42 



CULTURE MEDIA. 





of the best French gelatin to the flesh-peptone solution above de- 

 scribed. This is the standard gelatin medium, but more or less 

 gelatin may be added to serve a special purpose. Thus, in Havana 

 during the summer months the writer used a medium containing 

 twenty per cent of gelatin, because when but ten per cent was used 

 the gelatin was liquefied by the normal temperature of the atmo- 

 sphere. Ten-per-cent gelatin, of good quality and carefully pre- 

 pared, will stand a temperature of 20 to 22 C. (68 to 71.6 F.) 

 without melting. When twenty per cent of gelatin is used the 

 melting point is about 8 C. higher. It must be remembered that 

 exposure to a boiling temperature reduces the melting point of gela- 

 tin. It is therefore desirable to accomplish the operations of cook- 



FIG. 17. 



ing and sterilizing in as short a time as is practicable. The French 

 gelatin used comes in thin sheets ; this is broken up and added to 

 the flesh-peptone solution. 



Usually we prepare a litre of nutrient gelatin at one time, and for 

 this quantity one hundred grammes of gelatin will be required for the 

 standard preparation (ten per cent). It is well to allow it to soak for 

 a time in the liquid before applying heat for the purpose of dissolving 

 it. Then apply gentle heat until it is completely dissolved. The gela- 

 tin of commerce usually has an acid reaction, and it will be necessary 

 to carefully neutralize the medium after it has been added. A slightly 

 alkaline reaction is usually no disadvantage, but certain pathogenic 

 bacteria will not grow when there is a trace of acid present. The 

 next step consists in clarifying the nutrient medium. It is allowed 



