A Precision Photometer for the Study of Suspensions of Bacteria and Other Microorganisms

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Abstract
Suspensions of most microorganisms behave optically as suspensions of weak imperfectly figured lenses. A collimated light flux (Pm), incident on such a suspension, is divided by it into a flux (Pu), transmitted without deviation, and a scattered flux (Ps). Instruments are classified as "tyndallmeters," measuring the intensity of Ps in some given direction, and "densitometers," measuring Pm, and the sum of Pu and some fraction "q" of Ps, where q is a function of the angular distribution of Ps and the angular selectivity of the instrument. The optical density (D) is given by: D = log Pm/ (Pu + qPs) . Where qPs is small in relation to Pu, the relation D = sN holds over a considerable range of N, which is the number of organisms per unit volume. The proportionality constant s has the significance of a scattering coefficient per organism, and is mainly a function of its size, shape, and index of refraction. Various instruments which have been employed are discussed in the light of these expressions, and a highly sensitive and linear photoelectric cell densitometer, using vacuum tube amplification, is described. Its linearity for N, as a function of D, over the conc. range of 107 to 6 X 10S; for a suspension of Escherichia coli, is shown by a dilution series.