EUCAST News

28.09.2025

MH-F vs. MH-S for AST of fastidious organisms

The EUCAST preferred and recommended medium for AST of fastidious microorganisms is still MH-F, but MH-S can be used for most.

EUCAST has now systematically compared _MH-F and MH-S to see if the media are interchangeable. MH-F contains 5% mechanically defibrinated horse blood and beta-NAD and will grow also Haemophilus whereas MH-S contains 5% mechanically defibrinated sheep blood but no beta-NAD since MH-S will not grow Haemophilus spp.unless unreasonable amounts of beta-NAD is added.
Based on all results, and excluding Haemophilus, differences between the media were small and EUCAST draws the following conclusions:

  • Since MH-F supports growth of more species (in particular, Haemophilus) than MH-S, EUCAST will continue to consider MH-F the standard EUCAST medium for disk diffusion of fastidious organisms unless otherwise specifically stated. This will be evident from the methods section on the website and at the top of species tabs in breakpoint tables. For preparation of MH-F agar plates, see “Preparation of agar plates and broth for EUCAST AST” at https://www.eucast.org/ast_of_bacteria/media_preparation.
  • For almost all species and agents (the exception being Haemophilus spp), MH-S can be used instead of MH-F. The MH-S agar plates must contain 5% mechanically defibrinated sheep blood and have the same agar depth as MH-F (4.0 mm with a random variation of ± 0.5 mm). However, EUCAST will not validate MH-S to the same extent as MH-F, and when using MH-S instead of MH-F, it is the responsibility of the user to validate the quality and usability of the medium, for all disks and species included in disk diffusion testing at the laboratory. This is primarily done by use of QC strains but also by comparing local distributions with those available from EUCAST.
  • When using MH-S instead of MH-F, all other aspects of the EUCAST disk diffusion method (e.g. inoculum preparation, inoculation of plates, application of disks, disk potencies, incubation and reading of zone diameters) should be adhered to.
  • Reading of zone diameters on MH-S should be performed according to EUCAST standard reading instructions for fastidious organisms, except for Streptococcus groups A, B, C and G and trimethoprim-sulfamethoxazole, for which growth within the zone should be ignored and the outer zone edge read.
  • EUCAST encourages the involvement of NACs in investigations into the national availability and comparative costs of MH-F and MH-S. NACs can aid by coordinating testing and trouble shooting. NACs may decide to recommend the use of MH-F or MH-S on a national basis. We encourage users to perform further comparisons and validation of MH-S, using the MH agar and disks available locally and to report problems to EUCAST. The EDL can assist in trouble shooting.
  • EUCAST will, for each new organism or antimicrobial agent developed for testing on MH-F, perform a parallel, less extensive validation of MH-S to disclose any major discrepancies between the two media.
  • Problems with using MH-S, when noted and confirmed by EUCAST, will be listed as a warning on the EUCAST website (https://www.eucast.org/ast-of-bacteria/warnings).
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