Detection of decreased penicillin susceptibility in viridans group streptococci

One hundred consecutive clinically significant viridans group streptococcal isolates had their susceptibility to penicillin determined by the penicillin E-test method. The ability of penicillin 2 and 10 unit disks and the oxacillin 1 microg disk to detect reduced penicillin susceptibility, ie; MIC > or = 0.25 microg/ml, in viridans group streptococci was determined by comparing the zone diameters against the penicillin E-test MICs. The sensitivity, specificity and predictive 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 penicillin. The previous 1993 NCCLS interpretative criteria for the penicillin 10 unit disk, ie; resistant < or = 27 mm failed to detect 16 of 37 (43%) isolates with reduced penicillin susceptibility. The 1 microg oxacillin disk using existing meningococcal interpretative criteria, ie; resistant < or = 10 mm, failed to detect 11 of 37 (40%) isolates with reduced penicillin susceptibility. When the oxacillin 1 microg disk pneumococcal interpretative criteria were used, ie; resistant < or = 19 mm, all the isolates with reduced penicillin susceptibility were detected but 42 of 63 (67%) susceptible isolates were misclassified as resistant. Based on our data, we set new interpretative criteria to detect all isolates with decreased penicillin susceptibility for each of the three disks. Using our proposed zone diameters to detect decreased penicillin susceptibility of < or = 27 mm for the penicillin 2 unit disk, < or = 35 mm for the penicillin 10 unit disk, and < or = 17 mm for the oxacillin disk, 34 (54%), 44 (70%),and 21 (33%) of the 63 susceptible isolates, respectively, were misclassified as having decreased penicillin susceptibility. Our data show that the oxacillin 1 microg disk is able to detect decreased susceptibility to penicillin in viridans group streptococci with greater specificity than either penicillin 2 or 10 unit disks.