The Kirby-Bauer method and the dilution method are two types of antibiogram tests used to determine the susceptibility of bacterial strains to antibiotics. The Kirby-Bauer method involves testing the bacterial strain’s susceptibility to various antibiotics using circular paper discs, while the dilution method determines the minimum inhibitory concentration (MIC) of an antibiotic that will inhibit the growth of the bacterial strain. The MIC value helps evaluate the changing resistance of bacterial strains and sets limits on antibiotic therapy for infections.
An antibiogram test can be used for two different purposes. The Kirby-Bauer method is used to test the susceptibility of a bacterial strain to various antibiotics. Susceptibility testing can also determine the minimum inhibitory concentration (MIC), or the smallest concentration of an antibiotic that will inhibit the growth of the 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. Testing is done in Petri dishes, which are shallow, round, coated dishes filled with an agar medium or a gelatinous substance infused with nutrients to promote bacterial growth. The bacterial strain to be tested is uniformly distributed on the surface of the plate. Several circular paper discs, each infused with a different antibiotic, are spread evenly across the surface of the plate and gently pushed into the agar to make contact with the bacteria. The plates are then allowed to grow overnight in an incubator.
After overnight incubation, a circular area free 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 whether the bacterial strain tested is resistant, intermediate or susceptible to each of the different antibiotics. A large zone of inhibition would mean that the bacterial strain is susceptible to the antibiotic on the test disc.
The dilution method is a susceptibility test used to determine the most effective antibiotic concentration to use against a strain of bacteria. Start by growing or growing a new batch of the test bacterium overnight and then checking the purity of the new cultures or purifying the culture. Two control or comparison bacteria are also prepared. The concentration of the bacterial strain is determined using a spectrophotometer and the concentration is adjusted to a range appropriate for the dilution method.
A serial dilution of the antibiotics to be tested is prepared by making increasing and gradual changes in the concentration in different vials. A series of different concentrations of antibiotic is inoculated with quantities equal to the bacterial strain under test and the other series are inoculated with the control bacterial strains. All 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 concentration of the antibiotic at which no visible bacterial growth is observed. An MIC value is a controlled way to evaluate the changing resistance of bacterial strains and sets limits on antibiotic therapy for infections.
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