Eleven structurally similar ketolide antibiotics were tested at a concentration of 1 microg/ml for their relative inhibitory effects on growth and ribosome activities in Staphylococcus aureus cells. Ten of the compounds examined had an inhibitory effect on protein synthesis at this concentration and eight of the 11 compounds were also effective inhibitors of the formation of the 50S ribosomal subunit. All of the drugs tested inhibited protein synthesis to a greater extent than they affected 50S subunit formation. The decline in growth rate and cell number was proportional to the effect on ribosome formation and function. The growth of an ermC erythromycin-resistant strain of S. aureus was also significantly inhibited by nine ketolide compounds, suggesting that they were not inducers of methylase gene expression. These inhibitory activities can be related to structural differences between these ketolide antibiotics.

The translational functions of the bacterial ribosome are the target for a large number of antimicrobial agents. The 14- and 16-membered macrolides, the lincosamides, and the streptogramin B type antibiotics are thought to share certain inhibitory properties, based on both biochemical and genetic studies. We have shown previously that the 14-membered macrolides, like erythromycin, have an equivalent inhibitory effect on translation and the formation of the 50S ribosomal subunit in growing bacterial cells. To extend this work, we have now tested the 16-membered macrolides spiramycin and tylosin, the lincosamides lincomycin and clindamycin, and 3 streptogramin B compounds pristinamycin I(A), virginiamycin S, and CP37277. Each of these was a specific inhibitor of 50S subunit formation, in addition to having an inhibitory effect on translation. By contrast, two streptogramin A compounds, virginiamycin M1 and CP36926, as well as chloramphenicol, were effective inhibitors of translation without showing a specific effect on the assembly of the large ribosomal subunit. A combination of an A and B type streptogramin (virginiamycin M1 and pristinamycin I(A)) demonstrated a synergistic inhibition of protein synthesis without exhibiting a specific inhibition of 50S subunit formation. These results extend our observations on 50S assembly inhibition to the entire class of MLS(B) antibiotics and reinforce other suggestions concerning their common ribosome-binding site and inhibitory functions.

Macrolide antibiotics like erythromycin can induce the synthesis of a specific 23S rRNA methyltransferase which confers resistance to cells containing the erm gene. Erythromycin inhibits both protein synthesis and the formation of 50S subunits in bacterial cells. We have tested the idea that the 50S precursor particle that accumulates in antibiotic-treated Staphylococcus aureus cells is a substrate for the methyltransferase enzyme. Pulse-chase labeling studies were conducted to examine the rates of ribosomal subunit formation in control and erythromycin-induced cells. Erythromycin binding to 50S subunits was examined under the same conditions. The rate of 50S subunit formation was reduced for up to 30 min after antibiotic addition, and erythromycin binding was substantial at this time. A nuclease protection assay was used to examine the methylation of adenine 2085 in 23S rRNA after induction. A methyl-labeled protected RNA sequence was found to appear in cells 30 min after induction. This protected sequence was found in both 50S subunits and in a subunit precursor particle sedimenting at about 30S in sucrose gradients. 23S rRNA isolated from 50S subunits of cells could be labeled by a ribosome-associated methlytransferase activity, with (3)H-S-adenosylmethionine as a substrate. 50S subunits were not a substrate for the enzyme, but the 30S gradient region from erythromycin-treated cells contained a substrate for this activity. These findings are consistent with a model that suggests that antibiotic inhibition of 50S formation leads to the accumulation of a precursor whose 23S rRNA becomes methylated by the induced enzyme. The methylated rRNA will preclude erythromycin binding; thus, assembly of the particle and translation become insensitive to the inhibitory effects of the drug.

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The effects of the everninomicin antibiotic evernimicin (SCH27899) on growing Staphylococcus aureus cells were investigated. Cellular growth rates and viable cell numbers decreased with increasing antibiotic concentrations. The rate of protein synthesis, measured as 35 S-amino acid incorporation, declined in parallel with the growth rate. Significantly, the formation of the 50S ribosomal subunit was inhibited in a dose-dependent fashion as well. 30S ribosomal subunit synthesis was not affected over the same concentration range. Evernimicin did not stimulate the breakdown of mature ribosomal subunits. Pulse-chase labeling experiments revealed a reduced rate of 50S subunit formation in drug-treated cells. Two erythromycin-resistant strains of S. aureus that carried the ermC gene were as sensitive as wild-type cells to antibiotic inhibition. In addition, two methicillin-resistant S. aureus organisms, one sensitive to erythromycin and one resistant to the macrolide, showed similar sensitivities to evernimicin. These results suggest a use for this novel antimicrobial agent against antibiotic-resistant bacterial infections.In the struggle to keep up with the current increase in the number of antibiotic-resistant infectious organisms, both new antimicrobial agents and new cellular targets must be found (10). A compound identified 35 years ago has recently been reinvestigated as a new and potentially effective antibiotic. The everninomicins are a group of complex, sugar-derived antibiotics isolated from Micromonospora carbonacea (25, 26). They were described and characterized many years ago, but very few studies have been conducted to examine their modes of action (12, 21; A. K. Ganguly and A. K. Saksena, Communications to the editor, J. Antibiot. (Tokyo) 28:707-709, 1975). Avilamycin, a polysaccharide antibiotic with similarities to the everninomicins, was shown to affect protein synthesis by interacting with the 30S ribosomal subunit (27). This compound has been used as an antimicrobial agent in animal feed (1).Recently, another everninomicin, evernimicin (SCH 27899), has been examined in more detail (13,14,24 We have identified a novel target for macrolide antibiotics in bacterial cells, the assembly of 50S ribosomal subunits (3-5). Macrolide and ketolide antibiotics have equivalent inhibitory effects on both translation and 50S subunit formation in S. aureus (7,8). Since it has been suggested that evernimicin inhibits protein synthesis by interacting with the 50S subunit (Adrian and Klugman, 38th ICAAC), we decided to investigate its inhibitory effects on translation and subunit assembly in a systematic fashion. We found that both translation and 50S subunit formation were targets for inhibition in wild-type S. aureus cells and in both MRSA and erythromycin-resistant mutant strains. The significance of these findings are discussed in terms of the effects both on 50S subunit formation and on the potential clinical use of this antimicrobial agent. MATERIALS AND METHODSMeasurements of cell growth, subunit assembly, and...