Turbidity Changes in Suspensions of Gram-Positive Bacteria in Relation to Osmotic Pressure

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 M-NaC1 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. It was reported in preceding communications (Mager, Kuczynski, Schatzberg & Avi-Dory 1956; Avi-Dor, Kuczynski, Schatzberg & Mager, 1956), that variations in the osmotic pressure of the suspending medium elicited corre-sponding changes in turbidity of Gram-negative bacteria. The light-scattering ability of suspensions of Gram-positive micro-organisms was found, however, to be independent of the prevailing tonicity. The object of the present study was to investigate the cause of the different response of these two classes of micro-organisms to changes in the osmotic pressure. METHODS Culture media. Escherichia coli was grown on nutrient agar (Difco) at 37". Bacillus megaterium was cultivated in a 2 yo (w/v) solution of an enzymic casein hydrolysate (N.B.C.), fortified with 0-1 yo (w/v) Marmite and incubated at 30 ' with shaking. Staphylococcus aureus was grown under the same condi-tions as B. meguterium with the addition of 1 % (w/v) glucose to the culture medium. Inoculum. A volume of 0.1 ml. of 24 hr. slope culture of bacteria suspended in 5 ml. saline was used for inoculation of 100 ml. medium