148 BACTERIA 



1880 by the great German bacteriologist, Koch, and has 

 been an indispensable means of developing the vast 

 modern science of bacteriology. The various technical 

 methods of making pure cultures are based on the fact 

 that if a small quantity of a liquid containing bacteria 

 is diluted with a large excess of sterile water and the 

 bacteria evenly distributed through it, each drop of 

 the mixture will contain only a few or even a single 

 organism. Such a drop added to a sterile culture 

 medium will probably give rise to a growth consisting 

 of one sort of bacillus only. In practice it is more 

 convenient to immobilise the bacteria in the diluted 

 mixture by making it with a warm gelatine or agar 

 jelly, which will set when it is cold. The individuals 

 then multiply and give rise to visible " colonies/' 

 each originating from one organism, which can be trans- 

 ferred to other sterile media, and the forms of the cells 

 and the effects of their growth on the medium can be 

 studied at leisure. 



Genera, Species and " Strains " of Bacteria. The 

 original " genera " of bacteria, based on form names 

 such as bacillus (rod), coccus (sphere), spirillum (spirally 

 curved rod), are still to some extent retained, but have 

 been considerably added to as knowledge has increased, 

 and the present classification is partly based on form 

 and partly on activity. Thus rod forms are still called 

 Bacterium and Bacillus, the genera of cocci are dis- 

 tinguished by prefixes (Micrococcus, Streptococcus- 

 cells arranged in chains, Staphylococcus cells arranged 

 like bunches of grapes, etc.). Many of the simpler 

 spiral forms are placed in Vibrio and Spirillum, while 

 the long spiral forms which move by contractility of 

 the body protoplasm and have no flagella are placed 

 in Spirochczte, Other genera are Sarcina (a coccus 



