Assuming the typical colon bacillus and the typical typhoid bacillus to represent the types of this group that present the greatest divergences in biological peculiarities, we conclude, as others also have done, that there is a series of closely related forms that may be regarded as intermediate or transitional and which serve to establish a biological relationship, either near or remote, between these two typical members.
From our own studies we are inclined to regard the typical colon bacillus as the type of this group, for the reason that its functional equilibrium, as observed in the intestine, is so permanent a quality that it may readily be perpetuated under what is ordinarily regarded as favorable artificial circumstances, and that with the continuance of such conditions there is no conspicuous tendency on the part of this organism to deviate from what we regard as its norm; whereas, on the other hand, with all the other members of this group with which we have worked, there is not only a lack of uniformity in the adjustment of the functions, but such as exists is readily disturbed under artificial environment; though it must be borne in mind that even with the typical colon bacillus we have also shown functional modifications to be possible under particular conditions.
When the members of the colon group are cultivated under circumstances favorable to the development of both the function of fermentation and that of proteolysis, fermentation invariably takes precedence and no evidence of proteolysis is manifested until after fermentation has ceased.
The cultivation of all the members of the colon group under circumstances that favor the development of one function, viz. that of proteolysis, at the expense of another, viz. that of fermentation, results first in an apparent increase of vigor; but this is of temporary duration and is quickly followed by the decline and death of the cell.
The result of this increased activity of the proteolytic function is the formation of much larger amounts of indol by typical colon cultures than has ever been obtained by us by any other method.
By the method of experimentation through which we were enabled to accentuate the proteolytic activity of the typical colon bacillus, as caused by an increase of indol formation, we have also induced the function of indol formation not only in atypical colon bacilli that had been devoid of it, but in every specimen of typical typhoid bacilli to which we had access as well.
We feel justified in regarding one of the differential tests between the typhoid and colon bacillus, namely that of indol formation on the part of the latter and the absence of this function from the former, as of questionable value, for the reason, as shown above, that by particular methods of cultivation indol production has been shown to accompany the development of a number of specimens that we have every reason to regard as genuine typhoid bacilli.
As a result of our own experiments, together with the observations of others, there can be no doubt that the bacillus coli communis at times possesses pathogenic properties, and that by artificial methods of treatment it may often be brought from a condition of benignity to one of virulence.
The spleen of a typhoid patient has always been regarded as the only trustworthy source from which to obtain the typical typhoid bacillus. While we believe this to be true, still our investigations show that other members of the colon group may also be present in this viscus; in fact, from such spleens we have isolated practically all of the varieties of this group with which we are acquainted.
From our experience, the value of the serum test for the differentiation of typhoid and colon bacilli would seem to be questionable. We are inclined, however, to attribute the irregularities recorded above as due more to the method of application than to defects of the principles involved; for, as stated, by the use of dried blood, as in our experiments, it is not possible to make the test with constant and accurate, or even approximately accurate, dilutions of the serum. Our irregularities may be in part due to this defect. We therefore lay less stress upon this than upon the other features of our work.