After the importance of the subject had been demonstrated by Pasteur, others turned their attention in the same direction, either for the purpose of verification or refutation of Pasteur’s views.  The advance was not very rapid, however, since bacteriological experimentation proved to be a subject of extraordinary difficulty.  Bacteria were not even yet recognised as a group of organisms distinct enough to be grouped by themselves, but were even by Pasteur at first confounded with yeasts.  As a distinct group of organisms they were first distinguished by Hoffman in 1869, since which date the term bacteria, as applying to this special group of organisms, has been coming more and more into use.  So difficult were the investigations, that for years there were hardly any investigators besides Pasteur who could successfully handle the subject and reach conclusions which could stand the test of time.  For the next thirty years, although investigators and investigations continued to increase, we can find little besides dispute and confusion along this line.  The difficulty of obtaining for experiment any one kind of bacteria by itself, unmixed with others (pure cultures), rendered advance almost impossible.  So conflicting were the results that the whole subject soon came into almost hopeless confusion, and very few steps were taken upon any sure basis.  So difficult were the methods, so contradictory and confusing the results, because of impure cultures, that a student of to-day who wishes to look up the previous discoveries in almost any line of bacteriology need hardly go back of 1880, since he can almost rest assured that anything done earlier than that was more likely to be erroneous than correct.

The last fifteen years have, however, seen a wonderful change.  The difficulties had been mostly those of methods of work, and with the ninth decade of the century these methods were simplified by Robert Koch.  This simplification of method for the first time placed this line of investigation within the reach of scientists who did not have the genius of Pasteur.  It was now possible to get pure cultures easily, and to obtain with such pure cultures results which were uniform and simple.  It was now possible to take steps which had the stamp of accuracy upon them, and which further experiment did not disprove.  From the time when these methods were thus made manageable the study of bacteria increased with a rapidity which has been fairly startling, and the information which has accumulated is almost formidable.  The very rapidity with which the investigations have progressed has brought considerable confusion, from the fact that the new discoveries have not had time to be properly assimilated into knowledge.  Today many facts are known whose significance is still uncertain, and a clear logical discussion of the facts of modern bacteriology is not possible.  But sufficient knowledge has been accumulated and digested to show us at least the direction along which bacteriological advance is tending, and it is to the pointing out of these directions that the following pages will be devoted.