8o METHODS OF STERILISATION 



For the destruction of germs a number of methods are available, and may be 

 classified into two principal groups : the one physical and the other chemical. 

 The former may be subdivided into germ-killing by warmth, electricity, light, 

 mechanical concussion, or, finally, by gas under high pressure. We will confine 

 ourselves to the first of these five methods, the employment of the remaining 

 four being, for the purposes of the mycologist in general and of the fermentation 

 physiologist in particular, either too costly or too cumbersome. In so far, how- 

 ever, as their influence is of general biological interest, we have already reviewed 

 them in the preceding section. 



On the other hand, sterilisation by heat is the method always resorted to, 

 unless found undesirable on other grounds. Before giving it more detailed con- 

 sideration, we must first ascertain which group of organisms exhibits the greatest 

 tenacity of life and is able to longest withstand influences adverse thereto. This 

 group alone has to be borne in mind in testing the etficacy and general applica- 

 bility of a method of sterilisation ; since if the same is capable of destroying the 

 organisms exhibiting the greatest power of resistance, it will certainly, and much 

 more quickly, deprive all the remaining weaker ones of life. On the other hand, 

 when the contrary is not proved, it must always be assumed that the object to 

 be sterilised is infested with organisms of the highest resisting power. 



These hardy organisms we are already acquainted with, namely, the bacterial 

 endospores, which in this respect have no equal, and can therefore be made to 

 serve as test objects for determining the reliability of any germ-destroying pro- 

 cess coming under examination. It has been already stated, in 53, that great 

 differences exist in the resisting powers of the spores of the various species of 

 bacteria ; but of course we have only to take the strongest into consideration. 

 According to the investigations hitherto made, these are : among the non-patho- 

 genic varieties, those species commonly known as the hay and potato bacilli ; 

 and among the pathogenic bacteria, the anthrax bacilli. Bearing this in mind, 

 ROBERT KOCH (I.), the eminent medical bacteriologist, employed as reagent for 

 testing the efficacy of various disinfectants spores of anthrax bacilli, which, for 

 greater convenience in application, he allowed to dry on silk threads. 



The articles of metal or glass to be sterilised are placed in a case, formed on 

 the plan of the drying-ovens used in chemical laboratories, wherein they are 

 heated to 150 C. for an hour. During this time no diminution of temperature 

 is permissible, because if such a fall occurs, the labour will have been bestowed in 

 vain. KOCH and WOLFFHETGEL (I.) have shown that there are bacterial spores 

 that are killed only after an exposure to air at 140 0. for three hours. How- 

 ever, by an exposure to 150 for one hour we may be sure that all the germs 

 present have been killed ; and air-filters fitted with cotton-wool (freed from fat) 

 are also sterilised by the same treatment, the cotton-wool assuming thereby a 

 yellowish to brownish coloration. Both the apertures of such a filter must have 

 been previously closed with plugs of cotton-wool, which must not be removed 

 until the filter is about to be used. It is necessary that glass articles should be 

 dry before they are introduced into the hot-air sterilising apparatus, since other- 

 wise they will crack. 



Small metal instruments, such as forceps and inoculating needles, as well as 

 the glass stoppers used for closing Pasteur flasks, can be conveniently purified 

 in the flame of a Bunsen burner or spirit-lamp. 



77. Destroying- Germs by Moist Heat. 



The opinion expressed in a former chapter, that the seat of the high powers 

 of resistance enjoyed by bacterial spores is to be sought in their membrane, is 

 supported by their behaviour towards the influence of warmth, in so far as has 



