6o BACTERIA 



mind a point that is systematically neglected, namely, that 

 the bacteriological examination of a water which is suspected 

 of having conveyed the disease is from a variety of circum- 

 stances conducted too late to detect the causal bacteria. The 

 incubation period of typhoid we may take at fourteen days. 

 Let us suppose a town water supply is polluted with some 

 typhoid excreta on the 1st of January. Until the 14th of 

 January there may be no knowledge whatever of the state 

 of affairs. Two or three days are required for notification of 

 cases. Several more days elapse generally before bacterio- 

 logical evidence is demanded. Hence arises the anomalous 



B. CoLi Communis 



position of the bacteriologist who sets to work to examine a 

 water suspected of typhoid pollution three weeks previously. 

 There can be no doubt that these difficulties are very real 

 ones. The solution to the problem will be found in Dr. 

 Klein's dictum that ** a water in which sewage organisms 

 have been detected in large numbers should be regarded 

 with suspicion " ^ as the vehicle of typhoid, even though no 

 typhoid bacilli were discoverable. The chief of these sewage 

 bacteria are believed to be Proteus vulgaris^ B. coli, P. zenkeri^ 

 and B. enteritidis, and they are all nearly related to B. typhosus. 

 The presence of the B. coli in limited numbers is not suffi- 



' Harben Lectures, 1896. 



