Fungi

Fungi

Mycology is the study of fungi.

Fungi are aerobic or facultatively anaerobic absorptive chemoheterotrophs.

Most fungi are decomposers, and a few are parasites of plants and animals. Of the more than 1.5 million species about 300 are pathogenic, however the number of serious fungal infections is increasing.

Characteristics of Fungi

Molds and fleshy fungi consist of long filaments of cells joined together (hyphae).

A fungal thallus (body) is the vegetative, nonreproductive part of the organism and consists of a tangled mass of hyphae called a mycelium.

Septate hyphae contain cross-walls (septa), which divide them into uninuclear cell-like units.

Septa usually have openings that allow the cytoplasm of adjacent “cells” to be continuous.

Coenocytic hyphae have no cross-walls and look like one long, filamentous, multinucleated cell.

The vegetative hyphae obtain nutrients; the reproductive part of the fungus consists of reproductive or aerial hyphae.

Yeasts are unicellular fungi. To reproduce, fission yeasts divide symmetrically, whereas budding yeasts divide asymmetrically.

Buds that do not separate from the mother cell form pseudohyphae.

Pathogenic dimorphic fungi are yeast like at 37°C and moldlike at 25°C. (Figure 12.4)

Fungi reproduce asexually by fragmentation or production of asexual spores.

Two types of spores can be produced asexually:

1. Conidiospores (conidia) – a unicellular or multicellular spore that is not enclosed in a sac. Conidia are produced in a chain at the end of aerial hyphae called conidiophores. Aspergillus Arthrospores (arthroconidia) – Formed by fragmentation of septate hyphae into single cells. Coccidioides immitis Blastospores (blastoconidia) – Buds from parent cell, found in some yeasts. Cryptococcus Chlamydospores (chlamydoconidia) – Thick-walled spore, formed by rounding and enlargement within a hyphael segment. Candida albicans 2. Sporangiospores – formed within a sporangium (sac). Sporangia are formed at the end of aerial hyphae called sporangiophores. Rhizopus

Fungi are taxonomically classified according to the type of sexual spore that they form, but clinically, since most only exhibit asexual spores in the lab, are classified by asexual spore type.

Sexual spores are usually produced in response to special circumstances, often changes in the environment.

Phases of sexual reproduction:

Plasmogamy – the haploid nucleus of a donor (+) cell penetrates the cytoplasm of a recipient (-) cell.

Karyogamy – nuclei fuse to form diploid zygote nucleus.

Meiosis – forms haploid nuclei (sexual spores) that are genetic recombinants.

Nutritional Adaptations:

Fungi grow in aerobic environments; molds are aerobic, most yeasts are facultative anaerobes.

pH optima of about 5, too low for most bacteria.

Can grow in low moisture conditions, which don’t support most bacterial growth.

Can grow in relatively high sugar or salt concentrations (resist osmotic pressure gradients).

Usually require less nitrogen than bacteria.

They are able to metabolize complex carbohydrates, can grow on substrates bacteria cannot use.

Culture media:

Sabouraud’s agar - A culture medium for fungi containing neopeptone or polypeptone agar and glucose, with final pH 5.6; it is the standard, most universally used medium in mycology and is the international reference.

Modified Sabouraud's agar (Emmons modification) with less glucose is better for pigment development in the colonies. Synonym: French proof agar.

Medically Important Phyla of Fungi

Zygomycota have coenocytic hyphae.

Sexual spores are zygospores.

Asexual spores are sporangiospores.

Examples:

Rhizopus

Mucor

Both are ubiquitous in terms of habitat, cause systemic mycoses, and are considered opportunistic pathogens.

Ascomycota have septate hyphae.

Sexual spores are ascospores produced in a sac-like structure called an ascus.

Asexual spores are usually conidiospores although Trichophyton produces arthroconida.

Examples:

Penicillium (although it mutated and lost its ability to reproduce sexually and is now considered strictly an anamorph)

Aspergillus

Blastomyces dermatitidis

Histoplasma capsulatm

Microsporum

Trichophyton

Basidiomycota (club fungi) have septate hyphae and includes fungi that produce fruiting structures called mushrooms.

Sexual spores are basidiospores formed externally on a base pedestal called a basidium.

Asexual spores in some are conidiospores.

Examples:

Cryptococcus neoformans

Amanita phalloides (Death Angel, deathcap)

Psilocybe spp. (P. cubensis, P. mexicana , P. tampanensis and P. atlantis for example)

Anamorphs produce asexual spores only and have septate hyphae. Spore types are conidia, arthroconidia, and chlamydoconidia . 

Examples:

Pneumocystis carinii

Epidermophyton

Sporothrix schenckii, Stachybotrys

Coccidioides immitis

Candida albicans

Penicillium (formerly Ascomycota, mutation in the telomorphic form produced the anamorph.

Teleomorphic fungi produce sexual and asexual spores; the phyla Zygomycota, Ascomycota, and Basidiomycota are all teleomorphic, anamorphic fungi produce only asexual spores.

Deuteromycota was formerly used as a holding category for fungi without a known sexual spore type but ribosomal RNA sequencing identifies most as anamorphic phases of Ascomycota, a few are basidiomycetes.

Fungal Diseases

Systemic mycoses are fungal infections deep within the body that affect many tissues and organs.

Coccidiodomycosis – Coccidioides immitis

Histoplasmosis – Histoplasma capsulatum

Blastomycosis – Blastomyces dermatitidis

Meningitis - Cryptococcus neoformans

Ergot poisoning - Claviceps purpurea

psilocybin "poisoning" - Psilocybe cubensis, Psilocybe mexicana , Psilocybe tampanensis and Psilocybe atlantis

Aflatoxin poisoning - Aspergillus flavus

Mycotoxin poisoning (phalloidin and amanitin) - Amanita phalloides

Subcutaneous mycoses are fungal infections beneath the skin.

Sporotrichosis – Sporothrix schenckii

Chromomycosis – Fonsecaea, Phialophora, Cladosporium

Cutaneous mycoses (or dermatophytoses - ringworm, caused by dermatophytes) affect keratin-containing tissues such as hair, nails, and skin.

Tinea capitis – ringworm of the scalp

Tinea cruris – ringworm of the groin (jock itch)

Tinea pedis – ringworm of the feet (athlete’s foot)

Organisms

Epidermophyton – affects only skin and nails

Trichophyton – can affect hair, skin, or nails

Microsporum – usually affects only hair or skin

Dermatomycoses – cutaneous mycoses caused by other fungi, most often Candida

Superficial mycoses are localized on hair shafts and superficial skin cells.

Tinea versicolor – Malassezia furfur

Tinea nigra – Cladosporium werneckii

Opportunistic mycoses are caused by normal microbiota or fungi that are not usually pathogenic.

Opportunistic mycosis can infect any tissues, however, they are usually systemic.

AIDS patients susceptible to Cryptococcus, Pneumocystis, and Penicillium infections.

Mucormycosis - caused by Rhizopus and Mucor.

Aspergillosis, caused by Aspergillus.

Candidiasis, caused by C. albicans, can be dermal, oral (thrush) or vaginal.

Economic Effects of Fungi

Food production

Aspergillus niger – citric acid for foods and beverages

Saccharomyces – bread, wine, beer, and used as a protein supplement and vitamin B source

Torulposis –protein supplement

Trichoderma – produces cellulase, removes cell walls to produce clear fruit juice

Biological Control of Pests

Candida oleophilia can grow on harvested fruit and prevent the growth of spoilage fungi.

Mold spoilage of fruits, grains, and vegetables is more common than bacterial spoilage of theses products.

Diseases in Plants

Cryphonectria parasitica – killed off most of the chestnut trees

Ceratocystis ulmi – Dutch elm disease

Claviceps purpurea - grain infestation (rye)

Medical

Taxomyces produces taxol