Meet the Microbes
Microbeworld Radio
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Fungi

Fungi straddle the realms of microbiology and macrobiology.

They range in size from the single-celled organism we know as yeast to the largest known living organism on Earth — a 3.5-mile-wide mushroom.

Dubbed “the humongous fungus,” this honey mushroom (Armillaria ostoyae) covers some 2,200 acres in Oregon’s Malheur National Forest.

Dead Treethe humongous fungus at base of tree
The only above-ground signs of the humongous fungus are patches of dead trees and the mushrooms that form at the base of infected trees. Courtesy of the USDA Forest Service.

The only above-ground signs of the humongous fungus are patches of dead trees and the mushrooms that form at the base of infected trees. Courtesy of the USDA Forest Service.

It started out 2,400 years ago as a single spore invisible to the naked eye, then grew to gargantuan proportions by intertwining threads of cells called hyphae.

Under a microscope, hyphae look like a tangled mass of threads or tiny plant roots. This tangled mass is called the fungal mycelium, and is the part of the famous honey mushroom that spreads for miles underground.

If mushrooms and other fungi can get so huge, why mention them on a site about microorganisms?

Visible fungi such as mushrooms are multicellular entities, but their cells are closely connected in a way unlike that of other multicellular organisms.

Plant and animal cells are entirely separated from one another by cell walls (in plants) and cell membranes (in animals). The dividers between fungal cells, however, often have openings that allow proteins, fluids and even nuclei to flow from one cell to another. A few fungal species have no cell dividers: just a long, continuous cell dotted by multiple nuclei spread throughout.

Illustration showing the multicellular nature of mushrooms
This illustration shows the multicellular nature of mushrooms, and how they spread throughout the forest floor. © Bert Dodson.
Catenaria anguillulae zoospores
The zoospores have no cell wall, are uniflagellated, and may swim for 24 hours on endogenous energy reserves. On contact with a suitable surface (e.g., a nematode cuticle), the zoospore encysts by withdrawing its flagellum and surrounding itself with a thick cell wall and then adhering to the surface.

Courtesy of MicrobeLibrary.org.
A. oligospora shown trapping a nematod
The fungi Arthrobotrys oligospora can capture a nematode when it merely touches the outside of its trap.

Courtesy of MicrobeLibrary.org.
Animation (Low Resolution, 256K QT)

Classification

Fungi are eukaryotic (you-carry-ah-tick) organisms—their DNA is enclosed in a nucleus. Many of them may look plant-like, but fungi do not make their own food from sunlight like plants do.

Yeast
Yeast cell (blue highlights scars where buds formed)
Microbiol. Rev. 54:381-431, 1990

Friendly Fungi

Some fungi are quite useful to us. We've tapped several kinds to make antibiotics to fight bacterial infections. These antibiotics are based on natural compounds the fungi produce to compete against bacteria for nutrients and space. We use Saccharomyces cerevisiae (sack-air-oh-my-seas sair-uh-vis-ee-ay), aka baker's yeast, to make bread rise and to brew beer. Fungi break down dead plants and animals and keep the world tidier. We're exploring ways to use natural fungal enemies of insect pests to get rid of these bugs.

Fungal Enemies

There are some nasty fungi that cause diseases in plants, animals and people. One of the most famous is Phytophthora infestans (fie-tof-thor-uh in-fes-tuhns), which caused the Great Potato Famine in Ireland in the mid-1800s that resulted in a million deaths. See the news story on Phytophthora in the News section. Fungi ruin about a quarter to half of harvested fruits and vegetables annually.

Many more interesting facts about fungi can be found throughout the Microbe website, so keep clicking and reading.

You can also get a lot of details about fungi and see some cool images at The Microbial World website.

Fungal Growth and Reproduction

As the “humongous fungus” shows, fungi can grow to enormous mass if unimpeded.

Hyphae grow by adding cells at the tip. Hyphae are very tiny, measuring only a few microns in diameter in some cases. But they can also be incredibly strong, punching through not only the soft membranes of animal cells, but also the tough, woody walls of plant cells and the hard chitin that makes up insect bodies.

Fungi usually reproduce without sex. Single-celled yeasts reproduce asexually by budding. A single yeast cell can produce up to 24 offspring.

Fungi that make hyphae can reproduce asexually as well. Bits of the hyphae can break off and continue to grow as separate entities, or can form stalks containing seed-like spores.

Although less common, fungi can produce spores sexually. Two mating cells from hyphae of different strains of fungi can mate by fusing together and forming a spore stalk.

fruiting body of Pilobolus
The fruiting body of the fungus Pilobolus. This specimen was found in cow dung.
Courtesy of MicrobeLibrary.org.
When the spore caps at the end of spore stalks fully mature, they burst. The spores may simply drop in the same area, or be carried by the wind or rain to new spots.

Where they land, spores will germinate like seeds. But if they don’t land on a suitable food source or in ideal conditions, the spores can survive in a dormant state for extended periods, waiting for more favorable conditions or to be carried to a better spot.

Fungal hyphae
Fungal hyphae
Courtesy of Alex Hausler, Givaudan Roure

When you hear the word fungus, you probably think of mushrooms. Did you know bread mold is a kind of fungus, too? And that the itchy burning of athlete's foot is, yes, caused by another fungus? And that when you take penicillin, you're taking a medicine made by a fungus?

Fungi come in a variety of shapes and sizes and different types. They can range from individual cells to enormous chains of cells that can stretch for miles.