Quinupristin is an antibiotic, often used in combination with dalfopristin under the trade name Synercid. It is active against Gram-positive bacteria, atypical but not Gram negatives. They inhibit protein synthesis of the cells.
Quinupristin & dalfopristin are not active against Enterococcus faecalis and they need to be given in combination with other antibacterials for mixed infections which also involve Gram negative organisms.