Cellular charge of Cryptococcus neoformans: contributions from the capsular polysaccharide, melanin, and monoclonal antibody binding

Abstract

Cryptococcus neoformans is a human pathogenic fungus which is unusual in two respects: it has a polysaccharide capsule similar to that found in encapsulated bacteria and it can produce melanin. Capsular and melanization phenotypes are associated with virulence. In this study we analyzed the contributions of the capsular polysaccharide, melanization, and antibody binding to the capsule to the cellular charge of C. neoformans. Cell charge was inferred from measurements of zeta potential. The results indicate that (i) C. neoformans cells are significantly more negatively charged than Saccharomyces cerevisiae cells, (ii) the polysaccharide capsule of C. neoformans is responsible for the high negative charge of the cells, (iii) C. neoformans melanin is negatively charged, (iv) melanization in C. neoformans is associated with an increased negative charge per cell, and (v) antibody binding to the capsule of C. neoformans significantly alters the cell charge. These results suggest that alterations in cell charge attributable to polysaccharide capsule formation, melanization, and antibody binding may affect C. neoformans virulence given that macrophage phagocytosis is effected by the zeta potential of microorganisms.