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Journal of Clinical Microbiology, January 2000, p. 341-344, Vol. 38, No. 1
0095-1137/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Antifungal Susceptibility Testing of Dermatophytes:
Establishing a Medium for Inducing Conidial Growth and Evaluation
of Susceptibility of Clinical Isolates
C. J.
Jessup,
J.
Warner,
N.
Isham,
I.
Hasan, and
M. A.
Ghannoum*
Mycology Reference Laboratory, Center for
Medical Mycology, Department of Dermatology, Case Western Reserve
University, and University Hospitals of Cleveland, Cleveland, Ohio
Received 21 July 1999/Returned for modification 16 September
1999/Accepted 9 October 1999
A standardized reference method for dermatophyte in vitro
susceptibility testing is lacking. In a previous study, Norris et al.
(H. A. Norris, B. E. Elewski, and M. A. Ghannoum,
J. Am. Acad. Dermatol. 40(6, part 2):S9-S13) established the
optimal medium and other growth variables. However, the earlier study
did not address two issues: (i) selection of an optimal medium for
conidial formation by dermatophytes and (ii) validation of the method
with a large number of dermatophytes. The present study addresses these two points. To select which agar medium best supported conidial growth,
representative isolates of dermatophytes were grown on different agars.
Preliminary experiments showed that only oatmeal cereal agar supported
the production of conidia by Trichophyton rubrum. We tested
the abilities of 251 T. rubrum isolates to form conidia
using three different cereal agars and potato dextrose agar. Overall,
oatmeal cereal and rice agar media were comparable in their abilities
to support T. rubrum conidial growth. Next, we used the
oatmeal cereal agar for conidial formation along with the optimal
conditions for dermatophyte susceptibility testing proposed by Norris
et al. and determined the antifungal susceptibilities of 217 dermatophytes to fluconazole, griseofulvin, itraconazole, and
terbinafine. Relative to the other agents tested, terbinafine possessed
the highest antifungal activity against all of the dermatophytes. The
mean ± standard error of the mean MICs of fluconazole,
itraconazole, terbinafine, and griseofulvin were 2.07 ± 0.29, 0.13 ± 0.01, 0.002 ± 0.0003, and 0.71 ± 0.05 µg/ml,
respectively. This study is the first step in the identification of
optimal conditions that could be used for the standardization of the
antifungal susceptibility testing method for dermatophytes. Inter- and
intralaboratory agreement as well as clinical correlations need to be established.
*
Corresponding author. Mailing address: Center for
Medical Mycology, Mycology Reference Laboratory, University Hospitals
of Cleveland, 11100 Euclid Ave., Cleveland, OH 44106-5028. Phone: (216)
844-8580. Fax: (216) 844-1076. E-mail: mag3{at}po.cwru.edu.
Journal of Clinical Microbiology, January 2000, p. 341-344, Vol. 38, No. 1
0095-1137/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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