Susceptibility Testing

Susceptibility is a term used to describe the condition in which microorganisms are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing determines the potential effectiveness of an antimicrobial agent on the organism causing an infection and/or determines if the organism has developed resistance to certain antibiotics. The results of this test can be used to predict the potential effect in the patient. 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 antimicrobial agents 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 your culture is positive for the presence of pathogens. A culture of the infected area must first be performed on a sample from the site of suspected infection to see if any bacteria or fungi are present that may be causing your infection. (For more about cultures, see the 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 identified using biochemical, enzymatic, or molecular tests. Once they 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).

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.

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

No test preparation is needed.