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.