636 PATHOGENIC MICROORGANISMS 



agglutinate its homologous strains in dilutions of one to 1000 will often 

 fail to agglutinate other races of Bacillus coli in dilutions of 1 : 500 

 or 1 : 600. 



The normal serum of adult animals and man will often agglutinate 

 this bacillus in dilutions as high as 1 : 10 or 1 : 20 a phenomenon pos- 

 sibly referable to its habitual presence within the body. Corroborating 

 this assumption is the observation of Kraus and Low, 18 that the serurn 

 of new-born animals possesses no such agglutinating powers. The 

 fact that agglutinins for the colon bacillus are increased in the serum 

 of patients convalescing from typhoid fever or dysentery is probably 

 to be explained, partly by the increase of the group agglutinins pro- 

 duced by the specific infecting agent, and partly by the invasion of 

 colon bacilli, or the absorption of its products induced by the diseased 

 state of the intestinal mucous membrane. 



Varieties of the Colon Bacillus. During the earlier days of bac- 

 teriological investigations, a large number of distinct varieties of colon 

 bacilli were described, many of which may now be dismissed as based 

 simply upon a temporary depression of one or another cultural charac- 

 teristic of Bacillus coli communis, while others can be definitely included 

 within other closely related, but distinct groups. 



That secondary features, such as dimensions, motility, and luxuri- 

 ance of growth upon various media, may be markedly altered by arti- 

 ficial cultivation is a common observation. It has not, however, been 

 satisfactorily shown that cardinal characteristics, such as the forma- 

 tion of indol from pepton, or the power to produce gas from dextrose 

 and lactose, can be permanently suppressed without actual injury or 

 inhibition of the normal vitality of the microorganism. Such alter- 

 ation is, in fact, contrary to experience, which demonstrates that 

 whenever such changes do occur, they are purely temporary and a few 

 generations of cultivation under favorable environmental conditions 

 will regularly restore the organism to its normal activity. 



Distinct and constant varieties of the Colon Bacillus or, at least, 

 close biological relatives do occur. It is necessary to consider the organ^ 

 isms as a group for this reason, since, in sanitary work, it is of the utmost 

 importance to recognize forms which should properly be classified under 

 this category. It may be well, therefore, to reiterate the criteria for 

 identification of the group established by the American Public Health 

 Association Committee on Standard Methods of Water Analysis. 19 

 This report defines the general characteristics of the group, as follows: 



15 Kraus und Low, Wien. klin. Woch., 1899. 



