Bacteria are the most abundant of all organisms. Bacteria are unicellular organisms (made of one cell) and are very small in size, ranging from 0.5 to 5.0 micrometres (µm).
1.0 micrometre (µm) = 0.001 millimetres (mm); 1000 micrometres (µm) = 1mm. The symbol for the micrometre is µm. µ is a Greek letter pronounced ‘mu'.
Bacteria reproduce asexually. This means that they don't need a partner to reproduce, but simply divide into two, producing two new bacteria. There are pathogenic bacteria capable of causing human illness and food spoilage, but there are also beneficial species of bacteria that are essential to good health and a healthy environment. For example, beneficial bacteria live in our gut and help us digest our food; some bacteria are used to produce foods such as yoghurt and cheese; and others break down wastes in the environment.
Some bacteria are capable of forming highly resistant and endurable structures called spores. Bacterial spores are resistant to heat, freezing, drying, chemicals and other adverse environments. This means the spores can survive the normal processes of food storage and preparation. Two examples of spore-forming bacteria important in food contamination are Bacillus and Clostridium.
Temperature, humidity, oxygen and water are important for bacteria to grow and multiply. Under favourable conditions a growing bacterial population can double at regular intervals ranging from about 15 minutes to several hours. This means that the numbers of bacteria in food can increase rapidly and soon become hazardous to health, particularly if the food has a favourable temperature and water content. In the next section, we look in detail at factors that can promote or delay bacterial growth in our food.