222 BACTERIOLOGY. 
The Methods of Obtaining and Studying Pure Cultures of 
Single Species of Bacteria, 
In order to study bacteria, both in culture media 
and in the living body, we must separate those devel- 
oped from one organism from all others and study 
them by themselves in pure cultures. In order to do 
this we have to take the greatest precautions to insure 
that the materials that we make use of for the growth of 
bacteria, the flasks and tubes that hold these materials, 
and the instruments with which we transfer the bac- 
teria are sterile. We also carefully try to prevent any 
bacteria entering from the air or elsewhere. 
The Cleansing and Sterilization of Apparatus. 
In bacteriological work sterilization is practically 
always done by means of dry and moist heat, for no 
antiseptic substances can be allowed to remain in any 
of the media used for the growth of bacteria or on any 
of the apparatus which would come in contact with 
them, as such substances would inhibit the growth of 
the bacteria which we desired to study. 
The platinum wires and loops used in transferring 
bacteria are sterilized by holding them for a moment 
until red-hot in a gas or alcohol flame. They should 
not be used until time enough has elapsed for them to 
cool sufficienly not to injure the bacteria touched by 
them. Knives, instruments, ete., are, after thorough 
cleansing, placed in boiling 1 per cent. soda solution 
for three to five minutes. Hypodermic needles are 
sterilized by boiling in soda solution, or, when this is 
impossible, they are first frequently rinsed with boil- 
ing or with very hot water and then filled with a 5 per 
