Classification of Fungi and Pathogenesis

Classification of fungi on the basis of sexual spores

Zygomycetes

Zygomycetes are popularly known as bread molds and these are fast growing, terrestrial, mostly saprophytic fungi. Hyphae are coenocytic and mostly aseptate. Asexual reproduction is exhibited by Sporangiophores may be a simple or branched type. Sexual reproduction involves producing a thick-walled sexual resting spore known as zygospore .Some o f the medically important genera in this phylum include:

Mucor, Rhizopus , Rhizomucor, Absidia , Saksenaea, etc

Basidiomycetes

Basidiomycetes exist as saprobes and parasites of plants and this phylum includes those fungi that reproduce by the formation of basidiospores on a specialized structure known as basidia. The basidiomycetes are generally planted pathogens or environmental organisms that rarely cause disease in humans.Like ascomycetes, they also possess septate hyphae .Most members of basidiomycetes are the mushrooms which are fleshy fungi and their spores are borne in unusually fleshy fruiting bodies.Asexual spores are uncommon in this group.

Medical importance include:

1. Telemorph ofCryptococcus neoformans, which isFilobasidiella neoformans
2. Agents of basidiomycete
3. Mushroom poisoning byAmanita, Lepiota, Coprinus and Psilocybe.

Ascomycetes

Ascomycetes exist as saprophytes and parasites of plants and hyphae are septate with septal pores. Asexual reproduction is through conidia. Sexual reproduction is by the formation of endogenous ascospores, mostly eight ; in an ascus. Medically important genera of ascomycetes include the followings :

1. Teleomorphs of known pathogenic fungi e.g. Arthroderma (of Trichophyton and Microsporum),Ajellomyces dermatitides(ofBlastomyces dermatitides)
2. Agents of mycetoma, such asLeptosphaeria
3. Agents of black piedra, such asPiedraia hortae.

Deuteromycetes

Deuteromycetes are also commonly known as Fungi imperfecti due to an absence of sexually reproducing forms and are characterized by their asexual reproductive structures,primarily conidia . There are three classes of Deuteromycetes.

i.Blastomycetes:This class includes asexual budding forms of Cryptococcus, Candida, Torulopsis, and Rhodotorula. Based on the presence of melanin in their cell walls, they may be non-dematiaceous or dematiaceous.

ii.Hyphomycetes:These class of mycelial molds , reproduce asexually by conidia on hyphae. Hyphae are septate. This class includes the majority of medically important fungi. Dematiaceous fungi produce dark brown, green-black, or black colonies. Hyaline hyphomycetes include those conidial fungi, which are not darkly pigmented; colonies may be colorless or brightly colored. These contain the agents of hyalohyphomycosis, aspergillosis, dermatophytosis and the dimorphic pathogens, such asHistoplasma capsulatum.

iii.Coelomycetes:This class produces acervuli, which are tightly bound mats of hyphae on which the conidia are produced.

Entry

In healthy immunocompetent people, fungi rarely cause disease. Fungi entry and grows causes a disease to host by penetrating host barriers, in a person of immunologicdeficientt or in a host having other debilitating conditions.Fungi get intrance in a human host and infect the body through several portals of entry.The yeast C. albicans encounterwhile passing through the vaginal canal during the birth of a child. It is the first exposure to fungi that human experience. During this process, the fungus colonizes the buccal cavity and portions of the upper and lower gastrointestinal tract of the newborn, where it maintains a life-long as commensals.

Adaptation and Propagation

Fungi has capsule and ability to grow at 37^oC which shows both virulence mechanisms. Fungi exist in different morphologic forms such as yeasts, hyphae, spherules, and sclerotic bodies that provide support for their multiplication within the host.

Fungal Factors

Pathogenesis of fungal diseases (mycosis)

Most of the fungi are saprophytic or parasitic to plants and are fitted to their natural environment. Infection in humans is an event which occurs only when the condition is favorable. Except few fungi like the dimorphic fungi that cause systemic mycoses and dermatophytes, which are primary pathogens, the others are only opportunistic pathogens. The human body is a hostile environment and provide great resistance to fungal invasion. Most of the fungi are saprophytic and their enzymatic pathways function is more efficient at the redox potential of non-living substrates than at the relatively more reduced state of living metabolizing tissue.similarly, Some fungi such asCandidaandMalasezziahave adapted to the human environment and they exist as commensals. The complex relationship between fungal virulence factors and host defense factors will determine if a fungal infection will cause a disease or not . Infection depends on inoculum size and the immunity of the host.

Fungal Pathogenicity (virulence factors)

Followings are the virulence factors that causes fungal diseases in plants and animals:

• Ability to adhere to host cells by way of cell wall glycoproteins
• Production of a cytokine called GM-CSF byCandida albicans that suppresses the production of complement.
• Ability to acquire iron from red blood cells as inCandida albicans
• Ability to damage host by secreting enzymes like keratinase, elastase, collagenase
• Ability to resist killing by phagocytes
• Ability to produce mycotoxins
• Having a unique enzymatic capacity
• Exhibiting thermal dimorphism
• Surface hydrophobicity

Host defense factors

Followings are the agents or factors that inhibit the fungal diseases;

• Physical barriers, such as skin and mucous membranes
• The fatty acid content of the skin
• The pH of the skin, and body fluids
• Chemical barriers,
• Natural Effector Cells and the Professional Phagocytes (monocytes and macrophages)

REFERENCE

D, Grenwood, Slack RCB and Peutherer J. Medical Microbiology. Dunclude Livingstone: ELBS, 2001.

Cheesbrough, M.Medical Laboratory Manual for Tropical Countries.Vol 2. ELBS London, 2007.

Tille, P.Diagnostic Microbiology.13th. Elsevier, 2014.