1. It has been shown that a fairly constant fraction of the total number of bacteria in a fresh-water culture of E. coli can reproduce on direct transfer to a saline medium with a definite NaCl concentration, as judged from the viable count determinations in such a medium.

2. The absolute value of this fraction depends on a number of factors other than the salt content of the test medium, such as the hydrogen ion and yeast autolysate concentrations, aeration, and the physiological condition of the bacteria.

3. A method for testing the degree and rate of adaptation of the bacteria to saline environment, depending on the analysis of changes in the value of the salt-viable fraction, was developed.

4. Maximum adaptability to saline environments was found during the early stationary phase of NaCl-free cultures. Low adaptability accompanied the logarithmic phase and the senescence of the cultures.

5. The limits of variation could be extended by treatment of non-dividing cells with gradually increasing concentrations of salt or by subjecting them to a single intermediate NaCl concentration. This acclimatization was independent of reproduction. The number of bacteria becoming capable of reproducing in a hitherto unfavorable environment increased with the period of exposure to intermediate salt concentrations until a maximum value was reached.

6. This maximum value was shown to depend on the salinity of the test medium, the age of the bacterial culture, and the method of preliminary treatment. "Optimal acclimatization" could be effected by subjecting the organisms to a single fairly low intermediate NaCl concentration.

7. The rate of the individual acclimatization process was shown to be greater at higher than at lower temperatures.

8. Acclimatized bacteria rapidly lost their increased ability to reproduce in saline media upon return to a salt-free environment, although no reproduction of the cells could be detected. This was interpreted as an indication that the processes involved are readily reversible.

9. Studies on the reproduction of E. coli in strongly saline broth indicated that only those cells originally acclimatized to the salt concentration of the medium could divide. All cells produced in such a medium could continue to reproduce. The propagation in the altered medium was not accompanied by any further acclimatization throughout five subcultures.

10. Both the division rate and the maximum crop of cultures in saline broth were considerably lower than of those in a fresh-water medium. No change in either occurred throughout five successive subcultures. The morphology of the organisms was also altered by the presence of salt.

11. The division rate, maximum crop, morphology, and adaptive power returned immediately to normal on re-transfer of bacteria grown in an NaCl-containing medium to "salt-free" broth.

12. The entire adaptive response of the bacteria to a considerable increase in the salinity of the environment could thus be separated into two components: an acclimatization, independent of reproduction, and a selection of those cells with the widest range of potentialities.

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