Test Overview

A skin or wound culture is a test to find germs (such as bacteria or a fungus) that can cause an infection. A sample of skin, tissue, or fluid is added to a substance that promotes the growth of germs. If no germs grow, the culture is negative. If germs that can cause an infection grow, the culture is positive. The type of germ may be identified with a microscope or chemical tests. Sometimes other tests are done to find the right medicine for treating the infection. This is called sensitivity testing.

Most bacteria can grow in oxygen. They are called aerobic bacteria and usually are found in wounds close to the skin surface (superficial). Bacteria that cannot grow in the presence of oxygen (anaerobic) usually are found in deeper wounds and abscesses. A wound culture can find out whether bacteria are aerobic or anaerobic.

A fungal culture is done to find out if an infection is caused by a fungus. A viral culture can be done to find out whether an infection is caused by a virus.

Some types of bacteria that normally live on or in the body can cause an infection if they go to parts of the body where they are not normally found. For example, E. coli bacteria are normally found in the colon and anus. But if E. coli bacteria spread from the anus to the urethra, the bacteria may cause a urinary tract infection (UTI).

Culture samples may also be collected from the ear or eye, from open or closed sores, or from nails and hair.

Why It Is Done

A skin or wound culture is done to:

• Find the cause of an infection in a sore, burn, surgical wound, or injury. An injury includes animal bites, human bites, marine stings or scrapes, cuts, and puncture wounds that are more likely to get infected.
• Make decisions about the best treatment for an infection. This is called sensitivity testing.

How To Prepare

In general, there's nothing you have to do before this test, unless your doctor tells you to. If you are taking or have recently taken antibiotics, tell your doctor.

How It Is Done

To collect a tissue or fluid sample from a wound, a sterile swab is inserted into the wound. The health professional collecting the sample may press around the wound and gently turn the swab to collect as much tissue or fluid as possible. The swab is then placed into either an aerobic or anaerobic culture tube or both, depending on the type of organism suspected.

A needle may be used to collect fluid from a wound that is covered (scabbed-over) or from an abscess. The fluid is then placed in the culture tube.

Your doctor may need to remove a sample of skin or tissue (biopsy) for testing. If collecting the sample is likely to cause pain, you may be given a shot to numb the area (local anesthetic) first.

Once a sample is collected, it is placed in a container with a substance (called growth medium or culture medium) that helps bacteria, fungus, or viruses grow.

• Bacteria usually need about 1 to 2 days to grow.
• Fungi usually need several days to grow.
• Viruses need to be placed in a container with living cells and can take weeks to grow.

Any bacteria, fungi, or viruses that grow will be identified with a microscope, chemical tests, or both. If sensitivity testing is done to help make decisions about treatment, more time will be needed.

How It Feels

If you have a sample of fluid or tissue collected from a wound, you may feel some pain when the sample is collected. You may feel a short, sharp sting if you are given a shot of anesthetic to numb the area where the culture sample will be taken.

Risks

There is a very slight risk of spreading some infections if a biopsy is needed to collect the sample.

Results

Some types of bacteria, fungi, and viruses grow quickly in culture, and some grow slowly. Test results may take from 1 day to several weeks, depending on the type of infection suspected.

If test results are positive, sensitivity testing may be done help make decisions about treatment.