Virginiamycin B

Quinupristin-dalfopristin (RP 59500) is an injectable streptogramin antibiotic. It possesses a wide spectrum of activity against Gram-positive bacteria including methicillin-resistant staphylococci, glycopeptide-resistant. Enterococcus faecium and penicillin-resistant pneumococci. Quinupristin-dalfopristin has activity against some anaerobes and selected Gram-negative pathogens. Quinupristin-dalfopristin, by way synergism of between its 2 components, is unaffected by most forms of bacterial resistance. Rare forms of macrolide-lincosamide-streptogramin group B resistance may affect its activity; however, at present the incidence of strains with this type of resistance remains low. Quinupristin-dalfopristin is bactericidal against streptococci and staphylococci but has weak or no bactericidal activity against enterococci. In a compassionate use programme, 67% of 95 evaluable patients with vancomycin-resistant Gram-positive infections or intolerant of vancomycin showed improvement with eradication of infection.

Macrolide antibiotics (Mac) consist of a 12- to 16-membered lactone ring combined with a sugar moiety, and they inhibit protein synthesis via binding to 23S ribosomal RNA in bacteria. The 14- and 16-membered Mac are used for treating infectious diseases caused by Gram-positive and other bacteria; e.g., Haemophilus influenzae, Bordetella pertussis, Legionella pneumophila, Campylobacter, Treponema pallidum and Mycoplasma. Resistance to macrolide, lincosamide, and streptogramin-B (MLS) antibiotics in staphylococci is known to have the following mechanisms: 1) alteration of the target on ribosome due to dimethylation of a specific adenine residue in the 23S ribosomal RNA by the product of the erm gene, and consequently a decrease in binding of MLS antibiotics; 2) inactivation of streptogramin-B (STG-B) and lincosamide by the products of the sbh (encoding streptogramin B hydrolase) and linA' (encoding 3-lincomycin 4-clindamycin O-nucleotidyltransferase) genes, respectively; and 3) active efflux of Mac and STG-B antibiotics determined by the msrA and msrB genes in Staphylococcus epidermidis and Staphylococcus xylosus, respectively, both of which appear to act as an ATP-dependent efflux pump. I have shown that Staphylococcus aureus 8325(pEP2104) exhibits inducible resistance to PMS (partial macrolide and streptogramin B)-antibiotics [the 14-membered macrolides, erythromycin (EM), and oleandomycin (OL), and the 16-membered macrolide mycinamicin (MCM) and STG-B]. The sequence of the N-terminal amino acid residues of a 63 kDa protein (MsrSA) that appeared in the membrane of PMS-resistant strains was identical to that of an MsrA polypeptide related to enhanced efflux of [14C]EM. Ribosomes from PMS-resistant strains showed a similar affinity for EM to those from the PMS-sensitive host strain NCTC8325, and no inactivation of EM by 8325(pEP2104) was observed. In the present study, I showed the DNA sequence of the msrSA region on the constitutive PMS-resistant plasmid pMC38, PMS-inducible resistant plasmid pEP2104 and PMS-sensitive mutant plasmid pSP6, and the region that is essential for inducible expression in PMS resistance. In addition, I investigated the relationship between PMS resistance and intracellular accumulation of EM.

Synergimycins A and B act synergistically in vivo; the mixture of the two compounds is more powerful than the individual components and their combined action is irreversible. Type A (virginiamycin M, VM-like) components inactivate the donor and acceptor sites of peptidyltransferase, thus interfering with the corresponding functions of the enzyme. They block two of the peptide chain elongation steps: aminoacyl-tRNA (AA-tRNA) binding to the A site of ribosomes, and peptide bond formation with peptidyl-tRNA (pep-tRNA) at the P site. A tight (non-exchangeable) linkage of tRNA derivatives with the two ribosomal sites requires a stable interaction of their aminoacyl component with peptidyltransferase. Such interaction is prevented by VM, hence the release of AA-tRNA from the A site and of pep-tRNA from the P site upon translocation; ultracentrifugally unstable particles (60S) are thus formed. A new model for peptidyltransferase has been proposed, to account for the interference of VM with the two sites of the enzyme. The action of this antibiotic is partly due to its presence on the ribosome, and partly to the conformational alterations triggered by its binding. Type B synergimycins (VS-like) and the related 14-membered macrolides (erythromycin) have a more complex action, as revealed by copolymer-based models of cell-free protein synthesis. These antibiotics produce an inhibition of peptide bond formation, and a release of incomplete peptide chains, which processes are both template-dependent (i.e. linked to the polymerization of basic amino acids and proline). The functional interference of VS with peptidyltransferase is explained by the location of the corresponding binding site at the base of the central protuberance of 50S subunits. When ribosome.VS complexes are incubated with erythromycin, the former antibiotic is replaced by the latter; such a replacement does not occur in the presence of VM, which reduces ribosome affinity for macrolides and increases that for type B synergimycins. A study of these complex ribosomal interactions by stopped-flow spectrofluorimetry had allowed a mapping of the binding sites for the MLS antibiotics (macrolides, lincosamides, type B synergimycins) within the peptidyltransferase domain. The active component of these binding sites is represented by segments (loop V and domain II) of 23S rRNA, as indicated by protection and mutation mapping experiments, L proteins increasing the affinity of fixation and its specificity.