John Miller
Mushrooms are susceptible to contamination at all stages of growth. The leading cause of crop failure is contamination, which can come in the form of bacteria, mould, and animal pests. The human body is the greatest source of contamination in mushroom cultivation, as human hands and clothes contain lots of bacteria and spores that can ruin a crop. Other sources of contamination include the air, cultivation tools, and the mushroom cultures or substrates themselves. Incomplete sterilisation or improper pasteurisation of the bulk substrate before inoculating with a mushroom culture can also lead to contamination. Contamination can be identified by discolouration, such as green, blue, grey, or black patches, or by strange odours.
| Characteristics | Values |
|---|---|
| Common contaminants | Trichoderma, Bacillus spp., black bread mold, bacteria, molds, animal pests, fungi, yeasts |
| Appearance of contamination | Discoloration (green, blue, grey, black, or yellow patches), slime or crust, white mycelium, soft decay |
| Odor | Mouldy and musty smells |
| Sources of contamination | Air, cultivation tools, human body (hands, breath, clothes, skin, hair), mushroom cultures or substrates |
| Prevention | Clean environment, careful handling, sterilization of equipment and substrates, pasteurization, use of laminar flow hood or still air box |
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What You'll Learn
- Human bodies, clothes, skin, and hair can carry contaminants
- Incomplete sterilisation or improper pasteurisation of the bulk substrate
- Contaminants spread through the air, such as mould spores
- Bacterial endospores can be heat-resistant and survive the pressure-cooking process
- Trichoderma is a common contaminant that produces an aggressive white mycelium
Human bodies, clothes, skin, and hair can carry contaminants
Mushroom cultivators should be aware of the potential for bacterial contamination, particularly Bacillus spp., also known as "wet spot" or "sour rot." Bacterial endospores can be heat resistant and survive the pressure cooking process, so it is important to soak the grains for 12 to 24 hours before hydrating or sterilizing them. This allows the endospores to germinate and then be killed during sterilization. Without this soaking step, the endospores can lay dormant and survive the sterilization process, ruining the crop.
In addition to bacteria, fungi reproduce through spores, which are microscopic cells released from mushrooms and dispersed through the air. These spores can remain viable for long periods, even indefinitely, and can contaminate mushrooms. The best way to reduce undesired spores and other airborne contamination is through the use of a laminar flow hood.
Human hair can also carry contaminants. Psilocin, a compound derived from the Psilocybe species of mushrooms, has been identified in human hair and is a key indicator of magic mushroom abuse. However, interpretation of hair tests for psilocin is challenging due to its low concentration and sensitivity to water, light, air, and temperature. Skin contact with certain species of mushrooms, such as Suillus americanus and S. luteus, can cause allergic contact dermatitis, resulting in symptoms like reddening, swelling, and itching. These reactions can be avoided by wearing gloves and washing hands after handling mushrooms.
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Incomplete sterilisation or improper pasteurisation of the bulk substrate
Mushroom contamination can occur due to various factors, and incomplete sterilisation or improper pasteurisation of the bulk substrate is a significant concern. Here's a detailed guide to help you understand and address this issue:
Understanding Sterilisation and Pasteurisation:
Firstly, it's important to understand the difference between sterilisation and pasteurisation. Sterilisation aims to kill all bacteria and pathogens present in the substrate, creating a completely sterile environment for the mushrooms to grow. On the other hand, pasteurisation doesn't aim for total bacteria elimination but rather reduces the number of competing organisms within the substrate. Pasteurisation creates an environment that is toxic to many microorganisms, giving the cultivated mushrooms an advantage in the race to colonise the substrate.
Incomplete Sterilisation:
Incomplete sterilisation can occur due to various reasons, such as inadequate equipment or improper techniques. High-nutrient substrates, which are commonly used for mushroom cultivation, require effective sterilisation to prevent contamination. This can be achieved through pressure cooking, ensuring that the substrate is submerged in water and heated according to the manufacturer's instructions. A pressure of 15 PSI is generally recommended to eliminate potential contaminants.
Improper Pasteurisation:
Improper pasteurisation can occur when the pasteurisation process fails to reduce the number of competing organisms in the substrate effectively. This may be due to incorrect techniques or the use of inappropriate substrates for pasteurisation. For example, straw, wood chips, or sawdust are less nutritious substrates that may require pasteurisation instead of sterilisation. However, if the pasteurisation process is not carefully executed, it may not effectively reduce the number of harmful microorganisms.
Signs of Contamination:
To identify if your mushrooms have been contaminated due to incomplete sterilisation or improper pasteurisation, look out for signs of discoloration or decay. Common colours indicating contamination include green, blue, grey, or black patches on or within the fruiting box or substrate. Additionally, the presence of slime may suggest bacterial contamination.
Preventing and Managing Contamination:
To prevent contamination, it is crucial to choose the appropriate sterilisation or pasteurisation method for your chosen substrate. Additionally, maintaining a clean environment and practising good hygiene during the process are essential. If contamination occurs, it is vital to act quickly to prevent it from spreading. Contaminated cultures should be separated from non-contaminated ones, and sanitisation of the affected area is recommended.
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Contaminants spread through the air, such as mould spores
Mushrooms are extremely vulnerable to contamination, and it is nearly impossible to create a 100% sterile environment. The air is a very likely source of contamination for mushroom growers. Mould spores and other contaminants, though invisible to the naked eye, are everywhere, just waiting to ruin your crop.
Mould spores are microscopic and can float in the air, finding their way into the mushroom cultivation area. They can remain viable for long periods of time, and in some cases, indefinitely. Once they land on the substrate, they can quickly colonize and spread, causing significant damage to the crop. The substrate is the nutrified material that the mushroom grows from, and mould spores can contaminate it during inoculation.
To reduce airborne mould spores and other contaminants in your workspace, you can use a laminar flow hood. This is a workbench that includes a filter pad, fan, and a HEPA (High-Efficiency Particulate Air) filter built into it. This allows for a clean stream of air and can effectively eliminate airborne contaminants if used correctly. Another way to prevent mould spores from contaminating your crop is to inoculate your substrate in a still air or glove box in front of the laminar flow hood.
To limit the chances of encountering cobweb mould, make sure your grains/substrate aren't overhydrated, as cobweb mould prefers high humidity and stagnant air. By lowering the humidity and providing more air circulation, you can help limit the potential for and spread of cobweb mould contamination. Hydrogen peroxide (H2O2) spray is another weapon in the battle against mould because it will not harm your mycelium but will kill any mould spores.
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Bacterial endospores can be heat-resistant and survive the pressure-cooking process
Mushroom cultivators often soak their grains before pressure-cooking to prevent bacterial contamination. Bacterial endospores can be heat-resistant and survive the pressure-cooking process. Soaking the grains causes any bacterial endospores on the grains to awaken and germinate. Once the grains reach sterilisation temperatures during pressure cooking, the activated endospores are killed. Without the soak, the endospores could remain dormant and survive the sterilisation process, only to germinate after the grains have been inoculated, ruining the crop.
Bacterial endospores are highly resistant, metabolically inactive cell types produced by bacteria when environmental conditions become unfavourable, especially in response to nutrient deprivation. The endospore is released from the degenerating vegetative cell and becomes an independent cell called a spore. The endospore will remain dormant until it senses the return of more favourable conditions. The formation of endospores allows the bacterium to produce a dormant and highly resistant cell to preserve the cell's genetic material in times of extreme stress. Endospores can survive environmental assaults that would normally kill the bacterium, including high temperatures, high UV irradiation, desiccation, chemical damage, and enzymatic destruction.
The extraordinary resistance properties of endospores mean they are not readily killed by many antimicrobial treatments. They are resistant to antibiotics, most disinfectants, and physical agents such as radiation, boiling, and drying. The impermeability of the spore coat is thought to be responsible for the endospore's resistance to chemicals. The heat resistance of endospores is due to a variety of factors. Calcium dipicolinate, abundant within the endospore, may stabilise and protect the endospore's DNA. Small acid-soluble proteins (SASPs) saturate the endospore's DNA and protect it from heat, drying, chemicals, and radiation. The cortex may osmotically remove water from the interior of the endospore, and the resulting dehydration is thought to be very important in the endospore's resistance to heat and radiation. Finally, DNA repair enzymes contained within the endospore are able to repair damaged DNA during germination.
The presence of metal ions (Ca2+, Mg2+, or Mn2+) in the sporulation medium is the main extrinsic factor contributing to an increase in the heat resistance of spores. If the presence of these ions decreases, the thermal resistance of the spores also decreases. A diminution of certain fatty acids during sporulation can increase the heat resistance of spores, and a lack of glucose in the sporulation medium can reduce their heat resistance. The presence or absence of some amino acids makes the heat resistance of spores vary. Other factors that can influence the heat resistance of spores during the heating process are NaCl concentration, water activity (aw), the presence of sugar or some germinant such as lysozyme that can help to increase the apparent heat resistance, and sublethal heating of spores prior to the thermal process.
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Trichoderma is a common contaminant that produces an aggressive white mycelium
Mushroom cultivators face the constant risk of contamination, which can spread quickly and cause significant economic losses. The human body is the greatest source of contamination in mushroom cultivation, as human hands and clothes carry bacteria and spores that can ruin crops. Discolouration is a tell-tale sign of contamination, with invasive moulds often displaying distinct and bright colours like green, blue, grey, or black.
One of the most common contaminants is Trichoderma, a genus of microscopic fungi known for its rapid growth and ability to compete with other fungi for nutrients and space. Trichoderma is particularly problematic in myciculture, where it is perceived as an aggressive contaminant that can cause total destruction of the substrate. It is known to produce an aggressive white mycelium, which causes soft decay in mushrooms before sporulating into a vibrant green. This white mycelium is characteristic of the early stages of Trichoderma growth and can be difficult to distinguish from the mycelium of cultivated mushrooms. However, once it sporulates, Trichoderma is nearly impossible to misidentify due to its distinctive green colour.
The rapid growth and reproduction of Trichoderma allow it to outcompete other fungi for nutrients and space. It can quickly absorb to the roots of crops, forming a protective layer and inhibiting the growth of pathogenic fungi. This characteristic makes it valuable in agriculture, where it is used to control soil-borne diseases, improve root health, and enhance plant resistance. However, in mushroom cultivation, Trichoderma can lead to significant crop losses and malformed mushrooms.
Identifying and eradicating Trichoderma early is crucial to saving mushroom crops. Once it takes hold, Trichoderma is challenging to contain and can quickly spread to neighbouring dishes, jars, bags, and tubs. While some sources suggest using salt to combat Trichoderma, others recommend cutting losses and starting over with a thorough sanitization process to prevent future contamination.
To prevent Trichoderma contamination, mushroom growers must maintain a clean environment and handle their crops carefully. Understanding the typical appearance of their mycelium can help growers identify unhealthy or contaminated patches. Additionally, practices such as showering, using hand sanitiser, and wearing clean clothes and face masks can reduce the risk of contamination from human sources.
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Frequently asked questions
Discolouration is one of the most obvious signs of contamination. If you spot green, blue, grey, or black patches on or in your fruiting box, your culture is most likely contaminated. However, blue stains may be bruising and not mould. Other signs include slime, crust, or an unusual odour.
The leading cause of mushroom contamination is the presence of bacteria, mould, and animal pests. The human body is also a common source of contamination, as human hands and clothes contain lots of bacteria and spores that can ruin a crop.
To prevent mushroom contamination, it is important to maintain a sterile environment and use sterile processes. This includes sterilising all tools, such as scalpels, blades, and syringes, before use. It is also recommended to shower and wear clean clothes before handling mushrooms.
