An antibiogram test can be used for two different purposes. The Kirby-Bauer method is used to assess the susceptibility of a bacterial strain to various antibiotics. Antibiotics 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. The tests are performed in Petri dishes, which are shallow, round, covered plates filled with agar medium or a gel-like substance infused with nutrients to promote bacterial growth. The bacterial strain to be tested is spread evenly across the surface of the plate. Several circular paper discs, each infused with a different antibiotic, are evenly spaced on the surface of the plate and are gently pushed into the agar to make contact with the bacteria. Plates are grown overnight in an incubator.
After overnight incubation, a circular area devoid of bacterial growth will surround each paper disk. This area is called the zone of inhibition. The diameter of the zone of inhibition is measured for each disc and compared with 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 that 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 use against a strain of bacteria. You start by growing or growing a fresh batch of the test bacteria 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 in an appropriate range for the dilution method.
A serial dilution of the antibiotics to be analyzed is prepared by making gradual and increasing changes of the concentration in different vials. One series of varying concentrations of antibiotics is inoculated with equal amounts of the test bacterial strain, and the other series is 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. MIC is the concentration of antibiotic 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.
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