Food accessory substances in bacterial growth. III. Their fate in bacterial cultures.

Since the previous observations have shown that bacterial cultures containing tomato extract undergo a rapid change in hydrogen ion concentration leading to a considerable increase in pH, it was decided to determine whether the pH of surrounding fluid would have anything to do with the fate of growth promoting factors in bacterial cultures.

Preliminary experiments had shown that during the first 24 hours of multiplication of B. Shiga in plain broth of various initial pH no growth promoting nor growth inhibiting substances can be detected in the surrounding fluid which could vitiate the interpretation of the following results.

Cultures of B. Shiga containing tomato extract and of initial pH 5.4, 7.0 and 8.6 and control cultures of the same pH in plain broth were centrifuged at various intervals of incubation time for a period of 24 hours, the pH of these samples recorded, adjusted to pH 8.2 and inoculated with B. Shiga. These experiments demonstrated that the growth promoting factors are present in fluids derived from cultures of initial pH 5.8 and 8.2 for the period of 24 hours of observation, whereas they completely disappear from tomato extract broth cultures with initial pH 7.0 as soon as the latter reaches 5.2 (10 to 14 hours). It may be pointed out that the culture of initial pH 8.2 does not reach a pH higher than 5.8 during 24 hours of incubation, while that of initial pH 5.8 naturally reaches pH 5.2 very quickly (2 to 3 hours).

What, then, are the conditions that lead to the disappearance of growth promoting factors from bacterial cultures with initial pH 7.0?

1. The phenomenon of disappearance of growth-promoting factors does not depend on the amount of bacteria in these cultures, since these factors persist in tomato extract cultures of initial pH 8.2 which, as was shown before, 1 contain a more abundant growth than the cultures under discussion.