Carbohydrate fermentation by bacteria as influenced by the composition of the medium

The value of the fermentation test among the methods at our disposal for identification of bacteria is generally accepted. And yet, every bacteriologist must have encountered in his experience a number of instances of apparently inexplicable inconsistencies in the results obtained by this test. Not only does it often happen that a given strain producing a large amount of acid or gas will occasionally produce very little, but at times indeed it produces none at all. In fact, the amount of gas produced by a bacterium at different times varies so widely, that at present it is suggested by some bacteriologists that the amount of gas produced by a given culture has no diagnostic significance. This point of view owes its existence merely to the fact that the amount of gas produced by a given culture has no diagnostic significance. This point of view owes its existence merely to the fact that the amount of gas produced by bacteria depends on too many factors to attempt to control them. In our work we came across inconsistencies in the amount of acid and gas production, but discovered that these inconsistencies were very often due to variations in the composition of the media. The study is indeed not finished, but even in its present stage it is quite convincing. Omitting the details of the experiment, which will be published in full later, we shall state here merely the general plan and the results obtained.

The experiment consisted in growing a strain of B. coli, which, in the original culture, produced very little acid or gas, upon a medium consisting of peptone-phosphate-lactose-water with the addition of an indicator permitting direct reading of hydrogen ion concentration developed in the growing culture. 1