Microbial Food Spoilage- Types of microorganisms with examples

Food spoilage is the deterioration of the quality, safety, and acceptability of food due to the growth and activity of microorganisms. Microorganisms are microscopic living organisms that can be found everywhere in nature. They include bacteria, fungi, protozoa, algae, viruses, and prions. Some microorganisms are beneficial for food production, such as those involved in fermentation, while others are harmful and cause food spoilage.

Food spoilage can result in various changes in the appearance, texture, flavor, odor, and nutritional value of food. Some of these changes may be visible to the naked eye, such as mold growth, discoloration, or slime formation, while others may not be easily detected, such as toxin production or pH reduction. Food spoilage can also pose a health risk to consumers if they ingest contaminated food that contains pathogenic microorganisms or their toxins.

The extent and rate of food spoilage depend on several factors, such as the type and number of microorganisms present in the food, the intrinsic and extrinsic properties of the food, and the storage and handling conditions of the food. Intrinsic properties are those that are inherent to the food itself, such as water activity, pH, nutrient content, and natural antimicrobial compounds. Extrinsic properties are those that are related to the environment surrounding the food, such as temperature, humidity, oxygen availability, and light exposure.

Different types of microorganisms have different preferences and requirements for growth and survival in food. Therefore, different types of food may be susceptible to different types of spoilage microorganisms. In general, foods that have high water activity (aw), neutral or slightly acidic pH (6-7), and high nutrient content (such as proteins and carbohydrates) are more prone to bacterial spoilage. Foods that have low water activity (aw), acidic or alkaline pH (2-8.5), and low nutrient content (such as fats and oils) are more prone to fungal spoilage. Foods that have intermediate water activity (aw), acidic pH (3-5), and high sugar or salt content (such as jams or pickles) are more prone to yeast spoilage.

In this article, we will discuss the different types of microorganisms that cause food spoilage and provide some examples of foods that they affect. We will also briefly mention some methods to prevent or control food spoilage by microorganisms.

Bacteria are the most common microorganisms that cause food spoilage and foodborne illnesses. They are single-celled organisms that can grow in a wide range of conditions, such as temperature, pH, water activity, and oxygen availability. Some bacteria can form spores, which are resistant to heat and other adverse factors, and can germinate when conditions are favorable. Some bacteria can also produce toxins, which are harmful substances that can cause illness even if the bacteria are killed by cooking or other methods.

Bacteria can be classified into two groups based on their cell wall structure: gram-positive and gram-negative. Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall, which retains a purple stain when treated with a special dye. Gram-negative bacteria have a thin layer of peptidoglycan and an outer membrane, which does not retain the stain and appears pink. This distinction is important because gram-positive and gram-negative bacteria have different characteristics and effects on food.

Some of the common gram-positive bacteria that cause food spoilage are:

• Staphylococcus aureus: This bacterium is found on the skin and nasal passages of humans and animals, and can contaminate food through poor hygiene practices or cross-contamination. It produces a heat-stable toxin that can cause nausea, vomiting, diarrhea, and abdominal cramps within a few hours of ingestion .
• Bacillus spp: These bacteria are spore-formers that can survive high temperatures and low water activity. They can spoil foods such as cereals, grains, beans, nuts, spices, dairy products, meat, and fish. They can produce toxins that can cause diarrhea, vomiting, nausea, and abdominal pain .
• Clostridium spp: These bacteria are also spore-formers that can grow in anaerobic conditions (without oxygen). They can spoil foods such as meat, poultry, fish, eggs, dairy products, vegetables, and canned foods. They can produce toxins that can cause botulism (a serious paralytic illness), gas gangrene (a tissue infection), or food poisoning .
• Lactic acid bacteria (LAB): These bacteria are responsible for the fermentation of foods such as yogurt, cheese, pickles, sauerkraut, kimchi, and sourdough bread. They produce lactic acid as a by-product of their metabolism, which lowers the pH and inhibits the growth of other spoilage microorganisms. However, they can also cause spoilage of foods such as milk, meat, vegetables, and fruits by producing off-flavors, off-odors, gas bubbles, slime formation, or discoloration.

Some of the common gram-negative bacteria that cause food spoilage are:

• Salmonella spp: These bacteria are found in the intestinal tract of humans and animals, and can contaminate food through fecal contamination or cross-contamination. They can cause salmonellosis (a type of food poisoning) characterized by diarrhea, fever, abdominal cramps, and sometimes vomiting within 6 to 72 hours of ingestion .
• Escherichia coli: These bacteria are also found in the intestinal tract of humans and animals, and can contaminate food through fecal contamination or cross-contamination. They can cause various types of infections depending on the strain involved. Some strains can cause diarrhea, hemorrhagic colitis (bloody diarrhea), hemolytic uremic syndrome (a kidney disorder), or urinary tract infections .
• Vibrio spp: These bacteria are found in marine environments and can contaminate seafood such as fish, shellfish, oysters, and shrimp. They can cause vibriosis (a type of food poisoning) characterized by diarrhea, vomiting, abdominal pain, fever, and sometimes skin infections within 24 hours of ingestion .
• Campylobacter jejuni: This bacterium is found in the intestinal tract of poultry, cattle, pigs, and other animals, and can contaminate food through fecal contamination or cross-contamination. It can cause campylobacteriosis (a type of food poisoning) characterized by diarrhea, fever, abdominal pain, and sometimes bloody stools within 2 to 5 days of ingestion .

These are some examples of bacteria that cause food spoilage and foodborne illnesses. However, there are many other types of bacteria that can affect food quality and safety. Therefore, it is important to follow good hygiene practices, proper storage methods, and adequate cooking temperatures to prevent bacterial growth and contamination in food.

Fungi are the most abundant group of microorganisms that play an important role in food spoilage. Fungi are osmotrophic, meaning they obtain their nutrients by absorption. Fungi can be divided into two types: molds and yeasts.

Molds

Molds are the most common food spoilage-causing microorganisms. Molds grow on the surface of food (they require free oxygen for growth) and in a wide range of pH values (from 2 to 8.5), but the majority of them prefer acidic pH. Molds can grow at very low water activity levels (0.7–0.8) on dried foods (e.g. grains, beans, peanuts, and some spices).

The most common food spoilage causing molds are Mucor, Aspergillus spp, Rhizopus spp, Penicillium spp, Alternaria spp, Bothrytis, Byssochlamys, Fusarium spp. These molds cause off-flavors, mycotoxins contamination, discoloration, rotting, and are externally visible in the food.

Yeasts

Compared to bacteria and molds, yeasts play a minor role in food spoilage. Yeasts can grow with or without oxygen and are used for beneficial fermentation in bread and alcoholic drinks production. They often spoil food that has high sugar or salt content (e.g. maple syrup, pickles, jams, soy sauce, and sauerkraut).

Yeasts require a water activity level of 0.90–0.95 for growth and they can grow in a wide range of pH (3 – 8) but in general, they prefer acidic pH (4.5-5.5). The most commonly food spoilage causing yeasts are Zygosaccharomyces spp, Saccharomyces spp., Candida spp, Dekkera spp. These yeasts cause a change in color, a change in texture, an unpleasant odor, or an undesirable taste in food.

Molds are the most common food spoilage-causing microorganisms. Molds grow on the surface of food (they require free oxygen for growth) and in a wide range of pH values (from 2 to 8.5), but the majority of them prefer acidic pH. Molds can grow at very low water activity levels (0.7–0.8) on dried foods (e.g. grains, beans, peanuts, and some spices) .

The most common food spoilage causing molds are Mucor, Aspergillus spp, Rhizopus spp, Penicillium spp, Alternaria spp, Bothrytis, Byssochlamys, Fusarium spp . This mold causes off-flavors, mycotoxins contamination, discoloration, rotting, and is externally visible in the food.

Some molds are also used to produce certain foods, such as blue cheese and country-cured ham. However, these molds are safe to eat and do not cause spoilage .

Some tips to prevent mold growth on food are:

• Store food in a cool and dry place
• Refrigerate or freeze perishable food
• Use airtight containers or plastic wrap for leftovers
• Discard any food that shows signs of mold
• Do not sniff moldy food as it may cause respiratory problems

Compared to bacteria and molds, yeasts play a minor role in food spoilage. Yeasts are single-celled fungi that can grow with or without oxygen and are used for beneficial fermentation in bread and alcoholic drinks. They often spoil food that has high sugar or salt content (e.g. maple syrup, pickles, jams, soy sauce, and sauerkraut.)

Yeasts require a water activity level of 0.90–0.95 for growth and they can grow in a wide range of pH (3 – 8) but in general, they prefer acidic pH (4.5-5.5) . Most commonly food spoilage causing yeasts are Zygosaccharomyces spp, Saccharomyces spp, Candida spp, Dekkera spp . These yeasts cause a change in color, a change in texture, an unpleasant odor, or an undesirable taste in food.

Yeasts can also produce toxins such as ethyl carbamate, which is a carcinogen, and biogenic amines, which can cause headaches, nausea, and hypertension. Yeasts can also cause allergic reactions in some people.

Some examples of foods spoiled by yeasts are:

• Fruit juices: Yeasts can cause fermentation and gas production in fruit juices, resulting in off-flavors and loss of carbon dioxide.
• Honey: Yeasts can cause fermentation and foaming in honey, resulting in loss of quality and flavor.
• Cheese: Yeasts can cause discoloration, gas production, and surface defects in cheese, especially soft cheeses.
• Meat products: Yeasts can cause slime formation, off-odors, and spoilage of vacuum-packed meat products.

To prevent yeast spoilage of foods and beverages, some control measures are:

• Reducing the water activity by drying, salting, or adding sugar or preservatives .
• Lowering the pH by adding acids or fermenting with lactic acid bacteria .
• Refrigerating or freezing the food to slow down yeast growth .
• Pasteurizing or sterilizing the food to kill yeasts .
• Using proper hygiene and sanitation practices to avoid cross-contamination .

Protozoa are one-celled microorganisms without a rigid cell wall and the transmissible form of these organisms is termed cysts.

Protozoan parasites are highly associated with foodborne and water-borne outbreaks of disease. The water and food act as a carrier for transmission of protozoan parasites from one host to another.

The most common foodborne parasites are:

• Giardia lamblia: This parasite causes giardiasis, a diarrheal illness that can last for weeks or months. It is transmitted by consuming contaminated water or food, especially raw or undercooked meat, fruits, and vegetables. Symptoms include abdominal cramps, gas, nausea, and weight loss.
• Entamoeba histolytica: This parasite causes amebiasis, an infection of the intestines and sometimes the liver. It is transmitted by consuming contaminated water or food, especially raw or undercooked meat, fruits, and vegetables. Symptoms include diarrhea, dysentery, abdominal pain, fever, and liver abscesses.
• Cyclospora cayetanensis: This parasite causes cyclosporiasis, a diarrheal illness that can last for weeks or months. It is transmitted by consuming contaminated water or food, especially fresh produce such as raspberries, basil, and lettuce. Symptoms include watery diarrhea, loss of appetite, weight loss, nausea, and fatigue.
• Toxoplasma gondii: This parasite causes toxoplasmosis, an infection that can affect the brain, eyes, and other organs. It is transmitted by consuming contaminated water or food, especially raw or undercooked meat, unpasteurized milk, and cat feces. Symptoms include fever, headache, muscle pain, and swollen lymph nodes. In pregnant women and immunocompromised people, it can cause severe complications such as miscarriage, stillbirth, blindness, and brain damage.
• Trichinella spiralis: This parasite causes trichinellosis, an infection of the muscles and other tissues. It is transmitted by consuming raw or undercooked meat from animals infected with the parasite, especially pork and wild game. Symptoms include diarrhea, nausea, vomiting, fever, muscle pain, swelling of the face and eyes, and difficulty breathing.

These protozoan parasites can be prevented by:

• Boiling or filtering water before drinking or using it for cooking
• Washing hands with soap and water before eating or preparing food
• Washing fruits and vegetables thoroughly before eating them
• Cooking meat to a safe internal temperature
• Avoiding contact with cat feces or litter boxes
• Freezing meat for several days before cooking it

Algae are photosynthetic organisms that can be found in freshwater or marine habitats. They are a source of various nutrients and bioactive compounds, and they are used as food or food additives in some cultures. However, some algae can also cause food spoilage or food poisoning by producing toxins or affecting the quality of water.

Some of the algae that cause food spoilage are:

• Cyanobacteria or blue-green algae: These are bacteria that can perform photosynthesis and produce oxygen. They can grow rapidly in nutrient-rich water and form blooms or scums that can affect the taste, odor, and color of water. Some cyanobacteria can also produce toxins that can accumulate in fish and shellfish and cause illness in humans or animals. Examples of cyanobacterial toxins are microcystins, anatoxins, saxitoxins, and cylindrospermopsins.
• Dinoflagellates: These are single-celled algae that have two flagella for movement. They can also form blooms in marine water and produce toxins that can cause paralytic shellfish poisoning (PSP), diarrhetic shellfish poisoning (DSP), neurotoxic shellfish poisoning (NSP), or ciguatera fish poisoning (CFP). Examples of dinoflagellate toxins are saxitoxins, okadaic acid, brevetoxins, and ciguatoxins.
• Diatoms: These are single-celled algae that have silica cell walls and can form chains or colonies. They can also produce toxins that can cause amnesic shellfish poisoning (ASP) or domoic acid poisoning (DAP). Domoic acid is the main toxin produced by diatoms and it can affect the nervous system of humans and animals.

Some of the algae that are used as food or food additives are:

• Spirulina: This is a type of cyanobacteria that is cultivated as a dietary supplement or a natural colorant. It is rich in protein, vitamins, minerals, and antioxidants. It is considered safe for human consumption when produced under controlled conditions.
• Chlorella: This is a type of green algae that is also cultivated as a dietary supplement or a natural colorant. It is also rich in protein, vitamins, minerals, and antioxidants. It is considered safe for human consumption when produced under controlled conditions.
• Seaweed: This is a general term for various types of macroalgae that grow in marine water. They are used as food or food ingredients in many cuisines, especially in Asia. They are also rich in protein, vitamins, minerals, fiber, and bioactive compounds. Some examples of edible seaweeds are nori, kelp, wakame, kombu, dulse, and agar.

Viruses are microscopic parasites that need living cells to multiply and cause disease. Unlike bacteria, viruses cannot grow in food, but they can be carried by food and water. Viruses can also contaminate food through contact with infected people or animals .

Foods most commonly associated with foodborne viruses include shellfish, which are harvested near human sewage outlets, undercooked meats, and fruit and vegetables that are grown on animal waste fertilized grounds . Even if sewage systems are treated, the virus removal and the removal efficiency depend on how much viral load is present.

Some of the common foodborne viruses are:

• Norovirus: This virus causes acute gastroenteritis, with symptoms such as nausea, vomiting, diarrhea, and stomach cramps. It is highly contagious and can spread through contaminated food, water, surfaces, or person-to-person contact .
• Hepatitis A virus (HAV): This virus causes inflammation of the liver, with symptoms such as fever, fatigue, loss of appetite, jaundice, dark urine, and clay-colored stools. It can be transmitted through contaminated food or water, especially shellfish, or contact with an infected person .
• Hepatitis E virus (HEV): This virus also causes liver inflammation, with similar symptoms as HAV. It is mainly transmitted through contaminated water or undercooked pork products. It can cause severe complications in pregnant women and people with chronic liver disease .
• Astrovirus (AstV): This virus causes mild to moderate gastroenteritis, with symptoms such as diarrhea, vomiting, fever, and abdominal pain. It mainly affects children and the elderly. It can be transmitted through contaminated food or water or contact with an infected person.
• Rotavirus (RV): This virus causes severe diarrhea and vomiting in infants and young children. It can lead to dehydration and death if not treated. It can be transmitted through contaminated food or water or contact with an infected person.
• Coronavirus: This virus causes respiratory infections that can range from mild to severe. The most recent strain of coronavirus, SARS-CoV-2, causes COVID-19, a pandemic disease that has affected millions of people worldwide. It can be transmitted through respiratory droplets or contact with an infected person or surface .
• Sapovirus (SaV): This virus causes gastroenteritis similar to norovirus, but less severe. It mainly affects children and the elderly. It can be transmitted through contaminated food or water or contact with an infected person.

To prevent foodborne viral infections, it is important to follow good hygiene practices such as washing hands before and after handling food, cooking food thoroughly, avoiding cross-contamination between raw and cooked foods, refrigerating food promptly, and avoiding contact with sick people or animals .

Prions are infectious disease-causing agents that are composed of misfolded proteins. Unlike other microorganisms, prions do not have any genetic material and cannot replicate on their own. They can only infect other normal proteins and cause them to change their shape and function.

Prions can cause fatal neurodegenerative diseases in animals and humans, such as bovine spongiform encephalopathy (BSE) or mad cow disease in cattle, scrapie in sheep and goats, chronic wasting disease (CWD) in deer and elk, and Creutzfeldt-Jakob disease (CJD) and variant CJD (vCJD) in humans. These diseases are collectively known as transmissible spongiform encephalopathies (TSEs) because they cause sponge-like holes in the brain tissue.

Prions can be transmitted through food when people consume meat or meat products from infected animals. The most infectious parts of the animal are the brain, spinal cord, eyes, spleen, tonsils, and lymph nodes. Cooking or freezing does not destroy prions, so they can remain infectious even after processing.

The symptoms of prion diseases may take years or decades to appear after exposure. They include memory loss, personality changes, dementia, muscle twitching, difficulty walking, and seizures. There is no cure or treatment for prion diseases and they are always fatal.

To prevent prion diseases from spreading through food, several measures have been taken by governments and health authorities around the world. These include banning the use of animal by-products in animal feed, testing animals for BSE before slaughter, removing high-risk tissues from the food chain, and restricting the import and export of meat and meat products from affected countries.

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