SUGGESTIONS FOR USE OF THE MANUAL 3 



Acetylmethylcarhinol not produced. Methyl red test positive. Salts of citric acid may 

 or may not he used as a sole source of carbon. This places the organism in Genus I 

 Escherichia. 



We turn now to the key to the species of Genus I, Escherichia, page 335. On 

 tracing our organism in this key we find that its characters correspond with those 

 of Escherichia coli and turn to the description of this species for a final confirma- 

 tion of this identification. 



It is self evident that where the characters of the original culture have not 

 been determined accurately or completely, the identity of the unknown can- 

 not be determined positively. 



A second difficulty in the use of a key comes from inexperience in the use of 

 technical terms; that is, the student may not thoroughly understand the meaning 

 of the statement in the key and so cannot follow a route through the key with 

 certainty. For example in the keys used here, the student must know the dif- 

 ference between (1) chains of cells which are composed of dividing cells which 

 do not separate at once, and (2) trichomes which are composed of dividing cells 

 which remain more permanently together and are normally flattened against 

 each other on adjacent sides. The trichomes may show some differentiation into 

 holdfast cells and reproductive cells (conidia) . Both chains of cells and trichomes 

 are to be distinguished from the mycelial threads found in Actinomycetaceae : the 

 latter are unseptate and show true branching. 



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 may vary 

 within a species. Sometimes these variations are due to slight, possibly un- 

 recognized 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 with respect to the physiological activities of these micro- 

 organisms but also to such detectable structural features as the number and 

 position of flagella. Dark-field movies of motile cells and photographs taken with 

 the electron microscope are revealing new and heretofore unsuspected facts 

 regarding structural features. 



Source and habitat data are frequently helpful in aiding the student to rec- 

 ognize 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. 



