Fungi constitute an extremely diverse group of organisms and are generally classified as either molds or yeasts.

General CharacteristicsFungi: Mitochondria, Eukaryotic, Nuclear Membrane, True Nucleus, Cell Wall Chitin (must need a host)Bacteria: ProkaryoticPlants: Chlorophyll, Cell Wall Cellulose (doesn’t need a host)

Fungi: - obligate aerobes with neutral ph 7.0 (although can tolerate wide range of ph- Moisture is necessary for growth- spores and conidia survive in dry conditions

Yeast: - single vegetative cells-form smooth, creamy, bacterial like-colony - without aerial hyphae- both micro and macroscopic are similar- identification of yeast is based on biochemical testing- produce by budding with production of blastoconidium (daughter cell)- process involves lysis of the yeast cell wall to form blastoconidium- as structure enlarges, nucleus is passed into blastoconidium, a septum forms and blastoconidium breaks free.

Moulds: - fuzzy or woolly appearance because of mycelia- mycelia are made up of many log strands of tube-like structures called hyphae. (either aerial or vegetative) - Mycelia support reproductive structures that produce conidia- conidia can be used to identify different fungal genera- Aerial mycelia extends above surface of colony and responsible for fuzzy or woolly appearance- Vegetative mycelia extends downward to absorb nutrients from the medium (media)- The microscopic appearance often aids in identification of moulds

Types of hyphae: Spiral hyphae are tightly coiled spiralsAntler hyphae have swollen branching tips that resemble moose antlers.Racquet hyphae have enlarged, club-shaped areasRhizoids hyphae are root like structures, might see in some Zygomycetes, and their presence and placement can assist with identification.Fungal hyphae are described or referred as septate or sparsely septate. septate hyphae show frequent cross-walls occurring perpendicularly to outer walls of the hyphaesparsely septate hyphae have few cross-walls at irregular.aseptate hyphae meaning absence of septations, has

historically been used to describe the hyphae of Zygomycetes. Microscopic examination of hyphae associated with Zygomycetes often reveals occasional septations; therefore these hyphae are correctly termed sparsely septate as opposed to aseptate.

HYALINE (MONILIACEOUS) VS. DEMATIACEOUS: - pigmentation is useful in identification- Hyaline ormoniliaceous hyphae are either nonpigmented or lightly pigmented.- Dematiaceous hyphae are darkly pigmented because of melanin in the cell wall.- It depends on the amount of melanin present; the hyphae will appear pale to dark brown or nearly black. The dark hypha seen in tissue section is dark-colored because of stains that enable better visualization of fungal elements in tissue and not melanin.

Stain used: Gomori methylene stain = fungal elements appears black. Fontana-Masonstain = stains melanin causing dematiaceous hyphae appear brown whereas hyaline hyphae remain pink to red (another stain used in hyphal pigmentation in tissues)

DIMORPHISM VS. POLYMORPHISM: Dimorphism refers to the ability of some fungi to exist in 2 forms depending on growth conditions.- Dimorphic fungi include a mould phase and either a yeast or spherule phase.- Yeast, spherule or tissue state is seen in vivo or when the organism is grown 37 C w/ increased CO2.- Mould phase is seen when the organisms grown at room temperature (22-25 C) in ambient air conditions.- Thermally dimorphic fungal species associated w/ human disease include: Blastomycesdermatitidis, Coccidioidesimmitis, Histoplasmacapsulatum var. capsulatum, Paracoccidioidesbrasiliensis, Sporothrixschenckii, andPenicilliummarneffei.- mycoses = infections due to fungi- Polymorphic fungi have both yeast and mould forms in the same culture. This characteristic occurs despite growth conditions and is best observed in Exophiala spp. where yeast phase is observed initially followed by mould phase as colony ages.

REPRODUCTIONFungi can reproduce either asexually (imperfect) or sexually (perfect).- Asexual reproduction results in formation of conidia (singular:conidium) following mitosis. - It is carried out by specialized fruiting structures known as conidiogenous cells.- conidiogenous cellsform conidia which contains all genetic material necessary to create a new fungal colony.

- Two common conidiogenous cells are the phialide which are vase like structures that produce phialoconidia and the annelideare ringed structures that produce anneloconidia.Both form their conidia blastically (budding) like many yeasts: the parent cell enlarges and septum forms to separate the conidial cell.Another is arthroconidia which formed by fragmentation of fertile hyphae.- In clinical laboratory, most mould identification is based on the structures formed as a result of asexual reproduction. - Sexual reproduction requires the joining of two compatible nuclei followed by meiosis.Teleomorph– fungus that reproduce sexually but ocassionally will reproduce asexually and this asexual form is termed anamorph. - Synanamorphs– if more than one anamorph is present for the same teleomorph.The best example phenomenon is the teleomorphPseudallescheriaboydii, which has two anamorphs: Scedosporiumapiospermum and Graphium sp. These two anamorphs are synanamorphs to each other.

TAXONOMY: Most of the etiologic agents of clinical infections are found in four groups of fungi. They consist of the divisions Zygomycota, Ascomycota, Basidiomycota, and form-division Fungi Imperfecti (Deuteromycota).

ZYGOMYCOTAMembers of class Zygomycetesrapidly growing organisms found in soilopportunistic pathogens in immunocompromised hosts.gray to white, aerial myceliumsparsely septate hyphaeasexual reproduction of Zygomycetes is characterized by the presence of sporangiophoresand sporangiospores. The asexual spores (sporangiospores) are produced in structure called sporangium which develops from a sporangiophore.Some zygomycetes are capable of sexual reproduction resulting in the production of zygospores.- Common Zygomycetes are Mucor, Rhizopus, and AbsidiaASCOMYCOTA- production of sexual spores known as ascospores.- formed within saclike structure known as ascus.- identified on the basis of characteristic asexual structures.- Microsporum spp. Trichopytonsppand Pseudallescheriaboydii(teleomorph)

BASIDIOMYCOTA- are few and the only known major pathogen is Filobasidiellaneoformans – perfect form of Cryptococcus neoformans var. neoformans. - Basidiomycetous moulds recovered in laboratory remain sterile, complicating the identification process.- presence of clamp connection is one clue if a mould is basidiomycete. Clamp connection occur at the septations in the vegetative hyphae and easily visible with light microscopy.

- portion of hypha on one side of the septations grows out and connects to hypha on the other side of septations, bypassing the septation. - Basidiomycetous moulds are being recovered in increasing numbers in laboratory, but their role in infectious disease is not understood.

FUNGI IMPERFECTI or DEUTEROMYCOTA- contains the largest number of organisms that are etiologic agents of mycoses, including cutaneous, subcutaneous, and systemic disease. - Organisms with no mode of sexual reproductionhave been identified,-identified on the basis of characteristic asexual reproductive structures.