The Influence of Temperature on the Motility of

SUMMARY: Grown a t 22 " Pasteurella pseudotuberculosis was motile and had flagella and H antigen; grown a t 37 " it was non-motile and had neither flagella nor H antigen. The change occurred over a narrow range of temperature about 30". Cells grown at 37 " when transferred to 22 " developed flagella without cell division; cells grown a t 22" when transferred to 37 " became non-motile, due t o an unexplained adverse effect indicated by a high death-rate. It appeared that different metabolic processes were used at the two temperatures; it was not necessary to postulate the selection of a naturally occurring mutant and no evidence was obtained for the existence of an extracellular enzyme which would destroy flagella a t 37". In addition to the 'short-term ' effect there was a 'long-term ' effect of temperature shown by a difference in degree of motility and rate of growth (at 22") of cultures grown for several months at 22 or 37". This may have been an extension of the 'short-term ' effect, a period of training being required for the optimal functioning of the metabolic processes concerned. The long-term effect could be reversed by prolonged cultivation of the 22 " substrain a t 37 " and vice versa