408 TYPHOID FEVER 



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. dysenteries, 

 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 muta- 

 tions under experimental conditions. 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 constitution 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 altera- 

 tions in the naked-eye appearances of surface colonies and the 

 development upon them of papillae, the bacteria in these excres- 

 cences 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. 



