Ultrastructure of the capsule of Klebsiella pneumoniae and slime of Enterobacter aerogenes revealed by freeze etching

Summary The capsule of Klebsiella pneumoniae type I and slime of Enterobacter aerogenes strain A3 (SL) was examined by electron microscopy using the freeze etch technique. The capsules of K. pneumoniae were found to be composed of several layers of polysaccharide 10 nm thick; while the polysaccharide slime of E. aerogenes strain A3 (SL) was found to be composed of a diffuse network of fibrils. This work represents the first effort to visualize the replica of the unfixed, partially hydrated bacterial capsule or slime in the electron microscope. The slime of E. aerogenes strain A3 (SL) which was purified, and then freeze etched, resembled the layered structure of the capsule of K. pneumoniae. It is suggested that the charge or dielectric constant of the slime polysaccharide polymers was altered during purification, thereby permitting the layering to occur.Paper presented at the Annual Meeting of the American Society for Microbiology, Philadelphia, Pa. (U.S.A.), 1972.