23.02.2025
Streptococcus pneumoniae and benzylpenicillin susceptibility testing.
We are frequently asked questions regarding (1) EUCAST Warning No. 7: is this warning specifically directed at benzylpenicillin gradient tests (EtestTM, bioMérieux, and MTSTM, Liofilchem) when used for Streptococcus pneumoniae or does it apply to other bacteria, (2) is it still valid or have we forgotten to remove the warning and (3) since there is no warning directed at ampicillin or amoxicillin (and many others) – does this mean that EUCAST did not identify a problem with these gradient tests when used for S. pneumoniae or any other species?
Warning No. 7 states that both gradient tests underestimate benzylpenicillin MIC values by one to two dilutions, most prominently and with the most serious consequences, in the area around the R-breakpoint. The evaluation of the performance was repeated in 2022 with the same result - so the warning is still valid. As to whether gradient tests for ampicillin and amoxicillin can be trusted, EUCAST has not offered an opinion.
However, with breakpoint table 15 (1 January, 2025), a change in the benzylpenicillin breakpoints and the introduction of the benzylpenicillin 1U disk as part of the extended testing of oxacillin screen positive isolates of S. pneumoniae has obviated the need for MIC testing. This will simplify distinguishing between benzylpenicillin I and R.
Gradient tests(like other tests, including disk diffusion) need to be developed and validated (calibrated) for eachagent and speciesfor which their use is intended. Manufacturers and users are responsible for making sure this is the case.
EUCAST takes responsibility for standardised methods developed by EUCAST but does not have the resources to systematically investigate other tests. We react to “signals” from colleagues and then investigate and issue warnings when appropriate. We have investigated the quality of disks and media used for disk diffusion (see Warnings 2, 8 and 9) and we invest considerable efforts into identifying target values for QC strains for every relevant agent. This is to provide means to measure the accuracy in disk diffusion and to provide manufacturers and users a means to measure accuracy.
But, most tests, outside EUCAST reference broth microdilution and standardised EUCAST disk diffusion testing are not routinely investigated. The absence of a Warning cannot be interpreted to mean that EUCAST approves.
