THE BACTERIA AND VIRUSES 353 



believed to have been generated de novo in the putrefying substance itself. 

 Despite a number of experiments in the eighteenth and nineteenth centuries 

 which showed that if organic matter were first heated and then protected 

 from the air no decomposition and no generation of organisms resulted, 

 the belief in spontaneous generation persisted almost to our own time. 

 Pasteur, however, gave the idea its death-blow by his clear demonstration 

 that decomposition depended invariably on the presence of material particles 

 (his " germs ") which came from the air. When the air was filtered it lost 

 its power of starting decomposition, while among the particles filtered from 

 the air were many microscopic organisms of the same kinds as those which 

 appeared in the decaying substances. 



Pasteur's views and experiments struck the imagination of the great 

 English surgeon Lister, and his application of these resulted in the develop- 

 ment of antiseptic surgery. He showed that the inflammation or sepsis 

 of wounds was due to Bacteria which gained entrance either from the air 

 or from contamination by dirt, or even from the surgeon's instruments or 

 his skin. He therefore introduced the practice of washing all his instruments 

 in carbolic acid (phenol) and dressing the wound with material soaked in 

 the same substance, thus killing the Bacteria. The air of the operating 

 theatre was likewise disinfected by keeping up a continuous spray of carbolic 

 acid during an operation. 



The introduction of these practices saved innumerable lives and removed 

 for ever the fear of surgical gangrene, which had formerly meant a deadly 

 risk in even the simplest operation. The use of chemical antiseptics had 

 serious drawbacks, not only from the point of view of the surgeon, but also 

 in the retardation of healing in the patient. Nowadays the aseptic method 

 is used instead. Everything in the operating theatre which will be in contact 

 with the patient is sterilized beforehand by heat and the air is filtered. The 

 only antiseptic used is for cleaning the patient's skin at the site of the incision. 

 Even the surgeon himself is entirely wrapped in sterilized coverings and he 

 wears a sterilized mask and rubber gloves. 



Cultivation of Bacterl\ 



The possibility of a science of Bacteriology rests on the power of isolating 

 the different species and examining their properties separately. All the 

 early work was done by cultivation in liquid media, which made isolation 

 well-nigh impossible. The first pure culture, that of a milk organism, was 

 achieved by Lister in 1878, but it was Robert Koch who, in 1881, introduced 

 the technique of culture on a solid substratum, which made isolation simple. 

 He used beef extract containing gelatine, which was poured out on to glass 

 plates and allowed to solidify under a cover. When a liquid containing 

 Bacteria was thinly distributed over a gelatine plate, each individual cell 

 settled on the solid surface and remained stationary, multiplying until a 

 visible mass, or colony, was formed (Fig. 341), consisting of a pure strain, 

 all the offspring of one individual. If part of a colony was then transferred 



