SUMMARY: Three phases of , A, B and C, are distinguishable by cellular and colonial morphology, and to some extent serologically. Phase A is the modal form of freshly-isolated strains; it has a uniform bacillary morphology, swarms intermittently on nutrient agar, and forms stable suspensions in 0·ot85% saline. Phase B strains, though possessing flagella, are usually non-motile, non-swarming, and highly pleomorphie; usually form unstable suspensions. Phase C strains are uniformly filamentous, motile, often swarm in a continuous film, and stability in saline varies with the strain.

The somatic surface of phase A strains is characterized by a dominant type-specific antigen, and traces of a non-specific and a possibly strain-specific antigen. In phase B strains the type-specific antigen is largely lost, and the other two antigens dominate the surface. The antigenic surface of phase C strains does not differ markedly from that of phase A.

The phase variations A → B and A → C are reversible. Phase B resembles a partially R form, in which the mouse-virulence is significantly less than that of the parent phase A strain.


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