We are performing antimicrobial test using different antimicrobial agent. We treat bacteria at mid-log phase to detect the effectiveness of our antimicrobial agent. But we are not getting growth inhibition even at high concentration.
Some antimicrobial agents exhibit an inoculum effect, meaning that the effectiveness is lower when more cells are used (in the same volume).
I concur with both previous answers. Antibiotic effectiveness is often culture density-dependent. It's also media dependent. Richer medias can sometimes make antibiotics look less effective than nutrient-poor medias.
Usually grow culture to O.D(600) 0.5, and then do 1:20 dilution to the final solution
The population recommended in ANTIBIOTICS IN LABORATORY MEDICINE by VICTOR LORIAN is 104 UFC/ml
For E. coli you can take 50ul of 0.1 OD bacterial inoculum plus 50ul of 2x concentartion of the antibiotic required
It depends whether you are using a rich media or defined media. An O.D. of 0.5 should be good to use.
An O.D of 0.5 of the bacterial incoculum is acceptable and should yield good results.
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