Streptogramins. A unique class of antibiotics.

Streptogramin antibiotics represent a unique class of antibacterials in that each member of the class consists of at least 2 structurally unrelated molecules: group A streptogramins (macrolactones) and group B streptogramins (cyclic hexadepsipeptides). Both group A and group B streptogramins inhibit protein synthesis at the ribosomal level, and they act synergistically against many isolates, their combination generating bactericidal activities and reducing the possibility of emergence of resistant strains. The mechanisms of acquired resistance to group B streptogramins are similar to those induced by erythromycin, but group A streptogramins remain unaffected by target modifications and active efflux. The pharmacokinetic parameters of group A and group B streptogramins in blood are quite similar. In addition, both the A and B groups penetrate and accumulate in macrophages and in the bacterial vegetations of experimental endocarditis. There are important structural and biological differences between the streptogramins and the macrolides. The main differentiating features are the rapid anti-bacterial killing of streptogramins and the rarity of cross-resistance between the 2 groups of antibiotics.