Why do we do MIC, can disc diffusion assays be enough to check the antibiotic resistance pattern of a bacteria?
Dear Arnav.
Easy answer is no. Disc diffusion test is simple and cheaper method, but it take us qualitative results only. For treatment and optimal antibiotic dosing the information about real minimal inhibition concentration is necessary, beacause we know that for treatment we need 5-20 times higher concentration in area of infection (blood streem, tissue, urine ...) than MIC is. And this information you did not received from disc diffusion test.
You cannot determine MIC from disc diffusion because you can't know the exact amount of antimicrobial that has diffused into the agar. You can use it for screening compounds/extracts to see if they have any activity, and you can use it as a relative qualitative measure.
Similar methods to determine MIC are: agar dilution, broth dilution (macro or micro) and e-test.
If just classifying your test microorganism as 'resistant', 'intermediate' or 'susceptible' is sufficient, and you don't need numerical values of MIC, then yes, the disc diffusion method can be appropriate - providing that you adhere to the CLSI or EUCAST guidelines which, for example, state the inoculum density required, incubation period/temp, culture medium etc.
I have been doing disc diffusion tests for the past few weeks and after multiple issues I've now turned to agar dilution.
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