Additional observations on the rate of growth of Lactobacillus casei.

In the preceding paper (1) it was shown that the growth rate of Lactobacillus casei may be markedly affected by variations in the amount of asparagine present in the medium. This compound may be either stimulatory or inhibitory, depending upon the conditions existing; evidence was presented which indicates that asparagine is closely related metabolically to strepogenin. For example, strepogenin, in the form of trypsinized casein (2)) is completely inactivated by mild acid hydrolysis when the test medium does not contain asparagine, whereas its activity is not affected if asparagine is supplied to the organism. Moreover, a culture of Lactobacillus casei “trained” by appropriate subculture to grow rapidly without strepogenin was found to require asparagine for rapid development; another “trained” culture grew well in the absence of both, but was markedly inhibited by asparagine. This effect was overcome by trypsinized casein. As a result of these findings, experiments were undertaken to define more closely the nature of the relationship between asparagine and strepogenin, and to determine whether the rate of growth of Lactobacillus casei can be effectively altered by varying the level in the medium of known nutrients other than asparagine. Experiments along those lines have led to the development of a chemically completely defined medium which is capable of supporting rapid growth of Lactobacillus casei; maximum turbidity is obtained in about 24 hours and maximum acid production in about 30 hours. The rate of growth of the organism, under these conditions, is not appreciably affected by trypsinized casein. Upon omission of the appropriate substance, the medium is suitable for the rapid turbidimetric or titrimetric assay of folic acid, biotin, and serine, and should be adaptable for the assay of other compounds required by the organism.