30 BACTERIA 



of an idea which must always be present to the mind of the 

 bacteriologist, it may be desirable to make some passing 

 allusion to it. 



Chemical substances, perfect filtration, and heat are the 

 three means at our command in order to secure germ-free 

 conditions of apparatus or medium. The first two, though 

 theoretically admissible, are practically seldom used, the 

 former of the two because the addition of chemical substances 

 annuls or modifies the operation, the latter of the two on 

 account of the great practical difficulties in securing perfec- 

 tion. Hence in the investigation involved in bacteriological 

 research heat is the common sterilising agent. A temperat- 

 ure of 70° C. (158° F.) will kill all baciUi ; even 58° C. will 

 kill most kinds. Boiling at 100° C. (212° F.) for three min- 

 utes will kill anthrax spores, and boiling for thirty to sixty 

 minutes will kill all bacilli and all spores. This differ- 

 ence in the thermal death-point between bacilli and their 

 spores enables the operator to obtain what are called " pure 

 cultures " of a desired bacillus from its spores which may be 

 present. For example, if a culture contains spores of an- 

 thrax and is contaminated with micrococci, heating to 70° 

 C. (158° F.) will kill all the micrococci, but will not affect the 

 spores of anthrax, which can then grow into a pure culture 

 of anthrax bacilli. Fractional or discontinuous sterilisation 

 depends on the principle of heating to the sterilising point 

 for bacilli (say 70° C.) on one day, which will kill the bacilli, 

 but leave the spores uninjured. But by the following day 

 the spores will have germinated into bacilli, and a second 

 heating to 70° C. will kill them before they in their turn 

 have had time to sporulate. Thus the whole will be sterilised, 

 though at a temperature below boihng. 



Successful sterilisation, therefore, depends upon killing 

 both bacteria and their spores, and nothing short of that 

 can be considered as sterilisation. The following methods 

 are those generally used in the laboratory. For dry heat 



