Semi-Centennial Volume. 25 



Ehrlich has done much in the discovery and application of stains. Others 

 of course have helped. Staining, especially vi^hen differential or con- 

 trasting so that the bacterium will be differently colored from its sur- 

 roundings, has enabled experimenters to see germs where they would 

 otherwise have been invisible. 



Koch himself performed a strictly fundamental thing when he in- 

 vented the use of gelatine as a part of media. This rendered rather 

 easily possible in many cases the isolation of a pure culture by en- 

 ti-apping the germs, and requiring them to develop at points isolated 

 from one another. Other media having a similar purpose are now made. 

 Our own Professor Barber went a step farther and to a nicer conclu- 

 sion in making possible the isolation of a single germ by his method of 

 the very small pointed capillary pipette. It may interest some to know 

 that Professor Koch did not have much faith in Professor Barber's 

 device until he was shown it in use. Then he gracefully acknowledged 

 his sin of unbelief and atoned by ordering some of the improved ap- 

 paratus. 



Many different practices are now followed in obtaining pure cultures. 

 These include animal inoculation where, in specific cases, the animal will 

 destroy all but the one kind of germ and it will then be found in pure 

 culture in some part of the animal body. Other methods have for their 

 purpose the same end through a gradual elimination of those germs 

 not wanted and of the enrichment of the culture in the one desired, as in 

 the use of malachite green-agar by Loffler to secure typhoid germs. 

 Then there are differential media, such as Endo's, for the differentiation 

 of B. typhosus from B. coli. 



However, isolation is not the whole matter. It is often very difficult 

 to make bacteria grow at all, and a vast amount of study and experi- 

 mentation has resulted in a very great variety of media and of much 

 better understanding of the nature of bacteria and of the food require- 

 ments of various kinds. It may be noted that the requirement varies 

 from the purely mineral to practically a need for the living protoplasm 

 of some very closely limited species of plants or animals. 



And after recognizing the task assumed in applying the first two 

 of Koch's laws we are still not through with our difficulties. Often a 

 germ is harmless to an animal unless some peculiar method of inocula- 

 tion is used or some more or less exceptional condition of the animal is to 

 be induced. Tetanus germs are ordinarily harmless when taken into 

 the stomach, and again they may prove harmless when placed in the 

 flesh in pure culture. When accompanied by aerobes in the latter plan 

 they are much more serious. Typhoid bacteria fed to one person could 

 easily cause typhoid fever, but in one who for any reason is immune 

 to it they would produce no noticeable effect. Koch's laws are good 

 laws, but their demonstration is often very difficult, and still the un- 

 doubted causes of many diseases have been proved and these laws yet 

 observed. 



The use of gelatine media was announced in 1882. In May, 1881, 

 another milestone in this .science had been reached. Pasteur, working 

 near Melun, France, inoculated fifty sheep (really some were goats) 



