IDENTIFICATION OF BACTERIA 59 



a dark brown. When the color ceases to darken it may be assumed 

 that all the surrounding oxygen has been absorbed. 



IDENTIFICATION OF SPECIES 



When it is desired to identify an unknown organism already 

 isolated in pure culture a study is made of its general characteris- 

 tics. 



A. Morphology, method of grouping, motility, spore formation, 

 and staining reactions. 



B. Cultural reactions. 



C. Effect on animals. 



A. Morphology and Staining reaction may be determined by 

 film preparations and motility by means of a hanging drop made 

 from a twenty-four-hour broth culture. To test for spore forma- 

 tion a film may be made from a forty-eight-hour culture and 

 stained by the method already described, or the culture may be 

 heated to 75 C. for half an hour, after which a subculture should 

 be made and incubated. No vegetative forms will be found to 

 resist that temperature. 



B. Cultural Reactions. Ordinarily, young cultures of twenty- 

 four hours' growth should be observed and the following points 

 noted : 



(1) Growth on agar plates. Size of colony, outline, transpar- 

 ency, texture, color. (The colonies should be observed 

 with a hand lens or under the low-power objective of the 

 microscope.) 



;(2) Surface growth on agar slant at 37 C. and 22 C. Scanty 

 or abundant, smooth or irregular, moist or dry, slimy 

 or brittle. 



(3) Growth in stab culture. Most abundant at the top or 



bottom, extension of growth into medium (an indica- 

 tion of motility). Growth a continuous line or beaded. 



(4) Growth in broth. Cloudy at upper or lower level or 



throughout, pellicle or sediment formation. 



