CULTUEAL METHODS 87 



mine whether or not it is still pure. Inasmuch as micro- 

 organisms are common in the air there is always a chance 

 that some of the air forms may gain entrance when tubes 

 of media are opened. Many ser'ious errors have been 

 made by students and investigators because they were 

 dealing not with pure cultures but with mixtures of va- 

 rious species. 



Study op Growth Characters 



It is relatively difficult to tell many kinds of microorgan- 

 isms apart, consequently every observable point of differ- 

 ence should be noted. Wherever possible bacteria are dif- 

 ferentiated by observation of size, shape, motility, spore 

 production, and other morphologic characters. However, 

 these frequently fail, and species must be diffei;entiated 

 from each other by their appearance when grown upon 

 culture media, and by differences in their behavior, that is, 

 in their physiologic characteristics. 



For the purpose of arranging in logical order the most 

 important of the morphologic, cultural, and physiologic 

 characteristics of an organism, a committee of the Society 

 of American Bacteriologists has evolved a descriptive chart. 

 A copy of this chart is included on pages 88 to 90. First 

 on the chart is the name of the the organism, the correct 

 genus and species to be given together with the authority, 

 when this can be determined. Next, it will be noted, a 

 statement is inserted concerning invigoration of culture. 

 Bacteria that have been grown for some time on unsuitable 

 laboratory media sometimes need rejuvenation, that is, 

 cultivation upon some unusually suitable medium for a 

 period in order that they may regain their vigor. 



The cultures to be studied are usually slant or streak 

 cultures on solid media, stab cultures and plate cultures, 

 in gelatin or agar, or cultures in a liquid medium such as 

 broth or milk. 



