Do streptomycin solution in sterilized double distilled water. While performing anti-microbial studies for your compounds don't forget to maintain one control also (add DMSO alone). If you observe any zone of inhibition in control, then subtract the diameter of control with your test sample then that gives you the exact activity of your sample. For standard also do the same thing.

Good luck

It seems that compounds you are studying for antifungal activities are water insoluble. Only DMSO or T-80 at concentration used to dissolve the test compound may be used as negative control. For positive control streptomycin or Nistatin may be used, but it is to be dissolved in the water. It is also advisable to screen the sensitivity of fungal strains to DMSO or T-80 so that you can keep the DMSO ot T-80 below this level in your future studies.

Good luck.

Streptomycin dissolves in water, distilled and sterilized. For all antibiotics You can find data about solvents and diluents in CLSI Performance Standards for Antimicrobial Susceptibility Testing.

you can use microbroth dilution method for MIC

Other, solutions for insoluble substances is use other solvents with better vapor tensión as ethanol (96%) also acetone, they avapore easyer that DMSO.

Did you dilute your compounds in pure DMSO? I don't know what bacterial/fungal strains you are using, but usually they don't tolerate more thatn 10% of DMSO without affecting their growth. We usually dilute our compounds in a mix of broth and DMSO. As someone mentioned, you should use DMSO in a disc, exactly the same concentration as you used on your compounds, as a control. Also, DMSO will not evaporate just like that. It is a solvent very difficult to remove.

streptomycin is actually called as streptomycin hydrochloride.... so add 10 ul of HCL and sterilize by filteration. before starting the experiment first do it with a ATCC starin and a known Positive control strain. If there is any zone of inhibition in control, then you need to subtract the diameter of neg control from that of the diameter of test sample

Try to avoid DMSO by using solvent like methanol, acetone or water. If this is not possible then prepare highly concentrated stock solution in DMSO then dilute with solvent like methanol, acetone or water. Try to keep concentration of DMSO low enough in the dilutions to avoid its antimicrobial effect on the test organism. Run a control with this DMSO dilution without the test compound(s). Prepare streptomycin solution in water using streptomycin sulfate.

Hi, I think DMSO is used in agar well method for the better diffusion of the metabolite/antibiotic through agar. Streptomycin is easily dissolve in Methanol if you use disc method.

Why don't you try ethanol or methanol to dissolve your compounds? Also consider that DMSO may inhibit growth of fungal strains

Dissolve it in 100% methanol or 50% methanol. This would be the best solvent. You can use the methanol as the negative control. The protocol can be found in the following papers:

http://www.ncbi.nlm.nih.gov/pubmed/22519438

http://www.ncbi.nlm.nih.gov/pubmed/22129274

I would suggest you to follow NCCLS which is a sensitive microtitre plate assay. This assay is useful as it uses only less amount of the test compound compared to the disc method. The reference to the article is:

http://www.ncbi.nlm.nih.gov/pubmed/17560320

Keep in mind that for a certain antimicrobial there is a solvent and there is a diluent that are recommended/acceptable. Some compounds are just not soluble in water or any other buffer, such as phosphate. For those compounds, it is acceptable you dissolve them in DMSO, acetone, alcohol... but, you will need to dilute them with water/buffer before you test them, meaning that even the stock solution should be a mix of solvent:diluent. Remember that any solvent that you use is likely to interact with the bacteria, fungal or any component of the broth you are using for your testing. Some compounds when pure may even interact with the disc. Just make sure that any inhibition you see is due to your compound and not what you dissolve it in, so run appropriate controls.

I would suggest that you use MIC standard protocols to test the antibacterial properties of your compounds since it is an easy method to apply and it allows you to dissolve you compounds in different solvents.

here is a website for one protocol-

http://cmdr.ubc.ca/bobh/methods/MICDETERMINATIONBYMICROTITREBROTHDILUTIONMETHOD.htm

Dear Rajender Reddy

Negative control is DMSO, if is instance use in experimental as a solvent, But about antibiotic discs as positive controls, in disc diffusion method usually they have standard concentration and do not need to make concentration of antibiotics and their exact concentration are not routinely mentioned in the papers. However, for example it is 10 mcg of Gentamicin, 10 units (10 IU) of Penicillin, 30 mcg of Neomycin and 300 mcg of Nitrofurantion antibiotic. But about make of the concentration for antibiotic discs, should not similar certainly solvent of the samples and antibiotics. However, all solvents should be test for antibacterial activates as negative control.

Best regards

I use 5% DMSO as a negative control. I have worked with about 16 strains of clinical isolates and in all cases, i did not get zone of inhibition with 5% DMSO suggesting that it has not antibacterial activity at this minimal concentration.  If used 100%, you may get! DMSO is generally preferred as compared to other solvents like methanol, ethanol etc. as they induce negative cellular effects by acting on the cell membranes.  Another factor is volatility. We keep atleast for 12 hours to 16 hours in the incubator at 37oC for antimicrobial tests and measurement of zones of inhibition.  

Good Luck!