Yeasts are unicellular fungi which usually appear as oval cells 1-5 µm wide by 5-30 µm long. They have typical eukaryotic structures. They have a thick polysaccharide cell wall. They are facultative anaerobes. The yeast Candida is said to be dimorphic in that it can grow as an oval, budding yeast, but under certain culture conditions, the budding yeast may elongate and remain attached producing filament-like structures called pseudohyphae (Walsh and Dixon 2006). C. albicans may also produce true hyphae similar to molds.
In this case long, branching filaments lacking complete septa form. The pseudohyphae and hyphae help the yeast to invade deeper tissues after it colonizes the epithelium. Asexual spores called blastoconidia (blastospores) develop in clusters along the hyphae, often at the points of branching. Under certain growth conditions, thick-walled survival spores called chlamydoconidia (chlamydospores) may also form at the tips or as a part of the hyphae (Barron and Madinger, 2008).
Reproduction of yeasts
Yeasts reproduce asexually by a process called budding. A bud is formed on the outer surface of the parent cell as the nucleus divides. One nucleus migrates into the elongating bud. Cell wall material forms between the bud and the parent cell and the bud breaks away. A few yeasts, such as Candidaalbicans, also produce clusters of asexual reproductive spores called blastoconidia (blastospores) and thick-walled survival spores called chlamydoconidia (chlamydospores). Yeasts can also reproduce sexually by means of sexual spores called ascospores which result from the fusion of the nuclei from two cells followed by meiosis. Sexual reproduction is much less common than asexual reproduction but does allow for genetic recombination (Cotran et al, 2005).
TYPES OF FUNGAL INFECTIONS
SUPERFICIAL FUNGAL INFECTIONS:
Superficial fungal infections attack tissues on the surface of the body, which include the skin, nails, or hair. Some common examples are ringworm, athlete's foot, jock itch, and yeast infections. These affect the outer layers of the skin, the nails and hair. The main groups of fungi causing superficial fungal infections are: Dermatophytes (tinea), Yeasts i.e. candida, malassezia, and piedra (Rapini et al, 2007).
Superficial fungal infections are somewhat contagious and pass from person to person through direct contact or, less commonly, through clothes or contact with surfaces of other objects in the environment (Ryan and Ray, 2004).
Subcutaneous fungal infections:
These involve the deeper layers of the skin (the dermis, subcutaneous tissue and even bone). The causative organisms normally live in the soil living on rotting vegetation. They can get pricked into the skin as a result of an injury but usually stay localised at the site of implantation (Rapini et al, 2007).
Deeper skin infections
These affect the deep fascia and include: Mycetoma, Chromoblastomycosis. These infections unlike skin, nail, and hair infections have the ability to illicit immune response due to the site of infection being close to blood supply (James et al, 2006).
SYSTEMIC FUNGAL INFECTIONS
Systemic mycoses may result from breathing in the spores of fungi, which normally live in the soil or rotting vegetation or as opportunistic disease in immune compromised individuals (Walsh and Dixon, 2006).
Some fungi are normally present in the body, kept under control by the body's immune system. If the immune system is abnormally weak, however, the fungi can grow out of control and cause illnesses. These are termed opportunistic infections. Repeated infection may occur.
Among the most common opportunistic fungal infections that affect people are: Candidiasis, Phycomycosis, Aspergillosis (found everywhere), Mucormycosis, Cryptococcosis (where there are pigeon droppings) Trichosporonbeigelii,Pseudallescheriaboydii(Kourkoumpetis et al 2010).
Other fungi cause illnesses even in otherwise healthy people with normal immune systems. In these cases, people inhale the spores (immature form) of fungi that normally live in the environment, especially the soil, in certain geographical areas. Examples include: Histoplasmosis which occurs in the United States in the east and midwest, in Mexico, and in Central America. The fungus that causes it often grows on chicken droppings and bat guano. Coccidioidomycosis, or Valley fever, is found in the southwestern United States. North American blastomycosis is found throughout North America, and outbreaks recently have been reported in Africa as well. South American blastomycosis, which is caused by a different fungus, is seen in South and Central America (Cotran et al, 2005).
Since we will be dealing solely on yeasts, it is imperative that we will look at yeasts of poultry which can also attack the farmers.