1 8 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



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 gnw best at one temperature, others at another, the limits 

 for the great majority of organisms being about i6° C. and 40° C. 

 In practice two temperatures 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., 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 practi- 

 tioner, for the temperature of many rooms is not 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, and remains constant for long periods whatever be the 

 external temperature. It is hardly necessary for the practitioner 

 to purchase one of these. Careful search in the house will 

 usually reveal some cupboard or corner in which the temperature 

 will remain sufficiently near 20° C. for a sufficiently long period ; 

 it is more important that it should not rise above 22° C. than that 

 it should not fall below 18° C, as the former temperature may 

 melt the gelatin, while the latter will only delay the growth of the 

 colonies. It will probably be necessary to find one such place in 

 the hot weather {e.g., the cellar) and another one in the winter 

 {e.g., a cupboard not far from the hot-water pipes). 



It is necessary that cultures which are being incubated should 



