2.36 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
hand, the other l)etween the second and third fingers, the plugs pro- 
jecting outward. Flame the mouths of the tubes and test coolness 
of the platinum wire by plunging it for a distance of about 1 centi- 
meter into the sterile medium. 
(e) Remove some material from the infected tube by dipjjing the 
tip of the wire in it, and transfer to the sterile tube. 
(/) Replace the plugs in their respective tubes and sterilize the wire 
before laying it down. 
2. The Isolation of Pure Cultures of Bacteria. — .J . Aerobic Organisms. 
—It is the exception rather than the rule that bacteria exist in Nature 
or in many pathological processes in pure culture; that is, that a single 
kind of organism alone is present. From 
such mixtures of bacteria it is frequently 
necessary to isolate one or more organisms 
in a pure state, uncontaminated by other 
microorganisms. A common and efficient 
method of separating bacteria from mixtures 
is to distribute them in melted gelatin or 
agar,^ in such a manner that individual cells 
are somewhat widely separated . The medium 
is then allowed to harden. The organisms 
are immobilized in or upon the medium and 
surrounded by nutrients; the descendants of 
each individual organism thus develop locally 
and apart from the descendants of other organ- 
isms. Under favorable conditions the des- 
cendants of individual cells become so num- 
erous they may be seen with the unarmed 
eye as spots or colonies, each of which is 
made up of the progeny of a single organism. 
It is a simple matter to touch such a colony 
with a sterile, cool platinum needle, and infect 
sterile media with the adherent bacteria. In 
this manner pure cultures are obtained. The 
technique of the isolation of aerobic and facul- 
tatively anaerobic bacteria is technically termed plating, or streaking, 
depending upon the apparatus used. 
1. Plate Method.— Three tubes of nutrient agar or gelatin are 
melted and cooled to 42° to 45° C. A platinum wire, previously 
sterilized and cooled, is dipped to a depth of about 0.5 cm. in a mixture 
of bacteria in fluid media, or touched to a growth in solid media, and 
then rotated two or three times in a tube of the sterile melted medium. 
Without sterilizing the needle, the process is repeated in the second 
and third tubes. Each tube is then rotated between the palms of 
1 Agar melts at about 95° C. and solidifies at about 40° C. It is necessary to work 
rapidly with melted and cooled agar, to carry out the technique of inoculation before 
solidification takes place. 
Fig. 23. — Platinum needle 
and platinum loop. 
