Strain variation in composition and molecular size of the capsular polysaccharide of Cryptococcus neoformans serotype A

Abstract

The capsule of Cryptococcus neoformans is an important virulence factor. In this investigation capsular polysaccharides (CPSs) were isolated by ethanol precipitation from culture filtrates of C. neoformans serotype A strains 6, 15, 98, 110, and 145. Capsule sizes on India ink examination ranged from barely perceptible (strain 15) to greater than the diameter of the yeast cell (strain 6); the others were intermediate in size. On ion-exchange chromatography on DEAE-cellulose each CPS eluted at 0.2 M NaCl; CPS of strain 15 had two major peaks, designated III and IV. On gel-permeation chromatography CPSs of strains 6, 98, 110, and 145 eluted at the void volume of Sepharose CL-2B in the presence or 0.1 M EDTA, while the CPS of strain 15 eluted in two peaks. Sephacryl S-1000 resolved CPSs of all five strains in the following order, from largest to smallest molecular size: 145 greater than 110 greater than 98 greater than 6 much greater than 15. All five CPSs contained mannose, xylose, and glucuronic acid, while the carboxyl-reduced CPS of strain 110 also contained a large percentage of an inositol-like compound. The CPS of strain 110 contained approximately 30% uronic acid by weight, while the others had 15 to 20%. The composition of peak IV from the CPS of strain 15 resembled those of the other strains; peak III of strain 15 contained a substantial amount of galactose. Each CPS contained less than 0.2% protein by weight. The significant differences in molecular size and sugar composition among CPSs of these strains of C. neoformans serotype A may partially explain strain differences in virulence and biological properties of the organism.