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# Streptogramins

A pair of antibiotics (type A and type B) that bind the 50S ribosome and together kill multidrug-resistant gram-positive bacteria.

Updated Nov 1, 2026

## What they are

Semisynthetic derivatives of products of Streptomyces pristinaespiralis. The only systemic streptogramin in human use is the fixed combination of quinupristin (type B) and dalfopristin (type A). It was an option against vancomycin-resistant Enterococcus faecium, now displaced by better tolerated drugs.

## Mechanism of action

Each component alone is bacteriostatic. Dalfopristin binds the 50S, changes its shape and blocks chain formation; that change increases the binding of quinupristin, which sits in the macrolide site and halts elongation. The cooperative binding makes the pair bactericidal against many streptococci and staphylococci.

Target: 50S ribosomal subunit: peptidyl transferase centre (type A) and exit tunnel (type B)

## Key points

- Synergy: type A (dalfopristin) + type B (quinupristin) in a 30:70 ratio.
- Active against vancomycin-resistant E. faecium, but not E. faecalis (intrinsic resistance).
- Inhibits CYP3A4: raises levels of ciclosporin, tacrolimus, warfarin and other substrates.
- Typical toxicity: phlebitis in the infusion vein, and arthralgia and myalgia.
- Discontinued in the US; mostly an exam concept.

## Drugs in this class

- [**Quinupristin + dalfopristin** IV combination of two streptogramins that, bound to the 50S ribosome, kill resistant gram-positives such as… Bacterial ribosome, 50S subunit (peptidyl transferase centre and exit tunnel)](https://durgical.com/en/pharmacology/quinupristin-dalfopristin/)

## Others in the class

- pristinamycin

## Test yourself

Think of the answer, then tap the question to see it.

## Why is quinupristin-dalfopristin bactericidal when each component alone is bacteriostatic?

Dalfopristin changes the 50S conformation and increases quinupristin binding; the cooperative binding of both kills the bacterium.

## Which vancomycin-resistant enterococcus does it cover, and which not?

It covers E. faecium, but not E. faecalis, which is intrinsically resistant.

## Which pharmacokinetic interaction is typical of the streptogramins?

They inhibit CYP3A4 and raise the levels of its substrates, such as ciclosporin and tacrolimus.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Quinupristin and Dalfopristin Injection: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a603007.html) (MedlinePlus (U.S. National Library of Medicine), 2026).

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