LESSON IX. 



CLASSIFICATION AND IDENTIFICATION OF BAC- 

 TERIA. 



No thoroughly satisfactory classification of the bacteria has as yet been proposed 

 and will probably be wanting until our knowledge of their characteristics is sufficiently 

 developed to render certain their true generic relations. The older classifications, 

 dealing with the so-called oligomorphous varieties, were based entirely upon the shape 

 of the individual bacterial cells, a system continued to the present by a number of 

 writers; prominent functional characteristics, as the power to liquefy gelatine, serving 

 in a measure to subdivide the major groups. It is not to be supposed that there does 

 not exist sufficient natural basis for arrangement of these forms of life into orders, 

 families, genera, and species, as is done in case of the higher vegetables and animals ; 

 but it must be acknowledged that from the insufficiency of our methods of study there 

 is not as yet that amount of information which will permit a stable arrangement such 

 as is established in the studies of higher botany and zoology. Whatever classification 

 is adopted must for the present, therefore, be regarded as tentative. Probably the 

 best classification for working purposes thus far suggested is that proposed by Migula 

 (System der Bakterien, 1900) ; and this, with certain emendations by Chester (Deter- 

 minative Bacteriology, 1901), and further minor modifications approaching more closely 

 to the arrangement of Migula, is adopted in these pages. It is unfortunate that its 

 generic system is based upon a structural feature of bacteria wh'ich is difficult of demon- 

 stration and apparently not invariable throughout the life-history of the individual 

 cells or under all conditions viz., the presence, number, and position of flagella. It 

 is possible, however, that in the future much of this objection may be relieved by the 

 discovery of more reliable methods of demonstrating these fiagellar appendages than 

 those now known, when perhaps it will be found that these organs are not so variable 

 as would to-day appear. Or it is possible that with the advance of our knowledge 

 of the chemistry of bacterial constitution some more constant feature may be found 

 upon which to base a classification of convenient genera. Until the acquirement 

 of some such desiderata, however, the writer is disposed to regard such an arrangement 

 as that of Migula with much appreciation, as of no little use in reducing to system 

 a study which has been marked by much confusion of excellent as well as careless 

 work. 



Recognizing the entire group of the schizomycetes or fission fungi as an order, 

 the classification adopted would divide it into two suborders, the Eubacteria (corre- 

 sponding roughly with the oligomorphous bacteria of various writers), the individual 

 cells of the members of which have no granules of sulphur or bacteriopurpurin in their 

 constitution; and the Thiobacteria (roughly, the pleomorphous bacteria of writers), in 

 the cells of which are to be seen red or violet granules of sulphur or bacteriopurpurin. 



The eubacteria or true bacteria are divided into five families from morphologic 

 peculiarity of the cells: (I) Coccacece, spherical forms; (II) Bacteriacea, straight, rod- 



262 



