28 METHODS OF CULTIVATION OF BACTERIA. 



nique of the separation of micro-organisms from mixtures of 

 these, and the maintaining of pure cultm'es when the latter have 

 been obtained. We shall here find that different methods are 

 necessary according as we are dealing with aerobes or anaerobes. 

 Each of these methods will be considered in turn. 



The Methods of Sterilisation. 



To exclude extraneous organisms, all food materials, glass 

 vessels containing them, wires used in transferring bacteria from 

 one culture medium to another, instruments used in making 

 autopsies, etc., must be sterilised. These objects being so 

 different, various methods are necessary. The foods comprise 

 meat infusions, jellies, potatoes, etc., and a method suitable for 

 their sterilisation evidently may not be suitable for the sterilisa- 

 tion of, say, a glass flask'. Bacteria may be killed by various 

 methods. Many chemicals will kill them, but the difficulty of 

 subsequently removing such chemicals, so that they may not 

 interfere with the growth of the microbes we wish to cultivate, 

 makes their use inapplicable. We therefore in practice take 

 advantage of the principle that all bacteria are destroyed by 

 heat. The temperature necessary varies with different bacteria, 

 and the vehicle of heat is also of great importance. The two 

 vehicles employed are hot air and hot water or steam, The 

 former is usually referred to as "dry heat," the latter as "moist 

 heat." As showing the different effects of the two vehicles, 

 Koch found, for instance, that the spores of bacillus anthracis, 

 which were killed by moist heat at 100° C, in one hour, 

 required three hours' dry heat at 140° C. to effect death. Both 

 forms of heat may be applied at different temperatures ; in 

 the case of moist heat above 100° C., a pressure higher than 

 that of the atmosphere must of course be present. 



A. Sterilisation by Dry Heat. 



A. (i) Red Heat or Dull Red Heat. — Red heat is used for 

 the sterihsation of the platinum needles, which, it will be found, 

 are so constantly in use. A dull heat is used for cauteries, the 

 points of forceps, and may be used for the incidental sterilisa- 

 tion of small glass objects (cover-slips, slides, occasionally when 

 necessary even test-tubes), care of course being taken not to melt 

 the glass. The heat is obtained by an ordinary Bunsen burner. 



