The liquefaction of gelatin is generally recognized and employed as a fundamental criterion for the differentiation of bacterial species. Unfortunately the methods in vogue for observing this property are crude and unreliable. Usually nutrient gelatin is inoculated by stabbing, kept at a temperature below its gelation point, and compared with controls to detect any liquefaction. Measurement of the rate of liquefaction is sometimes attempted by inoculating the entire surface of a tube of gelatin

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... and recording the depth of liquid formed, after different periods of storage, but comparable results are rarely obtained. The first step in the liquefaction of gelatin is peptization or transformation from the gel to the sol state; from a very viscous to a more fluid condition. Davis and Oakes (1922) , in a recent paper on the physical characteristics of gelatin solutions have shown that the transformation from the gel to the sol state in 4 per cent gelatin solution takes place at 38.03°C. Above this temperature, the viscosity remains constant on ageing or decreases if the temperature is sufficiently high to cause hydrolysis. Below this transition point, viscosity increases with age. At the meeting of the Society of Bacteriologists in 1919, William M. Clark (1920) reported some observations on proteus gelatinase in which he determined the solidification time by a modification of the method of Palitzsch and Walbum. Unfortunately the complete paper has not been published and the abstract does not give full details.