Dear Sverre,
Enterococci (and other bacteria) can take up thymine or thymidine from
the surounding medium. In doing so, they do not longer depend on their
own thymine synthesis and are therefore resistant to the metabolic block
by trimethoprim and sulfonamides. In order to avoid false-resistant
results with these compounds, you should use Mueller-Hinton agar for
antimicrobial susceptibility testing. The NCCLS recommends to use
inhibition of Enterococcus faecalis ATCC 29212 or ATCC 33186 by a
sulfamethoxazole/trimethoprim disk as a control for each new lot of
Mueller-Hinton agar (satisfactory lots should give essentially clear
zones of inhibition of at least 20 millimetres).
--
Andre P. Burnens MD
Institute for Veterinary Bacteriology, University of Berne
Laenggass-Strasse 122, CH-3012 Berne, Switzerland
Tel. +41 31 631 24 85 Fax. +41 31 631 26 34
