The size and shape of bacteria by light scattering measurements

Abstract

Experimental measurements show that the Rayleigh-Gans approximation for calculating the intensity of scattered light at angles up to 30° applies surprisingly well to particles as large as bacteria. Theoretically, this is because the index of refraction of the organism is nearly that of the medium, and also because the phase shifts of light scattered by different parts of a single bacterium, which are neglected in the Rayleigh-Gans treatment, largely compensate for one another when averaged over the population of different sizes and orientations. Experimental measurements were made with a simple apparatus based on an unenclosed, disjunct Zeiss PMQ II spectrophotometer. With this instrument, measurements only 1° from the primary beam could be made. Two sizes ranges of growing bacteria were studied. The light scattering measurements indicated a volume factor of about 4-fold between the two types of cultures whereas particle counter measurements indicated a factor of 7-fold. Test indicates that this discrepancy is neither due to a failure of the Rayleigh-Gans approximation nor to heterogeneity in index of refraction of the particles, but to the effect of cell walls. It is proposed that low angle light scattering can be useful in estimating the average volume of the bacterial cells in populations.