SUMMARY: The turbidity of suspensions of Gram-negative micro-organisms is known to be influenced by the tonicity of the suspending medium. In contrast to this, suspensions of Gram-positive micro-organisms have the same optical density in 0·2-NaCl as in distilled water. At higher salt concentrations, however, occasionally some batches were found to display changes in turbidity. Correlation was shown between the concentration of the amino acid pool of the organisms and their response to changes in osmotic pressure of the suspending medium. Depletion of the amino acid pool resulted in an increase in the changes in turbidity, with the simultaneous suceptibility of Gram-positive bacteria to plasmolysis. A possible correlation between this optical effect and the separation of cytoplasmic membrane from the bacterial cell wall is discussed.


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