Vernamycin B

Inducible resistance to macrolide, lincosamide, and streptogramin type B antibiotics in Streptomyces spp. comprises a family of diverse phenotypes in which characteristic subsets of the macrolide-lincosamide-streptogramin antibiotics induce resistance mediated by mono- or dimethylation of adenine, or both, in 23S ribosomal ribonucleic acid. In these studies, diverse patterns of induction specificity in Streptomyces and associated ribosomal ribonucleic acid changes are described. In Streptomyces fradiae NRRL 2702 erythromycin induced resistance to vernamycin B, whereas in Streptomyces hygroscopicus IFO 12995, the reverse was found: vernamycin B induced resistance to erythromycin. In a Streptomyces viridochromogenes (NRRL 2860) model system studied in detail, tylosin induced resistance to erythromycin associated with N6-monomethylation of 23S ribosomal ribonucleic acid, whereas in Staphylococcus aureus, erythromycin induced resistance to tylosin mediated by N6-dimethylation of adenine. Inducible macrolide-lincosamide-streptogramin resistance was found in S. fradiae NRRL 2702 and S. hygroscopicus IFO 12995, which synthesize the macrolides tylosin and maridomycin, respectively, as well as in the lincosamide producer Streptomyces lincolnensis NRRL 2936 and the streptogramin type B producer Streptomyces diastaticus NRRL 2560. A wide range of different macrolides including chalcomycin, tylosin, and cirramycin induced resistance when tested in an appropriate system. Lincomycin was active as inducer in S. lincolnensis, the organism by which it is produced, and streptogramin type B antibiotics induced resistance in S. fradiae, S. hygroscopicus, and the streptogramin type B producer S. diastaticus. Patterns of adenine methylation found included (i) lincomycin-induced monomethylation in S. lincolnensis (and constitutive monomethylation in a mutant selected with maridomycin), (ii) concurrent equimolar levels of adenine mono- plus dimethylation in S. hygroscopicus, (iii) monomethylation in S. fradiae (and dimethylation in a mutant selected with erythromycin), and (iv) adenine dimethylation in S. diastaticus induced by ostreogrycin B.

Using dimethyl sulfate and kethoxal, we have probed antibiotic-ribosome complexes, and identified sites of interaction of chloramphenicol, erythromycin, carbomycin, vernamycin B and viomycin with 23S rRNA. Chloramphenicol, erythromycin, carbomycin and vernamycin B protect overlapping nonequivalent sites in the central loop of domain V. From the known functional effects of these drugs and their protection patterns, we infer that peptidyl transferase is inhibited as a result of binding antibiotics proximal to A-2451, whereas antibiotics bound proximal to A-2058 interfere with growth of the nascent polypeptide chain. Vernamycin B also strongly protects A-752, implying that this region of domain II is proximal to the central loop of domain V. Viomycin, which affects translocation and subunit dissociation, protects U-913 and G-914.

A total of 332 staphylococcal and 263 streptococcal isolates from three hospital microbiology laboratories were tested with erythromycin, clindamycin, and vernamycin B alpha to determine the prevalence of macrolides-lincosamides-streptogramin B resistance. Constitutive resistance was detected in 28 Staphylococcus aureus isolates (15.5%), 53 coagulase-negative staphylococci (35.1%), and 20 streptococci (7.6%). Inducible resistance was observed in 13 S. aureus isolates (7.2%), 25 coagulase-negative staphylococci (16.6%), and 2 streptococci (0.8%). Eleven coagulase-negative staphylococci (7.3%) exhibited a novel phenotype, namely inducible resistance to erythromycin and vernamycin B alpha but not clindamycin. Among the staphylococci, two variants of the inducible phenotype detected with the agar diffusion assay correlated with the presence of classical ermA or ermC genes, respectively, by dot-blot analysis. The prevalence of the staphylococcal phenotypes were different in the hospitals surveyed, and there was an apparent inverse correlation between the resistance observed and the use of erythromycin in each hospital.