The relation of agglutination by specific serum to agglutination by acid

The Relationship between Morphology, Colonial Appearance, Agglutinability, and Virulence to Mice of Certain Variants of Bacterium aertrycke

1. Analysis of a series of 135 strains of B. aertrycke indicates that at least four well-defined types may be recognised on the basis of morphology, colonial formation, virulence to mice, and to a less extent agglutination by salt, serum, and acids. These types comprise: (i) type A—the normal smooth virulent form; (ii) type B—a smooth form of low virulence; (iii) type C—a roughish, so-called ichthyotic, form of low virulence; (iv) type D—the really rough avirulent form. Between these well-defined types certain transitional forms may sometimes be recognised, the virulence of which is frequently of an order intermediate between that of the A type on the one hand, and the B, C, or D types on the other.

2. Evidence is brought to show that if a virulent strain is sub-cultured daily, under conditions known to lead to a fall in virulence of the whole culture, a gradual replacement occurs of the virulent type A bacilli by the avirulent B, C or D types. So long as an adequate proportion, which is very small, of type A bacilli persists in the culture the virulence of the whole culture remains fairly high; but when all type A bacilli have been replaced by bacilli of the B, C or D types the virulence of the whole culture falls considerably.