Rogers, C. G. (Department of National Health and Welfare, Ottawa, Ontario, Canada), and W. B. Sarles. Isolation and identification of enterococci from the intestinal tract of the rat. J. Bacteriol. 88:965-973. 1964.-Surface inoculation was employed in a comparison of three selective media, M-enterococcus (M) agar, thallium acetate-glucose (TITG) agar, and KF-streptococcus (KF) agar, for enumeration and isolation of enterococci from contents of the digestive tract of the rat. Similar yields of enterococcus colonies were obtained with M, TITG, and KF agar. When cecal contents were examined, species identification of 120 isolates, 40 from each medium, revealed only minor differences in selective activity. Streptococcus faecalis and its variety zymogenes represented 55 to 63% of the isolates from each medium; the remaining strains resembled S. durans or S. faecium. More than 99% of the isolates failed to survive the heat-tellurite tolerance test. Periodic analyses of intestinal contents from young rats indicated that enterococci became established in the gut from 10 to 14 days after birth. The species isolated most frequently on M agar was S. faecalis var. zymogenes. When rats were fed a purified diet with casein as the source of nitrogen, S. faecalis var. zymogenes predominated among isolates from contents of the small intestine plated on M agar. This species was largely replaced by others resembling S. durans or S. faecium when mixtures of l-amino acids provided the dietary nitrogen. Nineteen enterococcus isolates, representing all that met the Sherman criteria, required arginine, glutamic acid, histidine, isoleucine, leucine, methionine, tryptophan, and valine; of these, two strains required in addition threonine, glycine, and lysine; four required threonine and glycine; two, threonine and lysine; and one, threonine.