METHODS OF MEASURING ANTIBIOTIC ACTIVITY 75 



even though the macroscopic appearance of the culture did not show any 

 inhibition. Pneumococci and S. viridans show marked strain differences 

 by this method. In one experiment with Salmonella tyfhiy partial in- 

 hibition was obtained in a dilution of i : 1 0,000 j however, elongation 

 of the cells was detected in a dilution of i : 60,000, a concentration 

 which was considered as a therapeutic possibility (Table 8). 



The other modifications of this method either use different test cul- 

 tures, such as B. sub tilts (285), or have been developed to meet the re- 

 quirements of the clinician when only small amounts of blood or other 

 body fluids are available, in which case a very sensitive strain of S. 

 hemolyticus is used (751). The use of Klebsiella fnemnoniae for as- 

 saying streptothrycin and streptomycin permits the determination of as 

 little as 0.05 Mg/ml., giving somewhat more rapid and more accurate 

 results (191). 



Agar Diffusion {Cuf, Pafer Disc, Cylinder) Method (5, 173, 283, 

 285, 390) 



This method, first employed for measuring antiseptics qualitatively 

 (810), was later developed for quantitative use. A suitable agar me- 

 dium is inoculated with a test organism {S. aureus or B. subtilis), the 

 active agent being placed upon the agar within a groove or in a special 

 small glass cup with an open bottom from which the substance diffuses 

 into the medium. The rate of diffusion of the antibiotic is parallel to its 

 concentration. Potency can be calculated by measuring the zone of in- 

 hibition and comparing it with that of a known standard preparation. 

 Various modifications of this method have recently been introduced 

 (286, 937). This method has the advantage of simplicity and con- 

 venience, since it does not require sterile material and several prepara- 

 tions or duplicates can be tested on the same plate. The method also 

 possesses certain disadvantages, however, since it cannot be used for 

 comparing different substances but is limited to the measurement of 

 activity of only one type of substance; it cannot be used for the study 

 of unknowns until a standard has been established for each j it cannot 

 be used for substances that are not water soluble. 



Nutrient agar containing 5 gm. NaCl, 3 gm. meat extract, 5 gm. 



