Rate of Growth and Acid Production of Streptococcus lactis

Evidence has been published elsewhere which indicates that slow growth enables an organism to better adapt itself to its environment and hence to exhibit greater viability when exposed to deleterious environmental factors.It has also been pointed out that among bacteria and higher organisms those species which have become adapted to growth under conditions which are unfavorable for most forms of life have relatively slow rates of growth. 1If this hypothesis is correct, it is logical to expect that with any given organism the more slowly growing strains should show greater tolerance for the environmental impediments with which it has had to contend during its course of evolution.In the case of Streptococcus lactis, acidity is the most obvious limitation to growth under the only natural conditions we know it--in milk and milk products.?fTith this thought in mind, we have attempted to learn whether or not there does exist a relationship between rate of growth and total acid producing power (acid tolerance) in Streptococcus lactis.While we do not wish to draw sweeping conclusions from the small body of data contained in this note, it is presented for what it may be worth as a contribution to the general problem and, also, because it is barely possible that this line of approach might yield useful results in the selection and development of strains of Streptococcus lactis for certain dairy purposes.An obvious way to test the theory would be to isolate a number of cultures of Streptococcus lacti~ and determine the rate of growth and acid producing power of each.This procedure was not considered sound, and was not attempted, because in a given pure culture there may be slow and rapidly growing strains, one of which would give the culture the characteristic of fast growth and the other that of acid tolerance.(However, it may be mentioned incidentally that of the two cultures used in this study the one which grew more slowly had the ability to produce the greater amount of acid.)It is apparent that in any given culture, unless very old, the fastest growing strains would be present in greatest numbers, while the strains which have a slower rate of reproduction would naturally occur in smaller numbers.With this fact in mind it was reasoned that, if our working hypothesis is correct, two things should be demonstratable :