Also Known As
Sensitivity testing
Drug resistance testing
Formal Name
Susceptibility Testing
This article was last reviewed on
This article waslast modified on
15 January 2018.


The world is teeming with microorganisms - bacteria, fungi, parasites, and viruses. Our immune systems have evolved to fight those that threaten us and to co-exist peacefully with countless others. Every person has a balance of normal flora, a mixture of microorganisms that live on the surface of the skin and in the digestive tract. In general, these tiny life forms are helpful, aiding in the digestion of food and serving as a barrier against pathogenic (disease-causing) microorganisms. Most cause no problems unless there is a disruption in the balance, the patient’s immune system is depressed, or an injury or stressor creates a breach in the immune system’s protection. If this happens, there may be an opportunistic overgrowth of one type of normal flora, or one or more pathogenic microorganisms may find their way into the body and cause an infection.

When a patient’s immune system cannot rid itself of a pathogen, or restore its normal flora balance, then doctors turn to antimicrobials for assistance. These are drugs (antibacterial, antiviral, and antifungal) that have been developed to combat the offending microorganisms. They target different physical characteristics of the “bug” – such as its ability to multiply, its cell wall, or metabolism. Different antimicrobials are effective against different kinds of microorganisms. Some are narrowly focused, meant to eliminate one specific family of bacteria, for instance, without disrupting the balance of normal flora. Others are broad spectrum, developed to inhibit the growth of many microorganisms. When used, broad spectrum antimicrobials may affect both pathogens and normal flora.

Some microorganisms are resistant to certain antimicrobials, however. Susceptibility testing is often used to determine the likelihood that a particular drug treatment regimen will be effective in eliminating or inhibiting the growth of the infection. The remainder of this article explains how susceptibility (also called “sensitivity”) testing is performed and how drug resistance occurs, focusing exclusively on bacterial infections. (A companion piece focusing on viral resistance and susceptibility testing will soon follow.)

Accordion Title
More About Susceptibility Testing
  • Bacterial Susceptibility Testing

    When a doctor suspects that a patient has a bacterial infection, he requests a “culture and susceptibility test” to help determine the cause. (Your doctor may refer to this as a “culture and sensitivity,” - sensitivity is a more casual term for susceptibility that has been widely used.) Susceptibility is the likelihood that a particular antimicrobial will be effective in killing or sufficiently inhibiting the growth of specific pathogen that is causing an infection. Susceptibility testing measures whether or not the microorganism can grow when it is exposed to a variety of antimicrobials in a laboratory test.

    A culture of the infected area must be done to obtain the organism for identification and to allow susceptibility testing to be performed if warranted. Referred to by the type of body fluid or cells collected (such as: blood culture, urine culture, sputum culture, wound culture, etc.), the culture involves incubating a sample at body temperature in a nutrient- rich environment. This process promotes the growth (replication) of any microorganisms present in the sample. Samples from the skin, stool, or sputum will grow normal flora as well as pathogenic bacteria if they are present. Other body samples, such as blood and urine, are usually sterile; they will show little or no growth unless a pathogenic microorganism is present.

    The pathogenic bacteria are isolated (separated out from any other microorganisms present) and identified using biochemical and enzymatic tests. Each type of bacteria that may be clinically significant in the specimen (a pathogen) is tested individually to determine the ability of antimicrobials to inhibit its growth. Susceptibility testing is performed by growing the pure bacterial isolate in the presence of varying concentrations of several antimicrobials and then examining the amount of growth to determine which antimicrobials at which concentrations inhibit the growth of the bacteria. Results of the testing are reported as “Susceptible” (likely, but not guaranteed to inhibit the pathogenic microorganism), “Intermediate” (may be effective at a higher than normal concentration), and “Resistant” (not effective at inhibiting the growth of the organism). If there is more than one pathogen, the laboratory will report results for each one.

    Doctors choose an appropriate antimicrobial agent from those on the report that were categorized as “Susceptible.” If there are no “Susceptible” choices, then the doctor may select one categorized as “Intermediate.” This may mean a higher dosage of antimicrobial and may involve a longer duration of therapy and a higher risk for medication side effects. A pathogen may be “Resistant” to all of the antimicrobials that are usually used to treat that type of infection. If this is the case, then the doctor 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 due to resistant bacteria have proven very difficult to treat.

  • Bacterial Resistance

    Bacterial resistance may be innate (natural) or acquired. Some bacteria have an innate resistance to the specific target (bacterial multiplication, cell wall, or metabolism) of a particular type of antimicrobial. This resistance is part of the bacteria’s normal physical characteristics. Since bacteria 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 bacteria a survival advantage, it may be passed on to subsequent generations.

    An acquired resistance may develop through a selection process. When a patient is treated with an antimicrobial, the most susceptible bacteria are the ones that are killed first. If treatment is stopped before all of the pathogenic bacteria are killed, the survivors may develop a resistance to that particular antimicrobial. 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.

    Resistance can also develop when bacteria that are resistant share their genetic material with susceptible bacteria. This may occur more frequently in a healthcare setting, where many patients are treated with antimicrobials and many have compromised immune systems. For instance, resistant strains of bacteria, such as MRSA (meticillin resistant Staphylococcus aureus), have been a problem in hospitals for decades and are now becoming 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, and potentially across the world. Once a strain of bacteria has become resistant to one or more drugs, the only recourse is to try to inhibit its spread and to try to find another drug that will kill it. The second or third choice drugs that are available are often more expensive and toxic (associated with more side effects). This challenge is compounded by the fact that bacteria are becoming resistant faster than new antimicrobials are being developed.

  • Prevention

    Sooner or later, every antimicrobial will have one or more strains of bacteria that are resistant to it, and eventually its usefulness will diminish. The best that we can do is to try to push that time further into the future, to use antimicrobials appropriately, and prolong the time that they are effective. Steps that we can take to do this are listed below.

    Things that you can do:

    • Don’t ask your doctor for antibiotics when you don’t need them, such as when you or your child has a viral illness, such as the common cold or flu. (For a good source of information for parents on common illnesses in children, what causes them, and appropriate treatments, visit the following link:
    • When prescribed, take the full course of antibiotics as instructed.
    • Don’t take your own or anyone else’s left over antibiotics.
    • Limit the use of antibacterial products.
    • For multi-drug resistant tuberculosis, consider taking part in directly observed treatment (DOT) and commit yourself to the months of treatment prescribed.

    Things your community and doctor can do:

    • Don’t over prescribe antibiotics.
    • Track outbreaks of resistant bacteria; prompt identification, isolation, and treatment can minimize their spread.

    Things that are being done on a national level:

    • Reduction of the use of antibiotics in animal feed, only use what is necessary to keep animals healthy, not to promote faster growth. Avoid the use of antibiotics that are also commonly used to treat people. (This is a growing area of concern as resistant bacterial strains can arise in the treated animals and then share their resistance with pathogenic bacteria that affect humans).
    • Promotion of the wise usage of antibiotics.
  • Common Questions

    Would a doctor ever prescribe an antibiotic without or before performing a culture?
    Yes. In certain situations, a doctor may choose a therapy while a culture is incubating and in others, he may prescribe therapy without ever requesting a culture. 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 doctor may rely on his experience to deduce which “bug” is most likely to be causing the infection and to choose a drug 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 doctor chooses therapy to cover the most common pathogens that cause these infections.