50 mg/l  is OK. 

10 mg/L is working fine.

10 mg/L 

10-30 mg/L is used commonly and I have used 10mg/L and this concentration is O.K

50 mg/l is standard

Prepare 1000x stock solution in DMSO.

10 µg/ml is ok

You can also use streptomicyn 25 µg/ml or spectinomicyn 25 µg/ml. 

Don't use more than 50ug/ml. Keep your stock at -20 degreC and better use aluminum foil to cover the container. Don't autoclave it. Filter strile in laminar flow.

Thank you so much... for your suggestion.. i m using now 20mg/L of rifamipcin