Bacteriology & Antibiotics

Antibiotic sensitivity

Antibiotic sensitivity or antibiotic susceptibility is the susceptibility of bacteria to antibiotics. Because susceptibility can vary even within a species (with some strains being more resistant than others), antibiotic susceptibility testing (AST) is usually carried out to determine which antibiotic will be most successful in treating a bacterial infection in vivo. Testing for antibiotic sensitivity is often done by the Kirby-Bauer method. Small wafers containing antibiotics are placed onto a plate upon which bacteria are growing. If the bacteria are sensitive to the antibiotic, a clear ring, or zone of inhibition, is seen around the wafer indicating poor growth.

Ideal antibiotic therapy is based on determination of the aetiological agent and its relevant antibiotic sensitivity. Empiric treatment is often started before laboratory microbiological reports are available when treatment should not be delayed due to the seriousness of the disease. The effectiveness of individual antibiotics varies with the location of the infection, the ability of the antibiotic to reach the site of infection, and the ability of the bacteria to resist or inactivate the antibiotic. Some antibiotics actually kill the bacteria (bactericidal), whereas others merely prevent the bacteria from multiplying (bacteriostatic) so that the host's immune system can overcome them. Müeller-Hinton agar is most frequently used in this antibiotic susceptibility test.

An antibiogram is the result of an antibiotic sensitivity test. It is by definition an in vitro sensitivity, but the correlation of in vitro to in vivo sensitivity is often high enough for the test to be clinically useful.

Before starting this treatment, the physician will collect a sample from a suspected contaminated compartment: a blood sample when bacteria possibly have invaded the bloodstream, a sputum sample in the case of a ventilator associated pneumonia, and a urine sample in the case of a urinary tract infection. These samples are transferred to the microbiology lab, which looks at the sample under the microscope, and tries to culture the bacteria. This can help in the diagnosis.