Three psychrophilic micro-organisms (strains of Arthrobacter, Candida and ) were capable of growth for a period when exponential-phase cultures in chemically defined media were transferred from temperatures at or near the optima for growth (20, 10 and 15°, respectively), to 37, 25 and 30°, respectively. The latter temperatures were 3–5° above the maxima for the growth of the organisms in freshly inoculated cultures. Growth at the higher temperatures was greatest with the Candida and least with the Arthrobacter. Cultures of the Arthrobacter and Candida grew when transferred back to the optimum temperatures for growth, after a lag which increased with the length of time that the cultures had spent at the higher temperatures. cultures grew almost immediately after they were transferred back to the optimum growth temperature. Growth of the organisms at the higher temperatures was not affected by supplementing cultures with bacteriological peptone and/or yeast extract. There was a rapid decline in the viability and in the rates of respiration of endogenous reserves and of exogenous glucose and pyruvate when Arthrobacter and Candida cultures were transferred to the higher temperatures. But with the respiratory activities were not so markedly affected by the change in incubation temperature, while the viability of this bacterium increased slightly after the transfer of cultures to the higher temperature. The activities of many of the tricarboxylic acid cycle enzymes in Arthrobacter and Candida were diminished after the transfer of organisms from the optimum temperature to one above the maximum for growth; but the activities of these enzymes in were less affected by the change in incubation temperature. There was no marked intracellular accumulation or excretion of ultraviolet-absorbing compounds by the organisms after the transfer of cultures to the higher temperatures. The results are discussed in relation to the biochemical factors which may determine the maximum temperatures for growth of these organisms.


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