THE EFFECT OF METHIONINE AND METHIONINE PRECURSORS ON THE GROWTH OF STREPTOCOCCUS LACTIS1

During the course of a study on the amino acid requirements of a strain of Streptococcus lacti8, it was observed that the organism manifested adequate growth in the absence of added methi-onine. Further investigations with a number of strains of S. lactis revealed that all were able to manifest appreciable growth in the absence of added methionine. A detailed study using one of these strains showed that it responded favorably to the addition of amino acids previously shown to be precursors of methionine in the rat. It appeared that the organism readily synthesized methionine and that the pathway of the synthesis was strikingly similar to that established for the rat. EXPERIMENTAL METHODS The strain of S. lactis used in this study was from the departmental culture collection and was selected on the basis of its excellent growth in a synthetic medium. Throughout the experimental work the culture was carried in litmus milk and routinely transferred every seven days. The synthetic medium used was that of Henderson and Snell (1948). The modifications of this medium by Anderson and Elliker (1953) were not adopted in toto, the exception being the ascorbic level which was lowered to 20 mg per 100 ml of medium. The original medium was further modified to the extent that the citrate level of the medium was reduced to 10 mg per ml and acetate was deleted (Kizer and Speck, 1954). Each medium tested was inoculated initially from a 24 hr litmus milk culture using 0.05 per cent inoculum and incubation at 32 C for 24 hr. This was followed by two serial 24 hr subcultures in the same medium using 0.05 per cent of the previous 24 hr culture for inoculum. Cultures were tested in 2 ml of medium, and inoculatio