106 ELEMENTS OF WATER BACTERIOLOGY 



1900) found that 1.33 parts of phenol in 1000 materially 

 decreased the number of colon bacilli which would 

 develop, while i part gave very satisfactory results, 

 the plates showing pure cultures of B. coli. The addi- 

 tion of antiseptics in this way is always open to the 

 objection that weaker strains may be killed and lost. 



In Germany the Endo medium and the Conradi- 

 Drigalski medium have been extensively used for the 

 direct isolation of colon bacilH with excellent results. 

 The composition and use of these media have been 

 discussed in Chapter V. It does not appear that they 

 have sufficient advantages to compensate for the dif- 

 ficulty in preparing them. 



The Use of Preliminary Enrichment Media in the 

 Isolation of Colon Bacilli. The test for the colon 

 bacillus may be made more delicate by a preHminary 

 cultivation of the sample in a liquid medium for 24 

 hours at 37°, thus greatly increasing the propor- 

 tion of these organisms present before plating. As 

 suggested in the classic researches of Theobald Smith 

 (Smith, 1892), this method may be made approx- 

 imately quantitative by the inoculation of a series 

 of tubes with measured portions of the water. If, 

 for example, of ten tubes inoculated each with tdtt 

 of a cubic centimeter, four show B. coli, we may 

 assume that some 40 of these organisms were present 

 in the cubic centimeter. Irons (Irons, 1901), in a 

 comparative study of various methods for the isola- 

 tion of B. coli, showed that the preliminary enrichment 

 frequently gave positive results when the results of 

 the direct use of the agar plate were negative, and 



