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Journal of Clinical Microbiology, February 2001, p. 525-532, Vol. 39, No. 2
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.2.525-532.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Influence of Glucose Supplementation and Inoculum
Size on Growth Kinetics and Antifungal Susceptibility Testing of
Candida spp.
Manuel
Cuenca-Estrella,*
Teresa M.
Díaz-Guerra,
Emilia
Mellado, and
Juan L.
Rodríguez-Tudela
Servicio de Micología, Centro
Nacional de Microbiología, Instituto de Salud Carlos III,
28220 Majadahonda, Madrid, Spain
Received 22 September 2000/Returned for modification 7 November
2000/Accepted 1 December 2000
The influences of inoculum size and glucose supplementation on the
growth kinetics of 60 Candida spp. clinical isolates
(Candida albicans, Candida tropicalis, Candida parapsilosis,
Candida glabrata, Candida krusei, and Candida
lusitaniae [10 isolates each]) are assessed. The combined
influence of growth and reading method (visual or spectrophotometric)
on the determination of the MICs of amphotericin B, flucytosine,
fluconazole, itraconazole, ketoconazole, and voriconazole is also
analyzed, and the MICs are compared with those determined by the
National Committee for Clinical Laboratory Standards standard
microdilution method (NCCLS document M27-A). Glucose supplementation
and inoculum size had a significant influence on the growth cycles of
these yeasts, and a statistically significant denser growth (optical
density at 540 nm) was seen for both incubation periods, 24 and 48 h (P < 0.01). A longer exponential phase and shorter
lag phase were also observed. The A540 values
at 24 h of incubation with medium containing glucose and an
inoculum of 105 CFU/ml were >0.4 U for all species, with
the exception of that for C. parapsilosis
(A540 = 0.26 ± 0.025). The MICs at
24 h determined by testing with 2% glucose and an inoculum of
105 CFU/ml showed the strongest agreement (96.83%) with
MICs determined by the reference method. MICs were not falsely
elevated, and good correlation indexes were obtained. The
reproducibility of results with this medium-inoculum combination was
high (intraclass correlation coefficient, 0.955). The best agreement
and reproducibility of results for spectrophotometric readings were
achieved with endpoints of 50% growth inhibition for flucytosine and
azoles and 95% for amphotericin B. Supplementation of test media with
glucose and an inoculum size of 105 CFU/ml yielded a
reproducible technique that shows elevated agreement with the reference
procedures and a shorter incubation period for obtaining reliable MIC
determinations. The spectrophotometric method offers an advantage over
the visual method by providing a more objective and automated MIC determination.
*
Corresponding author. Mailing address: Servicio de
Micología, Centro Nacional de Microbiología, Instituto
de Salud Carlos III, Ctra. Majadahonda-Pozuelo Km. 2, 28220 Majadahonda
(Madrid), Spain. Phone: 34-91-5097961. Fax: 34-91-5097966. E-mail:
mcuenca-estrella{at}isciii.es.
Journal of Clinical Microbiology, February 2001, p. 525-532, Vol. 39, No. 2
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.2.525-532.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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