Journal of Antimicrobial Chemotherapy, Vol 42, 577-583, Copyright © 1998 by The British Society for Antimicrobial Chemotherapy
MF Palepou, AP Johnson, BD Cookson, H Beattie, A Charlett and N Woodford
The susceptibilities to mupirocin of 102 selected clinical isolates of
Staphylococcus aureus and of control strain S. aureus NCTC 6571 were
determined by disc diffusion (using discs containing 5, 15, 25, 30, 50 and
200 microg of mupirocin) and Etest and the results were compared with MICs
determined using an agar incorporation method. On the basis of agar
incorporation MICs, 42 isolates were sensitive to mupirocin (MIC < or =
4 mg/L), 39 showed low-level resistance (MICs = 8-128 mg/L) and 22 were
highly resistant (MICs > or = 256 mg/L) and contained the mupA
resistance gene. Using Stokes' criteria, none of the discs used gave major
errors (sensitive isolates classified as highly resistant) or very major
errors (highly resistant isolates classified as sensitive) in assigning a
category of susceptibility, but minor errors (a difference of one category)
were noted with all strengths. The best correlation with agar incorporation
MIC was obtained with 25 microg mupirocin discs, which classified correctly
98 (95%) isolates, while worse correlations were noted with 5 microg and
200 microg discs which are the only types currently available commercially,
for which there were 47 and 30 minor errors, respectively. The MICs found
by Etest were the same as, or lower than, those determined by agar
incorporation. Etests classified correctly all 42 mupirocin-sensitive
isolates, 19 (49%) low-level resistant isolates and 16 (73%) highly
resistant isolates. Two isolates that contained the mupA gene and showed
agar incorporation MICs of 256 mg/L and 512 mg/L were not classified as
highly resistant by any of the diffusion methods used. Agar incorporation
MIC determination, possibly supported by detection of the mupA gene, offers
the most effective means of identifying high-level mupirocin resistance in
S. aureus, although the Etest also proved to be reproducible. However, we
conclude that 25 microg discs warrant further evaluation for possible use
in clinical laboratories, as they appear to be more reliable than the discs
currently available.
ORIGINAL ARTICLES
Evaluation of disc diffusion and Etest for determining the susceptibility of Staphylococcus aureus to mupirocin
Laboratory of Hospital Infection, Central Public Health Laboratory, London, UK.
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