Microbes and food poisoning

Microbe categories

Microbes can be divided into the following categories:

  • Bacteria
  • Viruses
  • Fungi (Moulds and yeasts)
  • Protozoa

Microbes are very small organisms invisible to the naked eye of a human, but they exist everywhere around us. Microbes can be found in the water, air, soil, dust, in humans and animals, and in the food we eat. Not all microbes are harmful (in fact, some microbes are beneficial for humans and are used in the food industry), but it is important for a food operator to be aware of microbial growth conditions and food poisonings that contaminated food may cause. 

Bacteria are single-cell organisms that reproduce by dividing in two. Different types of bacteria require different conditions for survival. Some prefer warmer or colder environments. Bacteria can be divided into three groups according to their temperature preferences: psychrotrophs, mesophiles and thermofiles (see more info below). Some may need oxygen in order to survive, others don’t. Bacteria that need oxygen are called aerobic, while bacteria that survives without oxygen is called anaerobic. Facultatively anaerobic bacteria is a type of bacteria that can proliferate both in aerobic and anaerobic conditions. Most of food poisoning bacteria belong to this group. Some bacteria can produce spores that can resist unfavourable conditions and later, when conditions become favourable, again develop into reproductive cells. 

Compared to other microbe types bacteria is the type that needs more moisture to multiply. The more humid the environment, the better it is for bacterial multiplication. In terms of acidity, bacteria prefers neutral levels of pH 6-8.

Best foods for bacterial growth are protein-rich foods, such as, meat, poultry, fish and foods that contain them. 

Reproductive bacteria in food may be killed by sufficient heating/cooking, but in order to kill bacteria spores more efficient heat treatment is needed (such as heating over +110 °C in a steam cooker during food canning process or heating over +135 °C during milk sterilizing process).

Viruses in contrast to bacteria are no-cells organisms, which means that they are not living organisms as such. They consist of one or more molecules and need a living host cell in order to reproduce. Viruses tolerate cold environments and may survive for months at the temperatures below +10°C. Viruses also tolerate acidic environments, certain types of viruses (for example, norovirus, rotavirus and enterovirus) survive even at pH 3. However, viruses may be killed by heat - heating to +60°C kills most viruses.   

Yeasts and moulds are most commonly known fungi. Moulds grow in the form of multicellular filaments which together form a network called mycelium. For nutrition they use paper, wood and all foodstuffs. Moulds need oxygen for growth, that's why they grow on the surfaces of foodstuffs. Compared to other microbe types moulds need the least amount of moisture. They also grow in acidic environments (such as jams and juices). Their preferable range on the acidity scale is pH 3-5, and preferable temperature range is 20-45 °C. 

In the food industry some moulds have beneficial use, for example in the cheese production and in the production of penicillin. The harmful effects of moulds include food spoilage and food poisoning with mycotoxins. Heating to the temperatures of +70 - +80 °C will kill both the moulds and their spores. However, there is a risk that harmful mycotoxins will remain poisonous even after heat treatment. 

Yeasts are single-celled fungi. Yeasts require sugars for growth and thrive in jams, sugary juices and berries. Their preferred temperature range is 20-35 °C and pH 5 on the acidity scale. Yeasts require oxygen for multiplication, but they can also survive in anaerobic environment. Yeasts require higher moisture levels than moulds. Certain yeasts are used in the food industry (e.g. wine and beer production), however, yeasts can cause food to spoil. Yeast cells die at the temperatures over +45 °C. In order to kill yeast spores higher temperatures are needed (+70- +80 °C). 

Protozoa are single-celled organisms that exist in moist environments such as marine and fresh water as well as the soil. Protozoa occurs in two forms: trophosomes and cysts. Cysts are a durable form of protozoa that can resist unfavourable environmental conditions such as heat, dryness or chemical treatment and may survive for long periods of time. Protozoa food poisonings mainly occur due to faecal contamination of drinking water or contaminated irrigated water. 

Microbial reproduction

In food operation it is important to be aware of microbial reproduction requirements and of the ways to control them. This is important in order to prolong the shelf-life of foods. Microbial reproduction requirements vary for different types of microbes. If one of the necessary requirements for a microbe to reproduce is not met or significantly differs from the optimal value, the reproduction will slow down or stop completely. Foodstuffs represent a good growth medium for almost all microbes. 

Microbial growth depends on a certain combination of the following environmental conditions: presence of oxygen, nutrition, moisture, acidity and temperature.  

1) Presence of oxygen

Presence of oxygen is one of the requirements that several types of microbes need in order to reproduce. For some microbes low level of oxygen is enough while others may multiply even in the oxygen-free environment (e.g. Salmonella, Bacillus cereus and Listeria monocytogenes bacteria multiply both in the presence and absence of oxygen). Microbes that can multiply in the absence of oxygen (i.e. in anaerobic conditions) are called anaerobes. 

2) Nutrition

In order to be able to live and grow microbes need nutrients. Food and food residues on the kitchen equipment, utensils and surfaces provide favourable conditions for microbes to feed and grow. 

3) Moisture

Microbes need at least some amount of moisture to multiply. Moist areas such as bathrooms and kitchen surfaces are most suitable for bacterial growth. Water contained in foods also represents suitable conditions for microbial growth. The more other substances like sugar and salt are dissolved in the water, the less free water is available for microbes. Free water contained in foods, which is not bound to food molecules, is defined as water activity. Water activity and moisture is not the same thing. Certain foods can have same moisture level, but different levels of water activity. Examples of foods with low water activity, which prevents microbes from multiplying, are cookies, dried fruits, soy sauces and jams. Most food poisoning bacteria require high levels of water activity (over 0.95), and most food-borne bacteria require at least 0.90. Moulds are microbes that require the least amount of water activity in foods in order to multiply. That’s why mould can sometimes appear on the surface of a jam even though jam has low water activity.

4) Acidity

Environmental pH is an important factor for microbial growth. Acidity is measured with 1-14 pH scale where values below 7 represent increasing acidity and values above 7 represent increasing alkalinity. Values of 6-8 represent neutral pH – conditions when most types of bacteria prefer to multiply. 

Acidity scale

Certain types of bacteria are able to live in more acidic or more alkaline environments, but only a few bacteria can multiply at pH 4-5 or pH 9-10. Moulds, unlike bacteria, are able to multiply also in more acidic environments (at pH 3-8 on the acidity scale). Cleaning solutions are usually highly alkaline (dishwasher powder, soda) or acidic in order to eliminate bacteria that cannot survive in these environments. 

The multiplication of bacteria in highly acidic foods is typically hindered, therefore they can be stored longer without spoiling. In addition, preserving agents such as vinegar or citric acid can be added to food to prevent bacterial growth and increase storage time. 

The most vulnerable foods for microbial growth are pH neutral foods such as milk, eggs, fresh meat and fish, cooked rice, fresh butter and plain yogurt.

5) Temperature

While acidity, level of oxygen and amount of moisture in foods may be alternated only to a certain extent, the temperature is the main factor which helps to control microbial multiplication. 
Based on the temperature preferences microbes can be divided into three categories: psychrophiles, mesophiles and thermophiles.

Psychrophiles are microbes that prefer relatively low temperatures for growth. 

Psychrophiles tend to grow at temperatures from 0 °C to +35 - +40 °C. The subgroup of psychrophiles that can live in conditions above 20°C is called psychrotrophs. They prefer colder environment, but can exist also in higher temperatures. Optimal conditions for psychrophiles is +10 °C while for psychrotrophs it is +20°C. 

Mesophiles prefer room and body temperatures of +25 - +40 °C but can multiply also between +15 and +45°C. These bacteria can be found on human and animal skin and intestines, and in the soil. 

Thermophiles are organisms that tolerate high temperatures up to +75°C with the optimum conditions for growth between +50 and +70 °C.

In order to control microbial growth in foods it is necessary to store them at enough cold or high temperatures that will hinder multiplication of microbes. It is best to avoid storing foods in the temperatures ranging from +6 to +60 °C, which is often called a ‘danger zone’. Storing foods at low temperatures (for example, in the fridge) will slow down the multiplication of microbes, but will not kill them. Freezing and industrial deep-freezing will not kill microbes, but will stop the multiplication process.

Food poisoning bacteria

Bacteria High-risk foods Symptoms  Preventive measures

Listeria monocytogenes

Ready-to-eat deli-style meats and poultry, sausages, hot dogs, poultry, pâtés or meat spreads. Non-pasteurised milk and products made of non-pasteurised milk. Frozen vegetables. Smoked seafood and ready to eat cold salads containing ham, chicken or seafood.

Fever, stiffness in the neck, overall weakness, muscle aches, vomiting, sometimes diarrhea and nausea.

Incubation period of 3-70 days.

Raw meat, poultry and seafood should be well cooked through. Do not allow cross-contamination from raw foods to cooked meals. Wash hands after handling raw meat, poultry and seafood and eggs. Consume ready-to-eat and deli meats as soon as possible. 


Can occur in water, soil, food and contaminated surfaces. Can be transmitted through uncooked or contaminated food, swallowing contaminated water, or placing contaminated object in the mouth. 

Dehydration and weight loss, fever, nausea, diarrhea, stomach pain and vomiting. Incubation period of 2-10 days

Wash hands before eating, after bathroom, after handling animals and changing diapers. 

Escherichia coli O157:H7

Most often occurs in beef and ground beef, may occur in raw fruits and vegetables, unpasteurized milk and juice

Severe diarrhea, strong abdominal pains, vomiting. Incubation period of 3-4 days

Heat food (especially hamburgers and ground beef) to above +70°C. Wash raw vegetables and fruits before consumption. 

Clostridium perfringens

Occurs in soil and intestines. Meat, smoked fish, use of leftovers and hot buffets are risk food. Exists in anaerobic conditions only. 

Abdominal cramps, nausea, and diarrhea. Incubation period of 6 -24 hours.  Hot foods should be kept hot at internal temperature above 60 °C. Cold foods should be kept refrigerated. Perishable foods should be left at room temperatures no longer than a couple of hours. 

Clostridium botulinum

Vacuum packed food, canned food (due to insufficient sterilizing), tightly wrapped food, honey. Exists in anaerobic conditions only.  Toxin affects the nervous system. Vision is affected (blurred vision, tiredness). Dryness in the mouth, swallowing problems, muscle weakness. Incubation period of 18 to 36 hours.  Damaged, leaked or swelled canned or vacuum packed foods should be thrown away.
Hot foods should be kept hot at internal temperature above 60 °C. Cold foods should be kept refrigerated. Cold chain should be maintained.
Salmonella  Poultry and eggs, meat, unpasteurized milk and products made of non-pasteurised milk, other products of animal origin. Bacteria may also contaminate fresh vegetables and fruits. 

Fever, diarrhea, abdominal cramps.  Incubation period of 12 to 72 hours. 

Raw meat, poultry and eggs should be well cooked through. Heat poultry meat above +75  °C. Wash fruits and vegetables before consumption. Do not consume non-pasteurised milk and milk products. 


Salads is a high risk food due to poor personal hygiene of food handlers. Occurs due to faecal contamination. 

Diarrhea, fever, abdominal cramps. Incubation period of 1-2 days.

Good personal hygiene. Wash hands with soap after visiting bathroom, changing diapers, and before handling food. Heat food to above +70°C.

Staphylococcus aureus

Non-pasteurised milk and cheese, food buffets (self-service). Found on human and animal skin and in the noses. 

Diarrhea, nausea, abdominal cramps, vomiting.

Incubation period up to 6 hours. 
Toxins produced by this bacterium resist heating, therefore preventive measures such as good personal hygiene and food handling practices are particularly important. Hot foods should be kept hot at internal temperature above 60 °C. Cold foods should be kept refrigerated. Perishable foods should be left at room temperatures no longer than a couple of hours.

Raw seafood. Occurs in saline waters and seafood. 

Diarrhea, vomiting, stomach pain. Incubation period 1-7 days. 

Avoid eating raw oysters or other raw shellfish. Keep the cold chain and prevent cross-contamination. Shellfish should be refrigerated no longer than in 2 hours after cooking.
Yersinia Raw or undercooked pork. 

In young children - fever, abdominal pain, and diarrhea. In older children and adults -fever and pain on the right side of the abdomen. Incubation period of 2-6 days.

Good hand hygiene. Avoid eating raw or undercooked pork. Use separate utensils to avoid cross-contamination. 

Campylobacter jejuni

Contaminated surface waters, unpasteurized milk, raw or undercooked meat, poultry, and shellfish.

Diarrhea, fever, abdominal pains. Incubation period of 2-5 days. 

Avoid non-pasteurised milk and milk products. Thoroughly cook meat and poultry (to over 75°C). Keep good personal hygiene. 
Bacillus cereus Rice, soups, sauces, spices, milk. Foods kept in room temperature for too long. Food buffets, use of leftovers. 

Diarrhea, abdominal cramps, nausea, vomiting. Incubation period of 30 minutes to 6 hours. 

Refrigerate cooked food as soon as possible. Hot foods should be kept hot at internal temperature above 60 °C. Cold foods should be kept refrigerated.

Food poisoning viruses

Virus Cause Symptoms Prevention
Hepatitis A Faecal contamination of waters used for irrigation of vegetables or drinking water. Raw shellfish and oysters from contaminated waters, contact with an infected food handler or touching the surface an infected person has touched.  

Diarrhea, fever, dark urine, abdominal pain, headache, weight loss.

Incubation period may vary from 15 to as long as 50 days.

Careful hand washing and personal hygiene. Avoid eating raw shellfish and oysters. Tourists planning to travel to countries with high risks of hepatitis A are advised to get vaccinated. 


Farm products, ready-to-eat foods, food buffets, raw shellfish and oysters, leafy greens, salads, herbs and spices.

Foods contaminated with vomit or faeces of infected person. 

Nausea, abdominal pain, vomiting, watery diarrhea, muscle pain.

Incubation period of 12-48 hours. 

Careful hand washing, usage of disinfectants.

Bare-hand contact with ready-to-eat-foods by restaurant workers should be avoided. Fruits and vegetables should be washed prior to consumption. Avoid raw oysters and shellfish. 
Astrovirus Usually transmitted via human contact, sometimes via contaminated food or water. High-risk foods are berries and vegetables contaminated by irrigation or rinsing water or raw oysters and shellfish that filtrate contaminated water. 

Diarrhea, nausea, vomiting, weakness, abdominal pain.

Incubation period of 1-2 days.
Careful hand washing, washing and sufficient  heating of vegetables and berries. 


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