SUMMARY: Various substances protect dry Escherichia coli against oxygen. The concentration of these substances in the bacterial suspension, necessary to achieve a given degree of protection, is a function of the concentration of the bacteria in the suspension. The protector seems to act in the dry state. The viability of freeze-dried bacteria, unexposed to oxygen, may also depend on the concentration of the bacterial suspension. A common mechanism is suggested to explain the dependence of killing on population density, both during freeze-drying and during exposure of dried organisms to oxygen. The viability of bacteria during freeze-drying and of dried bacteria exposed to oxygen are both markedly affected by the presence of certain substances such as serum albumin or Bacto-protone.
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