The Effect of Chemicals on Microorganisms 



211 



Fig. 35. Paper disc method for the determination of penicillin con- 

 centration in a solution. The size of the inhibition zone is related to the 

 amount of penicillin in the paper discs. {Coiirtesij of Eli Lilly and 

 Company, Indianapolis, Indiana.) 



ganisms decisions can be arrived at within a normal working day. 

 Impregnated discs may be purchased in graded strengths, and the 

 relative merits of each antibiotic can be determined by comparing 

 the width of the zones of clearing. 



THE PHENOL COEFFICIENT TEST 

 An official method for evaluating the usefulness of disinfectants 

 is that prescribed by the Food and Drug Administration (F.D.A. 

 method). In brief, the procedure is a comparison of the relative 

 effectiveness of liquid disinfectants with that of pure phenol under 

 one set of highly standardized laboratory conditions. If the ulti- 



