14 CLASSIFICATION. 



alone. Thus, for example, there are among the spherical bacteria, or 

 micrococci, numerous well-established species which the most expert 

 microscopist could not differentiate by the use of the microscope 

 alone ; the same is true of the rod-shaped bacteria. The assump- 

 tion often made by investigators who are riot sufficiently impressed 

 with this fact, that two microorganisms from different sources, or 

 even from the same source, are the same because stained prepara- 

 tions examined under the microscope look alike, has led to serious 

 errors and to much confusion. As an example of what is meant we 

 may refer to the pus organisms. Before the introduction of Koch's 

 "plate method" micrococci had been observed in the pus of acute 

 abscesses. Some of these were grouped in chains streptococci 

 and some were single, or in pairs, or in groups of four ; but whether 

 these were simply different modes of grouping in a single species, or 

 whether the chain micrococci represented a distinct species, was not 

 determined with certainty. That there were in fact four or more 

 distinct species to be found in the pus of acute abscesses was not 

 suspected until Rosenbach and Passet demonstrated that this is the 

 case, and showed that not only is the streptococcus a distinct species, 

 but that among the cocci not associated in chains there are three 

 species which are to be distinguished from each other by their color 

 when grown on the surface of a solid culture medium. One of these 

 has a milk-white color, one is of a lemon-yellow color, while the third 

 is a golden-yellow. 



Those microorganisms which form pigment are called chromo- 

 genes, or chromogenic ; those which produce fermentations are 

 spoken of as zymogenes, or zymogenic ; those which give rise to dis- 

 ease processes in man or the lower animals are denominated patho- 

 genes, or pathogenic. We cannot, however, classify bacteria under 

 the three headings chromogenes, zymogenes, and pathogenes, for 

 some of the chromogenic species are also pathogenic, as are some 

 of the zymogenes. These characters must therefore be considered 

 separately as regards each species, and in studying its life history and 

 distinguishing characters we determine whether it is chromogenic 

 or non-chromogenic ; whether it produces special fermentations ; 

 and whether it is or is not pathogenic when inoculated into the 

 lower animals. In making the distinction between pathogenic 

 and non-pathogenic microorganisms we must remember that a 

 certain species may be pathogenic for one animal and not for an- 

 other. Thus the anthrax bacillus, which is fatal to cattle, sheep, 

 rabbits, guinea-pigs, and mice, does not kill white rats ; the bacillus 

 of mouse septicaemia kills house mice, but field mice are fully im- 

 mune from its pathogenic effects ; on the other hand, the bacillus of 

 glanders is fatal to field mice but not to house mice. 



