Staphylococcal infections - Causes of staphylococcal infections

There are many types of staphylococcus bacteria, but most infections are caused by Staphylococcus aureus (S. aureus).

S. aureus is common in humans. It's often found inside the nose and on the surface of the armpits and buttocks. In most cases, the bacteria do not cause any symptoms. If a person has bacteria living on their body but they do not experience any symptoms, they are said to be colonised by bacteria.

In England, it is estimated that 4 out of 5 people will be colonised by S. aureus at least once in their life, and that 1 out of 5 people are persistently colonised.

Skin infections

Staphylococcal (staph) bacteria can spread through the air, by person-to-person contact and they can also be picked up from contaminated surfaces.

Once the bacteria reach the skin, they can go on to cause infection if they become heavily concentrated on the surface of the skin or if they enter the body through a break in the skin.

Staph bacteria often enter the body through an inflamed hair follicle or oil gland. They can also enter through skin damaged by burns, cuts and scrapes, other infections or insect bites.

PVL-producing Staphylococcus aureus

A minority of S. aureus bacteria can produce a substance called Panton-Valentine leukocidin (PVL), which kills the white cells, causing the body to make more white cells to continue to fight the infection.

PVL-producing strains of bacteria are therefore more likely to cause repeated skin infections, such as boils and abscesses. They can also cause more serious conditions such as:

• sepsis – blood poisoning caused by bacteria multiplying in the blood
• pneumonia – swelling (inflammation) of the lungs caused by an infection

Invasive staphylococcal infections

In healthy people, the layers of skin and the immune system usually provide a good defence against a skin infection spreading further into the body. 

An invasive staph infection usually only develops if you:

• have a weakened immune system due to an underlying medical condition or due to a side effect of treatment
• use medical equipment that goes directly inside your body, and/or
• experience severe trauma to the skin, such as a deep wound or a major burn

Medical equipment that increases your risk of developing invasive staphylococcal infections include:

• equipment used to perform all types of dialysis
• urinary catheters (a tube used to empty the bladder)
• feeding tubes
• breathing tubes

Food poisoning

Staphylococcal food poisoning is usually caused by food contaminated with staphylococcal bacteria from the skin of someone who handles food.

If the food is not cooked thoroughly, or if it's not kept hot or cold, the bacteria will continue to reproduce when they come in contact with it. The bacteria then produce the toxin (poison) that causes food poisoning.

It is not possible for staph food poisoning to be spread from person-to-person.

Read more about the causes of food poisoning.