Fungi

General characteristics

  • Eukaryotes, whose cells contain membrane-bounded nuclei, mitochondria & other organelles.
  • Cell wall is composed of chitin, a polymer that consists of subunits of a nitrogen-containing sugar. Chitin is highly resistant to enzymatic breakdown.

Structure of Chitin:

  • Fungi lack chlorophyll and are nonphotosynthetic. Nonetheless, some species exist as symbiotic partners with algae in lichens.(See http://www.fs.fed.us/wildflowers/interesting/lichens/index.shtml )
  • Fungi undergo external digestion, in which they secrete digestive enzymes (and acids such as acetic acid) and then absorb predigested food through their cell walls and plasma membranes.
  • Most fungi have a filamentous body plan, in which filaments called hyphae permeate through the habitat in which they live. The entire tangled mass of hyphae is called a mycelium. The biomass of a mycelium can be quite extensive. This body plan allows for a maximal surface area-to-volume ratio.

 

Hyphae - Diagram

Hyphae - Photomicrograph

 

  • Some hyphae are coenocytic, in which the filaments are NOT divided by membranes. Other hyphae are divided by septa, which may or may not be perforated. One or more nucleus may reside in each chamber.
  • Most fungi reproduce by spores, which may be dispersed by wind, water or animals. They may be produced by asexual or sexual reproduction.
  • Above-ground structures tend to be reproductive structures.
  • Many fungi secrete compounds which are toxic to bacteria (antibiotics, e.g. Penicillium) or to animals (hallucinogens, e.g. Claviceps purpurea: see http://www.wisc.edu/botany/fungi/oct99.html ) .

Inhibition of bacterial growth by selected antibiotics:clockwise from top; Novobiocin; penicillin G, dark pink; streptomycin, white; tetracycline; chloramphencol, light yellow; erythromycin; fuscidic acid,green; methicillin. thanks to J.Deacon, University of Edinburgh; from http://www.sln.org/pieces/burr/pupilact1.htm#discovery .

 

Fungal phyla and notes

 

 

Chytridiomycota (chytridiomycetes)

Example:Allomyces

Asexual reproduction:Zoospores

Sexual reproduction:Flagellated gametes in some chytrids

Allomyces sp. life cycle

 

Allomyces sp. photomicrograph

 

Zygomycota (zygomycetes)

Example:Rhizopus (black bread mold)

Asexual reproduction:Nonmotile spores form in a sporangium

Sexual reproduction: Zygospores

Rhizopus sp. life cycle

 

Rhizopus sp. aerial hyphae

Rhizopus sp. above- and below-ground structures

Video clip of loaf colonized by Rhizopus:
http://www.youtube.com/watch?v=eXcLVjHLl_o

Video clips of individual hypha & sporangium formation:
http://www.youtube.com/watch?v=PXwLddA4Ctw&NR=1

Ascomycota (ascomycetes or sac fungi)

Examples: Saccharomyces (yeasts); powdery mildews, molds, morels, truffles; Sordaria

Asexual reproduction: Conidia pinch off from conidiophores

Sexual reproduction: Ascospores

Sordaria sp. life cycle

Basidiomycota (basidiomycetes or club fungi)

Examples: Mushrooms, bracket fungi, puffballs, rusts, smuts

Sexual reproduction: Basidiospores

Deuteromycota (deuteromycetes or imperfect fungi)

Examples: Molds; Trichophyton interdigidale - Athletes foot or tinea pedis;††
C. Candida albicans - Candidiasis.
    This fungus is dimorphic, assuming a yeast-like or a hyphal form. Candidiasis typically is an irritating, itching condition which is more annoying than dangerous. Forms include vaginitis, and thrush.

Asexual reproduction: Conidia

Sexual reproduction: Not observed

From:http://www.mycology.adelaide.edu.au/Mycoses/Cutaneous/Dermatophytosis/

SOME OF THESE IMAGES MAY CAUSE ďABESĒ, ACQUIRED BURNING EYEBALL SYNDROME!DONíT SAY I DIDNíT WARN YOU!!

Tinea pedis

Infections by anthropophilic dermatophytes are usually caused by the shedding of skin scales containing viable infectious hyphal elements [arthroconidia] of the fungus. Desquamated skin scales may remain infectious in the environment for months or years. Therefore transmission may take place by indirect contact long after the infective debris has been shed.

Substrates like carpet and matting that hold skin scales make excellent vectors. Thus, transmission of dermatophytes like Trichophyton rubrum, T. interdigitale and Epidermophyton floccosum is usually via the feet. In this site infections are often chronic and may remain subclinical for many years only to become apparent when spread to another site, usually the groin or skin.

It is important to recognise that the toe web spaces are the major reservoir on the human body for these fungi and therefore it is not practical to treat infections at other sites without concomitant treatment of the toe web spaces. This is essential if a "cure" is to be achieved. It should also be recognised that individuals with chronic or subclinical toe web infections are carriers and represent a public health risk to the general population, in that they are constantly shedding infectious skin scales.


Tinea pedis caused by T. rubrum. Sub-clinical infection (left)
showing mild maceration under the little toe and more severe
infection showing  extensive maceration of all toe web spaces


"Moccasin-type" tinea pedis caused by E. floccosum (left) and
vesicular type tinea pedis caused by T. interdigitale (right).


"Tinea incognito" or steroid modified tinea pedis
caused by T. rubrum .


Tinea is transmitted via the feet by desquamated skin
scales in substrates like carpet and matting.

Tinea cruris

Tinea cruris refers to dermatophytosis of the proximal medial thighs, preum and buttocks. It occurs more commonly in males and is usually due to spread of the fungus from the feet. Thus the usual causative agents are T. rubrum, T. interdigitale and E. floccosum.


Tinea of the groin showing typical erythematous lesions on the inner thighs .


Tinea of the buttocks caused by
T. rubrum granular strain .


Tinea of the buttocks caused by
T. rubrum downy strain.

Tinea unguium (dermatophyte onychomycosis)

Trichophyton rubrum and T. interdigitale are the dominant dermatophyte species involved. In countries like Australia, UK and USA the incidence of dermatophyte onychomycosis has been estimated to be about 3% of the population, increasing up to 5% in the elderly, with some subgroups such as miners, servicemen and sportsmen etc having an incidence of up to 20% due to the use of communal showers and changing rooms.

It is important to stress that only 50% of dystrophic nails have a fungal aetiology, therefore it is essential to establish a correct laboratory diagnosis by either microscopy and/or culture, before treating a patient with a systemic antifungal agent.


Tinea of the nails caused by T. rubrum.

Dermatophyte onychomycosis may be classified into two main types; (1) superficial white onychomycosis in which invasion is restricted to patches or pits on the surface of the nail; and (2) invasive, subungual dermatophytosis in which the lateral, distal or proximal edges of the nail are first involved, followed by establishment of the infection beneath the nail plate. Distal subungual onychomycosis is the most common form of dermatophyte onychomycosis. The fungus invades the distal nail bed causing hyperkeratosis of the nail bed with eventual onycholysis, and thickening of the nail plate.

As the name suggests, lateral subungual onychomycosis begins at the lateral edge of the nail and often spreads to involve the entire nail bed and nail plate. In proximal subungual onychomycosis, the fungus invades under the cuticle and infects the proximal rather than the distal nail bed causing yellowish-white spots which slowly invade the lunula and then the nail plate.