A Numbers Game: Ribosome Densities, Bacterial Growth, and Antibiotic-Mediated Stasis and Death

• Type of Material: Article
• Authors: Bruce Levin, Emory University; Ingrid C. McCall, Emory University; Veronique Perrot, Emory University; Howard Weiss, Georgia Institute of Technology; Armen Ovesepian, University of Copenhagen; Fernando Baquero, Ramón y Cajal University Hospital
• Language: English
• Date: 2017-03-08
• Publisher: American Society for Microbiology: Open Access Journals
• Publication Version: Final Published Version
• Copyright Statement: © 2017 Levin et al.
• License: Creative Commons Attribution 4.0 International
• Final Published Version (URL): http://dx.doi.org/10.1128/mBio.02253-16
• Title of Journal or Parent Work: mBio
• ISSN: 2150-7511
• Volume: 8
• Issue: 1
• Start Page: e02253-16
• End Page: e02253-16
• Grant/Funding Information: This research was supported by a grant (GM 091875) from the United States National Institutes of Health (B.R.L.) and by FIS PI15-00818 and CIBERESP (CB06/02/0053) from the Instituto de Salud Carlos III, Spain (F.B.).
• Keywords: inhibition of growth; Escherichia coli
• Research Categories: Biology, General; Health Sciences, General

Abstract

We postulate that the inhibition of growth and low rates of mortality of bacteria exposed to ribosome-binding antibiotics deemed bacteriostatic can be attributed almost uniquely to these drugs reducing the number of ribosomes contributing to protein synthesis, i.e., the number of effective ribosomes. We tested this hypothesis with Escherichia coli K-12 MG1655 and constructs that had been deleted for 1 to 6 of the 7 rRNA (rrn) operons. In the absence of antibiotics, constructs with fewer rrn operons have lower maximum growth rates and longer lag phases than those with more ribosomal operons. In the presence of the ribosome-binding “bacteriostatic” antibiotics tetracycline, chloramphenicol, and azithromycin, E. coli strains with 1 and 2 rrn operons are killed at a substantially higher rate than those with more rrn operons. This increase in the susceptibility of E. coli with fewer rrn operons to killing by ribosome-targeting bacteriostatic antibiotics is not reflected in their greater sensitivity to killing by the bactericidal antibiotic ciprofloxacin, which does not target ribosomes, but also to killing by gentamicin, which does. Finally, when such strains are exposed to these ribosome-targeting bacteriostatic antibiotics, the time before these bacteria start to grow again when the drugs are removed, referred to as the post-antibiotic effect (PAE), is markedly greater for constructs with fewer rrn operons than for those with more rrn operons. We interpret the results of these other experiments reported here as support for the hypothesis that the reduction in the effective number of ribosomes due to binding to these structures provides a sufficient explanation for the action of bacteriostatic antibiotics that target these structures.