Antibiotic susceptibility is a measurement of how sensitive a population of organisms is to antibiotics. Within a given population, some individuals are highly susceptible and some are highly resistant, and the goal is to find a method of treatment that will kill most of the organisms. Once the bulk of the infection has been addressed, the immune system is much more capable of attacking the infection and killing off any remaining individuals.
There are several techniques available for testing antibiotic susceptibility. One of the most basic involves growing a population of bacteria in culture and adding small discs coated with different kinds of antibiotics. If the organisms are sensitive to a given medication, a dead zone will appear around the disc because bacterial growth will be inhibited. Using this information, doctors can decide on the medications they will use to treat a patient.
Applications for antibiotic susceptibility testing are varied. These tests can be used while developing new antibiotic medications for the purpose of identifying organisms to target with the medications. They can also be used in the evaluation of a patient with a stubborn infection, to test susceptibility in vitro rather than in the patient, allowing doctors to prescribe an effective medication as quickly as possible. Studies on susceptibility to antibiotics are also used to learn more about how organisms evolve and respond to antibiotic exposure.
Some bacteria have very low antibiotic susceptibility. This can be a natural trait, and over time, as antibiotics are used and the organisms are exposed to them, the organisms with the most resistance survive. These go on to breed, contributing to an increase of resistance in the overall population. Antibiotic resistance is a concern in medical treatment around the world, especially when it comes to infections acquired in hospitals, where patients are already weak and less capable of fighting infection, and in patients who are immunocompromised, like AIDS patients.
Even in organisms known to have very high antibiotic susceptibility, there are still a few individuals with more resistance. It is important to complete courses of antibiotics to make it less likely that these individuals survive, thereby keeping the population generally susceptible and making it easier to treat infections with these organisms. If people fail to finish antibiotics or do not follow administration directions, there is a risk that they will contribute to the development of resistance in the population, making those drugs less effective for recurrence of the infection in themselves, as well as in other patients in the future.