Fungi (Mycosis)

Fungi (Mycosis)

They are a diverse group of saprophytic and parasitic eukaryotic organisms. Human fungal diseases (mycoses) are classified by the location on or in the body where the infection occurs. They are called cutaneous when limited to the epidermis, subcutaneous when the infection penetrates significantly beneath the skin, and systemic when the infection is deep within the body or disseminated to internal organs. Systemic mycoses can be further divided into those that are caused by true pathogenic fungi capable of infecting healthy individuals, and those that are opportunistic, infecting primarily those individuals who have predisposing conditions such as immunodeficiency or debilitating diseases. Fungi produce and secrete a variety of unusual metabolic products, some of which, when ingested, are highly toxic to animals, including humans. Thus, fungi can cause poisonings as well as infections. Lastly, fungal spores are important as human allergenic agents.
Characteristics of Major Fungal Groups
Fungi can be distinguished from other infectious organisms such as bacteria or viruses because they are eukaryotes (that is, they have a membrane-enclosed nucleus). Their characteristic structures, habitats, and modes of growth and reproduction are used to distinguish between different groups of fungi.
A. Cell wall and membrane components
The fungal cell wall and cell membrane are fundamentally different from those of bacteria and other eukaryotes. Fungal cell walls are composed largely of chitin, the fungal membrane contains ergosterol, rather than the cholesterol found in mammalian membranes.
B. Habitat and nutrition
All fungi are heterotrophs; that is, they require some preformed organic carbon source for growth. Fungi do not ingest food particles as do organisms such as protozoa, but depend upon transport of soluble nutrients across their cell membranes. To obtain these soluble nutrients, fungi secrete degradative enzymes (for example, cellulases, proteases, nucleases) into their immediate environment. It is this ability that enables fungi to live saprophytically on organic waste. Therefore, the natural habitat of almost all fungi is soil or water containing decaying organic matter. [Note: Some fungi can be parasitic on living organisms. However, these parasitic infections usually originate from the individual s contact with fungus-infested soil, an exception being Candida, which is part of the normal human mucosal flora
C. Modes of fungal growth
Most fungi exist in one of two basic morphologic forms (that is, either as filamentous mold or unicellular yeast). However, some fungi are dimorphic (that is, they switch between these two forms in response to environmental conditions).