The same enterococci; resistant/sensitiv in two media (trimethoprim)

Andre Burnens BURNENS at VBI.unibe.ch
Thu Apr 11 11:47:24 EST 1996

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

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