I am not so familiar with this assay, but I have done a lot of antifungal assays. From my experience, it is not all that uncommon for drugs to lose their activity. This could come from freeze-thaw, light exposure, etc. I would try a broth microdilution assay with the compounds to ensure that they are still active. Spectrophotometry is not even needed for this as just visual inspection of the plates will give you your answer. Also, just check that the range of drug concentrations is sufficient to observe decreased growth. The last thing, you could try taking the readings earlier. Maybe there is outgrowth after the antibiotics have had their effect and this could explain the same turbidity for all of the samples.
Edited by HOYAJM, 09 February 2012 - 08:02 PM.