Cecal enlargement and microbial flora in suckling mice given antibacterial drugs.

Enlargement and microbial colonization of the cecum were examined in neonatal mice suckling mothers drinking either water or an aqueous solution of penicillin. The full ceca increased in weight at the same rate in both drug-treated and control mice during the first 15 to 17 days after birth. Thereafter, cecal weight increased at a greater rate in the drug-treated animals than in the untreated controls. At weaning, the ceca in treated mice were two to three times the size of control organs and remained enlarged as long as penicillin was given. The enlarged ceca did not differ histologically from those in controls. From birth, the cecal microflora in the drug-treated mice differed qualitatively and quantitatively and in colonization pattern from the flora of control mice. The ceca of untreated animals were colonized primarily by large populations of lactobacilli during the first week after birth, small populations of coliforms and enterococci during the second week, and enormous populations of bacteroides and certain gram-negative fusiform-shaped anaerobic bacteria during the third week. In contrast, the organs of the treated mice were populated by large populations of coliforms and enterococci during the first week and enormous populations of clostridia and unusual gram-negative nonsporeforming bacteria during the third week. These large abnormal populations were present in the ceca as they enlarged during the third week after birth in the drug-treated animals. These findings confirm that only certain populations of anaerobic bacteria can act to maintain cecal size in normal animals.