When used in low concentration, formaldehyde increases the rate of autolytic disintegration of pneumococci whereas in large concentrations it completely inhibits autolysis and preserves both the morphological and staining characteristics of the cells.
Pneumococci treated with large concentrations of formaldehyde, then washed free of the antiseptic and resuspended in physiological solutions, rapidly undergo a change which renders them Gram-negative and smaller. The lysis is only partial, however, and is not accompanied by an actual disintegration of the cell. It is caused by the autolytic enzyme of the cell which remains inactive in the presence of an excess of formaldehyde but recovers its activity when the cells are resuspended in a neutral medium after removal of the antiseptic. If the autolytic enzyme is irreversibly inactivated by heating, or maintained inactive in acid or alkaline reaction, the formolized cells retain their staining characteristics and morphological integrity.
Formolized pneumococci which have become Gram-negative owing to the action of their autolytic enzyme, fail to elicit the type specific carbohydrate antibodies in rabbits. Formolized pneumococci in which the autolytic enzyme has been destroyed or maintained inactive, and which have retained their Gram-positive character, function as a very effective type specific antigen in the rabbit.
These observations emphasize once more the close relation between the Gram-positive structure of pneumococci and the capsular polysaccharide antigen of the cell. They can be used as a basis for the preparation of suspensions of formolized pneumococci which are stable and very effective as type specific antigens.