Electron microscopic observation of the antibody-induced capsular swelling phenomenon in Klebsiella pneumoniae

Abstract

The capsular swelling phenomenon of Klebsiella pneumoniae strain 277 was examined morphologically using the electron microscopy techniques of freeze- substitution. The capsules of strain 277 measured about 52 nm in thickness, and were composed of a number of fine fibers. After treating the bacteria with anti-capsular serum, the size of the capsules increased to about twice the normal size and they lost their electron density. The capsular fibers that are tightly packed in normal cells became loose and thus the identification of the individual capsular fibers was difficult in the swollen capsules. Capsule swelling was induced by washing the cells with phosphate- buffered saline. The removal of either divalent cations or some other materials might thus be important for maintaining the normal capsule structure. The mechanism of the swelling phenomenon was also discussed.