Escherichia coli cells challenged with low or high concentrations of hydrogen peroxide are killed via two different mechanisms and respond with morphological changes which are also dependent on the extracellular concentration of the oxidant. Treatment with low concentrations (<2.5mM) of H
2O
2 is followed by an extensive cell filamentation which is dependent on the level of H
2O
2 or the time of exposure. In particular, addition of 1.75mM H
2O
2 results in a growth lag of approximately 90min followed by partial increase in optical density, which was mainly due to the onset of the filamentous response. In fact, microscopic analysis of the samples obtained from cultures incubated with the oxidant for various time intervals has revealed that this change in morphology becomes apparent after 90min of exposure to H
2O
2 and that the length of the filaments gradually increases following longer time intervals. Analysis of the ability of these cells to form colonies has indicated a loss in viability in the first 90min of exposure followed by a gradual recovery in the number of cells capable of forming colonies. Measurement of lactate dehydrogenase in culture medium (as a marker for membrane damage) has revealed that a small amount of this enzyme was released from the cells at early times (<150min) but not after longer incubation periods (300min). Cells exposed to high concentrations of H
2O
2 (>10mM) do not filament and their loss of viability is associated with a marked reduction in cell volume. In fact, treatment with 17.5mM H
2O
2 resulted in a time-dependent decrease of the optical density, clonogenicity, and cellular volume. In addition, these effects were paralleled by a significant release in the culture medium of lactate dehydrogenase thus suggesting that the reduced cell volume may be dependent on membrane damage followed by loss of intracellular material. This hypothesis is supported by preliminary results obtained in electron microscopy studies. In conclusion, this study further demonstrates that the response of
E. coli to hydrogen peroxide is highly dependent on the concentration of H
2O
2 and further stresses the point that low or high concentrations of the oxidant result in the production of different species leading to cell death via two different mechanisms and/or capable of specifically affecting the cell shape.
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