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EUCAST disk diffusion: Can Mueller-Hinton agar with 5% sheep blood (MH-S) be an alternative medium for fastidious
Erika Matuschek1, Simon Hintze2, Lized Rodriguez1
1EUCAST Development Laboratory, Växjö, Sweden.
Objectives:
EUCAST recommends Mueller-Hinton (MH) agar with defibrinated horse blood and β-NAD (MH-F) for disk diffusion testing of fastidious organisms, but MH-F has been difficult to access in several geographical areas. The objective of this study was to evaluate MH with sheep blood (MH-S) as an alternative medium.
Methods:
Disk diffusion (DD) was performed according to EUCAST recommendations on MH-F and MH-S in parallel. All species with breakpoints on MH-F agar were tested, except for Haemophilus influenzae due to its inadequate growth on MH-S. Growth and zone diameters from parallel tests on MH-F and MH-S were compared and categorical agreement was calculated according to EUCAST recommendations. Essential agreement and bias were calculated adapting the recommendations for MIC evaluations in ISO 20776-2. Also, zone diameter distributions on MH-S produced from MH agar from three manufacturers were compared.
Results:
Growth on MH-S was equally good as on MH-F for all tested species. Correlation between DD results on MH-S and MH-F was good, with 61-85, 89-100 and 95-100% inhibition zone diameters within 1, 2 and 3 mm, respectively, for each species/organism group. Systematic deviation (bias) for MH-S was below ±30% when analysed per species and for 25 of 32 agents when analysed per agent. Specific reading instructions were required for trimethoprim-sulfamethoxazole vs. Streptococcus groups A, B, C and G when tested on MH-S. Inhibition zone diameters on MH-S were not significantly influenced by the three brands of MH investigated.
Conclusions:
MH-S can be used for DD for all species with EUCAST zone diameter breakpoints on MH-F (except H. influenzae, which does not grow on MH-S). Since MH-F supports growth of more species than MH-S, EUCAST will continue to recommend MH-F as the standard EUCAST medium for disk diffusion testing of fastidious organisms, unless otherwise stated. Updated information is available on the EUCAST website.
