480 BACTERIOLOGICAL ANALYSES 



TESTING OF DISINFECTANTS. 



Methods have already been detailed (page 310) for 

 the purpose of studying the vital resistance offered by 

 micro-organisms to the lethal effect of germicides. But 

 it frequently happens that the bacteriologist has to 

 determine the relative efficiency of " disinfectants" 

 from the standpoints of the sanitarian and commercial 

 man rather than from the research worker's point of 

 view. In pursuing this line of investigation, it is con- 

 venient to compare the efficiency, under laboratory 

 conditions, of the proposed disinfectant with that of 

 some standard germicide, such as pure phenol. In so 

 doing, and in order that the work of different observers 

 may be compared, conditions as nearly uniform as 

 possible should be aimed at. The method described is 

 one that has been in use by the writer for many years 

 past, modified recently by the adoption of some of 

 the recommendations of the Lancet Commission on the 

 Standardisation of Disinfectants particularly of the 

 calculation for determining the phenol coefficient. 



This method has many points in common with that 

 modification of the "drop" method known as the 

 Rideal- Walker test. 



General Considerations. 



These may be grouped under three headings : Test 

 Germ, Germicide, and Environment. 



i. Test Germ. B. coli. 



As disinfectants are tested for sanitary purposes, it is 

 obvious that a member of the coli-typhoid group should 

 be selected as the test germ. B. coli is selected on ac- 

 count of its relative nonpathogenicity, the ease with 

 which it can be isolated and identified by different ob- 

 servers in various parts of the world, the stability of its 

 fundamental characters, and evenness of its resistance 

 when utilised for these tests; finally since the colon 



