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.