A mold 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 called yeasts.
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 microbesand do not form a specific taxonomicor phylogeneticgrouping, but can be found in the divisions Zygomycotaand Ascomycota. In the past, most molds were classified within the Deuteromycota.
Molds cause biodegradationof natural materials, which can be unwanted when it becomes food spoilageor 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.
There are thousands of known species of molds, which have diverse life-styles including saprotrophs, mesophiles, psychrophilesand thermophilesand a very few opportunistic pathogensof humans.They all require moisture for growth and some live in aquatic environments. Like all fungi, molds derive energy not through photosynthesisbut from the organicmatter on which they live, utilising heterotrophy. Typically, molds secrete hydrolytic enzymes, mainly from the hyphal tips. These enzymes degrade complex biopolymerssuch as starch, celluloseand lignininto simpler substances which can be absorbed by the hyphae. In this way molds play a major role in causing decompositionof organic material, enabling the recycling of nutrients throughout ecosystems. Many molds also synthesise mycotoxinsand siderophoreswhich, together with lyticenzymes, 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 nucleusor 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, hydrophobicspores 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 dispersal. 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 pressure.
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 refrigeratedat 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 fuel.:22
Xerophilicmolds 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.
Common genera of molds include:
The Kōji (麹) molds are a group of Aspergillusspecies, 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 pasteand soy sauce. Kojimolds break down the starchin rice, barley, sweet potatoes, etc., a process called saccharification, in the production of sake, shōchūand other distilled spirits. Kojimolds are also used in the preparation of Katsuobushi.
Red rice yeastis a product of the mold Monascus purpureusgrown 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” cholesterolas effectively as certain commercial statindrugs.
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
Penicilliumspp. – various cheeses including Brieand Blue cheese
Rhizomucor miehei– microbial rennet for making vegetarian and other cheeses
Rhizopus oligosporus– tempeh
Rhizopus oryzae– tempeh, jiuqufor jiuniangor precursor for making Chinese rice wine
Alexander Fleming’s accidental discovery of the antibiotic penicillin involved a Penicillium mold called Penicillium notatum(although the species identity is disputed as possibly being Penicillium chrysogenumor 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 by the USDAand by Pfizer.
Several statincholesterol-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 Tolypocladium inflatum.
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. bathrooms, kitchens, 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 mycotoxinsincluding 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, penicillinadversely affects the growth of Gram-positive bacteria (e.g. Clostridiumspecies), certain spirochetesand certain fungi.
Mold growth, assessment, and remediation and Indoor air quality
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 known 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.
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- “Study: Red Rice Yeast Helps Cut Bad Cholesterol”. National Public Radio. 2008-07-01. Retrieved 2010-02-01.
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Lovastatin (also known as mevinolin) is produced by Aspergillus terreus
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Mold(American English) or mould(British English), also sometimes referred to as mildew, is a fungal growth that develops on wet materials. Mold is a natural part of the environment and plays an important part in nature by breaking down dead organic matter such as fallen leaves and dead trees; indoors, mold growth should be avoided. Mold reproduce by means of tiny spores. The spores are like seeds, but invisible to the naked eye, that float through the air and deposit on surfaces. When the temperature, moisture, and available nutrient conditions are correct, the spores can form into new mold colonies where they are deposited.There are many types of mold, but all require moisture and a food source for growth.
Mold are ubiquitous, and mold spores are a common component of household and workplace dust. In large amounts they can be a health hazard to humans, potentially causing allergic reactions and respiratory problems.
Some mold produce mycotoxins, chemical components of their cells walls, that can pose serious health risks to humans and animals. “Toxic mold” refers to mold which produce mycotoxins, such as Stachybotrys chartarum.Exposure to high levels of mycotoxins can lead to neurological problems and death. Prolonged exposure (for example, daily exposure) can be particularly harmful. Mycotoxins can persist in the indoor environment even after death of the fungi. They can adhere to dust particles and can can spread through the air attached to these dust particles or spores.There must be very specific temperature and humidity conditions in order for fungi to produce mycotoxins.
Symptoms of mold exposure may include nasal and sinus congestion; runny nose, eye irritation; itchy, red, watery eyes, respiratory problems, such as wheezing and difficulty breathing, chest tightness, cough, throat irritation, skin irritation (such as a rash), headache, and persistent sneezing.Immune-compromised people and people with chronic lung illnesses, such as obstructive lung disease, may get serious infections in their lungs when they are exposed to mold. These people should stay away from areas that are likely to have mold, such as compost piles, cut grass, and wooded areas.
Infants may develop respiratory symptoms as a result of exposure to Penicillium, a fungal genus. Signs of mold-related respiratory problems in an infant include a persistent cough or wheeze. Increased exposure increases the probability of developing respiratory symptoms during the first year of life. Studies have indicated a correlation between the probability of developing asthma and exposure to Penicillium.
Mold exposure has a variety of health effects, and sensitivity to mold varies. Exposure to mold may cause throat irritation, nasal stuffiness, eye irritation, cough and wheezing and skin irritation in some cases. Exposure to mold may heighten sensitivity, depending on the time and nature of exposure. People with chronic lung diseases are at higher risk for mold allergies, and will experience more severe reactions when exposed to mold. Damp indoor environments correlate with upper-respiratory-tract symptoms, such as coughing and wheezing in people with asthma.
Mold is found everywhere and can grow on almost any substance when moisture is present. They reproduce by spores, which are carried by air currents. When spores land on a moist surface suitable for life, they begin to grow. Mold is normally found indoors at levels which do not affect most healthy individuals.
Because common building materialsare capable of sustaining mold growth and mold spores are ubiquitous, mold growth in an indoor environment is typically related to water or moisture and may be caused by incomplete drying of flooring materials (such as concrete). Flooding, leaky roofs, building-maintenance or indoor-plumbingproblems can lead to interior mold growth. Water vapor commonly condenses on surfaces cooler than the moisture-laden air, enabling mold to flourish.This moisture vapor passes through walls and ceilings, typically condensing during the winter in climates with a long heating season. Floors over crawl spaces and basements, without vapor barriersor with dirt floors, are mold-prone. The “doormat test” detects moisture from concrete slabs without a sub-slab vapor barrier.Some materials, such as polished concrete, do not support mold growth.
Significant mold growth requires moisture and food sources and a substrate capable of sustaining growth. Common building materials, such as plywood, drywall, furringstrips, carpets, and carpet padding provide food for mold. In carpet, invisible dust and celluloseare food sources. After water damage to a building, mold grows in walls and then becomes dormantuntil subsequent high humidity; suitable conditions reactivate mold. Mycotoxin levels are higher in buildings which have had a water incident.
Mold is detectable by smell and signs of water damage on walls or ceiling and can grow in places invisible to the human eye. It may be found behind wallpaper or paneling, on the inside of ceiling tiles, the back of drywall, or the underside of carpets or carpet padding. Piping in walls may also be a source of mold, since they may leak (causing moisture and condensation).
Spores need three things to grow into mold: nutrients – cellulose (the cell wall of green plants) is a common food for indoor spores; moisture – To begin the decaying process caused by mold; time -mold growth begins from 24 hours to 10 days after the provision of growing conditions.
Mold colonies can grow inside buildings, and the chief hazard is the inhalation of mycotoxins. After a flood or major leak, mycotoxin levels are higher even after a building has dried out.
Food sources for mold in buildings include cellulose-based materials such as wood, cardboardand the paper facing on drywall and organic mattersuch as soap, fabrics and dust-containing skin cells. If a house has mold, the moisture may originate in the basement or crawl space, a leaking roof or a leak in plumbing pipes. Insufficient ventilation may accelerate moisture buildup. Visible mold colonies may form where ventilation is poorest and on perimeter walls (because they are nearest the dew point).
If there are mold problems in a house only during certain times of the year, the house is probably too airtightor too drafty. Mold problems occur in airtight homes more frequently in the warmer months (when humidity is high inside the house, and moisture is trapped), and occur in drafty homes more frequently in the colder months (when warm air escapes from the living area and condenses). If a house is artificially humidifiedduring the winter, this can create conditions favorable to mold. Moving air may prevent mold from growing, since it has the same desiccatingeffect as low humidity. Mold grow best in warm temperatures, 77 to 86 °F (25 to 30 °C), although growth may occur between 32 and 95 °F (0 and 35 °C).
Removing one of the three requirements for mold reduces (or eliminates) new mold growth: moisture; food for the mold spores (for example, dust or dander); and warmth since mold generally does not grow in cold environments.
HVACsystems can produce all three requirements for mold growth. The air conditioningsystem creates a difference in temperature, encouraging condensation. The high rate of dusty air movement through an HVAC system may furnish ample food for mold. Since the air-conditioning system is not always running, warm conditions are the final component for mold growth.
An observation of the indoor environment should be conducted before any sampling is performed. The area should be surveyed for odors indicating mold or bacterial growth, moisture sources, such as stagnant water or leaking pipes, and water-damaged building materials.This can include moving furniture, lifting (or removing) carpets, checking behind wallpaper or paneling, checking ventilation ductwork and exposing wall cavities. Efforts typically focus on areas where there are signs of liquid moisture or water vapor (humidity), or where moisture problems are suspected. Often, quick decisions about the immediate safety and health of the environment can be made by these observations before sampling is even needed.The United States Environmental Protection Agency(EPA) does not generally recommend sampling unless an occupant of the space has symptoms. In most cases, if visible mold growth is present, sampling is unnecessary.Sampling should be performed by a trained professional with specific experience in mold-sampling protocols, sampling methods and the interpretation of findings. It should be done only to make a particular determination, such as airborne spore concentration or identifying a particular species.
Before sampling, a subsequent course of action should be determined.
In the U.S., sampling and analysis should follow the recommendations of the Occupational Safety and Health Administration(OSHA), National Institute for Occupational Safety and Health(NIOSH), the EPA and the American Industrial Hygiene Association (AIHA). Types of samples include air, surface, bulk, dust, and swab.Multiple types of sampling are recommended by the AIHA, since each has limitations.
Air is the most common form of sampling to assess mold levels. It is considered to be the most representative method for assessing respiratory exposure to mold.Indoor and outdoor air are sampled, and their mold spore concentrations are compared. Indoor mold concentrations should be less than or equal to outdoor concentrations with similar distributions of species.A predominant difference in species or higher indoor concentrations can indicate poor indoor air quality and a possible health hazard.Air sampling can be used to identify hidden mold and is often used to assess the effectiveness of control measures after remediation.An indoor mold air sampling campaign should be performed over the course of at least several days as the environmental conditions can lead to variations in the day-to-day mold concentration.Stationary samplers assess a specific environment, such as a room or building, whereas personal samplers assess the mold exposure one person receives in all of the environments they enter over the course of sampling.Personal samplers can be attached to workers to assess their respiratory exposures to molds on the job.Personal samplers usually show higher levels of exposure than stationary samples due to the “personal cloud” effect, where the activities of the person re-suspend settled particles.There are several methods that can be used for indoor mold air sampling.
For more detailed information on air sampling methods for indoor mold, see Bioaerosol.
Surface sampling measures the number of mold spores deposited on indoor surfaces. With swab, a cotton swab is rubbed across the area being sampled, often a measured area, and subsequently sent to the mold testing laboratory. The swab can rubbed on an agar plate to grow the mold on a culture medium. Final results indicate mold levels and species located in the suspect area. Surface sampling can by used to identify the source of mold exposure. Molecular analyses, such as qPCR, may also be used for species identification and quantification. Swab and surface sampling can give detailed information about the mold, but cannot measure the actual mold exposure because it is not aerosolized.
Bulk removal of material from the contaminated area is used to identify and quantify the mold in the sample. This method is often used to verify contamination and identify the source of contamination.Dust samples can be collected using a vacuum with a collection filter attached. Dust from surfaces such as floors, beds, or furniture is often collected to assess health effects from exposure in epidemiology studies.Researchers of indoor mold also use a long-term settled dust collection method where a dust cloth or petri dish is left out in the environment for a set period of time, sometimes weeks.Dust samples can be analyzed using culture-based or culture-independent methods. Quantitative PCRis a DNA-based molecular method that can identify and quantify fungal species. The Environmental Relative Moldiness Index (ERMI) is a numerical that can be used in epidemiological studies to assess mold burdens of houses in the United States. The ERMI consists of a list of 36 fungal species commonly associated with damp houses that can be measured using qPCR.Like swab and surface sampling, bulk and dust sampling can give detailed information about the mold source, but cannot accurately determine the level of exposure to the source.
The first step in solving an indoor mold problem is to remove the moisture source;new mold will begin to grow on moist, porous surfaces within 24 to 48 hours. There are a number of ways to prevent mold growth. Some cleaning companies specialize in fabric restoration, removing mold (and mold spores) from clothing to eliminate odor and prevent further damage to garments.
The effective way to clean mold is to use detergent solutions which physically remove mold. Many commercially available detergents marketed for mold cleanup include an EPA-approved antifungal agent.
Significant mold growth may require professional mold remediation to remove the affected building materials and eradicate the source of excess moisture. In extreme cases of mold growth in buildings, it may be more cost-effective to condemn the building than to reduce mold to safe levels.
The goals of remediation are to remove (or clean) contaminated materials, preventing fungi (and fungi-contaminated dust) from entering an occupied (or non-contaminated) area while protecting workers performing the abatement.
The purpose of cleanup is to eliminate mold and remove contaminated materials. Killing mold with a biocideis insufficient, since chemicals and proteinscausing reactions in humans to remain in dead mold. The following methods are used.
- Evaluation: Before remediation, the area is assessed to ensure safety, clean up the entire moldy area, and properly approach the mold. The EPA provides the following instructions:
- HVAC cleaning: Should be done by a trained professional.
- Protective clothing: Includes a half- or full-face respirator mask. Goggles with a half-face respirator mask prevent mold spores from reaching the mucous membranesof the eyes. Disposable hazmat coveralls are available to keep out particles down to one micrometer, and protective suits keep mold spores from entering skin cuts. Gloves are made of rubber, nitrile, polyurethane, or neoprene.
- Dry brushing or agitation device: Wire brushing or sanding is used when microbial growth can be seen on solid wood surfaces such as framing or underlayment (the subfloor).
- Dry-ice blasting: Removes mold from wood and cement; however, this process may spray mold and its byproducts into surrounding air.
- Wet vacuum: Wet vacuuming is used on wet materials, and this method is one of those approved by the EPA.
- Damp wipe: Removal of mold from non-poroussurfaces by wiping or scrubbing with water and a detergentand drying quickly.
- HEPA(high-efficiency particulate air) vacuum: Used in remediation areas after materials have been dried and contaminated materials removed; collected debris and dust is stored to prevent debris release.
- Debris disposal: Sealed in the remediation area, debris is usually discarded with ordinary construction waste.
Equipment used in mold remediation includes: moisture meter: measures drying of damaged materials; Humidity gauge: often paired with a thermometer; borescope: Camera at the end of a flexible snake, illuminating potential mold problems inside walls, ceilings and crawl spaces; digital camera: Documents findings during assessment; personal protective equipment(PPE): Respirators, gloves, impervious suit, and eye protection; thermographic camera: Infrared thermal-imaging cameras identify secondary moisture sources.
During mold remediation in the U.S., the level of contamination dictates the protection level for remediation workers.Contamination levels have been enumerated as I, II, III, and IV:
- Level I: Small, isolated areas (10 square feet (0.93 m2) or less); remediation may be conducted by trained building staff;
- Level II: Mid-sized, isolated areas (10–30 square feet (0.93–2.79 m2)); may also be remediated by trained, protected building staff;
- Level III: Large, isolated areas (30–100 square feet (2.8–9.3 m2)): Professionals experienced in microbial investigations or mold remediation should be consulted, and personnel should be trained in the handling of hazardous materialsand equipped with respiratory protection, gloves and eye protection;
- Level IV: Extensive contamination (more than 100 square feet (9.3 m2)); requires trained, equipped professionals
After remediation, the premises should be reevaluated to ensure success.
According to the EPA, residential mold may be prevented and controlled by cleaning and repairing roof gutters, to prevent moisture seepage into the home; keeping air-conditioning drip pans clean and drainage lines clear; monitoring indoor humidity; drying areas of moisture or condensation and removing their sources; treating exposed structural wood or wood framing with an EPA-approved fungicidal encapsulation coating after pre-cleaning (particularly homes with a crawl space, unfinished basement or a poorly-ventilated; attic).
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