MILK AS A CULTURE MEDIUM. 55 



of this tube is placed a piece of cotton wool, which catches 

 any condensation water which may form. The wedge rests 

 on the constriction above this bulbous portion. The tubes, 

 washed, dried, and with cotton wool in the bottom and in 

 the mouth, are sterilised before the slices of potato are in- 

 troduced. After the latter are inserted, the tubes are 

 steamed in the Koch steam steriliser for one hour. An 

 ordinary test-tube may be used with a piece of sterile 

 absorbent wool in its bottom, on which the potato may 

 rest. 



The use of the potato as a medium is very important, 

 as in certain cases the growths of bacteria on it are very 

 characteristic. Potatoes ought not to be prepared long 

 before being used, as the surface is apt to become dry and 

 discoloured. It is well to take the reaction of the potato 

 with litmus before sterilisation, as this varies ; normally in 

 young potatoes it is weakly acid. The reaction of the 

 potato may be more accurately estimated by steaming the 

 potato slices for a quarter of an hour in a known quantity 

 of distilled water and then estimating the reaction of the 

 water by phenol-phthalein. The required degree of acidity 

 or alkalinity is obtained by adding the necessary quantity 

 of HC1 or NaOH solution (p. 45) and steaming for other 

 fifteen minutes. The water is then poured off and steri- 

 lisation continued for another half hour. Potatoes before 

 being inoculated ought always to be incubated at 37 C. 

 for a night, to make sure that their sterilisation has been 

 successful. 



Milk as a Culture Medium. 



This is a convenient medium for observing the effects 

 of bacterial growth in changing the reaction, in coagulating 

 the soluble albumin and in fermenting the lactose. It 

 is prepared as follows : fresh milk is taken, preferably after 

 having had the cream " separated " by centrifugalisation, as 

 is practised in the best dairies, and is steamed for fifteen 

 minutes in the Koch, it is then set aside in an ice chest or 

 cool place over night to facilitate further separation of 



