Molds (or moulds, see spelling differences) include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae. In contrast, microscopic fungi that grow as single cells are called yeasts. A connected network of these tubular branching hyphae has multiple, genetically identical nuclei and is considered a single organism, referred to as a colony or in more technical terms a mycelium.
Molds do not form a specific taxonomic or phylogenetic grouping, but can be found in the divisions Zygomycota, Deuteromycota and Ascomycota. Although some molds cause disease or food spoilage, others are useful for their role in biodegradation or in the production of various foods, beverages, antibiotics and enzymes.
There are hundreds of known species of molds, which include opportunistic pathogens, saprotrophs, aquatic species and thermophiles. Like all fungi, molds derive energy not through photosynthesis but from the organic matter, inside of which they live. Typically, molds secrete hydrolytic enzymes, mainly from the hyphal tips. These enzymes degrade complex biopolymers such as starch, cellulose and lignin into simpler substances which can be absorbed by the hyphae. In this way, molds play a major role in causing decomposition of organic material, enabling the recycling of nutrients throughout ecosystems. Many molds also secrete mycotoxins which, together with hydrolytic enzymes, inhibit the growth of competing microorganisms.
Molds reproduce through small spores, which may contain a single nucleus or be multinucleate. Mold spores can be asexual (the products of mitosis) or sexual (the products of meiosis); many species can produce both types. Some can remain airborne indefinitely, and many are able to survive extremes of temperature and pressure.
Although molds grow on dead organic matter everywhere in nature, their presence is only visible to the unaided eye when mold colonies grow. A mold colony does not comprise discrete organisms, but an interconnected network of hyphae called a mycelium. Nutrients and in some cases organelles may be transported throughout the mycelium. In artificial environments like buildings, humidity and temperature are often stable enough to foster the growth of mold colonies, commonly seen as a downy or furry coating growing on food or other surfaces.
Some molds can begin growing at temperatures as low as 2°C. When conditions do not enable growth, molds may remain alive in a dormant state, within a large range of temperatures before they die. The many different mold species vary enormously in their tolerance to temperature and humidity extremes. Certain molds can survive harsh conditions such as the snow-covered soils of Antarctica, refrigeration, highly acidic solvents, and even petroleum products such as jet fuel.
Xerophilic molds use the humidity in the air as their only water source; other molds need more moisture.
The koji molds are a group of Aspergillus species, notably Aspergillus oryzae, that have been cultured in eastern Asia for many centuries. They are used to ferment a soybean and wheat mixture to make soybean paste and soy sauce. They are also used to break down the starch in rice (saccharification) in the production of sake and other distilled spirits.
Several cholesterol-lowering drugs (such as Lovastatin, from Aspergillus terreus) are derived from molds.
Exposure to bacteria and fungus in indoor air has emerged as a significant health problem in residential environments as well as in occupational settings .
Mold spores can be allergenic, causing irritations of eye, nose, throat, and lungs. In response to this, environmental health research has yielded tests such as the MELISA test, which can determine whether or not a person is allergic to a specific mold.
Molds may excrete liquids or gases, not all of which can be detected by smell. Some molds generate toxic liquid or gaseous compounds, called mycotoxins. Molds that produce mycotoxins are sometimes referred to as toxic molds. It is thought that all molds may produce mycotoxins and thus all molds may be potentially toxic. Mycotoxins are not produced all the time, but only under specific growing conditions. Mycotoxins are harmful or lethal to humans and animals when exposure is high enough. Some of the most deadly chemicals on the planet are mycotoxins. Mycotoxins are then carried on the mold spore, mold fragments, and are found on the substrate of the surface they grow on. They can cause human illness through ingestion, but also through dermal exposure and inhalation of mycotoxin-contaminated debris or spores.
Dermatophytes are the parasitic fungi that cause skin infections such as Athlete's foot and Jock Itch. Most dermataphyte fungi take the form of a mold, as opposed to a yeast, with appearance (when cultured) that is similar to other molds.
Mold growth in buildings can lead to a variety of health issues. Various practices can be followed to mitigate mold issues in buildings, the most important of which is to reduce moisture levels that can facilitate mold growth. Removal of affected materials after the source of moisture has been reduced and/or eliminated may be necessary for remediation.