CHAPTER V. 



THE ISOLATION OF AEROBIC MICRO-ORGANISMS 

 IN PURE CULTURE. 



Introduction. 



Section I. Mechanical methods, p. 76. 



1. Dilution, p. 76. 2. Dissemination : (a) in liquefied solid media, p. 77 ; (6) on the 

 surface of a solid medium, p. 81. 

 Section II. Biological methods, p. 83. 



1. Heat, p. 84. 2. Cultivation at the optimum temperature, p. 84. 3. Cultivation 

 on special media, p. 85. 4. Animal inoculation, p. 85. 



BEFORE a study of the morphology and biology of any miero-organism can 

 be undertaken the organism must be obtained in pure culture, a culture, 

 that is, free from all other organisms or as they are technically called con- 

 taminations. The first step, therefore, in a bacteriological investigation will 

 be the preparation of a pure culture. 



It is obviously impossible in view of their exceedingly small size to pick out 

 individual micro-organisms and transfer them to tubes of culture media, so 

 that resort has to be had to more complicated methods. There are numerous 

 processes in everyday use for the isolation of organisms in pure culture ; 

 for convenience of description these may be divided into two groups according 

 as to whether in attempting to isolate an organism a purely mechanical 

 method depending upon dilution and dissemination is relied upon, or whether 

 advantage is taken of the biological properties of the organism. 



The former, the mechanical methods, will be more useful when every species 

 of organism present in a given material has to be isolated while the latter, 

 the biological methods, are more especially applicable when a particular 

 organism of which the chief characteristics are known beforehand has to be 

 isolated from material in which it is suspected to be present. 



Above all in attempting to isolate micro-organisms it is of the first import- 

 ance to distinguish between aerobic and anaerobic species, for according as 

 to whether the one or the other has to be isolated so the cultures will have 

 to be grown in the presence or absence of air. In the case of anaerobic 

 organisms the methods of isolation will be dealt with later (Chap. VI.). The 

 present chapter is devoted entirely to the methods available for the isolation 

 of aerobic micro-organisms. 



SECTION I. MECHANICAL METHODS. 

 1. Isolation by dilution. 



This method was originally devised by Lister and extensively adopted 

 by Nsegeli and by Miquel, but is now of very limited application. It gives- 

 very exact results but occupies much time and is exceedingly tedious. 



