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SUMMARY: Refractive index measurements were made on the spores and vegeta- tive cells of strains of Bacillus cereus, B. cereus var. mycoides and B. megaterium by phase contrast and interference microscopy with protein immersion. The refractive indices of the spores were found to be very high and comparable with that of dehydrated protein, suggesting that they contained very little water. Those of the vegetative cells were much lower, and indicated a solid content of about 30 yo, w/v. Because of their relatively high resistance to heat and other lethal agents, it has long been thought probable that bacterial spores contain appreciably less water than the vegetative forms. Only a few attempts to measure the water content of spores or vegetative cells have, however, been described (Virtanen & Pulkki, 1933; Henry & Friedman, 1937), and the methods used, all of which involved bulk weighing and varying degrees of desiccation, were necessarily rather crude and imprecise. Refractive index measurements can now be made on microscopical objects with considerable accuracy (e.g. Barer, Ross & Tkaczyk, 1953; Mitchison & Swann, 1953; Davies, Wilkins, Chayen & La Cour, 1954; Ross, 1954; Barer & Joseph, 1954), and, as was first pointed out by Davies & Wilkins (1952) and by Barer (1952a), there is a close correlation between the refractive index of a cell and its water and solid content. It has been shown that the majority of substances found in living cells have approximately the same specific refraction increment (a) of c. 0.0018. This means that their combined refractive indices increase by about 0.0018 for every 1 yo rise in their w/v concentration (Davies et al. 1954; Barer & Joseph, 1954). In the present work, the refractive indices of the vegetative cells and spores of three selected organisms were measured with a view to obtaining a more accurate indication of their water and solid content than hitherto.

The Water and Solid Content of Living Bacterial Spores and Vegetative Cells as Indicated by Refractive Index Measurements