METHODS OF MEASURING ANTIBIOTIC ACTIVITY 



75 



The agar cup method has also been utilized (869) for comparing 

 the disinfectant action of chemical antiseptics. S. aureus and B. suhtilu 

 were found to be most sensitive to the action of aliphatic alcohols. Vari- 

 ous modifications of this method have been introduced, including the 

 use of paper discs treated with known dilutions of the active preparation 

 (818,917,985). 



Turbidimetric Method 



End-point methods have long been recognized as having many limi- 

 tations. Since it is difficult to determine accurately the end point and 

 since it takes a relatively much larger amount of an antibiotic substance 

 to inhibit completely the growth of the test organism as compared with 

 only 50 or 99 per cent inhibition, the suggestion has been made that 

 partial inhibition of growth be measured and, from this, the concentra- 

 tion of the active substance be calculated in a manner similar to the 



Figure 9. Relation between penicillin concentration and inhibition of 

 Staphylococcus aureus. The penicillin preparation contained 42 Oxford units 

 per milligram, and the incubation period was sixteen hours at 37° C. From 

 Foster (281). 



