Fungi (thing)

Overview

Fungi are one of the five types of microbes¹.

Fungi are plant-like organisms, but unlike plants they do not have chlorophyll. Because of this they cannot make their own food. Instead, fungi absorb food from dead organic matter. The function of fungus in the ecosystem is to break down complex organic substances and recycle carbon and other elements. Some fungi are biotrophic, meaning they feeding from living cells; others are necrotrophic, meaning they kill cells and then feed on them. Most fungi feed on plant material rather than animal material.

The main types of fungus are:

• Moulds (also spelled mold), which have filaments; and
• Yeasts, which are usually single cells.

Some common fungal Diseases that affect humans are listed below.

Taxonomy

The approximately 100,000 species of the Fungus kingdom are divided into four phyla, based on sexual characteristics.

• Sac or Cup Fungi (Ascomycota) - 50,000 species including Hemiascomycetae (Brewer's yeast, bakers yeast, and nutritional yeast) and Truffle mushrooms;
• Club Fungi (Basidiomycota) - most other mushrooms (a.k.a. basidiocarp);
• Conjugation Fungi (Zygomycota) - including common bread mould;
• Imperfect Fungi (Deuteromycota) - fungi that are not easily classified, including most of the ones cited below under Diseases.

The lichens (Mycophycophyta) were formerly considered to be a fifth fungus pyhla, but this is no longer true.

Structure and form

The visible part of a fungus is usually a "fruiting body" such as a mushroom. This is usually a small portion of the overall organism. The main body of most fungi is a web called the mycelium. The mycelium is made of very fine threads called hyphae. The hyphae branch and intertwine throughout the fungus' food source (such as soil or wood). A fungal mycelium is generally too fine to be seen by the naked eye, unless the hyphae are packed densely together.

The cell wall of a fungus is made of chitin, which is also found in insects. This is something that distinguishes fungi from plants - the cell walls of plants are made of cellulose.

Reproduction (Replication)

Fungi are capable of two types of reproduction:

• Asexual:
  • Fungi reproduce asexually by forming spores. When conditions are right the spore starts to grow. A hypha grows from the spore, and begins to branch and weave to form the mycelium. If growing conditions continue to be are favorable, the mycelium develops fruiting bodies, which produce new spores.
  • Fungi can also reproduce asexually by fragmentation. If pieces of the mycelium or hyphae are broken off, those portions can grow into a complete fungus.
  • Yeasts, which are single cells, reproduce by yet another asexual process called budding. A yeast cell splits at the tip to produce a small offspring cell.
• Sexual

  Each of the fungi phyla have a different form of sexual reproduction.

  • Ascomycota make spores sexually inside a bag or sac called the ascus. In the ascus, meiosis and usually also a mitotic division take place, resulting in spores called ascospores. Asexual spores are made at the tips of the hyphae.
  • Basidiomycota make a structure called the basidium by fusing hyhae, and place spores on the outside. Reproduction results in the forcible discharge of a spore called a ballistospore. Most Basidiomycota do not reproduce asexually.
  • Zygomycota generate their sexual spores inside a thick-walled compartment (the zygosporangium) which is created when 2 hyphae of appropriate mating types fuse together, and then are walled off from the rest of the hypha. Asexual spores are made in a sporangium.
  • In Deuteromycota, the sexual structures are unknown or don't exist. This is a "leftover" category and lacks a single defining characteristic. Most reproduce asexually.

Diseases

The types of diseases caused by fungi can be divided into 3 major categories:

• Allergic reactions (sensitivity to fungal proteins);
• Toxicity reactions (consumption of fungal toxins); and
• Fungal infections (invasion of living tissue by a fungus)

The most well-known example of an allergic reaction to fungus is "toxic mould" and "sick building syndrome" in buildings. (This was in the news in April 2002. Celebrity Ed McMahon sued his insurance company over a botched repair that caused fungus to spread throughout his home, making his family ill and allegedly killing his dog, Muffin. On doctor's orders, the McMahons moved out of their six-bedroom home in September 2001. The suit was settled in May 2003 for $7.2 million US.) The resulting condition is Allergic fungal sinusitis which causes a variety of symptoms, and in extreme cases can result in anaphylactic shock. Note that while moulds are very common in buildings, and will grow anywhere that there is moisture, 'toxic' moulds such as Stachybotrys chartarum are very rare. More common indoor moulds include Penicillium (the basis for the drug Penicillin).

The most well-known examples of fungal toxins are toadstool poisoning. Toadstools such as the Death cap (Amanita phalloides) and Destroying angel (Amanita virosa, A. verna, A. bisporigera, A. ocreata) ultimately cause death due to kidney and liver failure. Another famous example is the Liberty Cap or Magic Mushroom ( Psilocybe semilanceata) which affects the central nervous system inducing a hallucinogenic response. Another example is ergotism, a syndrome which can cause gangrene, hallucinations, and death. It is caused by consumption of rye products contaminated with toxins produced by the mould Claviceps purpurea. This fungus' toxins may have caused illness and delirium in Salem, Mass. leading to the Salem witch trials!

Some fungal infections, called primary pathogens, can cause disease regardless of the previous health of the victim. Others, referred to as opportunistic, affect people with immune systems that are already weakened by other factors. Diseases caused by fungal infection include:

• Tinea including Ringworm (an infection of the hair, skin, or nails); jock itch (tinea cruris); and athlete's foot (tinea pedis) -- caused by mould fungi called dermatophytes
• Oral thrush, yeast infection, diaper rash -- Technical name candidiasis -- caused by Candida albicans
• Histoplasmosis -- Begins in the lungs, spreads through the body -- caused by Histoplasma capsulatum
• Pneumocystis pneumonia -- Lung infection² in people who are HIV-positive, especially children. -- caused by Pneumocystis carinii

Fungi are the major cause of disease in plants, accounting for over 70% of major crop diseases. The Irish Potato Famine was caused by a fungus an oomycete*, Phytophthora infestans. (The distinction between fungus and oomycete is fairly recent.) Although P. infestans did not infect humans, humans were certainly affected by it!

* oomycete correction by cordyceps.

Footnotes

1. The others are bacteria, viruses, protozoans, and helminths (worms).
2. Most pneumonia is bacterial or mycoplasmic, this is the only fungal pneumonia.

Primary references

• Other writeups in this node. This writeup began life as high school biology lecture outline. Some redundancies exist with other writeups in this node, both for completeness and because they were used as a source for this work. Care has been taken to minimize the redundancies.
• The remarkable Tom Volk's Fungi pages http://botit.botany.wisc.edu/toms_fungi/
• http://www.perspective.com/nature/fungi/
• http://www.mercksharpdohme.com/pro/fungal_disease/info/facts/home.html
• http://www.howe.k12.ok.us/~jimaskew/bfungi.htm
• http://www.achrnews.com/CDA/ArticleInformation/RegionalNews_Item/1,4149,75953-West,00.html
• http://helios.bto.ed.ac.uk/bto/microbes/applerot.htm#crest
• Salem: http://botit.botany.wisc.edu/toms_fungi/oct99.html
• Destroying angel: http://www.bluewillowpages.com/mushroomexpert/amanita_bisporigera.html