I am trying to determine the MIC of Silver Nitrate (AgNO3) against a number of bacterial strains using broth micro/macro dilutions method.
I prepared the AgNO3 in distilled water to a concentration of 1M. When I added this to my growth broth, it immediately forms a precipitate.
I initially used Tryptic soy broth and thought it was the salt content of the broth causing the precipitation, so I then tried Mueller-Hinton (MH) broth, and Mueller-Hinton-II (MH-II) broth (cation adjusted). Using my initially prepared AgNO3 solution, no precipitation occurred.
I then prepared a fresh stock of 1M AgNO3 solution in order to preform a MIC, but this solution formed precipitate upon addition to the MH broth.
The precipitate formed is white and heavy. This gives the impression it's AgCl, but with no NaCl in MH broths, I'm not sure how it could be forming.
Ideally, I need a broth that will not cause precipitation, or any additive I could use to prevent precipitation without having any adverse effects on the bacteria samples.
Any advice on this issue would be greatly appreciated!
If your solution is not acidic enough, Ag2O might get precipitated out.
We've used silver nitrate by diluting it in 28% w/v PEG 400, 26% w/v sorbitol to a maximum of 3% AgNO3 and used this for dilutions (up to 10-4) in MIC/MBC assays.
Ag2O is a dark black compound. May be you have another source of chloride ions different from NaCl or other halides (Br-, I-). You should check for these. Sometimes aminoacids are used in the form of chlorhydrate derivatives.
Thank you for all your suggestions!
Alessandro, you were correct. I looked into the MH broth composition further, it contains HCL hydrolysed casein as a amino acid supply which produces Cl- salts as part of its production process.
I will try to get in PEG 400 and sorbitol to prepare as suggested. I am also looking into the possible use of surfactants. As I have it already on use I will prepare a LB media with no/trace amounts of NaCl
Please keep us informed about developments for solving this problem.
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