SUMMARY: One hundred and seventeen strains of lactic acid bacteria from the rumen, representing four species of lactobacilli and four of streptococci, were examined for their requirements for vitamins. Of the lactobacilli only Lactobacillus plantarum (7 strains) was homogeneous in respect of its vitamin requirements. Within each of the other three species, strains differed in their requirements for individual vitamins, but on the whole the patterns of requirement were distinctive, and could be correlated with other specific characters. The vitamin requirements of three of the species (L. casei, L. plantarum and L. acidophilus) were broadly similar to those reported in a variety of non-rumen strains. Most of the rumen strains of L. fermenti required vitamin B, and riboflavin, in addition to the requirement for nicotinic acid, thiamine and pantothenic acid variously reported as typical of the species. Streptococcus faecalis and S. liquefaciens had distinctive patterns of vitamin requirements as did the unclassified group. The remaining species, S. bouis, had no requirement for exogenous vitamins under the anaerobic test conditions used. Under conditions of aerobic culture, however, strains of this species were found to require one or more vitamins for growth.In a study of the types of bacteria normally present in large numbers (lo9-l0l1/ml.) in the rumens of cows and calves, some 350 strains of streptococci and 300 of lactobacilli were isolated; about a quarter of these were taken as representative, and identified (Perry, Wilson, Newland & Briggs, 1955;Perry & Briggs, 1957). The present study was carried out to determine the patterns of vitamin requirements in these rumen organisms and to establish to what extent these patterns are characteristic of the species as differentiated on the basis of bacteriological tests. A longer-term objective is to study symbiotic interrelationships between the more important rumen species, and the nutritional factors which may determine the equilibrium between the assorted populations present.The nutrition of the lactic acid bacteria has been widely studied, and the vitamin requirements of representative strains have been worked out in detail (cf. Snell, 1948;Rogosa, Wiseman, Mitchell, Disraely & Beaman, 1953). So far, however, very few strains from the rumen have been included in these studies. Jensen, Smith, Edmondson & Merilan (1956) identified eight species of lactobacilli present in the bovine rumen at concentrations of lo6 or 107/ml. They concluded, on the basis of classical bacteriological tests, that none was indigenous to the rumen, and implied that their requirements for vitamins and
I8G. Microb. XVIXI