[Babcock_Hall]

I am collaborating with a microbiologist to test some compounds to see whether or not they possess antibacterial properties.  These compounds are either neutral or bear a single negative charge near pH 7, and they have aromatic rings.  The compounds we have tested so far are soluble in DMSO, but modestly soluble in ethanol (less than 10 mg/mL), and probably not soluble in unbuffered water.  The assay begins with adding the compound to a disk that is placed in contact with bacteria.

My question concerns solvents that would be appropriate to use to dissolve the compounds.  I assume that one control is to try the solvent without the compound of interest.  Are there any solvents that are known to be so toxic in this sort of assay that they are best avoided altogether?  If two solvents had about the same polarity, is it better to choice the more volatile or less volatile solvent?  Is there any other property that we should consider?  So far I looked in a microbiology laboratory manual, but I did not find anything.  Perhaps there is a good reference that someone could suggest.  Thank you.

[wildfyr]

You could try this alternate procedure

https://pubs.acs.org/doi/abs/10.1021/acsbiomaterials.6b00221. Bear in mind, that one is for a surface bound antimicobial material.

I think most solvents are going to be pretty nasty for bacteria. I would use a very volatile one (diethyl ether if possible?) and let it fully evaporate from the surface.

[Babcock_Hall]

Thank you; that is an interesting paper.  Inasmuch as we are working in collaboration, we are probably limited to disk diffusion for now.  Because the compounds are soluble in DMSO, I was going to try acetone next for one of them.  That way the acetone will simply evaporate, leaving the compound behind.  The other compound came into a solution of K₂HPO₄.  If we did our calculations correctly, we created a phosphate buffer.