THE COLON GROUP OF BACILLI 137 



at least a week. Granting that the lactose bile test 

 gave us the whole colon group and that the " tj-pical 

 B. coli " giving characteristic reactions in milk, peptone, 

 gelatin and the nitrate solution, only the more sensitive 

 members of the group, indicative of recent pollution, 

 was the extra information gained worth the additional 

 trouble? Under the conditions which generally obtain, 

 in the United States at least, it appears not. The colon 

 test at best is an approximate one, and its results 

 are usually only expressed in decimal fractions, positive 

 in lo C.C., I c.c, or .i c.c. for example. From 70 to 

 90 per cent of the bacteria which give the lactose bile 

 test prove to be " typical " B. coli on any of the defini- 

 tions ordinarily used. This makes a difference so slight 

 as to be almost negligible. We cannot condemn a 

 water because it contains 10 rather than 7 colon bacilli 

 in a given proportion. It may be that under tropical 

 conditions such as those described by Clemesha (to 

 be discussed in a following chapter) certain forms of 

 the colon group persist for a long time in stored waters 

 from which disease germs have disappeared. These 

 resistant forms must, however, be studied by a much 

 more elaborate procedure than the 7 tests in the old 

 American Standard method; and it seems clear that 

 they do not occur in large numbers in temperate cli- 

 mates. For our conditions the whole group of forms 

 which produce gas W la:tose bile should be absent from 

 safe waters. 



The Committee on Standard Methods of Water 

 Analysis in its last report (191 2) apparently takes 

 this ground, although its discussion of the problem 



