Food poisoning bacteria and viruses

Food poisoning is caused by bacteria, or in some cases viruses, contaminating the food we eat. While we tend to blame the last thing we ate it may have been something we ate several days ago.

Food isn’t sterile; it comes from animals or grows in soil. If we are healthy adults, our immune systems can deal with small numbers of bacteria and viruses but at higher levels they can make us quite sick. Vulnerable people such as pregnant women, the elderly or people with poor immune systems can get very ill or even die from food poisoning.

Food poisoning is more frequently caused by bacteria. Bacteria like to grow in the ‘temperature danger zone’ between 5°C and 60°C. Their numbers can double and redouble and in a few hours they can reach dangerous levels.

Some viruses that cause foodborne disease include Hepatitis A, Norwalk virus, Norovirus, Rotavirus and some of the Caliciviruses. Viruses don’t grow in food, and one particle may be enough to make you sick.

Common symptoms of foodborne diseases can be mild or severe and can include nausea, vomiting, diarrhoea, stomach cramps, headaches and fever etc. Generally the illness only lasts one or two days. Exceptions are Hepatitis A, Some Escherichia coli infections or typhoid which can be severe illnesses lasting for many weeks. Some foodborne illnesses can result in long term health issues involving the immune system, the gut or kidneys as described below. Here is advice on What to do if you get food poisoning.

Below is a list of some more common food poisoning bacteria:


Salmonella food poisoning (salmonellosis) is usually linked to consuming inadequately cooked meats or poultry, other foods contaminated by raw meats and poultry, as well as foods containing raw or undercooked eggs, unpasteurised dairy products such as raw milk or cheeses. But many other foods have been linked to outbreaks caused by Salmonella including mayonnaise (with raw eggs), fruits and vegetables, salads, milk, unpasteurised fruit juices, nuts, seeds and sprouted seeds. It gets into other foods by cross contamination from contact with raw foods, utensils, equipment and hands.

In Australia, salmonellosis tends to be more prevalent in the warmer, northern parts of the country and eating food that has been kept in the temperature danger zone for too long allowing the bacteria to grow is often the cause of the illness. However, even small numbers of Salmonella can cause foodborne disease. Sensitive individuals such as the elderly, young children and people with low immune systems, are much more likely to become ill after consuming only a small number of cells.

It usually takes 8 to 72 hours for symptoms of salmonellosis to occur, but can take up to a few weeks, so it is not necessarily the last meal you ate that caused it. Salmonella causes a ‘gastro-flu-like’ infection which in most cases lasts about two to five days. However, in some people it can lead to chronic conditions such as Reiter’s Syndrome or reactive arthritis.

Salmonella usually needs to grow to a sizeable population of bacteria to make healthy adults sick. However, in high fat foods like peanut butter, potato crisps and chocolate, and liquids which pass through the stomach quickly, such as unpasteurised juices, the presence of only a few bacteria can cause illness. It can also survive in fairly dry and mildly acidic foods for some time during food storage.

Because Salmonella is a natural resident in the gut of food production animals we should assume that it is in raw animal foods such as meat, poultry, milk and eggs. Vegetable foods can also be contaminated, for example if they come in contact with animal faeces or contaminated water. Most outbreaks occur through cross contamination from raw to cooked food, and contaminated food remaining in the temperature danger zone for too long. If cooking or reheating is inadequate then the bacteria will survive.


Campylobacter is present in the gut of a wide range of animals, especially birds. Outbreaks have been linked to the consumption of undercooked poultry, unpasteurised milk, water, and other foods that have been contaminated by raw foods like meat and poultry. Pets may also be a source of infection. Campylobacter doesn’t grow well in foods, but it only has to be present in food in low numbers to cause an infection. It is a problem because quite low numbers of the bacteria can cause illness.

Symptoms can take two to seven days to appear and are gastrointestinal, lasting for about five days. A fever may also be present. A small number of people are left with a chronic condition called Guillain-Barre Syndrome which can last for several weeks or months.

Listeria monocytogenes

Listeriosis is a comparatively rare form of foodborne illness, but it can be a very serious disease in pregnant women, people with poor immune systems and the elderly who will need to avoid certain foods see this additional Listeria advice. It has also caused occasional outbreaks of mild gastroenteritis in healthy people.

The symptoms are usually described as ‘flu-like’, although vomiting and discoloured urine can occur. Miscarriage can result if a pregnant woman is infected, even if she doesn’t show the symptoms. The time from infection to symptoms can be anywhere between 8 to 90 days.

Listeria is widely found in the environment so most raw foods are likely to be contaminated. Listeria is easily killed by heat, although cooked foods can easily become re-contaminated through poor food handling after cooking.

This is one of the few pathogens that can grow in the refrigerator, so ready to eat food should never be stored in the fridge too long. Although it can grow in the fridge, it will do so only very slowly so make sure your refrigerator is keeping your food at or less than 5 °C. Never eat packaged food after its use by date.

Escherichia coli (E.coli)

Many strains of E. coli are harmless and are found naturally in the gut of humans and animals. Traditionally its presence in foods has been an indication of faecal contamination of food or water. However, particular strains are pathogenic and traveller’s diarrhoea and haemolytic uraemic syndrome (HUS) are caused by E. coli strains. See Travelling overseas food safety. Although pathogenic types are rare, in the last few years there have been several food poisoning outbreaks from certain strains of E. coli both in Australia and overseas.

A wide variety of foods have been implicated in this outbreaks, including unpasteurised apple and orange juices, sprouted seeds, fruit, raw milk cheese, salads and meat and meat products, especially undercooked minced meat patties in hamburgers.

E. coli is easily killed by heating so cooking food properly is a basic method of control. Water can also be a source of the bacteria.

Staphylococcus aureus

Staphylococcus aureus, also known as ‘Golden staph’, is important from both a medical and food perspective. About half of us carry this organism in our skin and nasal passages. If you have an infected cut or sore, it can contain large numbers of Staph. Keep any cuts or sores well covered if you are handling foods.

Animals and poultry also carry this bacteria on their bodies and all raw meat and poultry products should be handled as though they are contaminated. Raw milk can also be a source of this bacteria. It likes to grow in salty and sweet foods, for example those containing custard, hams, frankfurters, salads, cream-filled bakery products etc.

The important thing to remember is that Staphylococcus aureus produces a heat stable toxin as it grows and it is the toxin that makes you sick. If it is allowed to grow in food the toxin will remain even if the food is cooked again. The toxin takes only a very short time to make you sick (one to six hours) and causes nausea, vomiting, abdominal cramps and diarrhoea as the usual symptoms.

Clostridium botulinum

Clostridium botulinum is one of the better known foodborne disease microorganisms due to the severe nature of the illness it causes. Fortunately, in Australia it is fairly rare. As Clostridium botulinum grows in food it produces a neurotoxin. This causes symptoms about 12- 36 hours after consumption, although this can vary. Early symptoms include nausea, diarrhoea and vomiting, but neurological symptoms follow. Infant botulism is commonly reported in some countries in children under one year old and the source of the infection is usually unknown. However, a common source is dust or soil.

C. botulinum forms spores when it is heated and when conditions become favourable the spores germinate. In the past it has been mainly associated with canned foods but it has recently also been associated with vegetables in oil and some other foods.

Clostridium perfringens

This is a less well known bacteria which causes severe stomach cramps and a mild form of diarrhoea that lasts only about 24 hours and therefore tends to go unreported. However, it is probably fairly common and can be fatal in the frail elderly or people who are already ill.

Symptoms begin about 8 to 22 hours after the food is eaten. Large numbers of the bacteria have to be eaten before you get sick, but because the bacteria can grow very fast (the number can double every 18 minutes) it doesn’t take long for large numbers to build up. The cause of the illness is a toxin that is produced when the bacteria forms spores in the gut. It’s the presence of the toxin that makes you sick.

Clostridium perfringens is widely found in soil and in intestinal tracts of humans. It is usually associated with food that has been allowed to stay warm for several hours. During cooking, which will kill most types of bacteria, Clostridium perfringens turns into another form called a spore. A spore is like a seed, it stays dormant in the food until conditions are favourable, then like a plant seed it will germinate and grow. The spores of Clostridium perfringens are very heat resistant and will withstand boiling for several hours.

Bacillus cereus and other Bacillus species

This pathogen can cause two types of foodborne illness—the diarrhoeal type and the emetic or vomiting type. Like Clostridium perfringens the illness is a mild one, but unpleasant nevertheless.

The diarrhoeal type occurs within 8 to 16 hours of eating the food and lasts for about 24 hours. Foods involved vary from starchy vegetables, meat products, cereal foods, sauces, puddings and spices. A much shorter time is required for symptoms of the vomiting type to appear (30 minutes to five hours). The most common food associated with the vomiting type is rice. Cooked rice should always be cooled and stored in the refrigerator.

Bacillus cereus can form heat resistant spores and a heat resistant toxin. If cooked food is allowed to cool slowly the spores can germinate. If growth occurs then the toxin can form under certain conditions. Reheating or lightly cooking the food will not destroy this toxin.

Although this bacteria can grow and produce toxin at refrigeration temperatures, it does so much more slowly than at room temperature. Precooked food should not be stored in the refrigerator for more than two to three days.


Viruses are not living organisms but bits of reproductive material that attack human cells and hijack them. An infected cell then starts making more viruses until it can’t make any more, breaking open and releasing the new viruses into the body to infect more cells. The most important viruses that cause foodborne disease are Hepatitis A, Norwalk virus, Norovirus, Rotavirus and some of the Caliciviruses. Viruses don’t grow in food, and one particle is enough to make you sick.

Symptoms can be severe gastroenteritis or similar to the ‘flu’. Generally the illness only lasts one or two days. The exception is Hepatitis A which can be a severe illness and last for many weeks.