Abstract
The previous observations, 5 (2528), have demonstrated that the tomato extract is not able to promote the growth of B. Shiga in broth of initial pH 5.2-6.2, that it has a moderate growth accelerating effect on this microorganism at initial pH 6.6-7.8 and that it has the best effect when added to broth having a pH 8.2-8.6. However, upon studying the end reaction of cultures containing tomato extract, it was observed that the final pH of these cultures, while always markedly acid, showed considerable difference, this difference depending upon the initial pH of the medium. Thus, for example, initial pH 8.2 of the tomato extract broth gives a final reading of 5.8, whereas initial pH 7.0 gives a reading of 5.2.
In this connection the work of Cohen and Clark 1 was of considerable interest. These authors found that on the acid borders of optimum zones of pH, slight change in pH produces a marked inhibitory effect upon the growth of bacteria (4.7 for B. Shiga), but the acid border shifts with the nature of the acid and a distinct difference between the effect of HCl and that of acetic acid was noticed. When adjustments were made with acetic acid, the narrow pH zone 5.5-5.7 may be considered critical, while when HCl was used, this point was 4.8-5.0.
In the author's experiments, the question, therefore, arose as to whether the organic acids contained in tombto extract shift the critical point of B. Shiga from 4.7 to some point quickly reached by a tomato extract culture of initial pH 6.6-7.8, but much slower, or perhaps never at all reached by a culture containing tomato extract of initial pH 8.2-8.6.
Investigation of this point demonstrated in fact that the tomato extract broth, from which the growth promoting factol has been remover by process described elsewhere, 2 is able to inhibit completely the growth of B. Shiga at pH 5.4-5.6 whereas plain broth becomes inhibitory only at pH 4.8.
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