Impact of the agar medium and disc type on disc diffusion susceptibility testing against teicoplanin and vancomycin

Susceptibility to teicoplanin and vancomycin was assessed by three disc types: two commercially available discs (NeoSensitabs and PDM disc (30 micrograms)) and one locally prepared 30 micrograms disc (SS disc) on four different medium types: Mueller-Hinton agar (MH medium), MH medium and PDM agar II supplemented with 5% horse blood (HMB medium and PDM medium, respectively), and Danish blood agar (DBA medium). Two previously studied groups of Gram-positive bacteria were tested: group B (N = 75) comprised miscellaneous cocci, and group C (N = 59) mostly rods. With NeoSensitabs, mean zone diameters were larger than with PDM and SS discs on all medium types, and mean zone diameters were larger on DBA medium than on MHB and PDM medium with all disc types. The impact of the medium type on the zone diameter was evaluated for 121 strains growing on MHB medium, PDM medium, and DBA medium. Bacterial groups B and C each divided into three MIC groups were analysed separately. We compared mean zone diameters for each specific group with the average zone diameter, i.e. the mean value for all zone diameters obtained. The smallest deviations from the average zone diameters were observed on PDM medium for both teicoplanin and vancomycin. Thirty-seven percent of strains failed to grow on MH medium, but supplementation of MH medium with horse blood significantly reduced the zone diameter for group B strains both for teicoplanin and vancomycin. Poor predictability of MIC from the zone diameter was found especially for strains with MICs < or = 1 microgram/ml. The medium type hardly affected the results of regression analysis. In contrast, the medium type markedly affected the results of error-rate bounded analysis. No errors were recorded with the SS disc on MHB medium for either teicoplanin or vancomycin, but no strains with MICs of vancomycin within the intermediate group could be correctly classified on DBA medium.