n8 METHODS OF STERILISATION. 



A knowledge of the nature of toxic action the progress of 

 which depends more or less upon chemists obtaining a clear idea 

 as to the constituents of the albuminoids is of the greatest im- 

 portance, both to the study of organic life in general, and to that 

 of the pathogenic and fermentative microbes in particular. It 

 is also important, as we shall soon see, for the technique of 

 sterilisation. 



The destruction of germs by heat in certain nutrient solutions 

 and food stuffs is often a very difficult task, because it necessitates 

 temperatures that damage the sample both as regards nutritive 

 value and palatability. Success may, however, be attained by 

 combining the influence of heat with that of poison, although the 

 isolated action of either is incapable of killing the germs. This 

 is the leading idea on which is based the process of mixed or 

 combined sterilisation, wherein the death of the micro-organism is. 

 caused by the simultaneous application of two factors; one of 

 which (the poison) is without influence on the chemical composi- 

 tion of the sample, whilst the other (heat) is too low to set up any 

 injurious decomposition. 



At first sight it may seem that the presence of poison restricts 

 the application of the process to such cases as the sterilisation, 

 pure and simple, of a liquid, and precludes its use when such 

 liquid is intended for the cultivation of micro-organisms or for 

 human consumption. On more mature deliberation, however, a 

 contrary conviction will be formed. 



Many of the substances named in the preceding paragraph 

 are in themselves innocuous to the health of man, provided the 

 quantity present is not too large; this is particularly the case 

 with alcohol and the organic acids, and it is precisely these acids- 

 that are generally employed for the preservation of numerous 

 food stuffs. A fuller account of this subject will be given in a 

 future chapter, so we will simply refer to it here and pass on 

 to the consideration of the second question : Is the combined 

 method also suitable for sterilising nutrient media intended for 

 mycological work ? 



Let us recall the observation that has been frequently made 

 in previous paragraphs with reference to the behaviour of micro- 

 organisms under the influence of physical and chemical forces. 

 Just as a certain degree of heat is fatal to one species, simply 

 retards the development of a second, is favourable to a third, and 

 insufficient to allow the cells of a fourth species to grow at all so 

 given amounts of poison may be fatal to one species of organism, 

 inert towards a second, and even stimulating to a third. In 

 other words, the constants of influence of a given poison vary 

 with different organisms. 



We are indebted to TH. SCHWANN (II.) for the first observa- 

 tions on the variations in behaviour thus exhibited, but to PASTEUR 

 (I.) for the first practical application thereof. Attention has 



