Acetylmethylcarhinol not produced. Methyl red test positive. Salts of citric acid may or may not be 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 confirmation 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 cannot 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 difference 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 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 with respect to the physiological activities of these microorganisms 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 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.
