Genus Cryptococcus

• phylum Basidiomycota

• nonfermentative yeast

• produces urease

• one pathogenic and 38 nonpathogenic species

Picture of Crypto

• round yeast with a capsule is Cryptococcus neoformans

• capsule is visualized with a mucicarmine stain, which stains the organism pink. clear areas are the capsule

• SEM of budding

Ecology of Cryptococcus

• var neoformans is found in soil contaminated by pigeon droppings

• var gattii is found in eucalyptus trees

Virulence factors

• ability to grow at body temperature

• capsule

• phenol oxidase, which produces melanin, a black pigment that absorbs free radicals

• mannitol synthesis

Birdseed agar

• the black pigment tells you that you are dealing with C. neoformans rather than any other species of Cryptococcus

Epidemiology of C. neoformans infection

• can cause disease in immunologically normal hosts

• however, vast majority of infections are still in AIDS patients

• in AIDS, C. neoformans var neoformans causes almost all the disease

Pathogenesis of C. neoformans

• organism is inhaled

• can cause pneumonia or disseminate directly

• infects meninges, occasionally skin

• in AIDS patients, can cause widespread disease

• serologic and skin testing reagents are cross-reactive, so incidence of asymptomatic or self-limited infection is unknown

Host defenses against Cryptococcus

• T-cell-mediated immunity

• NK cells

• macrophages

• ? whether anti-capsular antibody can be protective

Diagnosis of Crypto

• cryptococcal antigen, not antibody, in CSF or serum

• culture of the organism

Category: Microbiology Notes