30 - Yeasts Causing Infection

Abstract

The yeasts constitute a large group of unicellular fungal organisms. They are placed in the form class Blastomycetes or in three groups: Ascomycetes, Basidomycetes, and Fungi Imperfecti. A number of species are found as commensals in the gastrointestinal tract of humans and animals. Only a few species are the causes of disease in animals. A wider variety have been involved in infections in humans, perhaps because of the intensive and prolonged use of antibiotics. The normal vegetative forms of yeasts are round or oval cells with a diameter usually in the range of 2.5-6 μm. Their mode of reproduction is by budding and some possess capsules. The principal pathogenic yeasts of animals are Candida albicans and Cryptococcus neoformans. This chapter discusses Geotrichum candidum, although it resembles a mold more than a yeast. Yeasts are recovered occasionally from clinical materials. Most often, they are contaminants or are derived from the normal flora on mucous membranes. The kind of disease and/or lesion usually leads the clinician or pathologist to suspect a yeast infection. Also, the recovery of a potentially pathogenic yeast in nearly pure culture or in large numbers may indicate diseases because of a yeast. The repeated demonstration and recovery of a yeast from a lesion provides strong evidence of its significance. Yeasts are frequently seen on primary media such as blood agar and Sabouraud agar.