66 



MANUAL OF BACTERIOLOGY. 



communis, Part IV.)- To i liter of nutrient agar, add i gram of 

 dextrose and 0.5 gram of neutral red. Sterilize as usual. 



Potato. — ^The potatoes are washed, a slice is removed 

 from each end, and with an apple-corer or cork-borer a cylinder 

 is cut out. This cylinder is divided diagonally into two pieces. 

 The pieces are washed in running water for 

 several hours. They are placed in test- 

 tubes containing a little water to keep the 

 potato moist, and are supported from the bot- 

 tom on a piece of glass tubing about i to 2 

 cm. in length (or on cotton, or in a specially 

 devised form of tube with a constriction at the 

 bottom) . The tubes are plugged, and steriHzed 

 as with other media. Sterilization, however, 

 must be thorough on account of the danger 

 of contamination with the extremely resistant 

 spores of the potato bacillus. Potato is best 

 when freshly prepared; it is likely to become 

 dry and discolored with keeping. It is a very 

 useful medium; certain growths on it, like 

 those of the bacillus of typhoid fever or of 

 glanders, and those of chromogenic bacteria, 

 are very characteristic* 



Milk. — -Milk fresh as possible is placed in 



a covered jar, sterilized for fifteen minutes, 



and then kept on ice for twenty-four hours. 



At the end of that time the middle portion 



is removed by means of a siphon. The upper and lower layers 



must not be taken; the upper part contains cream, and the 



lower part particles of dirt, both of which are to be avoided. 



Fig. 17.— Tube 

 Contain ing 

 Potato. 



* Bolton. The Medical News. Vol. I., 1887. P. 318. A Method of Pre- 

 paring Potatoes for Bacterial Cultures. Roux. De la Culture sur Pomme de 

 Terre. Annales de I'Institut Pasteur. T. II., 1888. P. 28. Globig. Ueber 

 einen Kartoffel-Bacillus mit ungewohnlich widerstandsfahigen Sporen. Zeil- 

 schrijt fur Hygiene. P. 322. 1888. 



