The surface of the skin is covered by millions of micro- organisms including bacteria, yeasts and mites. These resident bacteria are usually harmless and particularly like to live in areas which are moist, hairy and rich in sebaceous glands - oil glands.

The proportions of the different types of bacteria present varies from one individual to another and also according to different areas of the body, but remains fairly constant. These commensal bacteria help to defend the skin against infection by other pathogenic bacteria.

Bacteria are everywhere. They live on the skin of humans naturally without causing any infection. This is when they are called commensal. Literally this means eating together, but in reality they live harmlessly on the skin without any ill effect to the human.

However, when the skin is broken and damaged from a wound or a skin condition such as eczema, the bacteria can take hold in the skin and cause an infection.

Skin infections can be caused by three main types of bacteria:
Streptococci - a common cause of cellulitis, causes erysipelas and necrotising fasciitis.
Staphylococci - impetigo, ecthyma, boils, carbuncles and staphylococcal scalded skin syndrome.
Mycobacteria - infections include – cutaneous tuberculosis and leprosy.