My group is testing propolis extracts against a particular strain common in my region but when we used the broth microdilution method we saw dark green spots (similar to the propolis colour) at the bottom of the microtiter plates after 24h incubation at 37°C.

The extracts were diluted in 20% w/v in ethanol and the bacterial strain control has a different colour and spotting that the ones on the ones con the concentration rage, could it be because of the natural products being too concentrated? We do have a propolis extract control and it does not have the same spotting.

I can provide with pictures in case of need, thank you so much for your time.