NUTRIENT MEDIA 37 



Agar jelly remains solid at 40 C., and only melts com- 

 pletely at 99 C. ; hence this medium is well adapted for the 

 higher incubating temperatures. Nutrient agar is often 

 quite clear when hot, but is always slightly opalescent on 

 cooling. 



Glycerin Agar. Nutrient agar containing 5 per cent, 

 of glycerin. 



Glucose Agar. The addition of 1 to 2 per cent, of 

 glucose to nutrient agar is useful for the cultivation of 

 anaerobic bacteria. The tubes for this purpose are filled 

 two- thirds full. The medium should not be heated more 

 than is absolutely necessary during preparation and 

 sterilisation, or it becomes dark. 



Urine Gelatin and Agar. Fresh urine thickened with 

 10 per cent, of gelatin, or 2 per cent, of agar, with the 

 addition of 1 per cent, of peptone and -- per cent, of sodium 

 chloride, is rendered feebly alkaline and filtered. 



Peptone Water. Ten grammes of peptone and 5 

 grammes of sodium chloride are dissolved in 1,000 c.c. 

 of distilled water; the solution is then well boiled, and 

 neutralised carefully in the usual manner. The solution 

 is again boiled and filtered. The solution is then run into 

 tubes, and steamed for fifteen minutes on three successive 

 days. 



Glucose and Lactose Peptone Waters. The addition 

 of 1 to 2 per cent, of glucose or lactose to peptone water is 

 very useful when determining the fermentative power of 

 organisms. The medium may be tinged with litmus, in 

 order to show production of acid or alkali, sufficient of a 

 neutral solution of litmus being added for this purpose. 

 These media and their corresponding broths are preferably 

 introduced into Durham's tubes, which consist of the 

 ordinary test-tubes into which small inverted tubes have 

 been introduced. The small tubes during the process of 

 sterilisation becoma filled with the medium, and then serve 

 as gas-holders, should the sugar be fermented with the 

 production of gas. If the inner tubes do not fill during 

 sterilisation, two grease-pencil marks should be made 

 on the outer tube to show the volume of air left in, or the 

 Durham tubes may be put in a water-bath and heated 

 to boiling for ten minutes, when the air bubble will dis- 

 appear. As a general rule the production of gas can be 

 observed without using an inner tube, as a few small 



