Protein synthesis is catalysed by ribosomes and cytoplasmic factors. Bacterial ribosomes (70S) are made up of 2 subunits (50S and 30S) containing ribosomal RNA (rRNA) and ribosomal proteins: the 30S binds messenger RNA and begins the ribosomal cycle (initiation), whereas 50S binds transfer RNA (tRNA) derivatives and controls elongation. The key reaction, peptide bond formation, is promoted by the catalytic centre of 50S (the peptidyl transferase centre), and the growing peptide chain (peptidyl-tRNA) attached at the donor P site undergoes peptide linkage with an aminoacyl-tRNA at the acceptor A site. This reaction is inhibited by several antibiotics, the best known being chloramphenicol, and the macrolide-lincosamide-streptogramin (MLS) group. These inhibitors have a reversible action, except for streptogramins that are composed of A and B components, which are bacteriostatic alone, but bactericidal when combined.
The peptidyl transferase centre has been identified at the 50S surface, and the binding sites of inhibitors have been mapped within this domain: some of these sites overlap (e.g. those of macrolides, and type B streptogramins, which compete for binding to ribosomes). Chloramphenicol blocks the catalytic portion, and A streptogramins the substrate sites of the peptidyl transferase centre. Macrolides and type B streptogramins interfere with the formation of long polypeptides and cause a premature detachment of incomplete peptide chains. The synergism between types A and B streptogramins is due to induction by type A streptogramins of an increased ribosome affinity for type B streptogramins.
Microbial resistance to antibiotics mainly involves inactivation of inhibitors and modification of targets (mutations of ribosomal proteins or rRNA genes). Alterations of rRNA bases can induce resistance to a single inhibitor or to a group of antibiotics (e.g. MLSB). The impact of resistance in chemotherapy is less important for streptogramins than for other inhibitors, because the synergistic effect of A and B streptogramins also applies to strains resistant to the MLSB group. It is proposed that mutations and modifications of rRNA bases induce conformational ribosomal changes that prevent antibiotics binding to the target. Conformational changes are also triggered by type A streptogramins: they are responsible for their synergism with type B streptogramins.