The results of agar dilution are easily reproduced and they can be monitored at a much cheaper cost than what is required of other dilution methods. Advantages Īgar dilution is considered to be the gold standard of susceptibility testing, or the most accurate way to measure the resistance of bacteria to antibiotics. The lowest concentration of antibiotics that prevented bacterial growth is considered to be the minimum inhibitory concentration of that antibiotic against that bacterium. : 374 After incubation, the plates are examined to determine if bacterial growth has occurred in the inoculated spots. The plates are then incubated for sixteen to eighteen hours, although incubation time may be less for bacteria populations that divide quickly. The dilution plates are then incubated at a temperature of 37 degrees Celsius. Necessary controls include a control plate that does not receive any antibiotics and bacterial spread plates demonstrating that the bacterial included are in the correct concentration range. This technique allows for replicate spots of one bacterial type to be tested or spots of different bacteria so that the MIC of the antibiotic against multiple types of bacteria can be tested. After this, bacteria prepared to a standard concentration are added as a spot to each plate, with 10 4 colony forming units (CFU) per spot. An appropriate volume is then combined with melted agar to produce plates in which the final antibiotic concentrations represent a 2-fold dilution series.
The antibiotic to be tested is diluted with water to produce a series of concentrations.
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