74 



LABORATORY EXERCISES IN BACTERIOLOGY. 



(a) Dish Cultures. 



Exercise 23. Select potatoes of as regular shape as possible, without 

 blemish and with as few eyes as possible, each student preparing two 

 potatoes. Wash them in running water, using a stiff hand brush for 

 effectiveness. The eyes and any suspicious spots are next cut out freely, 

 after which the potatoes are immersed in a dish containing i : 1000 solu- 

 tion of bichloride of mercury for an hour. They are then put in the steam 

 sterilizer in a tin bucket with perforated bottom ("potato bucket") and 

 cooked for three-quarters of an hour or an hour at 100 C. They are 

 not to be removed from the sterilizer, but are again steamed at the same 

 temperature for fifteen minutes upon the second and third days. A 

 potato dish should in the mean while be prepared for their recep- 

 tion at the close of the process 

 of sterilization upon the third day 

 (i>. instructions, Ex. 22). The 

 hands of the operator are now to 

 be thoroughly washed with hot 

 water, soap, and brush, and soaked 

 for five minutes in a i : 1000 solu- 

 tion of mercuric chloride and a 

 potato knife, a flat-bladed knife 

 such as is commonly sold for use 

 in the household for paring pota- 

 toes (Fig. 22), is sterilized by flam- 

 ing (i>. instructions, Ex. 6) if it has 

 not previously been prepared by 

 baking or boiling. The opera- 

 tor, holding the potato with the thumb and fingers of the left hand 

 grasping its shortest diameter, divides it into halves, drawing as little 

 of the blade as possible through it and retaining the blade in position 

 between the cut surfaces without separating the two halves. An assistant 

 raising the cover of the culture dish, the potato is deposited therein in 

 position so that, as the two halves are separated by proper movement 

 of the knife, the cut surfaces will fall apart and remain uppermost. The 

 dish is now temporarily closed, and the second potato is in like manner 

 to the first divided and placed in the dish with its cut surfaces exposed. 

 In these operations no more exposure of the interior of the dish is allowed 

 for entrance of organisms from the air than can be avoided ; and obviously 

 the procedure should not be performed where any atmospheric draughts 

 prevail. In order that the student may obtain some idea of the objec- 

 tionable features of such cultures and the value of the method, this first 



FIG. 22. SECTION OF POTATO INTENDED FOR 

 DISH CULTURE. 



