398 APPLIED BACTERIOLOGY 



O'l to 0-5 of a c.c. of the water under examination is then 

 added to the tubes, the contents well mixed, and the tubes 

 again returned to the incubator. If, after twenty-four 

 hours' incubating at blood-heat, any of the tubes appear to 

 be turbid, they are submitted to ordinary plate-cultivation, 

 and the resulting colonies carefully examined in sub- 

 cultures. Frankland states that when only a few typhoid 

 bacilli are present, the incubation must be prolonged for 

 forty-eight or even seventy-two hours. 



The great objection to the use of phenolated broth is 

 that when cultivated at blood-heat the colon bacillus and 

 its allies multiply at from two to five times as quick as the 

 typhoid bacillus, even if this is not suppressed altogether. 

 This objection also applies, but in a less degree, to phenolated 

 plates, the surface of which may be covered by the ex- 

 panded colonies of the colon bacillus. 



It must be remembered that the preliminary concentra- 

 tion does not improve matters numerically, and, as has 

 been pointed out by Stoddart, it is not unreasonable to 

 suppose that the somewhat violent treatment may have a 

 more injurious action upon the typhoid bacilli than upon 

 the hardier forms. Since by no method at present known 

 can the ratio of the typhoid to the colon bacillus and the 

 common water saprophytes be increased, and the general 

 tendency is for them rather to decrease, the difficulty is best 

 met by increasing the area of the plates rather than by any 

 method of concentration. This numerical difficulty has, 

 perhaps, not been fully appreciated by many workers on 

 this subject, but it has been forcibly presented by Laws and 

 Andrewes in their recent report (' Keport upon the Micro- 

 organisms of Sewage,' presented to the London County 

 Council,' 1894). In the case, for instance, of a moderately 

 polluted water containing 50,000 microbes per cubic centi- 

 metre: of these, possibly 90 per cent, may be suppressed 



