The density and size of air-borne Serratia marcescens.

During an investigation of air-borne microorganisms, the net electric charges carried by individual cells at various relative humidities were studied using a Millikan apparatus (1947), a device in which the vertical rates of movement of the cells under gravity and under imposed electric fields were measured. Cell size and charge could have been calculated directly from such measurements if the exact cell density had been known. Lacking this information, an approximate density was first employed and then corrected by making use of the fact that the cells must have an indicated charge equivalent to whole numbers of electron charges. Thus it was possible to calculate actual values for both cell density and cell size. This information is presented because the behavior of the microorganisms under various humidity conditions should be of interest and because density and size are quite important in other microbiological studies, e. g., sedimentation studies.