Pus for Culture and Sensitivity?
Pus for Culture Aerobic Wound Culture, Anaerobic Wound Culture. A bacterial wound culture is a test that detects and identifies bacteria that cause infections (pathogenic) in a wound. Any wound may become infected with a variety of bacteria. A culture helps to determine whether a wound has become infected, which type(s) of bacteria are causing the infection, and which antibiotic would best treat the infection and help heal the wound.
Wounds may be superficial breaks in the skin such as scrapes, cuts, and scratches or may involve deeper tissues such as incisions, bites, punctures, or burns. (Read the article on Wound and Skin Infections.) A culture is performed by collecting a sample of fluid, cells or tissue from the wound and placing it on or in appropriate nutrient media. The media encourages the growth of bacteria that may be present, allowing for further testing and identification.
Typically, only one kind of pathogenic bacteria is causing the infection in a wound. However, there may be several types of normal skin bacteria present in the culture. Separating the various types of bacteria and identifying the pathogenic bacteria requires one or more days to perform.
A Gram stain is usually performed to help determine the type of bacteria present and provide a rapid result to the healthcare practitioner. The shape and color (morphology and staining characteristics) also help determine what other tests may need to be performed to definitively identify the cause of infection.
Because the results of the stain read under the microscope are not definitive, further tests such as biochemical reactions or mass spectrometry must be performed to identify the bacteria. Mass spectrometry using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) can provide identification to the genus and species level in less than an hour after the bacterial colony is grown on the culture media. This technique significantly decreases the time needed to identify bacteria from traditional biochemical reactions that require overnight incubation.
For many of the pathogens identified in wound cultures, testing is done to determine which antibiotics will be effective in inhibiting the growth of the bacteria (see Antibiotic Susceptibility Testing). The Gram stain of the wound, the culture, and susceptibility testing all contribute to informing the healthcare practitioner which pathogen(s) are present and what antibiotic therapy is likely to inhibit their growth.
Why is sensitivity analysis done?
Many bacteria are resistant to common antibiotics. This means that the drug can’t kill the bacteria. Sensitivity analysis is a useful tool to help quickly determine if bacteria are resistant to certain drugs.
Examples of antibiotic-resistant infections include:
- a persistent sore throat
- a recurring urinary tract infection (UTI)
- an unresponsive case of pneumonia
Sensitivity analysis may be ordered if your infection doesn’t respond to treatment.
This can help your doctor to see if the bacteria that’s causing your infection has developed resistance. Your doctor can then determine which drug would be more effective in treating the infection.
How is sensitivity analysis performed?
Sensitivity analysis starts with a bacterial sample. Your doctor will get this sample by sampling the infected area. Your doctor can sample any area that has an infection.
Samples may be taken from:
- sputum (spit)
- inside the cervix
- a pus-containing wound
Your doctor will send the sample to a laboratory, where it’ll be spread on a special growing surface. The grown bacteria is known as a culture, and bacteria in the culture will grow and multiply.
The bacteria will form colonies, or large groups of bacteria, that will each be exposed to different antibiotics.
These colonies can be susceptible, resistant, or intermediate in response to the antibiotics:
- Susceptible means they can’t grow if the drug is present. This means the antibiotic is effective against the bacteria.
- Resistant means the bacteria can grow even if the drug is present. This is a sign of an ineffective antibiotic.
- Intermediate means a higher dose of the antibiotic is needed to prevent growth.
Why Get Tested?
To detect and identify the bacteria causing an infection of your wound and to help guide treatment of your wound infection
When To Get Tested?
When you have signs and symptoms of wound infection, such as a wound that is tender, red, swollen, draining fluid or pus, and slow to heal
A sterile swab may be used to collect cells or pus from a superficial wound site. From deeper wounds, aspirations of fluid into a syringe and/or a tissue biopsy are the optimal specimens to allow for the recovery of aerobic and anaerobic bacteria.
How is it used?
A bacterial wound culture is primarily used, along with a Gram stain and other tests, to help determine whether a wound is infected and to identify the bacteria causing the infection.
If a culture reveals that a wound is infected, susceptibility testing is done to determine which antibiotic will inhibit the growth of the bacteria causing the infection. This is often performed automatically as part of the original order on the wound specimen so that antimicrobial susceptibility results can guide appropriate therapy as soon as possible.
A wound culture may also sometimes be ordered for an individual who has undergone treatment for a wound infection to determine whether the treatment was effective. It may also be ordered at intervals for a person who has a chronic infection to help guide further treatment.
If a fungal infection is suspected, then a fungal culture of the wound specimen may be ordered along with the bacterial wound culture. Yeast and some fungi may grow on the same media as bacteria, but many fungi are slow-growing. The media used to recover fungi inhibits bacterial growth and supports fungal growth for several weeks.
What are the results for a sensitivity analysis?
Once the bacterial cultures have been grown and tested with antibiotics, your doctor can analyze the results. These results can help determine the best antibiotic to treat your infection.
Your doctor will usually choose an appropriate drug from the report that was listed as “susceptible,” meaning it can fight the bacteria.
You’ll be prescribed a drug from the “intermediate” group if there are no known drugs available in the susceptible group.
You’ll likely have to take a higher dosage and for a longer time period if you’re taking a drug from the intermediate group. You may also experience medication side effects.
An antibiotic that bacteria has shown resistance to shouldn’t be used to treat your infection. Your doctor will decide which drug is best if several antibiotics are shown to be effective in killing the microorganism causing your infection.
You may be prescribed a combination of antibiotics if a bacterium is “resistant” to all of the drugs that are usually used to treat an infection. This combination of drugs is meant to work together to fight the bacteria.
Drugs in this category can be more expensive and may have to be given intravenously (through a needle in your vein). You’ll also likely have to take the combination of drugs for an extended time period.
Some infections may require further testing because it’s known that the drugs normally used to treat the bacteria or fungi causing the infection aren’t always effective.
It’s also possible for the sample taken from the infection to have more than one microorganism.
Susceptibility testing may be used to figure out which antibiotic or combination of antibiotics will be most effective in treating the different types of bacteria causing the infection.