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Although silicic acid jellies are inferior to agar, gelatin, blood serum or egg white in the preparation of solid media for the majority of bacteriological requirements, there are certain specialized purposes, such as the isolation of autotrophic bacteria, for which an inorganic jelly is necessary. Theoretical advantages from the utilization of silica gel as a specialized substitute for organic gels have not been realized in practice, largely as a result of the erroneous assumption that inorganic gels are inert in the medium. The present study deals with the influence of time, temperature, dialysis and autoclaving on the pH and the rigidity of silica jellies prepared for bacteriological purposes. Many modifications of two classical methods for the preparation of silicic acid jellies have been described by bacteriologists. The "sol" method, first applied in bacteriology by Kuhne (1890) and later used by Winogradsky (1891) and by Omeliansky (1899), was adequately reviewed by Bojanovsky (1925) and recently employed by Boltjes (1935) in obtaining pure cultures of nitrifying bacteria. This procedure has a distinct advantage in that the dialyzed and sterilized sol is gelled after addition of the bacterial suspensions to the plates. However, it was not considered of general utility because of the special apparatus required for the preparation of the dialyzed sol and because the liquid of syneresis exuded on the surface of the plates permits a film of contaminants to cover the pure colonies which may develop in the depths of the gel. Boltjes overcame this difficulty with the aid of a micromanipulator for colony picking.

The Preparation of Silicic Acid Jellies for Bacteriological Media