46 BACTERIA 



stain the organisms one colour and the tissue in which they 

 are situated a contrast colour. Some of the details of these 

 methods are mentioned in the Appendix. 



4. Sub-culture. The plate method was really introduced 

 by Koch in order to facilitate isolation of species. In a 

 flask it is impossible to isolate individual species, but when 

 the growth is spread over a comparatively large area, like 

 a plate, it is possible to separate the colonies, and this being 

 done by means of a platinum wire, the colonies may be 

 replanted in fresh media ; that is to say, a sub-culture may 

 be made, each organism cultivated on its favourite soil, and 

 its manner of life closely watched. We have already men- 

 tioned the chief media which are used in the laboratory, and 

 in an investigation many of these would be used, and thus 

 pure cultures would be obtained. Let us suppose that a 

 water contains six kinds of bacteria. On the plate these 

 six kinds would show themselves by their own peculiar 

 growth. Each would then be isolated and placed in a sep- 

 arate tube, on a favourite medium, and at a suitable tem- 

 perature. Thus each would be a pure culture ; i. e., one 

 and only one, species would be present in each of the six tubes. 

 By this simple means an organism can be, we say, cultivated, 

 in the same sort of way as in floriculture. From day to 

 day we can observe the habits of each of our six species, 

 and probably at an early stage of their separated existences 

 we should be able to diagnose what species of bacteria we 

 had found in the water. If not, further microscopic examin- 

 ation could be made, and, if necessary, secondary or ter- 

 tiary sub-cultures. 



5. Inoculation of Animals. It may be necessary to 

 observe the action of supposed pathogenic organisms upon 

 animals. This is obviously a last resource, and any abuse 

 of such a process is strictly limited by law. As a matter of 

 fact, an immense amount of bacteriological investigation can 

 be carried on without inoculating animals ; but, strictly 



