12 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 



