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Microorganisms are tiny single cell living organisms. All humans have microorganisms on their body. Sick people have more microorganisms or “germs”. Microorganisms are abundant on every surface and as they are not visible except under a microscope, it is easy to forget they are there.

Over 95% of all micro organisms are harmless to humans and many are essential to our well-being. The small group of micro-organisms which cause illness are commonly known as “germs”. The correct name for these microorganisms which cause human illness are pathogens.

There are several types of microorganisms:

Yeasts and moulds occur on food, mostly causing it to spoil. Yeasts and moulds are often able to be detected with an unattractive smell and look. Moulds are often visible as “fuzzy” or hairy.They are largely destroyed by a combination of heat and lack of oxygen.

Viruses are transferred from living organisms such as humans to food. They do not multiply in food but use it to transfer from person to person.

Bacteria are the microorganisms which are mostly responsible for food borne illness as they contaminate food with dangerous toxins. Many bacteria are harmless to people and can be useful in industrial and pharmaceutical processes. All humans and food items have a range of bacteria in and on them, at any one time.

Bacteria like other microorganisms grow well in the following conditions:

  1. protein rich food
  2. moisture-( sometime terms water activity, Aw)
  3. pH (bacteria prefer a neutral pH)
  4. warmth (5°C- 60°C)
  5. oxygen
  6. time

If these conditions are provided, bacteria will flourish and multiply over time.

Bacteria grow in moist low acid, protein foods such as meat, poultry, seafood, dairy foods, and cooked cereals such as rice. Dried products have less available water, so bacteria can survive but do not grow and multiply in dry conditions. The use of salt, sugar and alcohol in a product can also decrease the water activity of a food, reducing the growth of bacteria.

Food poisoning bacteria do not grow in acidic or alkaline foods; they prefer neutral pH foods with a ph from 7 down to 4.5. Adding an acid or alkali to foods can act as a preservative, as they inhibit the growth of dangerous food poisoning bacteria, as well as other microorganisms.

Bacteria generally multiply quickly in warm conditions. The temperature zone between 5°C - 60°C is known as the danger zone for food poisoning bacteria, as they multiply and grow to dangerous levels quickly at these temperatures.

Heat above 75°C kills most food poisoning bacteria and below 5°C, bacteria survive, but don’t grow and multiply. Refrigeration and freezing does not kill food poisoning bacteria, it only stops their growth.

Some food poisoning bacteria can survive above the danger zone, with a spore forming heat resistant coating. This means that some bacteria can start to grow again in cooked foods, once the food has time in the danger zone of 5°C -60°C. For this reason cooked foods must either remain hot, above 60°C or be chilled quickly, to prevent spores multiplying in the food when it is warm.

Some bacteria require oxygen to survive and do not survive when food is vacuum packed. Some bacteria do not require oxygen and can grow in sealed, packaged food items. These are known as anaerobic bacteria. Some bacteria can survive with or without oxygen; these are known as facultative bacteria.

Some common food poisoning bacteria are:

It is unfortunate that several food poisoning bacteria, such as salmonella, clostridium and staphylococcus can survive without oxygen and grow in vacuum packed and sealed food packages.

It is bacteria’s ability to divide and multiply rapidly in warm temperatures, their adaptability to survive high heat and no oxygen, which makes bacteria the major challenge of all the microorganisms for contamination and food borne illness.

Information on the conditions for growth and common food sources for each of these bacteria, that cause food borne illness can be found on the NSW Food Authority website (external website).

For effective control of microorganisms, the conditions of food, moisture, warmth and oxygen should be limited. Understanding the concept of temperature control, and minimising the time food spends in the “Danger Zone” of between 5°C - 60°C is central to keeping food safe, and minimising contamination.

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