Chemical Characterization of E. coli Capsules and Analysis of their Expression

Abstract

Escherichia coli causing extraintestinal infections counteract the unspecific host defense with the formation of capsules. These consist of acidic polysaccharides which form a gel-like extracellular structure with a very high water content. Fig. 1 shows an encapsulated E, coli after incubation with specific anticapsular antibodies. Due to their exquisite structure, capsules collapse during preparation of encapsulated bacteria for electron microscopy, unless they are stabilized. Today some 70 serologically distinct capsular polysaccharides are known which form capsules of similar appearance. The primary structures of about 40 of these have been determined (1). They consist of repeating oligosaccharide units and their structures determine the serological K specificity of encapsulated bacteria. Modern techniques, such as NMR spectrometry and computer-aided molecular modelling permits the analysis of secondary structures and thus the presentation of epitopes.