DETECTION OF DECREASED PENICILLIN SUSCEPTIBILITY IN VIRIDANS GROUP STREPTOCOCCI

One hundred consecutive clinically significant viridans group streptoc occal isolates had their susceptibility to penicillin determined by th e penicillin E-test method. The ability of penicillin 2 and 10 unit di sks and the oxacillin 1 mu g disk to detect reduced penicillin suscept ibility, ie; MIC greater than or equal to 0.25 mu g/ml, in viridans gr oup streptococci was determined by comparing the zone diameters agains t the penicillin E-test MICs. The sensitivity, specificity and predict ive values of previous, existing and proposed interpretative criteria to detect decreased penicillin susceptibility were determined. Thirty- seven per cent of the isolates had reduced susceptibility to penicilli n. The previous 1993 NCCLS interpretative criteria for the penicillin 10 unit disk, ie; resistant less than or equal to 27 mm failed to dete ct 16 of 37(43%) isolates with reduced penicillin susceptibility. The 1 mu g oxacillin disk using existing meningococcal interpretative crit eria, ie; resistant less than or equal to 10 mm, failed to detect 11 o f 37 (40%) isolates with reduced penicillin susceptibility. When the o xacillin 1 mu g disk pneumococcal interpretative criteria were used, i e; resistant less than or equal to 19 mm, all the isolates with reduce d penicillin susceptibility were detected but 42 of 63 (67%) susceptib le isolates were misclassified as resistant. Based on our data, we set new interpretative criteria to detect all isolates with decreased pen icillin susceptibility for each of the three disks. Using our proposed zone diameters to detect decreased penicillin susceptibility of less than or equal to 27 mm for the penicillin 2 unit disk, less than or eq ual to 35 mm for the penicillin 10 unit disk, and 17 mm for the oxacil lin disk, 34 (54%), 44 (70%), and 21 (33%) of the 63 susceptible isola tes, respectively, were misclassified as having decreased penicillin s usceptibility. Our data show that the oxacillin I mu g disk is able to detect decreased susceptibility to penicillin in viridans group strep tococci with greater specificity than either penicillin 2 or 10 unit d isks.