METHODS OF INVESTIGATING BACTERIA. 791 



These various nutrient substrata are more or less un- For patho- 

 suitable for the cultivation of the pathogenic bacteria, genie bacteria. 

 These organisms evidently require a certain quantity of 

 albumen and peptone, and sometimes also sugar, and 

 are very sensitive to variations in the composition of 

 the nutrient substratum. In the case of the majority of 

 the pathogenic bacteria, the most favourable medium 

 must be specially ascertained by experiment. 



The following nutrient solutions are the best : meat 

 infusion (prepared in the same manner as in the pre- 

 paration of the nutrient jelly) ; meat infusion, with 1 per 

 <ient. peptone and 2 per cent, dextrose ; solution of 

 meat extract (Liebig's meat extract, 1 per thousand), 

 with peptone and dextrose ; milk ; blood serum. We 

 have also a series of soft but solid nutrient substrata, 

 more especially boiled potatoes ; mixtures of the nutrient 

 solutions with solidifying agents, such as gelatine or agar ; 

 solidified blood serum. All these nutrient substrata 

 must be neutralised till the reaction becomes faintly 

 alkaline ; where the reaction is at first markedly acid, 

 this is done by the addition of concentrated soda 

 solution, where the excess of acid is slight it is best to 

 -employ a solution of the acid phosphate of soda. 



All the vessels and nutrient substrata must be sterilisation 

 thoroughly sterilised before their employment, i.e., and 

 they must be deprived of all living germs. This is done substrata. 

 by heating the vessels (test tubes, with their cotton wool 

 plug, &c.) in copper or iron cases kept at a temperature 

 of 150 to 180 C. for an hour ; at this temperature the 

 wool is slightly burned; as a rule the temperature 

 differs at various parts of the oven, and we must there- 

 fore ascertain in what part the proper temperature is 

 present. Larger dishes are washed out with a solution 

 of corrosive sublimate (1 to 2000), and then they are 

 repeatedly rinsed with distilled water which has been 

 sterilised by boiling, or with absolute alcohol. The 

 nutrient substrata as soon as they have been intro- 

 duced into the sterilised vessels are freed from their 

 germs by boiling them in a steaming apparatus ; accord- 

 ing to the amount of material they remain for fifteen to 



