84 ANTIBIOTIC ACTION OF ANTAGONISTS 



cases the serial dilution has been combined with the turbidimetric 

 method. The ability of penicillin to inhibit hemolysin production by 

 streptococci has also been utilized (8ooa). A comparison of the agar 

 diffusion, turbidimetric, and serial dilution methods led to the conclu- 

 sion that the last is the most suitable for routine clinical work (508). 

 Penicillin-containing material may also be spread over a given area of 

 a nutrient agar surface, allowing time for absorption of the liquid, and 

 streaking the surface with standard strains of S. aureus of known sensi- 

 tivity j on comparing with standard penicillin preparation, it is possible 

 to determine both the concentration of penicillin and the degree of sen- 

 sitivity to penicillin of the infecting agent (152). 



Fleming (266) proposed a method using a hemolytic streptococcus 

 as test organism and blood (preferably group O) from which leucocytes 

 have been removed or inactivated and treated with a clot-inhibiting 

 substance as a medium. Hemolysis of blood is measured either in a slide 

 cell or in a capillary tube. The penicillin concentration in the blood is 

 estimated by the serial dilution method. This method has been vari- 

 ously modified for clinical assays of penicillin. 



A convenient method for measuring the concentration of streptomy- 

 cin in body fluids is to use the agar diffusion method with an alkaline 

 medium, low in salt, and a carefully selected test organism (582, 879). 



