The streptogramin antibiotics include two distinct groups. Group A contain a 23-membered unsaturated ring with lactone and peptide bonds, and group B are depsipeptides (lactone-cyclized peptides). These antibiotics are included in the macrolide-lincosamide-streptogramin (MLS) group. However, whereas the individual A and B components act in a bacteriostatic fashion when used as sole agents, together the affinity for the ribosome is enhanced, causing them to be bactericidal. Streptogramins include virginiamycin. Streptogramins are used to treat vancomycin-resistant staphylococci and Enterococcus faecium.
