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

SUMMARY: Investigations on the size of inhibition zones in agar tube cultures of Staphylococcus aureu ~ (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 2740", nor with variations of inoculum size from 2 x 10s to 5 x los 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 B 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. Cooper & Woodman (1946) proposed, on theoretical grounds, a formula for the size of antibiotic inhibition zones in agar media and Cooper (1955) extended this formula by elucidating the nature of the factors which control the time during the growth of the organism a t which the size of the zone is determined. This critical time To is given by