A mold (US) or mould (UK / NZ / AU / ZA / IN / CA / IE) is a fungus
that grows in the form of multicellular filaments called hyphae.
In contrast, fungi that can adopt a single-celled growth habit are
Molds are a large and taxonomically diverse number of fungal species
in which the growth of hyphae results in discoloration and a fuzzy
appearance, especially on food. The network of these tubular
branching hyphae, called a mycelium, is considered a single organism.
The hyphae are generally transparent, so the mycelium appears like
very fine, fluffy white threads over the surface. Cross-walls (septa)
may delimit connected compartments along the hyphae, each containing
one or multiple, genetically identical nuclei. The dusty texture of
many molds is caused by profuse production of asexual spores (conidia)
formed by differentiation at the ends of hyphae. The mode of formation
and shape of these spores is traditionally used to classify molds.
Many of these spores are colored, making the fungus much more obvious
to the human eye at this stage in its life-cycle.
Molds are considered to be microbes and do not form a specific
taxonomic or phylogenetic grouping, but can be found in the divisions
Zygomycota and Ascomycota. In the past, most molds were classified
within the Deuteromycota.
Molds cause biodegradation of natural materials, which can be unwanted
when it becomes food spoilage or damage to property. They also play
important roles in biotechnology and food science in the production of
various foods, beverages, antibiotics, pharmaceuticals and enzymes.
Some diseases of animals and humans can be caused by certain molds:
disease may result from allergic sensitivity to mold spores, from
growth of pathogenic molds within the body, or from the effects of
ingested or inhaled toxic compounds (mycotoxins) produced by molds.
2 Common molds
3 Food production
4 Pharmaceuticals from molds
5 Health effects
6 Growth in buildings and homes
7 Use in art
8 See also
10 External links
There are thousands of known species of molds, which have diverse
life-styles including saprotrophs, mesophiles, psychrophiles and
thermophiles and a very few opportunistic pathogens of humans. They
all require moisture for growth and some live in aquatic environments.
Like all fungi, molds derive energy not through photosynthesis but
from the organic matter on which they live, utilising heterotrophy.
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 synthesise mycotoxins and
siderophores which, together with lytic enzymes, inhibit the growth of
competing microorganisms. Molds can also grow on stored food for
animals and humans, making the food unpalatable or toxic and are thus
a major source of food losses and illness. Many strategies for food
preservation (salting, pickling, jams, bottling, freezing, drying) are
to prevent or slow mold growth as well as growth of other microbes.
Molds reproduce by producing large numbers of 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 molds produce small,
hydrophobic spores that are adapted for wind dispersal and may remain
airborne for long periods; in some the cell walls are darkly
pigmented, providing resistance to damage by ultraviolet radiation.
Other mold spores have slimy sheaths and are more suited to water
Mold spores are often spherical or ovoid single cells, but
can be multicellular and variously shaped.
Spores may cling to
clothing or fur; some are able to survive extremes of temperature and
Although molds can grow on dead organic matter everywhere in nature,
their presence is visible to the unaided eye only when they form large
colonies. A mold colony does not consist of discrete organisms but is
an interconnected network of hyphae called a mycelium. All growth
occurs at hyphal tips, with cytoplasm and organelles flowing forwards
as the hyphae advance over or through new food sources. Nutrients are
absorbed at the hyphal tip. In artificial environments such as
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.
Few molds can begin growing at temperatures of 4 °C
(39 °F) or below, so food is typically refrigerated at this
temperature. When conditions do not enable growth to take place, molds
may remain alive in a dormant state depending on the species, within a
large range of temperatures. 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,
anti-bacterial soap and even petroleum products such as jet
Xerophilic molds are able to grow in relatively dry, salty, or sugary
environments, where water activity (aw) is less than 0.85; other molds
need more moisture.
Spores from green mold growing on an orange, 1000X wet mount
Common genera of molds include:
The Kōji (麹) molds are a group of
Aspergillus species, notably
Aspergillus oryzae, and secondarily A. sojae, 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. Koji molds
break down the starch in rice, barley, sweet potatoes, etc., a process
called saccharification, in the production of sake, shōchū and other
distilled spirits. Koji molds are also used in the preparation of
Red rice yeast
Red rice yeast is a product of the mold
Monascus purpureus grown on
rice, and is common in Asian diets. The yeast contains several
compounds collectively known as monacolins, which are known to inhibit
cholesterol synthesis. A study has shown that red rice yeast used
as a dietary supplement, combined with fish oil and healthy lifestyle
changes, may help reduce "bad" cholesterol as effectively as certain
commercial statin drugs.
Some sausages, such as salami, incorporate starter cultures of molds
 to improve flavour and reduce bacterial spoilage during curing.
Penicillium nalgiovense, for example, may appear as a powdery white
coating on some varieties of dry-cured sausage.
Other molds that have been used in food production include:
Fusarium venenatum – quorn
Geotrichum candidum – cheese
Neurospora sitophila – oncom
Penicillium spp. – various cheeses including
Brie and Blue cheese
Rhizomucor miehei – microbial rennet for making vegetarian and other
Rhizopus oligosporus – tempeh
Rhizopus oryzae – tempeh, jiuqu for jiuniang or precursor for making
Chinese rice wine
Pharmaceuticals from molds
Alexander Fleming's accidental discovery of the antibiotic penicillin
Penicillium mold called
Penicillium notatum (although the
species identity is disputed as possibly being
Penicillium rubens). Fleming continued to investigate
Penicillin, showing that it could inhibit various types of bacteria
found in infections and other ailments, but he was unable to produce
the compound in large enough amounts necessary for production of a
medicine. His work was expanded by a team at Oxford University;
Clutterbuck, Lovell, and Raistrick, who began to work on the problem
in 1931. This team was also unable to produce the pure compound in any
large amount, and found that the purification process diminished its
effectiveness and negated the anti-bacterial properties it had.
Howard Florey, Ernst Chain, Norman Heatley, Edward Abraham, also all
at Oxford, continued the work. They enhanced and developed the
concentration technique by using organic solutions rather than water,
and created the "Oxford Unit" to measure penicillin concentration
within a solution. They managed to purify the solution, increasing its
concentration by 45–50 times, but found that a higher concentration
was possible. Experiments were conducted and the results published in
1941, though the quantities of
Penicillin produced were not always
high enough for the treatments required. As this was during the
Second World War, Florey sought USA Government involvement. With
research teams in the UK and some in the US, industrial-scale
production of crystallized penicillin was developed during 1941–1944
USDA and by Pfizer.
Several statin cholesterol-lowering drugs (such as lovastatin, from
Aspergillus terreus) are derived from molds.
The immunosuppressant drug cyclosporine, used to suppress the
rejection of transplanted organs, is derived from the mold
Mold health issues
Molds are ubiquitous, and mold spores are a common component of
household and workplace dust; however, when mold spores are present in
large quantities, they can present a health hazard to humans,
potentially causing allergic reactions and respiratory problems.
Some molds also produce mycotoxins that can pose serious health risks
to humans and animals. Some studies claim that exposure to high levels
of mycotoxins can lead to neurological problems and in some cases,
death. Prolonged exposure, e.g. daily home exposure, may be
particularly harmful. Research on the health impacts of mold has not
been conclusive. The term "toxic mold" refers to molds that
produce mycotoxins, such as
Stachybotrys chartarum, and not to all
molds in general.
Mold in the home can usually be found in damp, dark or steamy areas
e.g. bathroom or kitchen, cluttered storage areas, recently flooded
areas, basement areas, plumbing spaces, areas with poor ventilation
and outdoors in humid environments. Symptoms caused by mold allergy
are watery, itchy eyes, a chronic cough, headaches or migraines,
difficulty breathing, rashes, tiredness, sinus problems, nasal
blockage and frequent sneezing.
Molds can also pose a hazard to human and animal health when they are
consumed following the growth of certain mold species in stored food.
Some species produce toxic secondary metabolites, collectively termed
mycotoxins including aflatoxins, ochratoxins, fumonisins,
trichothecenes, citrinin, and patulin. These toxic properties may be
used for the benefit of humans when the toxicity is directed against
other organisms; for example, penicillin adversely affects the growth
of Gram-positive bacteria (e.g.
Clostridium species), certain
spirochetes and certain fungi.
Growth in buildings and homes
Mold growth, assessment, and remediation
Mold growth, assessment, and remediation and Indoor air
Moldy housecorner from outside and inside
Mold growth in buildings can lead to a variety of health problems as
microscopic airborne reproductive spores, analogous to tree pollen,
are inhaled by building occupants. High quantities of indoor airborne
spores as compared to exterior conditions are strongly suggestive of
indoor mold growth. Determination of airborne spore counts is
accomplished by way of an air sample, in which a specialized pump with
a know flow rate is operated for a known period of time. Conducive to
scientific methodology air samples should be drawn from the affected
area, a control area, and the exterior.
The air sampler pump draws in air and deposits microscopic airborne
particles on a culture medium. The medium is cultured in a laboratory
and the fungal genus and species are determined by visual microscopic
observation. Laboratory results also quantify fungal growth by way of
a spore count for comparison among samples. The pump operation time
was recorded and when multiplied by the operation time results in a
specific volume of air obtained. Although a small volume of air is
actually analyzed, common laboratory reporting techniques extrapolate
the spore count data to equate the amount of spores that would be
present in a cubic meter of air.
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. Properly functioning air
conditioning (AC) units are essential to controlling levels of indoor
airborne fungal spores. Air filtration reduces the number of spores
available for germination, especially when a High Efficiency
Particulate Air (HEPA) filter is used. A properly functioning AC unit
also reduces the relative humidity, or the moisture inherent in the
air. The United States Environmental Protection Agency (EPA)
currently recommends that relative humidity be maintained between 30%
to 50% to preempt mold growth. Considering that fungal growth
requires cellulose, plant fiber, as a food source, using building
materials that do not contain cellulose is an effective method of
preventing fungal growth.
Eliminating the moisture source is the first step at fungal
remediation. Removal of affected materials may also be necessary for
remediation, if materials are easily replaceable and not part of the
load-bearing structure. Professional drying of concealed wall cavities
and enclosed spaces such as cabinet toekick spaces may be required.
Post-remediation verification of moisture content and fungal growth is
required for successful remediation. Many contractors perform
post-remediation verification themselves, but property owners may
benefit from independent verification.
Use in art
Del Nero artwork using mold
Various artists have used mold in various artistic fashions. Daniele
Del Nero, for example, constructs scale models of houses and office
buildings and then induces mold to grow on them, giving them an
unsettling, reclaimed-by-nature look. Staci Levy sandblasts enlarged
images of mold onto glass, then allows mold to grow in the crevasses
she has made, creating a macro-micro portrait. Sam Taylor-Johnson
has made a number of time-lapse films capturing the gradual decay of
classically arranged still lifes.
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