Multicellular Microscopic Fungi: Molds

Molds are a type of microscopic multicellular fungi. They have essentially the same growth requirements as bacteria, but they ordinarily grow much more slowly. The molds are also distinguished by their capacity to grow on materials that would seem to offer only small supplies of nutrients, for example, linen, cotton cloth, and tanned leather. Over 200 different kinds of molds have been described from mildewed fabrics; they grow in patches that often are brightly colored. Molds like mushrooms reproduce by spores. Airborne spores fall upon organic materials and begin growing, especially in warm and humid environments.

A common bread mold, Neurospora, forms red spores which give the characteristic color to moldy bread, though its relative, Aspergillus, is often present, too. The gray bread mold is Mucor. Bread molds viewed under a stereo microscope appear similar to miniature mushrooms: small balls on on tiny stalks. They have thread-like structure that cover the surface of the food and extend below the surface. We ingest penicillin, a chemical defense against bacteria first made by the green bread and fruit mold Penicillium long ago. The drug penicillin prevents infectious bacteria from making cell walls. Thus this fungus, by saving itself, has also saved millions of human lives. Although not always poisonous, some molds make food taste bad. On the other hand, humans eat some types of molds as food.

Molds growing in a petri dish
Molds growing from spores collected from the surface of a pillow case.
Photo by Larysa Johnston

Of the 100,000 known fungi only a small percentage is associated with the indoor environment. Indoor molds comprise a group of fungi that are pathogens of humans and cause a variety of allergies and respiratory illnesses. These molds produce abundant amounts of microscopic spores called conidia. All indoor environments have some mold spores, with the possible exception of strictest clean rooms. The molds get indoors by hitching a ride on shoes, pets, or wind currents through open windows and doors. Homes that are consistently cleaned, dusted, and vacuumed have a lot less molds compared to those homes that are cleaned infrequently.

Some of mold species may remain in the food even after extensive procedures. With regards to heat resistant molds, the species Neosartorya fischeri, Byssochlamys fulva , and also the genus Eupenicillium species, Talaromyces species and Eurotium species are commonly isolated. Some of the heat resistant molds can cause both spoilage of fruit products and produce toxic and sometimes carcinogenic compounds.1

Dishwashers: Friends or Foes?

Molds are found growing in surprising situations. A recent study identified 503 fungal strains inside 30 residential dishwashers. Irrespective of the sampled site, 83% of the dishwashers were positive for fungi. The most frequent pathogenic species were Exophiala dermatitidis, Candida parapsilosis sensu stricto, Exophiala phaeomuriformis, Fusarium dimerum, and the Saprochaete/Magnusiomyces clade. The black yeast Exophiala dermatitidis was detected in 47% of the dishwashers, primarily at the dishwasher rubber seals. Plumbing systems supplying water to household appliances represent the most probable route for contamination of dishwashers, as the fungi that represented the core dishwasher mycobiota were also detected in the tap water. The most contaminated sites were the kitchen drain and the dish drying rack. We conclude that dishwashers represent a reservoir of enriched pathogenic species that can spread from the dishwasher into the indoor.

Can Molds Think?

While less recognized than their animal counterparts, many non-neuronal organisms, such as plants, bacteria, fungi and protists, also have the ability to make complex decisions in difficult environments. The most incredible feats of problem-solving among non-neuronal organisms have been demonstrated by the unicellular slime mold Physarum polycephalum. This unicellular protist lacks a central nervous system and possesses no neurons, yet it has been demonstrated to solve convoluted labyrinth mazes to find shortest length networks and solve challenging optimization problems, to avoid revisiting areas it has already explored and even construct transport networks that have similar efficiency to those designed by human engineers! Without a brain or even neurons, what physical or biochemical mechanisms could be responsible for slime mold decision-making? The cell is composed of many small units. When one of these units senses attractants such as food, it oscillates faster, stimulating neighbouring units to do the same, and causing cytoplasm to flow towards the attractant. The reverse process is initiated when repellents such as light are perceived.3


  1. Survey of molds, yeast and Alicyclobacillus spp. from a concentrated apple juice productive process. Beatriz de Cássia Martins Salomão, Chalana Muller, Hudson Couto do Amparo, Gláucia Maria Falcão de Aragão. 2014
  2. The Black Yeast Exophiala dermatitidis and Other Selected Opportunistic Human Fungal Pathogens Spread from Dishwashers to Kitchens. Jerneja Zupančič, Monika Novak Babič, Polona Zalar, Nina Gunde-Cimerman. 2016
  3. Decision-making without a brain: how an amoeboid organism solves the two-armed bandit Chris R. Reid1, Hannelore MacDonald, Richard P. Mann, , James A. R. Marshall, Tanya Latty and Simon Garnier

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