BACTERIA 115 



believed to be bacteria, have been classed as flagellates by eminent 

 protozoologists. 



Physiologically the bacteria are quite closely related to the fungi, 

 and are frequently classed with them under the term ScUzomycetes. 



Artificial Cultivation op Bacteria* 



The introduction of methods of artificial cultivation marks the beginning of the 

 science of microbiology. These methods were developed by Pasteur and Koch and 

 are depended upon by the microbiologist of to-day as the foundation for most of his 

 work. It has been the aim of investigation to discover a more general culture 

 medium. So far it has been impossible to do this, but beef broth, made after a 

 formula suggested by Loeffler many years ago, forms the basis of nearly all of our 

 culture media. This beef broth, or nutrient bouillon, is made by extracting meat 

 free from fat in water, adding a small per cent of peptone, correcting the chemical 

 reaction, clarifying and sterilizing. To this broth various substances are added 

 for special purposes; gelatin and agar, in order to solidify the media, and various 

 sugars and other chemical substances for the purpose of determining the physiological 

 characteristics of various bacteria. One of the difficulties with the present methods 

 of the artificial cultivation of bacteria is the inconstancy of the composition of the 

 media, due to the fact that the extract of beef, the peptone, and other ingredients, 

 cannot be obtained chemically pure. If it should prove possible to use synthetic 

 substances, such as the polypeptids, it would mark a great step in advance, but it is 

 probably qu'te impossible to devise a single medium upon which all bacteria will 

 grow. Some bacteria, such as those which produce nitrification, refuse to grow on 

 ordinary media containing organic material. The cultivation of bacteria in pure 

 culture is dependent upon isolation, and the method of isolation suggested by Robert 

 Koch in i88o, and known as the plate culture method, has given eminent satis- 

 faction. This method is dependent upon the use of liquefiable solid media, such 

 as gelatin or agar. 



• Prepared by W. D. Frost. 



