T6r+-induced proteins and nucleic acids in Escherichia coli infected in the presence of streptomycin

Inhibition of DNA replication initiation by aminoglycoside antibiotics

The reinitiation of DNA replication induced by a temperature shift in a dnaC(Ts) mutant of Escherichia coli was markedly inhibited by aminoglycoside antibiotics around the MIC in a short period. Protein synthesis continued for several minutes after the addition of aminoglycosides but was immediately blocked by chloramphenicol, suggesting that the inhibition of initiation of replication by aminoglycosides is not a secondary effect due to the interruption of protein synthesis. Aminoglycosides did not significantly affect RNA synthesis, suggesting that primer RNA synthesis for DNA initiation is not blocked by the agents. The lethal action of habekacin was observed simultaneously with the inhibition of DNA reinitiation. DNA elongation demonstrated with a dnaE(Ts) mutant or toluene-treated cells of a polA mutant was not significantly affected by aminoglycosides. The oriC-membrane complex formation was markedly interrupted by habekacin in the dnaC(Ts) mutant, and the in vitro reconstitution of the oriC-membrane complex was completely blocked by aminoglycosides. The present studies show that aminoglycosides block initiation of DNA replication and suggest that the inhibition is caused by the interruption of oriC-membrane attachment.

Amikacin-induced alterations in the structure of gram-negative bacilli

Exposure of some species of Enterobacteriaceae and of Pseudomonas aeruginosa to amikacin results in structural alterations which coincide with a decrease in the number of colony forming units. Escherichia coli when exposed to amikacin at a concentration twice the minimum inhibitory concentration for 2 hr shows a reduction in the number of ribosomes in the center of the cell and an aggregation of nuclear material in a peculiar concentric pattern we called "tornado image." After 2-4 hr of exposure to amikacin at concentrations of two to five times the minimal inhibitory concentration, all species tested showed in addition to alterations in the distribution of ribosomes, ruptures in the cytoplasmic membrane, damaged cell walls, and in some instances complete loss of cellular shape. It appears that amikacin produces a lytic death of bacteria.