Measurement of gram-positiveness.

Several papers have been published (Kennedy and Barbaro, 1953; Barbaro and Kennedy, 1954; Bartholomew and Finkelstein, 1954; Kennedy and Woodhour, 1956; Barbaro et al., 1956) on methods of comparing degrees of gram-positiveness and the influence of certain factors on the quantity of crystal violet retained by the microorganisms. These papers compare the degrees of gram-positiveness by measuring the quantities of dye adsorbed by a given mass of cells. In experiments carried out with methylene blue and yeasts (Borzani and Vairo, 1958), it was verified that the adsorption of a dye by a microorganism follows a physico-chemical law (Freundlich's law) and logical measures of the degree of gram-positiveness could probably be derived if the adsorption of crystal violet by microorganisms also followed Freundlich's law. Later (Borzani and Vairo, 1959) attention was called to the fact that only adsorption of crystal violet by gram-positive microorganisms follows physico-chemical laws; the dye adsorption by gram-negative cells does not follow any law. Also, it was demonstrated that it is not correct to compare degrees of gram-positiveness by measuring the quantities of dye retained by a given mass of cells, and that Freundlich's law and Langmuir's law do not provide a method to compare degrees of gram-positiveness. Thus it appeared that the correct way to make comparisons was to measure the quantity of crystal violet adsorbed by a given area of cells. Freundlich's law and Langmuir's law considered in papers by Borzani and Vairo (1958, 1959) show: