The term fungus includes eukaryotic, spore-bearing organisms with absorptive nutrition, no chlorophyll, and that reproduce sexually and asexually. It refers to a wide range of life forms that grow on both living and dead organic materials. Mushrooms, yeasts and moulds are three large classes of fungi. Fungi are essential players in ecosystems, participating in the recycling of dead organic matter. Fungi are primarily terrestrial organisms, although a few are freshwater or marine.
The fungal branch of the eukaryotic kingdom includes both unicellular yeasts and filamentous, multicellular molds. Most of the important pathogenic fungi exhibit dimorphism - the ability to grow in either yeast or mould form. The yeast-to-mould or mould-to-yeast transition is frequently associated with infection.
Yeasts and moulds, which collectively constitute the fungi, have an important ecological role in breaking down plant and animal remains for reuse. They also make numerous antibiotics and are used in the manufacture of bread, beer, wine and cheese. Not so pleasant are fungal diseases, which range from relatively innocuous skin infections such as jock itch and athlete's foot to life-threatening Pneumocystis carinii pneumonia, a common cause of death among AIDS patients.
Human diseases called by fungi are called mycoses. The diseases are divided into three groups depending on where they occur on our body. Superficial mycoses are those that affect the skin, nails and hair, subcutaneous mycoses are responsible for infections in the deep layers of the skin and systemic mycoses which cause the most serious fungal infections are most commonly acquired by the inhalation of spores. Some spores will remain in the lungs and grow while others enter the bloodstream, travel around the body and infect other organs.
Most fungal infections are due to opportunistic pathogens, these affect people who are already ill or have a suppressed immune system (e.g. in patients who have been given an organ transplant or in AIDS patients). In a perfectly healthy person the fungus would not normally cause disease. True pathogens can cause disease in even the healthiest person.
2.0 FUNGI AND DISEASE
Although most fungal diseases are the result of accidental encounters with the agent, many fungi have developed mechanisms that facilitate their multiplication within the host. For example, the dermatophytes that colonize skin, hairs, and nails elaborate enzymes that digest keratin. Candida albicans as a commensal organism exists in a unicellular yeastlike morphology, but when it invades tissues it becomes filamentous; conversely, the systemic fungi Histoplasma capsulatum, Blastomyces dermatitidis and Paracoccidoides brasiliensis exist as molds in nature and change to a unicellular morphology when they cause disease. Other properties, such as capsule production by C neoformans and the adherence properties of Candida species to host tissues, also contribute to their pathogenicity. In general, the fungi that case systemic disease must be able to grow and multiply at 37C.
5.1 Host Factors
The high degree of innate resistance of humans to fungal invasion is based primarily on the various protective mechanisms that prevent fungi from entering host tissues. Fungal growth is discouraged by the intact skin and factors such as naturally occurring long-chain unsaturated fatty acids, pH competition with the normal bacterial flora, epithelial turnover rate, and the dessicated nature of the stratum corneum. Other body surfaces, such as the respiratory tree, gastrointestinal tract, and vaginal vault, are lined with mucous membranes (epithelium) bathed in fluids that contain antimicrobial substances, and some of these membranes are lined with ciliated cells that actively remove foreign materials. Only when these protective barriers are breached can fungi gain access to, colonize, and multiply in host tissues. Fungi gain access to host tissues by traumatic implantation or inhalation. The severity of disease caused by these organisms depends upon the size of the inoculums, magnitude of tissue destruction, the ability of the fungi to multiply in tissues, and the immunologic status of the host.
6.0 FUNGAL FACTORS
Most of the fungi that infect humans and cause disease are classified by tissue or organ levels that are primary sites of colonization.
6.1 Superficial Mycoses
Superficial mycoses involve fungal colonization of the hair, skin, or nails and infect only the surface layers. In general, these diseases are relatively benign and self-limiting. Some, such as Trichophyton infections of the feet (athlete's foot), are quite common. Spread is by personal contact with an infected person or by contact with contaminated surfaces such as bathtubs, shower stalls, or floors, or by contact with contaminated shared articles such as towels or bed linens. Treatment for severe cases is with topical application of miconazole nitrate or griseofulvin. Griseofulvin can also be administered orally. After entering the bloodstream, it passes to the skin where it can inhibit fungal growth.
6.2 Subcutaneous Mycoses
The subcutaneous mycoses are a second category of fungal infections. They involve deep layers of skin and a different group of organisms. One disease in this category is sporotrichosis, an occupational hazard of agricultural workers, miners, and others who come into contact with the soil. The causative organism is found as a ubiquitous saprophyte on wood and in soil. The lesions are usually initiated by fungal infection of a small wound or abrasion. Sporothrix schenkii can readily be isolated from the lesion and cultured in vitro. Treatment is with oral potassium iodide or oral ketoconazole.
6.3 Systemic Mycoses
The systemic mycoses are the third and most serious category of fungal infections. They involve fungal growth in internal organs of the body and are sub classified as primary or secondary infections. A primary infection is one resulting directly from the fungal pathogen in otherwise normal, healthy individuals. A secondary infection involves infection in hosts with a predisposing condition such as antibiotic therapy or immunosuppression.
A number of systemic fungal infections, including histoplasmosis and coccidioidomycosis, are especially serious and common in individuals whose immune systems have been impaired, for example, by acquired immunodeficiency syndrome (AIDS) or by immunosuppressive drugs. These are secondary fungal diseases because normal individuals either do not get the disease or generally have a less severe form. These fungi are known as opportunistic pathogens because of their particular ability to cause serious infections only in individuals with impaired defence mechanisms (in particular AIDs patients).