Skin and Wound Cultures
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
You do not need to do anything before you have this test. If you are taking or have recently taken antibiotics, tell your doctor.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form( What is a PDF document? ).
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.
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.
There is a very slight risk of spreading some infections if a biopsy is needed to collect the sample.
A skin or wound culture is a test to find and identify germs (such as bacteria, a fungus, or a virus) that may be growing on the skin or in a wound.
Some types of bacteria, fungi, and viruses grow quickly in culture, and some grow slowly. Test results may take from one day to several weeks, depending on the type of infection suspected.
No large numbers of harmful germs are found on the skin or in the wound. Normal culture results are negative.
Harmful germs are found on the skin or in the wound. Abnormal culture results are positive.
If test results are positive, sensitivity testing may be done help make decisions about treatment.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking or having recently taken antibiotics.
- Getting bacteria that is normally found on the skin in the tissue or wound sample.
What To Think About
- Types of bacteria that commonly cause infection in wounds are staph ( Staphylococcus), strep ( Streptococcus), and Clostridium perfringens, a bacterium found in soil and in stool (feces) that can cause gangrene. The most common type of fungus that causes infection in wounds is Candida albicans.
- Looking at fluid (such as pus) from a wound under a microscope can sometimes help identify the type of bacteria or fungi causing the infection before culture results are ready.
- A culture that does not grow any bacteria may not mean that you do not have an infection. Sometimes the amount of sample collected, the age of the wound (or skin problem), the type of culture done, and previous use of antibiotics can prevent the growth of bacteria in the culture.
- Testing for a virus may be done to detect and identify a viral infection in the body that is causing symptoms. To learn more, see the topic Viral Tests.
- Most types of fungi grow very slowly and may not show up in a culture for several weeks. Your doctor may recommend that you start treatment before your culture results come back if he or she thinks you could have a fungal infection.
- Sensitivity testing helps your doctor choose the best medicine to treat specific types of bacteria, viruses, or fungus.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.