Yeasts as Human Pathogens
Some yeast species are opportunistic pathogens that can cause infections in humans whose immune systems are compromised. Cryptococcus neoformans is a noteworthy such pathogen, causing the disease called cryptococcosis. Yeast cells are surrounded by a rigid capsule of polysaccharides, which helps to prevent them from being recognized by the immune system of humans. Yeasts of the genus Candida, another group of opportunistic pathogens, cause oral and vaginal infections in humans, a condition known as candidiasis. Candida occurs as a commensal yeast in the mucus membranes of humans. The same strains, however, can become pathogenic under certain circumstances.
Cultivation of Yeasts in the Laboratory
Yeasts can be grown in the laboratory on solid culture media or in liquid broths. Typical media used for the cultivation of yeasts include potato dextrose agar or broth, Wallerstein nutrient agar, yeast peptone dextrose agar, and yeast mold agar or broth.