NATURAL BACTERIAL SYSTEM 267 



(the genera Thermobaderium, Streptobacterium and Betabac- 

 terium) belonging to the lactic acid bacteria are by no means 

 always straight. They can be screw-formed and (especially Bac- 

 terium bifidum) bifurcated. Among the propionic acid bacteria, 

 which also form a natural family, we once more meet with both 

 sphere and rod forms, and among the latter very often club- 

 shaped and forked forms. 



I am glad to see that Breed, Conn and Baker (1918) in their 

 critique of the report of the Committee agree with me in the 

 view that 'Hhe shape of cell or form of body is not a funda- 

 mental character." It is so much the more strange that these 

 investigators nevertheless finish by setting up a purely morpho- 

 logical system. 



There is one further particular in which I must dissent from the 

 Committee, and that is in setting up new genera of pathogenic 

 bacteria (some of these being moreover named after persons), 

 and in this particular too Breed, Conn and Baker agree with me. 

 The pathogenic characters are not always so constant that they 

 can be used as specific characters; they are often difficult to 

 maintain when the bacteria are cultivated on artificial media. 

 Non-pathogenic species can become pathogenic (for instance, 

 certain streptococci and micrococci) through animal passages or 

 through mixed infections, or (in the case of certain coh bacteria) 

 simply by living in the intestinal canal. What is true of the 

 parasites of animals, will certainly also apply to those of plants, 

 and we thus know of moulds sometimes appearing as sapro- 

 phytes, sometimes as parasites. Many so-called pathogenic 

 species of bacteria ought more correctly to be considered as 

 saprophytes from which more or less virulent varieties are readily 

 developed, and although such species are more often met with 

 in one genus than in another, we must be very cautious in setting 

 up pathogenic genera. The interest which has been awakened 

 in the pathogenic bacteria described in medical literature has 

 hitherto left its trace in bacteriology to such an extent that it has 

 been attempted to group all known bacteria around these. This 

 is a step which must necessarily lead to the establishment of 

 systems as artificial as if in the animal and vegetable kingdoms 



