The streptogramin antimicrobials include two distinct groups: Group A, a 23-member unsaturated ring with lactone and peptide bonds, and group B, the depsipeptides (lactone-cyclized peptides). These antimicrobials 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.