Living Textbook MC610

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Streptogramins are isolated from streptomyces sp.and are composed of two components, each show feeble and bacteriostatic activity, but together they show synergistic effect. They are effective against Gram positive bacteria only, since they can not easily penetrate the Gram negative bacteria. They are used parenterally.

One agent that has recently been approved for use is Synercid® is composed of:

Type A, Dalfopristin , acts by binding to the 50S subunit and blocking the binding of the aminoacyl-tRNA to both the A and P site. It can only bind ribosomes that are not already involved in protein biosynthesis.

Type B, Quinupristin , inhibits peptide bond formation and elongation and can bind both free ribosomes and those involved in protein biosynthesis.

These effects result in complete and irreversible inhibition of protein synthesis, thus a bactericidal effect.

Synergism is due to a conformational change of the ribosome upon binding of Type A that enhances the binding of Type B. The fact that each agent act at a different stage of protein biosynthesis may also contribute to the synergistic effect.

Resistance is observed due to changes in ribosomal structure (MLS type resistance) and to lesser extent to a decreased permeability and inactivation of the antibiotic. The former mechanism affects the Type B agent as they share their binding site with Macrolides and Lincomycins.

It is mostly used in MRS and VRE (Vancomycin Resistant Enterococci) infections.