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I dissolved 2mg of Ellagic acid in 500 microliter of DMSO. I used 100 microliters of this solution to test for antimicrobial susceptibility against E.faecalis by adding it to 100 microliters of BHI media. After incubation for 24 hours in 96 well plate, I noticed precipitate at the bottom of the first row in the wells, which was confirmed not to be microbial growth . Another observation I made is that there was a distinct change in color(brownish) in the well. I tested other antimicrobial compounds as well but I had no issues. I repeated the experiment but got the same results, which lead to high optical density values. I maintained the ideal pH range mentioned for the media.
My question is what caused the formation of precipitate? Is there a reaction that takes between ellagic acid and the media?
EDIT: Ellagic acid is poorly soluble in water(0.82 g/L) hence DMSO(dimethyl sulfoxide) solvent is used. The media requires pH 7.4 ± 0.2 for its properties to be stable. The reason I posted this question here is I am not sure if there is any reaction taking place, or if any other factor is responsible for the formation of precipitate and also because it is about antimicrobial susceptibility testing.

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  • $\begingroup$ Please edit the following information into your question: 1) What is BHI media? 2) What is the solubility of Ellagic acid in water? 3) Is Ellagic acid's solubility dependent on pH and if so how does this relate to the pH of your media? Have you tried mixing equal volumes of Ellagic acid and your media? How about with just water? ——— Finally, if this is a reaction with the media, then this would be better moved to Chemistry — either delete from here before reposting or request migration because cross-posting is impolite. $\endgroup$
    – tyersome
    Commented Aug 6, 2022 at 18:39
  • $\begingroup$ Hint : look at the structure of Ellagic acid. Is it like tannins? How do tannins react with proteins? All in Wikipedia. $\endgroup$ Commented Aug 9, 2022 at 4:41

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