Agar dilution

Agar dilution is a method used by researchers to determine the resistance of pathogens to antibiotics. It is the dilution method most frequently used to test the effectiveness of new antibiotics.

Process

The antibiotic to be tested is added to agar, which is then placed in dilution plates and diluted with varying levels of water. After this, the pathogen to be tested is added to each plate, plus a control plate that does not receive any antibiotics. The dilution plates are then incubated at a temperature of 37 degrees Celsius. The plates are then incubated for sixteen to eighteen hours, although incubation time may be less for bacteria populations that divide quickly. After incubation, the plates are examined to determine if bacterial expansion has occurred. The lowest concentration of antibiotics that stopped the spread of the bacteria is considered to be the minimum inhibitory concentration of that bacteria.

Advantages

Agar 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 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. Additionally, up to thirty pathogen samples (plus two controls) can be tested at once, so agar dilution is useful for batch tests.

Disadvantages

Each dilution plate in agar testing has to be manually infected with the pathogen to be tested, so agar dilution testing is both labor-intensive and expensive. Unlike broth microdilution tests, agar dilution cannot be used to test more than one antibiotic at a time.