322 PRINCIPLES OF SOIL MICROBIOLOGY 



The constituents are sterilized separately and the final mixture sterilized at 

 S0°C. on three consecutive days. The reaction of the medium is adjusted to 

 pH 7.4. 



per cent per cent 



II. NaCl 0.06 MgS0 4 0.001 



KC1 0.001 Ammonium glycerophos- 



CaCl 2 0.002 phate 0.06 



Phenol red Trace 



As a result of a series of studies, Peters demonstrated that glucose and 

 ammonium lactate can serve as good sources of carbon and nitrogen for 

 Colpidium. In addition to these, phosphates and chlorides, as well as 

 potassium and magnesium are required. Amino acids can take the 

 place of ammonium salts as sources of nitrogen. Carbon sources con- 

 taining less than three carbon atoms in the molecule are not utilized; 

 glycerate is used, but not lactate or citrate. Purdy and Butterfield 41 

 found, however, by the use of pure, bacteria-free cultures of protozoa, 

 that the latter cannot exist in culture solutions containing organic 

 matter, but free from bacteria; these form the main food of the protozoa, 

 which develop at the expense of the bacteria. 



The concentration of the medium, reaction and temperature are of 

 great importance in the cultivation of protozoa. A ciliate and a bacte- 

 rium were cultivated 42 in a 0.1 per cent peptone solution, adjusted to pH 

 6.8, at 22° to 25°. At 24 hours, bacterial growth took place followed by 

 that of protozoa. The protozoa could be separated from the bacteria 

 electrolytically. Under the influence of the fall of potential, the protozoa 

 travel to the cathode and the bacteria to the anode. This has to be 

 repeated 6 times before cultures of protozoa free from bacteria are 

 obtained. This procedure combined with the destruction of bacterial 

 cells by heat were utilized by Oehler 43 for the purification of amoebae and 

 ciliates. By keeping dry cultures of protozoan cysts at 37° for 6 weeks, 

 the bacterial vegetative cells were destroyed. Water and a culture of 

 Saccharomyces were then added to the culture and the protozoa ex- 

 cysted. The yeasts can be killed at 60° for 24 hours. By the use of a 

 cataphoresis apparatus and unpolarized electrodes in 0.65 per cent 

 NaCl solution, the ciliates were found to travel to the cathode. How- 

 ever, the plate method was found to be best and the fact that protozoa 



41 Purdy, W. C, and Butterfield, C. T. The effect of plankton animals upon 

 bacterial death-rates. Amer. Jour. Public Health, 8: 499-505. 1918. 



42 Amster. Ein neues Zi'ichtungsverfahren fur Protozoen. Centrbl. Bakt. 

 I, Orig., 89: 166-168. 1922. 



"Oehler, 1924 (p. 319). 



