What is medical mycology? History, Transmission, Risk factors, Yeast, Mold
Introduction to mycology
Mycology is the study of fungi.
Medical mycology is a branch of mycology including fungal epidemiology, ecology, culture, pathogenesis, laboratory diagnosis, clinical manifestations, fungal diseases (occurring in humans), prevention, and control.
It also includes taxonomy, genetics, biochemical properties, and their use in humans as food, medicine, psychotropic substances, poisoning, or infections.
Nomenclature of mycology
Mycology is a derivate of the Greek words Mykes (mushroom) and logos (study or discourse) while fungi is the Latin word for mushroom.
History of Medical Mycology
Humans have been using fungi for thousands of years. Neolithic man Ötzi the Iceman, a 5,300-year-old mummy, was found carrying two species of polypore mushrooms - Fomes fomentarius and Piptoporus betulinus – which might have been used as tinder or medicine respectively.
In 1588, Italian scholar Giambattista della Porta first observed fungal spores. After the development of the microscope in the 17th century, Italian botanist Pier Antonio Micheli published Nova plantarum genera in 1729 where he observed spores and also showed it can be induced to grow into the same species of fungi under specific conditions.
Following the introduction of the binomial system of nomenclature in 1753, German mycologist Christiaan Hendrik Persoon was the first person to classify fungi and is known as the founder of modern mycology.
In 1835, Italian botanist Augustino Bassi isolated the fungi Baeveria basinia – known to cause infection in the silkworm Bombyx mori. Based on this finding, researchers predicted that fungi can cause disease in humans as well.
In 1910, French dermatologist Raymond Jacques Sabouraud published his work on dermatophytes which is an etiological agent of dermatphytosis. Hence Sabouraud is named the father of medical mycology.
Characteristics of fungi
The optimum growth conditions of fungi may range from obligatory aerobes to facultative anaerobes while none are strictly anaerobes. They require organic sources of carbon for their growth and metabolism.
Epidemiology of fungi
Fungi is one of the most common causes of infection, prominently in immunocompromised hosts. Although most fungi are saprophytes, some are capable of causing diseases. Among all the recognized fungi, around 200 to 250 species are found to be infective to humans.
Transmission of fungi
Fungi, especially of medical importance, infect via inhalation of spores or by introduction into host tissue as a result of trauma. Excluding dimorphic fungi, the accidental hosts (humans) are relatively resistant to fungal infections.
Risk factors
Risk factors that enhance fungal infection include:
Alteration in the immune system
Use of immunosuppressive drugs including antimetabolic therapy, ani-malignancy therapy, corticosteroids
The presence of debilitating diseases such as diabetes, AIDS
IV drug abuses
Longterm use of antimicrobial drugs
Gastrointestinal surgical procedures
Prolong use of caterers
Increased use of prosthetic devices
Types of fungi
Fungi are grouped into two groups – yeast and molds.
Yeast
Yeast are unicellular, oval, or round organisms whose size may range from 2 to 60 µm in diameter. Their colonies may be moist, creamy, opaque, or pasty on media. Some yeasts can be capsulated, which is a major virulence factor in pathogenic strains.
Most yeasts including Ascomycota, Basidiomycota, and Deuteromycota reproduce sexually and asexually. They produce blastoconidia (budding) and binary fission (mitosis) as asexual reproduction and sexually reproduce by production of ascospores and basidiospores.
Pathogenic strains of yeasts may exhibit yeast or yeast-like forms and filaments. These strains are called dimorphic fungi and if dimorphism is temperature dependent, it is called thermally dimorphic.
Some dimorphic fungi are as follows:
Histoplasma capsulatum
Blastomyces dermatitidis
Coccidioides immitis
Paracoccidioides brasiliensis
Sporothrix schenckii
Penicillium marneffei
Although a potential pathogen, yeasts such as Candida spp. Are unicellular, exhibiting pseudohyphae and/or true hyphae, and are not temperature dependent.
Molds
Molds are multicellular organisms that produce cottony, wooly, fluffy, or powdery colonies on media. They are made up of tube-like projections called hyphae. The hyphae, if present in a tangled mass, is known as mycelium.
Based on functionality, there are two types of hyphae – vegetative hyphae and reproductive hyphae.
Vegetative hyphae include the fungal main body which absorbs nutrients and water required for metabolism and growth.
Reproductive hyphae are required for spore formation. Aerial hyphae, which extend above the substrate, give rise to fruiting bodies from which asexual spores are produced.
Types of hyphae
Based on morphological differences, three types of hyphae are present:
Coenocytic hyphae: are septate – thus appear as long tubular thread-like structure
Dematiteous hyphae: septated and pigmented – pigmentation may range from dark brown, greenish grey, black, etc.
Hyaline hyphae: septate hyphae and are non-pigmented