CORRELATION OF TOBRAMYCIN-INDUCED INHIBITION OF PROTEIN-SYNTHESIS WITH POSTANTIBIOTIC EFFECT IN ESCHERICHIA-COLI

Scant data exist on intracellular events during aminoglycoside-induced postantibiotic effect (PAE). We examined DNA, RNA, and protein synthe ses after tobramycin exposure using [H-3]thymidine, [C-14]uracil, and [C-14]alanine incorporation in a clinical Escherichia coli strain. Lat e-log-phase bacteria in oxygenated minimal salts medium at 37 degrees C were exposed to tobramycin (7.5 mu g/ml) (twice the MIC) for 30 min. Tobramycin caused a kill of 2 log(10) CFU/ml prior to drug removal by filtration and a 5-h PAE, measured by viable counts. Excess amounts o f labelled precursors were added to tobramycin-exposed organisms durin g, immediately after, and at various intervals following exposure. In the presence of tobramycin, DNA, RNA, and protein syntheses were seque ntially inhibited within 1 generation time. Following drug removal, bo th DNA and RNA syntheses promptly resumed, suggesting readily dissocia ble nonspecific binding to DNA and RNA. However, total protein synthes is did not resume until 4 h later. beta-Galactosidase activity, a meas ure of functional enzymatic protein synthesis, was also inhibited for 4 h after drug removal. Bacterium length, measured by confocal microsc opy, increased during PAE. Two distinct populations eventually emerged : one that returned to control dimensions and one that remained excess ively elongated by the end of PAE (2.5 mu m versus 4.0 mu m; P < 0.05) . We hypothesize that only viable cells return to the control morpholo gy. Flow cytometry showed enhanced DNA complexity during PAE, consiste nt with either impaired cellular protein synthesis in viable cells or perturbations in dying cells. In summary, duration of PAE correlated w ith inhibition of total and functional protein synthesis but not DNA o r RNA synthesis.