Morphological evidence for penetration of anti-O antibody through the capsule of Klebsiella pneumoniae.

The role of the capsule in the reaction with anti-O or anti-K serum was examined morphologically by the techniques of freeze-substitution and immunoelectron microscopy in two strains of Klebsiella pneumoniae differing in virulence for mice. Strain Chedid (O1:K2), an encapsulated virulent strain, has a thicker capsule (150 nm) than the encapsulated avirulent strain 277 (O1:K2) (60 nm). Two morphologically recognizable domains in the capsule created by the arrangement of the capsular filaments were clearly seen in strain Chedid but were less evident in strain 277. Anti-O antibody could penetrate through the capsular layer of both strains. The capsule has no function as a barrier for the penetration of the antibody. Anti-K antibody reacted only on the surface of the capsule and induced swelling of the capsule.