VAKIETIES OF B. OOLI 397 



group is bacillus oxytocus perniciosus, which is said originally to have 

 been isolated from milk. This organism, along with the bacillus 

 vesiculosus and an organism denominated No. 71, were found by Mac- 

 Conkey to be of very common occurrence in human and animal faeces. 



In work of the kind with which we are dealing, two other organisms 

 are not infrequently observed which morphologically belong to the coli- 

 typhoid group, but neither of which is a lactose fermenter. These are 

 the bacillus fsecalis alcaligenes, and the bacillus celi anaerogenes. The 

 reactions of these will be found in the Table on p. 396. The latter 

 bacillus somewhat resembles the typhoid bacillus, but produces acid in 

 lactose and can be distinguished by agglutinating reactions. 



When any question arises regarding the relationships of an organism 

 isolated under saphrophytic conditions and resembling some definite 

 pathogenic type, important information can often be obtained by study- 

 ing its agglutinating reactions. In such a case the effect of sera produced 

 by the pathogenic type upon the unknown organism, and of sera pro- 

 duced by injection into animals of the pathogenic type in question, 

 ought to be studied. 



The Question of Mutation. — It is becoming more and more recognised 

 as our knowledge of pathogenic bacteria advances that around each 

 particular type form we must group a number of variants which closely 

 resemble it. This is specially true of some of the members of the coli- 

 typhoid group ; here the introduction of a variety of media makes the 

 recognition of variants comparatively easy. Thus, to take the b. dysenteric, 

 not only have different epidemics yielded different strains, but what is 

 somewhat perplexing, similar differences, even in the fundamental 

 character of behaviour towards mannite, have been observed in strains 

 isolated from different cases during the same epidemic. Such facts might 

 even raise doubts as to the etiological relationship of the organism to the 

 disease, and certainly make it necessary to consider whether the conditions 

 of growth existing in the animal body are capable of accounting for the 

 variations observed. 



Several facts bearing upon the question are now known. Neisser from 

 a non-lactose fermenter under his observation found a new strain capable 

 of fermenting lactose appear in his cultures. Of greater importance, 

 however, is the origination of such mutations under experimental con- 

 ditions. Thus, Twort found that by prolonged sub-culturing on a lactose- 

 containing medium the typhoid bacillus developed the capacity of 

 forming acid from this sugar, and Penfold has shown that this organism 

 can similarly produce acid from dulcite. Penfold has also observed 

 that the capacity of the b. coli to produce gas from various sugars can 

 be modified and in certain cases suppressed by a previous growth on 

 a medium containing monochloracetic acid. Similar results have been 

 obtained with other organisms, and the important fact has been 

 elicited that the changes in capacity are related to the chemical consti- 

 tution of the sugars employed, as for instance when the variant, while 

 unable to produce gas from certain pentoses, can to a certain extent 

 originate the change in hexoses. The isolation of variants is frequently 

 made possible by alterations in the naked-eye appearances of surface 

 colonies and the development upon them of papillae, the bacteria in these 

 excrescences being found to present different properties from those in the 

 flat part of the colony. The investigation of these mutations is not only 

 of great scientific importance, but may throw light on the multiplicity 

 of strains which has been observed under natural conditions. 



