The Effect of Inoculum Size on Inhibition Zones in Agar Media Using Staphylococci and Streptomycin

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Abstract
Investigations on the size of inhibition zones in agar tube cultures of Staphylococcus aureus (Mayo) due to diffusion of streptomycin have confirmed the formula previously suggested. No significant deviation from expectations has been found over a temperature range of 27-40[degree], nor with variations of inoculum size from 2 x 103 to 5 x 108 organisms/ml. The theoretical basis of the formula has been further elucidated and the meaning of critical population, critical concentration and critical time clarified. These constants are determined by the necessity of absorbing a critical amount of antibiotic on the organisms to inhibit growth within the zone. To maintain this absorption a minimum inhibitory concentration must remain in the surrounding medium.