Penicillin susceptibility testing of gonococci by disc diffusion

Abstract

This study investigated the use of various beta-lactam discs in the detection of penicillin sensitive, intermediate resistant and resistant strains (minimal inhibitory concentrations of penicillin G of less than 0.06, 0.06 to 0.25, and greater than 0.25 mg/l respectively). Discs containing 0.15 and 0.6 microgram of penicillin G, and 5 microgram of methicillin were the most useful in separating the three groups. Cut-off zones between sensitive, intermediate resistant and resistant strains are 12 and 24 mm for 0.15 microgram penicillin discs; 24 and 32 mm for 0.6 microgram penicillin discs; and 20 and 30 mm for 5 microgram methicillin discs. Oxacillin discs (1 microgram) were useful in separating resistant from both sensitive and intermediate resistant strains at a cut-off zone of 8 mm. Use of a combination of discs provides the best index of susceptibility group, and at least two should be used

Determination of Penicillin MICs for Streptococcus pneumoniae by Using a Two- or Three-Disk Diffusion Procedure

“…Statistical methods. Regression analysis of scatterplots was done by the leastsquares method, and correlation coefficients (r) were calculated for MICs versus MICs, MICs versus zone diameters, and MICs versus sums of zone diameters (13). MICs were calculated from zone diameters and sums of zone diameters with the BIOMIC system (Giles Scientific Inc., New York, N.Y.) algorithm (3).…”

“…MICs were calculated from zone diameters and sums of zone diameters with the BIOMIC system (Giles Scientific Inc., New York, N.Y.) algorithm (3). The method of summing zone diameters from several disks has been used in prior studies and provides greater precision in MIC prediction (13). The ratios of these calculated MICs to the mean MICs by the three methods and the ratios of the MICs by the three methods to each other were determined.…”

“…The results of MIC and disk diffusion tests were compared in order to determine if results from any single disk or a combination of disks could be used to accurately calculate MICs. The use of a combination of disks of varying penicillin content has been demonstrated previously to be an accurate method of determining penicillin MICs for Neisseria gonorrhoeae (13).…”