Scanning electron microscopy of encapsulated and non-encapsulated Cryptococcus neoformans and the effect of glucose on capsular polysaccharide release

Cryptococcus neoformans has a polysaccharide capsule composed primarily of glucuronoxylomannan (GXM, This study focuses on the morphology of both enca psulated and non-encapsulated organisms in the presence and absence of mono clonal antibodies (mAbs) and serum proteins, and the effect of glucose on c apsular polysaccharide release, Examination of the encapsulated C, neoforma ns strains 24067 and 34873 by scanning electron microscopy (SEM) revealed g lobular cells covered with a loose fibrillar network which was most promine nt during the early stationary phase. In the presence of GXM-binding mAbs o r serum the capsule border became distinct and bud scars were evident in th e fibrillar network. In contrast, SEM of strain 52817, a non-encapsulated m utant of 34873 revealed ovoid cells devoid of a fibrillar network with bud scars and small surface protrusions. mAb 2H1 bound to cells from strains 24 067 and 34873 but not 52817, No GXM was detected in supernatants of 52817 c ulture. For several strains: there was significantly more GXM in culture su pernatants using high glucose media. In summary, our results indicate, i) S EM methods for studying capsular structure in C, neoformans; ii) no reactiv ity by GXM-binding mAb with a non-encapsulated strain; iii) the presence of distinctive bud scars in both encapsulated and non-encapsulated cells, and iv) dependence of GXM concentration on glucose concentration in culture me dia. The implications of these results are discussed.