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Susceptibility Testing

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Also known as: Sensitivity Testing; Drug Resistance Testing; Culture and Sensitivity; C & S; Antimicrobial Susceptibility
Formal name: Bacterial and Fungal Susceptibility Testing

The Test Sample

What is being tested?

Susceptibility is a term used when microorganisms such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed primarily on bacteria but also on fungi that have been identified with a culture as causing an individual's infection. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.

Although viruses are microorganisms, testing for their resistance to antiviral drugs is performed differently, so this article is limited to the discussion of bacterial and fungal susceptibility testing.

Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective. Susceptibility testing is a way to determine if this is the case when a culture of a sample collected from the site of a suspected infection is positive for the presence of one or more pathogens. (For more about cultures, see specific articles: Blood Culture, Urine Culture, Wound Culture, AFB Smear and Culture, Fungal Tests).

During the culture process, pathogens – if present – are isolated (separated out from any other microorganisms present) and each is identified using biochemical, enzymatic, or molecular tests. Once the pathogens have been identified, a determination can be made as to whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. An example of this is strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus) that can be treated with penicillin.

Susceptibility testing is performed on each type of bacteria or fungi that may be clinically significant in the specimen and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and the antibiotic together in a growing environment, such as nutrient media in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria.

How is the sample collected for testing?

A sample is obtained from a pure culture of bacteria or fungi suspected of causing an infection.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.