92 "PURE MIXED CULTURES" OF AMOEBA 



cent, to 3-per-cent. solution and as vapour, the 

 time having been varied from simple washing to a 

 period of a week. On only a few occasions have 

 cysts been recovered which would excyst after the 

 formalin has been washed away. As living bacteria 

 have also been got from the surface of culture 

 media, exposed to 2-per-cent. formalin for a week, 

 we are inclined to think that those cysts which 

 escape have not been exposed to the full effect of 

 the chemical. There is thus the danger of examin- 

 ing pure mixed cultures of amceba made in this 

 way and finding them morphologically pure, 

 whereas they may be contaminated with the few 

 bacteria carried over with the unaffected cysts. 

 It is interesting to note that the cysts do not look 

 abnormal (e.g. as under the influence of heat), and 

 it is impossible to say whether they are alive or 

 dead, and whether their subsequent failure to 

 develop is due to coagulation of the ectocyst which 

 will protect the contents, or to death. 



(2) Mercuric Chloride. This substance was 

 employed because, having a high molecular weight, 

 it might be possible to kill the bacteria with it 

 before it diffused through the dense cyst walls and 

 destroyed the enclosed amceba. On one occasion 

 only were living cysts recovered after this treat- 

 ment, but the culture made from them was impure 

 on plating out colonies. The same remarks apply 

 here as in the case of formalin, and we should not 

 use either of these again in the preparation of pure 

 cultures. It appears as if strong solutions of germi- 

 cides which coagulate albumen rapidly cannot be 

 relied on to destroy all bacteria on the surface of a 

 culture medium. 



