I am preparing for a microbiology practical course where we would like to test the antimicrobial activity of S. griseus. I did perpendicular streak method with E. coli which worked well and microdilutions which did not work.
For microdilution I inoculated MHB broth with S. griseus and incubated for 4-5 days at 28 °C. The broth I used in microdilution (incubation at 37°C for 20 h against E.coli, and MRSA) to test its antibiotic properties. There was no inhibition of the tested strains.
Afterwards, I did this experiment again, freeze dried the S. griseus suspension after incubation and dissolved in a small amount of water. Again, there was no activity in microdilution.
I also tried the experiment with longer incubation (around 10 days) but the resuts were the same.
From your experience, do have an idea where the mistake is? Maybe the incubation time is too long and the streptomycin degrades?!
Thanks a lot!