Identification of two chemical types of K21 capsular polysaccharide from klebsiellae

Strains of Klebsiella pneumoniae of serotype K21 are frequently involved in outbreaks of nosocomial infections. The type strain of Klebsiella pneumoniae K21 (which we have renamed K21a) produces capsular polysaccharide that contains mannose, galactose and glucuronic acid in the ratio 2:2:1. In contrast, all eight of the randomly chosen isolates of Klebsiella pneumoniae that were initially typed as K21 were shown by paper chromatography and NMR spectroscopy to produce a different capsular polysaccharide. We have designated this new polysaccharide K21b. The K21b capsular material appears to have a closely similar immunodominant side chain to K21a. However, K21b polysaccharide has two molecules of rhamnose in the polysaccharide backbone replacing the two molecules of mannose found in the K21a capsule. Our results suggest that the K21b Klebsiella serotype may be more frequently distributed than the classical K21a type.