24 



THE BIOLOGY OF BACTERIA 



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 anthrax and is contaminated 

 with micrococci, heating to 70 C. 

 (158 F.) will kill all the micrococci, 

 but will not affect the spores of an- 

 thrax, which can then grow into a 

 pure culture of anthrax bacilli. Frac- 

 tional 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 boiling. 



Successful sterilisation, therefore, 

 depends upon killing both bacteria 

 and their spores, and nothing short 

 of that can be considered as sterilisa- 

 tion. The following methods are 

 those generally used in the laboratory. 

 For dry heat (which is never so in- 

 jurious to organisms as moist heat*) : 

 (a) the Bunsen burner, in the flame of 

 which platinum needles, etc., are steril- 

 ised; (b) hot-air chamber, in which 

 flasks and test-tubes are heated to a temperature of 150-170 C. 

 for an hour or more. For moist heat : (c) boiling, for knives and 



* It will be observed that there is a marked difference between the effects of dry 

 heat and moist heat. Moist heat is able to kill organisms much more readily than 

 dry, owing to its penetrating effect on the capsule of the bacillus. Dry heat at 

 140 C. (284 F.), maintained for three hours, is necessary to kill the resistant spores 

 of Bacillus anthracis and B. subtilis, but moist heat at forty degrees less will have the 

 same effect. It is from data such as these that in laboratories and in disinfecting 

 apparatus moist heat is invariably preferred to dry heat For with the latter such 

 high temperatures would be required that the articles being disinfected would be 

 damaged. Koch states the following figures for general guidance : Dry heat at a 

 temperature of 120 C. (248 F.) will destroy spores of mould fungi, micrococci, and 

 bacilli in the absence of their spores ; for the spores of bacilli 140 C. (284 F.), 

 maintained for three hours, is necessary; moist heat at 100 C. (212 F.) for fifteen 

 minutes will kill bacilli and their spores. 



FIG. 8. Koch's Steam Steriliser. 



