how to eliminate gram positive bacteria from a mixture of gram-negative bacteria?

Answer 1

Ok, basically you’re looking for a narrow spectrum antibiotic. I would recommend something like Vancomycin which should work on most gram-positive bacteria. Vancomycin resistance is becoming increasingly common now so if you don’t experience good results it may be worth trying something like Linezolid to see if that works.

Obviously as I don't know which bacteria you are dealing with I can't be confident that either of these will work, you should research both antibiotics yourself to ensure that they do what you want.

Answer 2

When a bacterial culture is contaminated by a different kind of bacteria - regardless of their gram staining classification - it is generally impossible to get rid of one without getting rid of the other. Reculture the organism of choice, with greater attention to sterile technique.