Susceptibility testing is used to determine which antimicrobial will inhibit the growth of the bacteria or fungi causing your infection. The results from this test will help your doctor determine which antibiotic will be most effective in treating your infection.

Some types of bacteria and fungi are known to be susceptible to certain antibiotics, 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 antibiotics 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 the different types of bacteria causing the infection. This may be true, for example, with wound infections.

This test is often ordered at the same time as a culture of a potentially infected site such as wound, urine, or blood. However, the test can only done when the results of the culture is positive for one or more pathogens. The test may also be ordered when your 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 your infection.

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; highly unlikely to be an appropriate choice for treatment

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

Your doctor will choose an appropriate antimicrobial agent from those on the report that were categorised as "Susceptible." If there are no "Susceptible" choices, then the doctor may select one categorised 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.

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.

Cultures usually require at least 24 hours before results are available. Once a pure sample of the microorganism is obtained, the susceptibility testing will another 16-72 hours depending on the method used and the speed of growth of the organism. Cultures for fungus and tuberculosis may take much longer — up to 6 to 8 weeks.

Yes. In certain situations, a doctor may choose a therapy while a culture is incubating and in others, they 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 doctor 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 such as meningitis or sepsis that must be treated immediately, with no time to wait for the results of a culture before commencing treatment. 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.

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 a patient 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, and potentially across the world. 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.

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) contains the mecA gene that confers resistance to the beta-lactam antibiotics including methicillin, dicloxacillin and cephalexin. Detection of the mecA gene using a molecular based test allows the rapid detection of MRSA prior to culturing the bacteria. The person carrying this organism in their nasal passages may be isolated from other patients in the hospital so that the resistant organism 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 and related antibiotics ineffective. This test can be used to determine whether the bacteria produce this enzyme and are therefore resistant to penicillin and other similar drugs.