The isoelectric point of bacterial cells.

This study was undertaken to compare the isoelectric points of gram positive and gram negative bacterial cells by the method of microelectrophoresis. Within recent years a number of workers have referred to differences in the isoelectric point of the gram positive and the gram negative groups of bacteria. This concept arose from the studies of Stearn and Steam (1924, 1925, 1928), Tolstoouhov (1929), and others who have used differential staining procedures at different pH levels. The apparent isoelectric point was taken to be the pH at which the cells retained equal amounts of basic and acidic dyes. Recent studies of the staining mechanism by Knaysi (1951) indicate that the basic dyes do not combine with the cell wall but stain the cytoplasmic membrane. The dye method would indicate then the properties of the cytoplasmic surface rather than the actual cell surface. It could be expected that the protoplasm of the gram positive cells with the high ribonucleic acid content would exhibit greater acidic properties than that of gram negative cells. However, the isoelectric point of the cytoplasmic membrane would correspond to the point at which equal amounts of basic and acidic dyes combined only if the dyes used possessed opposite charges of the same magnitude and if no other factors other than electrical charge influenced the combination of dyes with the cells. It has been demonstrated that in a series of structurally related acidic dyes, adsorption depends to a great extent in the configuration of the dye molecule (Harris, 1949). Furthermore the dye method as applied in the past depends upon the ability of the investigator to judge by microscopic examination when cells have been stained equally by the two