9.1: Introduction

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Fungi are eukaryotic organisms and include the yeasts, molds, and fleshy fungi. Yeasts are microscopic, unicellular fungi; molds are multinucleated, filamentous fungi (such as mildews, rusts, and common household molds); the fleshy fungi include mushrooms and puffballs.

All fungi are chemoheterotrophs, requiring organic compounds for both an energy and carbon source, which obtain nutrients by absorbing them from their environment. Most live off of decaying organic material and are termed saprophytes. Some are parasitic, getting their nutrients from living plants or animals.

The study of fungi is termed mycology and the diseases caused by fungi are called mycotic infections or mycoses.

In general, fungi are beneficial to humans. They are involved in the decay of dead plants and animals (resulting in the recycling of nutrients in nature), the manufacturing of various industrial and food products, the production of many common antibiotics, and may be eaten themselves for food. Some fungi, however, damage wood and fabrics, spoil foods, and cause a variety of plant and animal diseases, including human infections.

YEASTS

Yeasts are unicellular, oval or spherical fungi which increase in number asexually by a process termed budding (see Fig. \(\PageIndex{1}\)). A bud forms on the outer surface of a parent cell, the nucleus divides with one nucleus entering the forming bud, and cell wall material is laid down between the parent cell and the bud. Usually the bud breaks away to become a new daughter cell but sometimes, as in the case of the yeast Candida, the buds remain attached forming fragile branching filaments called hyphae (see Fig. \(\PageIndex{1}\)). Because of their unicellular and microscopic nature, yeast colonies appear similar to bacterial colonies on solid media. It should be noted that certain dimorphic fungi (see Lab 10) are able to grow as a yeast or as a mold, depending on growth conditions.

Fig \(\PageIndex{1}\): Uninoculated Plate of MacConkey Agar	Fig. \(\PageIndex{2}\): Escherichia coli Growing on MacConkey Agar	Fig \(\PageIndex{3}\): Klebsiella aerogenes (formerly known as Enterobacter aerogenes) Growing on MacConkey Agar	Fig \(\PageIndex{4}\): Proteus mirabilis Growing on MacConkey Agar

Note budding yeast (arrows)	Dimorphic Candida albicans switching from a yeast form to a filamentous hyphal form.	Note forming buds and bud scars
Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0	Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0	By Mogana Das Murtey and Patchamuthu Ramasamy - [1], CC BY-SA 3.0, https://commons.wikimedia.org/w/inde...curid=52254246	Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0

Developmental Biology Film Series episode 57. The reproduction of yeast cells by budding.

Yeasts are facultative anaerobes and can therefore obtain energy by both aerobic respiration and anaerobic fermentation. The vast majority of yeasts are nonpathogenic and some are of great value in industrial fermentations. For example, Saccharomyces species are used for both baking and brewing.

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)