V. 

 CULTURE MEDIA. 



To obtain a satisfactory knowledge of the biological characters 

 of the different species of bacteria, it is necessary to isolate them in 

 " pure cultures " and to study their growth in various culture media. 

 By a pure culture we mean a cultivation containing a single species 

 only ; and to be absolutely sure that we have a pure culture it is 

 desirable that all of the bacteria in a culture shall be the progeny of 

 a single cell. The methods of obtaining pure cultures will be given 

 later. At present we propose to give an account of the various cul- 

 ture media commonly employed by bacteriologists, and the methods 

 of preparing them for use. 



By a natural culture medium we mean one which, as obtained in 

 nature, contains the necessary pabulum for the development of one 

 or more species of bacteria. An artificial culture medium is one 

 which is prepared artificially by adding nutritive material to water. 

 A sterile medium is one which does not contain any living micro- 

 organisms. We may obtain natural media in a sterile condition, but 

 artificial media require sterilization, as they are infallibly contami- 

 nated with living " germs " from the atmosphere during the process 

 of preparing them. Sterilization is usually effected by heat. For- 

 ceps, glass tubes, etc. , may be sterilized by passing them through 

 the flame of an alcohol lamp or Bunsen burner. 



NATURAL CULTURE MEDIA. The most important natural cul- 

 ture medium is blood serum, which may be obtained from one of 

 the lower animals preferably from oxen or calves. This is to be 

 collected in a sterilized jar, with every precaution to insure cleanli- 

 ness, at the moment of slaughtering the animal. Or the blood of a 

 calf, sheep, or dog may be collected at the laboratory by a carefully 

 conducted operation, in which the femoral or carotid artery is con- 

 nected with a sterilized glass tube leading into a sterilized receptacle; 

 such as a Woulf 's bottle, into one neck of which a cotton plug has 

 been placed to permit the air to escape as the bottle fills with 

 blood through a tube which is secured in the other neck. When 

 blood is passed directly from an artery into a sterilized receptacle 

 the serum will not subsequently require sterilization. The writer is in 



