Results and Discussion

Survival of B. japonicum in HPM medium depended on its physiological state at the time of shock: stationary-phase cells were more resistant to NaCl than exponentially growing cells. Exponential-phase cells shocked with 0.1 or 0.4 M NaCl began to die after shock, whereas stationary-phase cells shocked with 0.1 M had a better survival than control cells, and cells shocked with 0.4 M NaCl showed a decrease in cell count similar to control cells. When these concentrations of NaCl are present in culture media from inoculation, growth is delayed or inhibited.

The number of exponential-phase cells counts increased from 2.2xl06 cells/g soil up to 2xl08 cells/g during the few days following inoculation. After 25 days, cell counts were 1.2-3.0xl08 cell/g soil either at 30% or 65% field capacity. In NaCl-supplemented microcosms inoculated with exponential cells microcosms, cell counts decreased to 7.0x103 and 3.9xl02 cells/g for 65% or 30% field capacity respectively. In microcosms without NaCl addition and inoculated with stationary-phase bacteria, cell counts did not change, irrespective of field capacity; but when NaCl was added, cell number decreased, and this decay was more pronounced at 30% field capacity.

These results suggest that different kinds of mechanisms of salt tolerance exist in B. japonicum. Some of them are needed to grow in the presence of salt and others allow for survival upon a sudden increase in NaCl or drought, but their operation is influenced by the environment. Most of the techniques used for physiological, biochemical and genetic studies in bacteria were designed for liquid media and many cannot be reproduced in microcosms. However, major differences in cell behavior in both environments are observed, and since microcosms would better resemble natural conditions, it is worthwhile to analyze also in microcosms the responses to stress.

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