A plate showing bacteria colonies

Colonies of bacteria from a swab sample growing on a laboratory plate. (Image: Monika Wisniewska /

Bacteria are tiny organisms that are too small to see with the naked eye. They are found in humans, animals, food, and the environment. Most bacteria are harmless but some can cause infections or disease.

Most bacteria in the body are harmless

The human body is a natural home for many bacteria. Some of these bacteria live harmlessly on the skin, in the mouth and intestines (gut) and perform useful functions in our bodies. Many different types of bacteria live in the gut (such as E. Coli or Escherichia coli) where they help with digestion, prevent harmful bacteria from growing, and make vitamins such as vitamin B7 (biotin) and vitamin K.

Some bacteria cause infections

Some bacteria can cause infections. For example, Streptococcus pneumoniae causes many respiratory tract infections including pneumonia and sinusitis. In addition to other chest infections, Klebsiella pneumoniae causes pneumonia and bronchitis.

Antibiotics fight bacteria

Antibiotics can be used to fight bacteria, and different antibiotics target different bacteria. Laboratory tests (for example throat swabs) will usually list the type of bacteria on the report. Knowing which bacteria are causing your infection can help your doctor to choose the best antibiotic treatment for you.

If you are generally healthy, your body’s immune system can usually fight minor bacterial infections (e.g. a 'strep' throat caused by streptococcal infection) without antibiotics. By not taking antibiotics when you don’t need them you will help reduce the spread of antibiotic resistance.

Preventing bacteria from spreading

Some bacteria (e.g. the bacteria that cause tuberculosis [TB]) and viruses (e.g. the cold viruses) are very infectious and can spread easily from person to person. When someone with the infection coughs, sneezes, laughs or talks, droplets carrying the infection become airborne and may infect other people.

Help to prevent the spread of respiratory tract infections and other infectious diseases by:

  • regularly washing your hands with soap and running water particularly before preparing and eating food and after blowing your nose
  • coughing and sneezing into a tissue then throwing it away
  • covering your mouth when sneezing or coughing
  • keeping your hands away from your eyes, nose and mouth
  • avoiding sharing cups, glasses and cutlery when eating or drinking.