Antibiotic Susceptibility Testing

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 care practitioner determine which drugs are likely to be most effective in treating a person’s infection.

Some 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, such as a wound infection. Susceptibility testing may be used to determine which antibiotic or antibiotic combinations will be most effective in treating all the different types of bacteria causing the infection.

Susceptibility testing is usually 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 drug would be more effective in treating the infection.

Results of the testing are usually reported as:

• Susceptible — likely, but not guaranteed to inhibit the pathogenic microbe; 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 in a laboratory test; may not be an appropriate choice for treatment

These categories are based on the minimum inhibitory concentration (MIC). This is the lowest concentration of an antibiotic that will be effective in inhibiting growth of the bacteria. Though results may be expressed as the MIC, in units such as micrograms/milliliter, 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 contain the results for each one.

A healthcare practitioner will choose an appropriate drug from those on the report that were categorized as “Susceptible.” If there are no “Susceptible” choices, or the patient is allergic to drugs in the susceptible category, 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 may have to be given intravenously, sometimes for extended periods of time. Some infections caused by 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 drug, unless the test is used to monitor the effectiveness of treatment.

Bacterial cultures usually require 24-48 hours to grow the pathogen and obtain a pure culture for further testing. Cultures for fungus and tuberculosis may take much longer — up to 6 to 8 weeks since these microbes grow more slowly. Traditional susceptibility testing assays require 18-24 hours of incubation; more rapid assays are becoming available that may provide results in less than 24 hours. Molecular tests to detect resistance genes vary from same day results to several days.

Since the wait time for results is a limitation of susceptibility testing, developing new, faster, susceptibility testing techniques is an active area of research. As the FDA approves more of these rapid tests, they are being added to the testing menu in clinical microbiology laboratories.

Yes. In certain situations, a health care practitioner may choose a therapy while a culture is incubating; and in other cases, the practitioner may prescribe therapy without ever ordering a culture based on knowledge and experience. While it is impossible to predict which microbe 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 E. coli to choose an antibiotic that is likely to be effective in most cases.

In addition, there are certain life-threatening infections, such as meningitis, 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 – inner ear infections. In these cases, the practitioner 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 microbe’s normal physical and genetic characteristics. Since microbes 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 microbe 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 drug, the most susceptible microbes 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 drug. 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 drug.

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

A resistant microbe may spread to other people throughout a community. Once a strain of bacteria has become resistant to one or more antimicrobial drug, 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 drugs that are available are often more expensive and associated with more side effects. This presents a challenge that is compounded by the fact that microbes are becoming resistant faster than new antimicrobial drugs are being developed.

Sometimes molecular methods are used to look for changes (mutations) in a microbe’s genetic material that enables it to grow in the presence of certain antimicrobial drugs. For instance, 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 microbe in their nasal passages can be isolated from other patients in the hospital so that the resistant staph are not transmitted to others.