GELATINE PLATE CULTURES 69 



Koch found it necessary in his investigations to devise a 

 medium that would be of such a composition that it would 

 afford food material for the growth of the greater number 

 of micro-organisms. It might be expected that it would be 

 an easy task to find a food to suit all the requirements of 

 the various bacteria, seeing that from one point of view 

 they are all so similar in character ; but, as a matter of fact, 

 there is the greatest diversity in their tastes, and media 

 which are suited to the growth of one organism are totally 

 unfit for the growth and nourishment of another. After a 

 great deal of investigation Koch found that meat-broth, 

 with the addition of salt, peptone, etc., thickened with 

 gelatine, gave the best results in practice. 



The methods of bacteriological study thus devised by 

 Koch have enabled us to separate and study the morpho- 

 logical and biological characters of each species of bacteria 

 free from the complications which led to such error and 

 confusion when the employment of liquid media was the 

 only available means of bacteriological study. 



In all investigations connected with the bacteria, it must 

 always be borne in mind by the student that our surround- 

 ings are always crowded with micro-organisms. Thus, it 

 will be seen that it becomes of paramount necessity that 

 every operation in connection with the study of the bacteria 

 should be conducted in such a way as to prevent the 

 possibility, or, at any rate, to reduce to a minimum the 

 chance, of the introduction of foreign organisms. The pre- 

 cautions to be taken in bacteriological work cannot be too 

 painstaking, and if in the descriptions of the preparation 

 of the various nutrient media and processes too much 

 emphasis may appear to have been laid on the necessity for 

 the most absolute care to be taken in the sterilisation of 

 vessels, and so on, to prevent contamination, it must be 

 remembered that by neglecting to take the most trivial 



