What Is an Antibiogram?

An antibiogram test can be used for two different purposes. The Kirby-Bauer method is used to test the susceptability of a bacterial strain to various antibiotics. Antibiograms can also determine the minimal inhibitory concentration (MIC), or the smallest concentration of an antibiotic that will inhibit the growth of bacterial strain, using the dilution method.

The Kirby-Bauer method is an antibiogram used to determine the best antibiotic to use against a particular bacterial strain. Tests are run in petri dishes, which are shallow, round, covered dishes filled with an agar medium or gel-like substance infused with nutrients to promote bacterial growth. The bacterial strain to be tested is evenly spread across the surface of the plate. Several circular paper discs, each infused with a different antibiotic, are evenly spaced over the surface of the plate and gently pushed down into the agar to make contact with the bacteria. The plates are then allowed to grow overnight in an incubator.

Following the overnight incubation, a circular area devoid of bacterial growth will surround each paper disc. This area is called the zone of inhibition. The diameter of the zone of inhibition for each disc is measured and compared to a control chart to determine if the bacterial strain tested is resistant, intermediate, or susceptible to each of the different antibiotics. A large zone of inhibition would mean the bacterial strain is susceptible to the antibiotic on the test disc.

The dilution method is an antibiogram used to determine the most effective antibiotic concentration to employ against a strain of bacteria. It begins by culturing or growing a fresh batch of the test bacterium overnight and then checking the new cultures for purity or purifying the culture. Two control or comparison bacteria are prepared as well. The concentration of the bacterial strain is determined using a spectrophotometer, and the concentration is adjusted into a range appropriate for the dilution method.

A serial dilution of the antibiotics to be tested is prepared by making increasing and graduated changes of the concentration in different vials. One series of the varying antibiotic concentrations is inoculated with equal amounts the test bacterial strain, and the other series are inoculated with the control bacterial strains. All the inoculated antibiotic concentrations are allowed to grow overnight in an incubator or until visible bacterial growth is observed in some of the vials. The MIC is the antibiotic concentration where no visible bacterial growth is observed. A MIC value is a controlled way to assess the changing resistance of bacterial strains and sets limits on antibiotic therapy for infections.