Hi Essam.. Gram negative bacteria from environmental samples can be eliminated by using Naldixic Acid. You can also use a combination of antimicrobials if you specifically want to eliminate a particular group of gram negative bacteria. However if you want to isolate gram positive bacteria then as stated above you should be more specific on what group of organisms you are intrested in, as there are different methods for isolation of actinomycetes when compared to other gram positive bacteria.