Patient Test Information

Susceptibility Testing

Also known as:

Sensitivity Testing; Drug Resistance Testing; Culture and Sensitivity; C & S; Antimicrobial Susceptibility

Formal name:

Bacterial and Fungal Susceptibility Testing

Related tests:

Urine Culture; Blood Culture; Bacterial Wound Culture; AFB Testing; MRSA; Fungal Tests; Sputum Culture; Stool Culture; Gram Stain; Body Fluid Analysis; Pleural Fluid Analysis; Pericardial Fluid Analysis; Peritoneal Fluid Analysis; Synovial Fluid Analysis; CSF Analysis

Why Get Tested?

To determine the likelihood that a particular antibiotic or antifungal drug will be effective in stopping the growth of the bacteria or fungi causing your infection

When to Get Tested?

As follow up to a positive bacterial or fungal culture; when you have an infection and one or more types of bacteria or fungi have been grown and isolated in a culture from a sample obtained from the site of suspected infection; when your infection is not responding to treatment

Sample Required?

A sample of a pure culture of bacteria or fungi grown and isolated from an infected body site

Test Preparation Needed?

None

How is it used?

Susceptibility testing is used to determine which antimicrobials will inhibit the growth of the bacteria or fungi causing a specific infection. The results from this test will help a health practitioner determine which drugs are likely to be most effective in treating a person's infection.

Like bacteria, viruses can develop resistance to the drugs used to treat them, but that type of testing is performed differently, so this article is limited to the discussion of bacterial and fungal susceptibility testing.

Some types of bacteria and fungi are known to be susceptible to certain antimicrobials, so routine testing may not always be necessary. For example, it is well known that most streptococci, including the type that causes strep throat, can be treated with penicillin, so these types of infections may be treated without susceptibility testing. However, if the usual drug of choice fails to treat the infection, then susceptibility testing may be necessary to determine a more effective drug.

Other types of infections may require testing because the bacteria or fungi isolated from an infection site are known to have unpredictable susceptibility to the drugs usually used to treat them. Some examples include staphylococci ("staph") and Pseudomonas aeruginosa. Sometimes there may be more than one type of pathogen isolated from an infected site. Susceptibility testing may be used to determine which antibiotic or antibiotic combinations will be most effective in treating all of the different types of bacteria causing the infection. This may be true, for example, with wound infections.

When is it ordered?

Susceptibility testing is often ordered at the same time as a culture of a potentially infected site, such as a wound, urine, or blood culture. However, the test will usually only be performed when the culture is positive for one or more pathogens. The test may also be ordered when an infection does not respond to treatment to see if the pathogen has developed resistance and to determine which antimicrobial agent would be more effective in treating the infection.

What does the test result mean?

Results of the testing are usually reported as:

  • Susceptible – likely, but not guaranteed to inhibit the pathogenic microorganism; may be an appropriate choice for treatment
  • Intermediate – may be effective at a higher dosage, or more frequent dosage, or effective only in specific body sites where the antibiotic penetrates to provide adequate concentrations
  • Resistant – not effective at inhibiting the growth of the organism; may not be an appropriate choice for treatment

Sometimes results are reported as minimum inhibitory concentration (MIC), in units such as milligrams per microliter (mg/mcL). This is the highest dilution (lowest concentration) of an antibiotic that will still be effective in inhibiting growth of the bacteria. Though results may be expressed this way, the laboratory will often include in the report an interpretation of what the results mean (e.g., susceptible, intermediate or resistant).

If there is more than one pathogen identified in a culture, the laboratory will report results for each one.

A health practitioner will choose an appropriate drug from those on the report that were categorized as "Susceptible." If there are no "Susceptible" choices, then the practitioner may select one categorized as "Intermediate." This may require a higher dosage and may involve a longer duration of therapy as well as a higher risk for medication side effects.

A pathogen may be "Resistant" to all of the drugs that are usually used to treat that type of infection. If this is the case, then the practitioner may prescribe a combination of antibiotics that work together to inhibit the bacteria when neither one alone will be effective. These drug therapies may be more expensive and have to be given intravenously, sometimes for extended periods of time. Some infections caused by resistant bacteria have proven very difficult to treat.

Is there anything else I should know?

A sample for culture and susceptibility testing should be collected before the start of any treatment with an antimicrobial unless the test is used to monitor the effectiveness of treatment. 

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.

  1. How long will it take to get my results?

    Cultures usually require 24-48 hours before results are available. Once the culture is complete and a pure sample of the microorganism is obtained, the susceptibility testing may take about another 24-48 hours depending on the method used. There are commercial tests available that offer rapid susceptibility testing and that may produce results in less than 24 hours. Cultures for fungus and tuberculosis may take much longer – up to 6 to 8 weeks.

  2. Would a doctor ever prescribe an antibiotic without or before performing a culture?

    Yes. In certain situations, a health practitioner may choose a therapy while a culture is incubating and in others, may prescribe therapy without ever ordering a culture based on knowledge and experience. While it is impossible to predict which microorganism is causing an infection unless a culture is performed, some organisms are seen more frequently than others. For instance, most urinary tract infections (UTIs) are caused by the bacterium Escherichia coli. Knowing this, a practitioner may rely on current susceptibility patterns for this bacterium to choose an antibiotic that is effective in most cases. In addition, there are certain life-threatening infections that must be treated immediately, with no time to wait for the results of a culture. In other instances, a culture would not be attempted because a specimen may not be obtainable (such as with otitis media - ear infections) or the pathogen may not be easily isolated from other flora in the specimen (such as with community-acquired pneumonia). In these cases, the practitioner chooses therapy to cover the most common pathogens that cause these infections.

  3. How do microorganisms become resistant to drugs?

    Resistance may be innate (natural) or acquired. Natural resistance is part of the microorganism's normal physical characteristics. Since microorganisms multiply very rapidly, they go through many generations in a short period of time. There is always the potential for antimicrobial resistance to arise through a genetic change (mutation). If this change gives the microorganism a survival advantage, it may be passed on to subsequent generations.

    An acquired resistance may develop through a selection process. When someone is treated with an antimicrobial agent, the most susceptible microorganisms are the ones that are killed first. If treatment is stopped before all of the pathogens are killed, the survivors may develop a resistance to that particular antimicrobial agent. The next time they are exposed to the same drug, it may be ineffective as the bacteria and their progeny are likely to retain resistance to that antimicrobial agent.

    Resistance can also develop when microorganisms that are resistant share their genetic material with susceptible ones. This may occur more frequently in a health care setting, where many patients are treated with antimicrobial agents. For instance, resistant strains of bacteria, such as MRSA (methicillin resistant Staphylococcus aureus), have been a problem in hospitals for decades and are increasingly common in the community.

    When a resistance trait arises in bacteria, for whatever reason, the resistant organism may spread to other people throughout a community. Once a strain of bacteria has become resistant to one or more antimicrobial agent, the only recourse is to try to inhibit its spread and to try to find another one that will kill it. The second or third choice antimicrobial agents that are available are often more expensive and associated with more side effects. This presents a challenge that is compounded by the fact that microorganisms are becoming resistant faster than new antimicrobial agents are being developed.

  4. Are there other ways of testing for resistance?

    Another way to test for resistance is by using molecular methods to test for changes (mutations) in a microorganism's genetic material that enables it to grow in the presence of certain antimicrobial agents. Methicillin-resistant Staphylcoccus aureus (MRSA) contain the mecA gene that confers resistance to the antibiotics methicillin, oxacillin, nafcillin, and dicloxacillin. Detection of the mecA gene using a molecular based test allows the rapid detection of MRSA prior to culturing the bacteria. Someone carrying this organism in their nasal passages can be isolated from other patients in the hospital so that the resistant staph are not transmitted to others.

    Another example of testing for resistance is a test for beta-lactamase, an enzyme produced by some bacteria that makes penicillin ineffective. This test can be used to determine whether the bacteria produce this enzyme and are therefore resistant to penicillin and other similar drugs. However, this test is rarely performed and only on certain groups of bacteria, for example, Haemophilus influenzae and anaerobic bacteria.