Interaction of amino acid dependent and independent strains of lactic acid bacteria.

Lactobacillus arabinosus strain 17-5 fails to grow in the absence of phenylalanine or tyrosine during an incubation period in which maximum growth is attained in the presence of these amino acids. However, good growth is observed in media deficient in these amino acids when the incubation time is extended (Dunn et al., 1947; Prescott et al., 1949; James, 1950; Atkinson and Fox, 1951; Borek and Waelsch, 1951; Holden, 1956). This delayed demonstration of independence is also observed with Streptococcus faecalis strain R in a tyrosine-deficient medium. The data of James (1950) and of Atkinson and Fox (1951) indicate that the cells of L. arabinosus which grow in the absence of these amino acids are mutants, stably independent of the amino acid requirement found in the cells of the parent culture. This report describes experiments showing that when adaptation occurs in the presence of low levels of these amino acids cultures of L. arabinosus and S. faecalis grow less than they do in the complete absence of the amino acids. MacLeod (1951) has independently observed this anomalous growth response using S. faecalis and a tyrosine-supplemented medium. Similar experiments have been reported previously with a tryptophan-independent culture of L. arabinosus (Wright and Skeggs, 1945) and a histidine-requiring mutant of Escherichia coli (Ryan and Schneider, 1948). The experiments of Ryan and Schneider (1949 a, b, and c) demonstrated that a histidine-independent back mutant of E. coli, which overgrows the parent histidine-dependent culture in the absence of histidine, is inhibited in the presence of histidine by the dependent cells whose growth is fostered by additions of the amino acid. The studies with L. arabinosus and S. faecalis described here indicate that a similar mechanism is responsible for the depression of adaptive growth of these organisms.