Killing mechanism of Listeria monocytogenes in activated macrophages as determined by an improved assay system.

Exposure of Listeria monocytogenes to gentamicin 5 mg/L for 4 h resulted in the killing of most extracellular bacteria, but had no effect on the survival of bacteria inside macrophages. Higher concentrations of gentamicin caused a reduction in the number of intracellular bacteria. This effect was associated with cellular uptake of gentamicin, but was unaffected by activation of macrophages by interferon-gamma and lipopolysaccharide. In experiments in which exposure to gentamicin 5 mg/L for 4 h was used to kill extracellular bacteria, killing by activated macrophages was impaired when O2- production was inhibited by superoxide dismutase, but not when nitric oxide production was blocked by NG-monomethyl-L-arginine. These data suggest that the reactive oxygen intermediates are more important than nitric oxide in the killing of L. monocytogenes, at least in macrophages activated in vitro.