10 BACTERIOLOGICAL APPARATUS 
of this suspension to a second tube of stemle water. This procedure 
is repeated until several concentrations of the sample aré secured. 
Portions of these are placed in sterile Petri dishes or under these condi- 
tions put into tubes of melted medium. ‘These are then put into the 
Petri dishes. 
Isolation of Pure Cultures. A pure culture may be defined in 
different ways but in this relation it may be regarded as the descend- 
ants of a single cell. Each single cell on the Petri dishes poured in the 
above paragraph will have developed into a colony. In most cases this 
colony will contain a pure culture if the plate does not contain too many 
colonies. When there are but a few colonies on the plates, each one 
that 1s isolated from the others will be pure. If it is found to be a 
mixed culture the plating operation must be repeated. In fact in 
careful work it is essential to replate two or three times, This makes 
the probability of a pure culture more certain and also serves to ‘‘ tune- 
up ” or strengthen the organism. 
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Fig 9—Bottcher’s Counting Chamber. 
It consists of an ordinary glass slide with a glass nng 20 mm in diameter and 8 mm high 
The cover glass 1s ruled to 100 squares, 19 of which are numbered 
The isolated colonies are picked by means of a sterile platinum 
wire. A small portion of the colony is removed to either an agar slant 
or broth tube. Before any extensive investigations are carried out 
with the organism, it should be studied in hanging drop and stained 
smear to demonstrate its purity. 
Transferring of Cultures. This procedure should be carefully 
carried out in a room with little dust. Sterile platinum wires should 
be used which should be sufficiently cooled. If a wire of too great 
diameter is used, it will retain its heat longer and there is then greater 
danger that the culture will be killed or altered. Also, if much trans- 
ferring is done at the same time there will be a tendency for some of 
the material to cake on the wire. This should be removed. A piece 
of emery paper or No. 00 sand paper will be found to be a valua- 
ble aid. 
Dilutions. These are essential in the quantitative enumeration of 
bacteria. The kind of containers to be used has been described above. 
