Mushroom Mystery: Bacteria's Role In Growth

Mushrooms are part of the fungal kingdom, which includes microorganisms such as yeasts and moulds. Fungi are distinct from plants, bacteria, and some protists due to the presence of chitin in their cell walls. While mushrooms have long been consumed as food, they have also been linked to bacterial food poisoning. Salmonella, Listeria, and Staphylococcus are common bacteria found in contaminated mushrooms, causing food poisoning outbreaks across the world. The presence of these bacteria can be attributed to mishandling, improper sterilisation of growing fertilizer, and inadequate processing temperatures or pH levels. Additionally, the harvesting and post-harvesting processes, as well as packaging and storage, provide opportunities for bacterial growth. While commercial farming of mushrooms ensures a degree of safety, foraging for wild mushrooms can be risky due to the possibility of misidentifying poisonous species. Understanding the complex relationship between mushrooms and bacteria is crucial to ensure food safety and harness the potential benefits of their interaction.

Bacteria and fungi have a symbiotic relationship, competing for nutrients and acting as pathogens

Bacteria and fungi are minuscule microbes that exist in the air, food, water, and shelter that humans share. Most of these microbes are harmless, but some are pathogens that can cause diseases. Fungi are more complex organisms than bacteria as they are eukaryotes, meaning they have cells. Fungi have two main types: environmental, which are yeast and mold that often live in the soil and do not generally cause infections, and commensals, which live on and inside the human body and usually do not cause harm. Commensal fungi may even play a beneficial role in human health.

Mushroom-forming fungi establish mutual beneficial interactions with plants and degrade organic waste. They also play an important role in human health. Fungi can cause yeast infections, valley fever, meningitis, and pneumonia. Certain environmental fungi reproduce spores that can enter the human body through the lungs or skin. These fungi can be especially harmful to people with weakened immune systems, as they can spread quickly and damage many organs. Other fungal infections can be caused by an overgrowth of commensal fungus.

Bacteria, on the other hand, can cause food poisoning, strep throat, urinary tract infections, and tuberculosis. Both bacteria and fungi can cause different forms of pneumonia, with symptoms varying depending on the type of microbe causing the infection.

The relationship between bacteria and fungi ranges from positive to negative symbioses. They are found in free-living microbial communities in the soil and host-associated polymicrobial biofilms. However, the impact of these symbioses on the organization and dynamics of microbial communities is uncertain due to knowledge gaps in understanding the genetic mechanisms of fungal-bacterial symbiosis. Despite the uncertainty, these interdomain partnerships are significant in the context of community ecology models.

Bacteria can cause mushroom-related food poisoning, with Salmonella, Listeria and Staphylococcus being common culprits

Mushrooms are part of the fungal kingdom, and while this is why they are of interest to food microbiologists, it is not the only reason. Bacteria can cause mushroom-related food poisoning, with Salmonella, Listeria, and Staphylococcus being common culprits.

Salmonella is one of the most well-known causes of mushroom-related food poisoning. In 2001, an outbreak of E. coli food poisoning in the UK led to the discovery of Salmonella Kedougou in a batch of mushrooms. While this did not result in any reported cases of food poisoning, it highlighted the potential risk of consuming raw mushrooms. More recently, in 2020, an outbreak of Salmonella Stanley was linked to imported dried wood ear mushrooms in the USA, affecting 55 people.

Listeria is another bacteria commonly associated with mushrooms. In 2020, an outbreak of Listeriosis occurred in North America due to Enoki mushrooms imported from Korea. This outbreak resulted in 36 cases, with 31 hospitalizations and four deaths. Listeria monocytogenes is a serious foodborne pathogen that can be found in agricultural environments and various animal species. It thrives in cool, wet conditions often present in mushroom growing and packing areas, making it a significant concern for the mushroom industry.

Staphylococcus, specifically methicillin-resistant Staphylococcus aureus (MRSA), is a challenging bacteria to treat and has been linked to mushrooms. While mushrooms themselves may not cause bacterial food poisoning, they can be contaminated with bacteria during production, handling, or processing.

To minimize the risk of bacterial contamination, the mushroom industry must remain vigilant. This includes implementing proper handling and sanitation practices, such as cleaning and sanitizing equipment and ensuring the safety of growing and packing environments.

Fungi produce antibiotics, which can be used to treat bacterial diseases

Fungi, including mushrooms, have been used for the production of antibiotics since the 1940s. The first antibiotic to be mass-produced, penicillin, was derived from Penicillium fungi. Fungi have also been used as a direct source of human food, in the form of mushrooms and truffles, and in the fermentation of various food products, such as wine, beer, and soy sauce.

Fungi are a potential goldmine for the production of pharmaceuticals. Researchers at Chalmers University of Technology have developed a method for finding new antibiotics from nature's own resources. The findings could prove very useful in the battle against antibiotic resistance. Fungi develop bioactive substances naturally as a way to protect themselves and survive in competitive environments.

More than 1000 pathways were discovered, showing the potential for fungi to produce a large variety of natural and bioactive chemicals that could be used as pharmaceuticals. In about 90 cases, the researchers were able to predict the chemical products of the pathways. For example, they followed the production of the antibiotic yanuthone and identified new fungi able to produce the compound, and that some species could produce a new version of the drug.

Fungi produce many antibiotics and other drugs for treating noninfectious diseases. For example, ciclosporin is commonly used as an immunosuppressant during transplant surgery, and fusidic acid is used to help control infection from methicillin-resistant Staphylococcus aureus bacteria. Fungi also produce statins (HMG-CoA reductase inhibitors), which are used to inhibit cholesterol synthesis.

Mushrooms, however, can become contaminated with bacteria. Soil bacteria like Listeria, Staphylococcus, and Salmonella are common culprits responsible for mushroom-related outbreaks. Contamination can occur from water, animals, improperly composted manure, and malpractices during and after harvest, storage, and transport. Cross-contamination from raw meat, poultry, or seafood can also occur.

Bacteria play a role in the composting of substrates for mushroom cultivation

The composting process for mushroom cultivation involves two phases, Phase I and Phase II, each with distinct goals. Phase I involves mixing substrates and wetting them to initiate self-heating, a process characterised by high microbial activity. During this phase, microbes grow and multiply, generating heat as they consume food and water. The compost pile is turned and mixed to expose all the material to biological and chemical processes, with water being added to maintain moisture. Phase I continues with the formation of ammonia and carbohydrates that the mushrooms will use as food. Phase II involves completing the composting process and converting ammonia and other substances into food for the mushroom.

The role of bacteria in the composting process for mushroom cultivation is important. Bacteria are involved in the defensive response to pathogens and the promotion of mycelia and mushroom growth. Certain bacteria, such as B. subtilis and Pseudomonas spp., have been found to inhibit the growth of harmful moulds and promote the growth of beneficial fungi. The bacterial community in the soil also influences the yield and diversity of mushrooms. For example, the abundance of Actinobacteria and Firmicutes increases during mushroom production, indicating their role in converting complex carbohydrates into simple sugars for the mushroom mycelium.

Compost bacteria have been found to belong to two phyla and three families, while casing bacteria belong to four phyla and eleven families. The diversity of bacteria in mushrooms lies between that of the two substrates. DNA sequencing studies have provided evidence of the presence of other microbes in compost and their functions, with a broad range of phyla involved in the microbial community during mushroom growth. Understanding the microbiology of substrates can help researchers design consortia of bacteria and fungi for bioaugmentation and substrate quality optimisation.

The pasteurisation process in Phase II aims to reduce or eliminate harmful microbes while minimising the loss of beneficial microbes. It involves heating the compost substrate to a specific temperature for a certain duration to achieve selective killing of pests and competitors. An effective pasteurisation process will eradicate harmful bacteria, insects, and other fungi while retaining beneficial microbes that aid in ammonia conversion and mushroom food production.

Bacteria can be introduced to mushrooms during the harvesting and post-harvesting processes

Mushrooms are cultivated from decaying organic matter, which naturally consists of many microorganisms, including bacteria. However, bacteria can also be introduced to mushrooms during the harvesting and post-harvesting processes, potentially causing contamination.

The human body is the greatest source of contamination in mushroom cultivation. Human hands and clothes can carry bacteria and spores that ruin crops. To prevent contamination, growers should shower and use hand sanitiser before handling mushrooms. Wearing clean clothes, including a lab coat or scrubs, along with a face mask, gloves, and refraining from talking are also recommended.

Another common form of bacterial contamination in mushroom cultivation is "wet spot" or "sour rot". This contamination is characterised by a dull grey slime with a sour-smelling odour that often forms in uncolonised patches along the bottom of grain jars. To prevent this, grains can be soaked for 12 to 24 hours at room temperature before sterilisation.

After being harvested, mushrooms are susceptible to deterioration, showing signs of discoloration, moisture loss, texture changes, an increase in microorganisms, and nutrient and flavour loss. To preserve mushrooms post-harvest, various techniques can be employed, such as precooling, refrigeration, washing with specific chemicals, edible coatings, modified atmosphere packaging, and the use of high-quality packaging materials.

Additionally, the grow room should be steamed after the substrate is removed, and the beds should be washed down to prevent pathogens from surviving post-crop pasteurisation. The temperature during steaming should be maintained at 150°F for at least 12 hours.

Frequently asked questions