fYPHOID FEVER 9; 



(4) Inoculate plates of Conradi-Drigalski culture-medium with the 

 culture thus obtained by charging a loop and drawing it in successive 

 strokes over the whole surface of the medium. Or, take a loopful or 

 two and spread it thoroughly over the medium, using as a spreader a 

 capillary pipette sealed at the tip and bent over at an angle about an 

 inch from the end. 



Conradi-Drigalski medium is not easy to prepare, and can be bought 

 ready for use. The formula is : 



Meat extract ... ... ... 20 grammes 



Nutrose ... ... ... 20 ,, 



Peptone ... ... ... 20 ,, 



Salt ... ... ... ... 10 ,, 



Agar ... ... ... 60 ,, 



Water ... ... ... 2 litres 



Boil, preferably in a steamer, until the ingredients are completely dis- 

 solved. This will take an hour or more. 



Boil 300 c.c. of a watery solution of litmus, and add 30 grammes of pure 

 lactose, and continue boiling fifteen minutes. Add this to the medium 

 already prepared, and if the mixture is not faintly blue, add enough 

 sterile soda solution to render it so. Then add 4 c.c. of a sterile solution 

 of sodium carbonate. Finally, dissolve ^ of a gramme of crystal-violet 

 B. in 100 c.c. of hot sterile distilled water, add 20 c.c. to the mixture, 

 mix well, and distribute into sterile flasks containing 50 to 100 c.c. 



For use, melt one of these flasks, taking care not to overheat it. 

 Pour the contents into as many Petri dishes as are required, making a 

 thick layer in each. Allow this to set, then place the dish, inverted 

 and slightly opened, in an incubator for half an hour or more to render 

 it surface-dry. This may be hastened by wiping the water of con- 

 densation off the lid with a pledget of sterile cotton-wool (the plug of 

 a sterile test-tube). 



The rationale of this medium is as follows : The crystal-violet acts 

 as a selective antiseptic, killing or inhibiting the growth of many 

 organisms, whilst allowing B. typhosus, B. coli, and others to grow 

 almost unchecked. The lactose is acted on by some of the organisms, 

 acid being formed, but is unaffected by others. The acid thus produced 

 acts on the litmus, causing the colonies to turn pink. Thus the use of 

 this medium allows us to pick out at a glance the members of the im- 

 portant group of non-lactose fermenters, most of which are pathogenic. 

 There is a slight difference between the colonies of the different 

 organisms suggestive to the trained eye. Thus, B. typhosus forms 

 transparent colonies with wavy edges, Paratyphoid A colonies are 

 small and seen with difficulty, whereas those of Paratyphoid B are 

 large and opaque. 



The next step is, to pick out a suspicious colony and make a second 

 culture on a Conradi plate. Several can be made on the same plate, 

 which is divided into grease-pencil lines on the glass underneath. This 

 second plating insures the purity of the culture and provides enough 

 material for the subsequent steps. Incubate twenty-four hours. 



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