How Does a Bacteria Culture Find an Infection?
A Bacteria Culture Test detects a sample of blood, urine, tissue or another body substance for bacteria that may be causing an infection.
In the lab, the sample is placed in a special substance called culture media. This creates the environment for bacteria to thrive. If there is growth, the laboratory professionals identify the organism.
The test may be used to investigate infections affecting the:
- Urinary tract
- Bloodstream
- Throat or lungs
- Digestive system
- Reproductive system
The findings of the result depend on where the sample came from. Bacteria found in a blood culture, for example, may be more significant than bacteria found in an area that naturally contains microorganisms.
Why the Sample Source Matters
There is no single sample used for every bacterial culture. Your healthcare provider chooses the type of sample based on where the infection may be.
Urine Culture
A clean catch midstream urine sample can be taken to test for bacteria linked with a urinary tract infection. A safe and correct sample collection avoids contamination from skin or genital area.
Blood Culture
Blood is collected using sterile techniques and placed in special culture bottles to check for bacteria in the bloodstream. More than one sample may be taken from different sites.
Throat Culture
A sterile swab is passed over the back of the throat and tonsils to collect a sample when a bacterial throat infection is suspected.
Sputum Culture
A sample of mucus coughed up from deep inside the lungs may be collected to investigate a lower respiratory infection. Saliva alone may not provide a suitable sample.
Wound Culture
A swab, fluid or tissue sample may be taken from a wound, abscess or infected area of skin. The surface may be cleaned before collection.
Stool Culture
A stool sample may be tested for certain bacteria that cause digestive illness. It should be collected without mixing it with urine or toilet water.
What to Know Before Sample Collection
Preparation depends on the specimen being collected.
Before providing a sample:
- Follow the instructions given for your specific sample
- Inform your healthcare provider if you are taking or recently took antibiotics
- Do not stop using prescribed antibiotics unless advised otherwise
- Use the sterile container, swab or collection kit provided
- Avoid touching the inside of the container or lid
- Return the sample within the recommended time
Antibiotics may reduce bacterial growth and affect the result. When appropriate, the sample may be collected before treatment begins. However, urgent treatment should not be delayed simply to obtain a culture.
Most cultures do not require fasting, although some sample types may need separate preparation.
How the Laboratory Grows and Identifies Bacteria
The sample may be placed on solid culture plates or into liquid culture media and kept under controlled conditions.
If bacteria grow, the laboratory may assess: how much bacteria grew, which types were found, and whether they are likely causing an infection or came from normal bacteria or contamination.
Some bacteria grow within a day, while others take longer. The reporting time depends on the specimen, the organism and whether further identification or sensitivity testing is required.
A preliminary report may sometimes be available before the result.
How Antibiotic Sensitivity Testing Helps
When clinically significant bacteria are identified, the laboratory may perform an antibiotic sensitivity test, also called antimicrobial susceptibility testing.
The report may describe the bacteria as:
- Susceptible: The antibiotic is likely to work when used correctly.
- Intermediate: The antibiotic may work in some cases. Or at a higher exposure.
- Resistant: The antibiotic is unlikely to work against the bacteria.
This information helps healthcare providers avoid antibiotics that may be ineffective. The infection site, symptoms and medical history are also considered before treatment is selected.
Patients should not start, stop or change antibiotics based only on the sensitivity report without medical advice.
When May a Culture Be Needed?
A bacterial culture may be requested when symptoms suggest an infection or when an existing infection is not improving as expected.
Possible reasons include:
- Pain or burning sensation while urinating
- Frequent urge to urinate
- Persistent sore throat
- Redness, swelling, pain or discharge from a wound
- Prolonged diarrhoea or blood in the stool
- Fever or chills
Urgent medical assessment may be needed for:
- State of confusion
- Having trouble while breathing
- Irregular heartbeat
- Severe weakness
- Low blood pressure
- Rapidly worsening symptoms
A culture can help identify bacteria, but people with signs of a serious infection should not wait for a routine laboratory appointment.
Understanding the Result and What Happens Next
A Bacteria Culture Test does not have one numerical normal range. Results are based on whether bacteria grew and what type of growth was found.
No Bacterial Growth
A negative culture or no growth result means that bacteria were not detected under the conditions used.
A negative result may mean that no bacterial infection was found, there were too few bacteria to detect, or antibiotics affected their growth. It may also mean that the sample did not contain the suspected bacteria or that the illness is caused by a virus, fungus or parasite, so further tests may be needed.
A negative result does not always rule out infection.
Bacterial Growth Detected
A positive culture means bacteria grew from the sample.
The report may tell you what type of bacteria was deteced, how much of them grew, whether they are likely causing an infection and what antibiotics might work against them.
A positive result does not automatically mean that treatment is required. The healthcare provider considers the specimen source, symptoms, amount of growth and whether the bacteria normally exist in that part of the body.
Mixed Growth or Possible Contamination
Mixed growth means that than one type of bacteria is present in the sample. This makes it hard to figure out which bacteria is causing the infection.
Mixed growth happens when bacteria, from our skin or mouth or genital area gets into the sample when it is being collected. A repeat specimen may be requested.
Normal Flora
Some parts of the body naturally contain bacteria that do not usually cause disease. These are known as normal flora.
The laboratory and healthcare provider consider:
- Where the sample came from
- The type and amount of bacteria
- Patient’s symptoms
- Whether the organism normally lives in that area
- Other laboratory findings
Finding a bacterial name on the report does not automatically confirm an infection.
Depending on the result, further steps may include:
- Repeating the culture
- Collecting a new or different sample
- Reviewing antibiotic sensitivity results
- Complete Blood Count or C-Reactive Protein test
- Imaging to look for an abscess or another source of infection
- Testing for viruses, fungi or parasites
Treatment depends on the infection site, organism and severity. Antibiotics do not treat viral infections and should only be used when prescribed.
Reference methods and reporting criteria vary depending on the specimen, laboratory and culture technique. Individuals must consult their healthcare provider for correct interpretation of the results.
Book An Appointment
Need a Bacteria Culture Test? Book an appointment with Genex Clinical Laboratories for specimen collection, bacterial identification and microbiology testing.
