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Motility is universally recognized as a significant biological character of certain bacteria, and is used as a fundamental basis of classification in many taxonomic systems. Significance is often attached to motility as an identifying character in the differentiation of species, and independent species have sometimes been established on the basis of lack or possession of motility. Relatively little attention seems to have been paid to variability in motility and the factors that influence it. Discordant statements and opinions about the presence or absence of motility in different species are, however, not difficult to find. Although the overwhelming weight of opinion today is that dysentery bacilli are non-motile (Lentz and Prigge, 1931; Gardner, 1929), both Shiga and Flexner originally described these organisms as motile and several experienced observers have since reported that motility either occurs spontaneously in certain strains or may be induced by the application of special methods.2 As regards other groups of bacteria, such as the so-called Morgan bacilli, quite divergent statements as to the presence or absence of motility are made. It is well known also that non-motile strains of predominantly motile species sometimes occur; Smith and Reagh's classical observations (1903) on the distinction between somatic and flagellar agglutinins were based on the discovery of a strain of the hog-cholera bacillus which was non-motile but in all other respects appeared to be identical with the motile type. The effect of environmental factors on motility seems to have been little studied. 1 The present investigation was aided by a grant to the University of Chicago from the Rockefeller Foundation. 2 It has been found, for example (Colquhoun, D. B., and Kirkpatrick, J., 1932), that many organisms, including a dysentery strain, that are non-motile in ordinary fluid or solid media develop motility when grown on semisolid medium.

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