Many food-poisoning bacteria have to multiply to high numbers in food before they are likely to cause illness. The four main requirements for bacterial growth are food, moisture, warmth and time.
The foods in which pathogens like to multiply are normally nutritious foods. These include meat, poultry, fish (particularly shellfish), cooked rice and pasta, milk products and eggs and also any foods which contain these as an ingredient such as meat pies, sandwiches, gravy, salads, etc.
Pathogens will grow in both raw and cooked foods. Many raw foods, particularly meat and poultry, will contain pathogens. Most such foods are cooked before consumption and thorough cooking will kill pathogens, making the food safe to eat. The consumption of foods which have not been cooked or heat-treated, however, may lead to food-poisoning.
The following foods have often been implicated in outbreaks:
It is, of course, possible to find bacteria in cooked food. Pathogens may be re-introduced to cooked foods by cross-contamination after cooking. There is also the opportunity for those bacteria which are able to produce spores when foods are cooked to become active bacteria again if the cooked food is kept in warm temperatures (within the growth zone). Cooked foods must therefore be protected from contamination and kept at the right temperature (either hot or cold).
* Note: Since raw eggs may sometimes contain food-poisoning bacteria, they should not be used as an ingredient in foods such as mousses and cold desserts which will not be cooked. It is now possible to buy and use pasteurised egg instead. This has been heat-treated to kill any pathogens. The Government advises that people vulnerable to infection such as babies, toddlers, older people, those who are already ill and pregnant women should only eat eggs that are thoroughly cooked until both the yolk and white are solid. They suggest that for other people there is little risk from eating cooked eggs.
Most foods naturally contain sufficient moisture to provide bacteria with the water they need in order to grow. Where moisture has been deliberately removed (e.g. in dehydrated foods such as milk powder, soup mixes, etc.), then bacteria will not grow whilst the food remains dry, but once water is added then bacterial growth may occur once more.
Warmth / Temperature
Bacteria have varying requirements in terms of the range of temperatures in which they will grow. Those which grow at low temperatures (usually below 20°C) are called psychrophiles and at high temperatures (above 45°C) are thermophiles. Some spoilage bacteria fall into these categories.
Most pathogens, however, like warmth and are known as mesophiles. They will grow at temperatures between 5°C and 63°C, commonly referred to as the growth or 'danger' zone and have an optimum temperature for growth of about 37°C. Listeria bacteria will grow very slowly below 5°C, but most pathogens become inactive (dormant) at low temperatures. They start to multiply more rapidly as the temperature rises.
At a temperature of about 37°C (human body temperature) pathogens multiply most quickly but as the temperature continues to rise, their rate slows down and they will stop growing altogether above 63°C. However, in order to destroy bacteria, temperatures must rise further. A temperature of 70°C for 2 minutes is recommended as a means of killing pathogens during the normal cooking process.
In ideal conditions (i.e. in moist foods at 37°C) bacteria will grow and multiply by dividing into two every 20 minutes. After 6 hours, in ideal conditions, one bacterial cell could become 131,072 bacteria.
Other Factors Affecting Growth
The acidity or alkalinity of foods will affect bacterial growth. Most bacteria like neutral conditions (pH value of 7) and will not grow in foods with a pH below 4.5, although if pathogens are introduced into an acid food, they may not necessarily die off immediately and could still cause illness.
Pathogens vary in their oxygen requirements. Those which require oxygen are called aerobes, e.g. Bacillus cereus. Those which do not need oxygen are called anaerobes, e.g. Clostridium perfringens and Clostridium botulinum. Those which will grow or survive with or without oxygen are known as facultative anaerobes and include Salmonella species and Staphylococcus aureus.
Where there are a number of different bacteria present in food, they compete for the same nutrients. Pathogens are often not as competitive as spoilage bacteria and unless present in high numbers, will usually die.