CRITERIA EMPLOYED IN THE CLASSIFICATION 

 OF BACTERIA OF THE GASTRO-ENTERIC 

 TRACT 



IT appears not out of place to refer briefly to the cri- 

 teria employed in classifying the bacteria of the gastro- 

 enteric tract although these standards of judgment do 

 not differ from those of bacteria generally. The cul- 

 tural characters of the growths upon ordinary media 

 must be regarded as relatively less important in form- 

 ing a judgment as to the character of given organisms 

 than was formerly the case, since in the early days of 

 bacteriology such cultural characters constituted our 

 chief reliance in identification. While the cultural char- 

 acters of a microorganism derived from the digestive 

 tract must in every instance be taken into account, 

 these do not usually suffice to give more than a suspicion 

 of the identity of the bacterium in question. In general 

 it may be said that the cultural characters do not pos- 

 sess a high grade of permanence and are, moreover, often 

 not specific within certain groups. For example, within 

 the group of colon bacilli one finds many organisms which 

 must be regarded as distinct but which nevertheless give 

 practically undistinguishable growths on the ordinary 

 culture media. Indeed, a strain of colon bacillus may in 

 time come to vary quite widely from its original appear- 

 ance upon a given medium. The cultural characters of 

 microorganisms are often much influenced by physical 



105 



