The fermentation of carbohydrates and other organic media by streptococci

The study of the fermentative powers of the streptococci has been carried out in England by merely noting the change of color in litmus media. In this country, Palmer and the writer, Broadhurst and Hilliard have used the more exact methods of titrating inoculated tubes and uninoculated controls, using phenolphthalein as an indicator and plotting the quantitative results obtained. The line of demarcation between fermenting and non-fermenting forms is drawn at the intermodal point between the peaks of the curve. A comparison of several hundred results obtained by English and American methods shows that the English method gives a uniformly higher proportion of fermenters, suggesting that a number of strains producing a very slight amount of acid and properly classed as non-fermenters, are recorded as positive by the English method.

A study of the correlations between action on different organic media shows that those substances tested may be arranged in a definite order of availability, such that a positive reaction in one medium usually implies that all those earlier in the series will be fermented, while failure to act on a given substance almost always implies that substances later in the series will not be fermented either. Among the streptococci dextrose comes first in order of availability, then the disaccharides, lactose and saccharose, and the glucoside, salicin (which easily yields a simple sugar). The starch-like body inulin and the alcohol mannit come next and the trisaccharide raffinose is least available of all. This order corresponds to the size of the molecule, whereas in the B. coli group, the configuration of the nolecule is the main thing, the aldehydic sugars being acted upon very readily, and the ketonic sugars less readily.