208 



PATHOGENIC BACTERIA. 



through the bottom of the inverted dish, will be more 

 than ever apparent. 



The colonies should be viewed from time to time in 

 their growth, drawings being made of the appearances, 

 so as to form a series showing the developmental cycle. 

 Most colonies will be found to originate as spherical, cir- 

 cumscribed, slightly granular, yellowish, greenish, or 

 brownish dots, and later to send out offshoots or filaments 

 or to develop concentric rings or characteristic liquefac- 

 tions. A few appear from the very first as woolly clumps 

 of entangled threads. 



Some of the most diverse forms of colonies are repre- 

 sented in the accompanying illustrations (Figs. 28-32). 



Figs. 28, 29, 30. — The various appearances of colonies of bacteria under the 

 microscope : a, colony of Bacillus liquefaciens parvus (Luderitz) ; b, colony 

 of Bacillus polypiformis (Liborius); c, colony of Bacillus radiatus (Luderitz). 



A pure culture, when obtained from colonies growing 

 upon a plate, must always be made from a single colony, 

 the transplantation being accomplished under a low power 

 of the microscope. The naked eye can rarely be depended 

 upon to recognize the purity of a colony or its isolation. 



Selecting as isolated, large, and characteristic a colony 

 as possible, it is brought to the centre of the field. A 

 platinum wire, securely fused into a glass handle about 

 8 inches long, is sterilized by being made incandescent 

 in a Bunsen flame, cooled, and then cautiously manipu- 

 lated until, while it is watched through the microscope, 



