MANUAL OF BACTERIOLOGY. 



FIG. 17. 



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 

 twelve to eighteen 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 sterilized 

 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, 



Tube contain- an( j t ] ien k t Qn j ce Qr twenty-four hours, 

 ing Jrotato. 



At the end of that time the middle por- 

 tion 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. About 7 to 10 c.c. are to be run into each 

 test-tube. The tube is plugged with cotton, and sterilized 

 as usual. When milk is contaminated with spores of the 

 hay or potato bacillus it is sometimes very difficult to steril- 

 ize, a fact of much importance in connection with the feed- 

 ing of children, where the fractional method of steriliza- 

 tion and the use of the autoclave are impracticable. 



