177 



4. Cool to 60 C. ; clarify with egg as for nutrient 

 agar (vide page 167). 



5. Filter through papier Chardin, using the hot- 

 water funnel. 



6. Tube, and sterilise as for nutrient agar. 



Peptone Water (Dunham) . 



1. Weigh out Witte's peptone, 10 grammes, and 

 salt, 5 grammes, and emulsify with about 250 c.c. of 

 distilled water previously heated to 60 C. 



2. Pour the emulsion into a litre flask and make up 

 to 1000 c.c. by the addition of distilled water. 



3. Heat in the steamer at 100 C. for thirty minutes. 



4. Filter through Swedish filter paper. 



5. Tube in quantities of 10 c.c. each. 



6. Sterilise in the steamer at 100 C. for twenty 

 minutes on each of three consecutive days. 



"Sugar" or "Carbohydrate" Media.- 



Formerly the ability of bacteria to induce hydrolytic 

 changes in carbohydrate substances was observed only 

 in connection with a few well-defined sugars, but of 

 recent years it has been shown that when using 

 litmus as an indicator these so-called "fermentation 

 reactions" facilitate the differentiation of closely 

 allied species, and the list of substances employed 

 in this connection has been considerably extended. 

 The media prepared with them are now no longer 

 regarded as special, but are comprised in the " stock 

 media" of the laboratory. The chief of these sub- 

 stances are the following, arranged in accordance with 

 their chemical constitution : 



Monosaccharides Dextrose (glucose), laevulose, galactose, . 



mannose, arabinose, xylose. 



Disaccharides Maltose, lactose, saccharose. 



Trisaccharides Raffinose (mellitose) . 



Poly sac char ides Dextrin, inulin, starch, glycogen, arnidon. 



Glucosides Arnygdalin, coniferin, salicin, helicin, 



phlorrhizin. 

 12 



