320 PRINCIPLES OF SOIL MICROBIOLOGY 



were left unaffected. As soon as the chlorine was removed, the amoebae 

 excysted and began to feed rapidly on the dead bacterial cells ; when the 

 food supply became exhausted, the protozoa encysted again. 35 



Robertson 36 ascribed the autocatalytic phenomenon observed in the 

 growth of protozoa to an accessory foodstuff, a soluble product of bac- 

 terial metabolism. According to Cutler it seems very uncertain that 

 autocatalysis occurs in protozoal growth, but, if the curve is autocatalytic 

 in nature, this is due to an increased food supply (bacteria) and not to an 

 accessory food as Robertson suggested. Certain species of bacteria, 

 like Bad. fluorescens and Bad. coli, may produce substances injurious 

 to the development of the protozoa. This injurious effect may be due to 

 the consumption of oxygen by the bacteria, or to a change in reaction 

 brought about by bacterial growth. 37 



Isolation of pure cultures of protozoa. By examining crude cultures of 

 protozoa from time to time, it is found that there is usually in any given 

 culture a more or less definite succession of animal forms. By selecting 

 the time and method of culture, it is possible to isolate pure protozoan 

 cultures. Several methods have been utilized 38 for isolation of amoebae, 

 the simplest of which consists in destroying the bacteria by means of 

 heat or disinfectants (also 2 per cent HC1 over night or 20 per cent 

 Na 2 C0 3 for 3 days) in encysted protozoan cultures; the more resistant 

 cysts survive. The use of the Barber pipette or Chambers' apparatus 

 has also been suggested. Oehler inoculated first a dish containing agar 

 or gelatin medium with a pure culture of a certain bacterium, then 

 a mixed protozoan culture was inoculated into the center of the dish. 



35 Further information on the cultivation of amoebae is given by Schaeffer, 

 A. A. Choice of food in amoeba. Jour. Anim. Behavior, Cambridge, 7: 220- 

 258. 1917; Arndt, A. Zur Technik der Amobenziichtung. Centrbl. Bakt. I, 

 Orig., 88: 417. 1922. The influence of bacteria upon the growth of protozoa is 

 discussed by Chatton, E., and Chatton, M. L'influence des facteurs bacteriens 

 sur la nutrition, la multiplication et la sexualite des infusoires. Compt. Rend. 

 Acad. Sci., 176: 1262-1265. 1923. The use of bacterial solutions for the cultiva- 

 tion of protozoa was reviewed by Cunningham, A., and Lohnis, F. Studies 

 on soil protozoa: I. The growth of protozoa on various media and the effect of 

 heat on active and encysted forms. Centrbl. Bakt. II, 39: 596-610. 1914. 

 Killer, J. Die Zahlung der Protozoen im Boden. Centrbl. Bakt. II, 37: 521- 

 534. 1913. A detailed review of the earlier literature on the cultivation of 

 protozoa is given by Kopeloff, Lint and Coleman, 1917 (p. 328). 



36 Robertson, T. B. The multiplication of isolated infusoria. Biochem. 

 Jour., 15: 595-611, 612-619. 1921. 



37 Oehler, 1921 (p. 319). 



38 Gordon, C. E. A method for obtaining amoeba. Science, 46: 212. 1917. 



