Antibiotic working concentratios. - (Mar/12/2002 )

I was wondering if anyone new the best working concentrations for a single genome insertion of Rifampicin, Streptomycin, and Trimethoprim resistance?

Do you mean insertion of resistance determinants at one copy per chromosome, or do you mean spontaneous resistance to these drugs? Either way, the level of resistance will vary greatly depending on what strain you're using (e.g. MICs of <0.032 ug/ml rifampicin are usual for sensitive S. aureus, whereas MICs of about 8 ug/ml are usual for sensitive E.coli), and the type of resistance determinant. If you are trying to select cells with a resistant phenotype from a background of sensitive cells, the best concentration to use is the lowest concentration you can get away with to reproducibly inhibit the background growth of sensitive strains. However, it is important to remember that using any of these drugs will result in selecting out spontaneous mutants from the background by point mutation in the target genes. This is unavoidable, and in most cases is not lessened by using a higher selective concentration.
If you are inserting resistance determinants into cells, be aware that streptomycin resistance is often a recessive phenotype.

Thank you!