in assuming normal functions of enteric flora after association with germ-free mice.

Strictly anaerobic and facultatively anaerobic bacteria were isolated from the intestinal tract of normal mice. Germ-free mice were associated with mixtures of varying complexity of pure cultures of these bacteria. The development of normal features in these animals was then determined on the basis of the following criteria: (i) size of the cecum, (ii) size of the Escherichia coli population in the cecum, (iii) histology of the intestinal tract, and (iv) development of a mucosa-associated flora in stomach and large intestine. Germ-free mice contaminated with cecal contents from conventional mice were used as controls to establish normal values for these parameters. Some strictly anaerobic bacteria could be implanted into germ-free mice only after prior implantation of an E. coli strain. E. coli was found in large numbers in stomach and cecum of mice monocontaminated with this organism. Use of restraining devices indicated that the E. coli population in the stomach was maintained by coprophagy and did not contribute significantly to the size of the cecal population. A mixture of 50 strictly anaerobic bacteria plus 80 facultative anaerobes rendered recipient animals normal with respect to the criteria tested. Other, less complex bacterial mixtures reduced the cecal size and the intestinal E. coli population to levels intermediate between those found in normal and germ-free mice. With all bacterial mixtures tested, the intestinal E. coli population decreased, if at all, within a period of about 10 days after introduction of other bacteria, and remained stable thereafter. This suggests that the intestinal E. coli population is controlled by a mechanism which reduces population size without affecting the growth rate.