



INCUBATION OF CULTURES. 25 



To make a stab culture take the other gelatin tube 

 and proceed as before until you get to step 5. When 

 you have passed the needle into the tube drive it steadily 

 into the medium, taking care not to deviate from the 

 axis of the tube. Finish the process as before. 



All this may seem involved. As a matter of fact it is 

 very simple and need not take more than a minute to 

 perform. But every step must be carried out, and the 

 whole process must be learnt so thoroughly that it is 

 performed automatically whenever a culture is made. 



INCUBATION OF CULTURES. 



The limits between which bacteria can live are very 

 wide ; some grow best at one temperature, others at 

 another, the limits for the great majority of organisms 

 being about 16 C. and 40 C. In practice two tem- 

 peratures are all that is used for ordinary work. The 

 lower, or so-called "room temperature" is about 20 C. 

 (68 F.) and is of most use for those bacteria which 

 grow naturally outside the body, i.e. t as saprophytes. 

 The higher, or body temperature, is about 37 C. 

 (98-6 F.), and is the best temperature for the majority 

 of germs which live within the body, i.e., the parasites. 

 It is obvious that gelatin cannot be incubated at this 

 high temperature as it melts at 25 C. or thereabouts ; 

 but all other media are available. 



The term "room temperature" must not mislead the 

 practitioner, for the temperature of most rooms is 

 rarely constant at or near 20 C. for periods sufficiently 

 long to permit of its use for incubating bacteria. In 

 the laboratory we use an incubator, the temperature of 

 which is regulated by means of an automatic regulator, 



