84 "PURE MIXED CULTURES" OF AMCEBA 



two bacteria present in the treated plates. Benzoic 

 acid, quinine sulphate, strychnine hydrochloride, 

 caustic soda, hydrochloric acid, heat, and drying 

 were then employed on slopes, plates, and sus- 

 pensions of the cysts in water. Of these substances 

 only hydrochloric acid and the action of drying 

 were successful (but the former is the best for 

 practical purposes), and it is only necessary to state 

 here that cysts were readily freed from the two 

 particular bacteria mentioned above by exposure to 

 2-per-cent. HC1 for forty-eight hours. The culture 

 made from these was found to be bacteriologically 

 pure with Bacillus fluorescens non-liquefaciens 

 (fig. 26). The strongest evidence we are able to 

 submit of having obtained pure mixed cultures of 

 amoeba is that the above mixture of two bacteria 

 rapidly liquefies a gelatin medium, whereas the 

 cysts after treatment with the HC1, when mixed 

 with a pure strain of a non-liquefying bacillus on a 

 1 2-per-cent. gelatin slope, develop into living 

 amoeba and encyst again, there being no liquefac- 

 tion of the gelatin at the end of a fortnight's 

 growth. When once one has obtained a pure 

 mixed culture with a particular bacillus reliance 

 must be placed on facts known to bacteriology as 

 regards the kind and quantity of germicide to 

 employ, as it is obviously impossible to make a 

 complete analysis of cultures with every bacterium 

 with which you place the cysts. 



If, for example, one obtains a culture with 

 Bacillus fluorescens liquefaciens, which is readily 

 destroyed by HC1, this can be used as a routine 

 method and an indefinite number of pure mixed 

 cultures made from it. This is the method we at 



