4 SUGGESTIONS FOR USE OF MANUAL 



The student should be warned not to take descriptions in the Manual 

 too literally or too rigidly. Descriptions are usually drawn to represent 

 average findings. Especially among bacteria, characters such as sugar 

 fermentations, gelatin liquefaction, presence or absence of flagella and 

 other things will vary. Sometimes these variations are due to slight, 

 possibly unrecognized variations in the techniques used in determining 

 these characters. Real knowledge of the characteristics of species may also 

 be very incomplete. This is true not only of the physiological activities 

 of these microorganisms; but also in regard to such detectable structural 

 features as the number and position of flagella. Dark field movies of motile 

 cells and photographs taken with the recently developed electron micro- 

 scope are revealing new and heretofore unsuspected facts regarding struc- 

 tural features. 



Source and habitat data are frequently helpful in aiding the student 

 to recognize species of bacteria and may indicate that the pathogenicity of 

 the culture in question may need to be tried on some specific animal or 

 plant. By habitat is meant the kind of a place in which the organism 

 normally grows; by source, the particular material and place from which 

 the culture was obtained. This source may or may not indicate the 

 natural habitat. The source of cultures is invariably more limited in scope 

 than the habitat as bacteria normally occur wherever their particular 

 habitat may be found in a world wide distribution. 



The student is also reminded that it is impracticable to note all exceptions 

 in keys. Bacteria like other living things are classified according to a 

 combination of characters, not according to some single character, and ex- 

 ceptions to the characters noted in the keys will occur in nature. These 

 may not be known to or may have been overlooked by the author of the key. 

 On the other hand, the importance of such exceptions should not be over- 

 emphasized and the student would do well to use the key as if there were no 

 exceptions. 



