2.1 WHAT IS MEDICAL MYCOLOGY?
The study of pathogenic fungi (i.e., fungal organisms that are of medical importance and cause disease in humans) is known as medical mycology. Pathogenic fungi are fungal organisms that cause disease in humans. Human mycoses may include infections of the outer skin layer, infections of the underlying tissues of the skin, systemic infections, and deep-seated infections or invasive mycoses. Other fungal infections are opportunistic in nature, and only cause disease in humans by chance. Opportunistic fungal infections rarely affect individuals whose immune system is still intact and strong. Opportunistic mycoses usually occur in immunocompromised individuals such as HIV/AIDS patients as well as in people with debilitating diseases like cancer. The elderly, patients on chemotherapy and individuals with other debilitating disease conditions are more prone to opportunistic fungal infections than people with strong immune system. Fungal organisms are major spoilage organisms of foods and other food products. Pathogenically, fungal infections can be exogenous or endogenous in nature.
Mycoflora are fungi that are normal flora of the human body. Some Candida species are mycoflora found in the human mouth and genital regions such as the female vagina. These mycoflora can become infectious when the body’s immune system becomes compromised or weakened. The main portals of entry of fungi that cause endogenous mycoses include the skin, mouth, GIT and the vagina. It is noteworthy that most fungal infections especially those caused by dermatophytes and even some endogenous mycoses are best prevented via an intact host skin which prevent pathogen entry from the outside.
Fungal infection in human population can cause serious pathological conditions in infected individuals especially in endemic regions. However, most fungal infections can be treated and managed clinically using antifungal agents. But personal and environmental hygiene also plays critical role in the control and prevention of fungal infection in human population. Good personal hygiene is also essential in preventing some fungal infections (for example, dermatophytosis). However, a broken or injured skin is the common site and portal of entry for environmental fungi to enter the body and become established. Fungal organisms elaborate both beneficial and non-beneficial effects in the natural environment. Beneficial fungi have been exploited by man since time immemorial to produce food, drugs and other products that are of economic and health importance. However, some fungal organisms termed as non-beneficial fungi cause a wide range of infections and diseases in human populations, as well as in plants and animals. More so, some fungi cause a variety of food spoilage which amount to great economic loss to food production companies (Figure 2.1). This chapter shall explain fungal species that are of medical importance and cause disease or infections in humans.
Figure 2.1: Illustration of fungal spoilage of bread by moulds (for example, Mucor species and Rhizopus species). Food spoilage caused by fungi is a major economic loss and the phenomenon negatively affects food availability.
2.2 EXOGENOUS AND ENDOGENOUS FUNGAL INFECTIONS
In exogenous fungal infections, the fungal infection is usually acquired externally from the environment via the inhalation of infectious fungal spores in aerosols or from dust particles. Infectious conidia or spores of fungi are ubiquitous in the environment especially in the soil, and they can easily cause human infection through inhalation. Inhalation of fungal spores in large doses can predispose humans to a variety of fungal infections. Fungi that cause exogenous infections in man penetrate the body via the respiratory tract system from where they reach the lungs and become disseminated to other parts of the body to cause systemic or deep mycoses. People living in endemic places of fungal infections or individuals who engage in certain type of job or activity (for example, construction workers, farmers) that cause fungal spores to become aerosolized can easily inhale infectious fungal spores. Endogenous fungal infections are caused by fungi that are members of the human normal microflora (for example, Candida species). Infections caused by endogenous fungi usually occurs when the body’s immune system is compromised or in a bad condition. Most fungal infections are restricted to a particular geographical area while others are distributed worldwide in the environment. The irrational use of antibiotics can displace normal bacterial flora that checkmate the growth of mycoflora. This allows fungi that are normal flora to overgrow and cause endogenous fungal infection in the affected individual.
2.3 GENERAL SIGNIFICANCE OF FUNGI
- Fungi are saprophytic organisms. They breakdown organic matter in the environment, and thus enhance nutrient recycling in the natural environment.
- They are used in the industry for food production. Fungi play critical roles in the production of alcoholic and non-alcoholic beverages through the process of fermentation (Figure 2.2). They are also used in the production of bread and organic acids. Saccharomyces cerevisiae (baker’s yeast) is a typical example of fungus used in the production of bread.
- Figure 2.2: Illustration of some food products produced by the fermentative action of fungi. MicroDok
- Some fungi such as mushrooms are edible. Not all mushrooms are fit for human consumption because some species are poisonous and can cause death when eaten. Caution is therefore required when harvesting mushrooms from the wild for human consumption.
- Fungi are involved in symbiotic relationships with a number of organisms. For example, fungi form symbiotic associations with green algae or cyanobacteria to form lichens.
- Lichens are macro-organisms comprising fungi and algae or cyanobacteria.
- Secondary metabolites produced by fungi and symbiotic associations formed by fungal organisms (for example, lichens) are important source of lead agents for the production of biopharmaceuticals, organic acids and other bioactive agents and antimicrobial agents.
- Fungi store food as lipids and glycogen.
- Most fungi are environmental organisms, and they grow on simple nutrient medium containing carbohydrates and nitrogen.
- Fungi also go into symbiotic associations with plant roots to form fungus roots known as mycorrhizae. Mycorrhizae contribute to plants nutrition by facilitating nutrient and water uptake by plant roots from the soil.
- Some fungi known as endophytes invade the upper parts of some plants where they confer both beneficial and harmful effects. Endophytes are endosymbionts or organisms that comprises of a fungus and a bacterium; and which live within plant tissues where they confer several benefits that is either mutualistic or parasitic.
- Most fungi are phytopathogens. Phytopathogens are fungal organisms that cause disease in plants especially agricultural crops. Fungi are important plant pathogens as there are more fungal organisms that parasitize crop plants than animals and humans.
- Some fungi (for example, Candida species) are members of the human normal flora, and can cause disease when the bacterial population of the normal flora is disturbed.
- Fungi are important tools for investigating the metabolic processes of eukaryotic cells. Neurospora species and Saccharomyces species are typical examples of fungi used by scientists to elucidate the biochemical and metabolic mechanisms of other eukaryotic cells.
- They are natural decomposers, and play critical role in carbon cycle and in the recycling of other nutrients in the ecosystem through their mineralization activities.
- Fungi are generally saprophytes, non-autotrophic and they mainly obtain their food by absorption. Thus fungi are chemoheterotrophs i.e., they breakdown organic compounds to inorganic compounds to obtain energy and carbon.
- Fungi can breakdown lignin (a complex and recalcitrant polysaccharide molecule).
- As pathogens, they cause diseases in man, animals and plants.
- Fungi are important sources of natural antibiotics (for example, penicillin and griseofulvin).
- Some fungal organisms synthesize organic acids (for example, citric acid) and enzymes that are of great industrial applications.
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