I2 BACTERIOLOGICAL DIAGNOSIS. 
PREPARATION OF CULTURE MEDIA. 
Bacteria are grown in the same way as other plants. 
A gardener who wishes to grow a plot of a particular 
plant will first prepare a soil suitable for the growth 
of that plant and free it as far as possible of all seeds, 
roots, &c. He will then sow it with the seeds of the 
plant in question and do what he can to expose them 
to a suitable temperature. An exactly similar process 
is adopted when we wish to cultivate the smallest of all 
plants. The soil which we prepare is called the culture 
medium, and differs in the case of different bacteria; 
the process of freeing this soil from bacteria and their 
spores is called sterilisation and we ensure a suitable 
temperature by means of an incubator, the heat of 
which is kept constant. 
The culture media which are used for special pur- 
poses are almost innumerable, but in the daily routine 
of the laboratory and for diagnostic purposes, broth, 
gelatin, agar-agar, and blood serum are all that are 
really necessary in the vast majority of cases. The 
blood-serum medium is difficult to prepare and can be 
replaced by ascitic agar in some cases. These media 
may all be bought from any firm of manufacturing 
chemists or from any bacteriological laboratory; and 
their purchase saves a great deal of work and is to be 
recommended for those who only wish to use them 
occasionally. They are sold in test-tubes which are 
kept sterile by being plugged fairly firmly with cotton- 
wool; this substance prevents the passage of bacteria 
as long as it is kept dry. The tubes are best stored in 
jars provided with tightly fitting lids, and it-is an 
