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Journal of Clinical Microbiology, March 2005, p. 1251-1255, Vol. 43, No. 3
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.3.1251-1255.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Analysis of the Influence of Tween Concentration, Inoculum Size, Assay Medium, and Reading Time on Susceptibility Testing of Aspergillus spp.

Alicia Gomez-Lopez, Amel Aberkane, Eva Petrikkou, Emilia Mellado, Juan Luis Rodriguez-Tudela, and Manuel Cuenca-Estrella*

Servicio de Micología, Centro Nacional de Microbiología, Instituto de Salud Carlos III, Madrid, Spain

Received 13 August 2004/ Returned for modification 14 September 2004/ Accepted 18 November 2004

The influence of several test variables on susceptibility testing of Aspergillus spp. was assessed. A collection of 28 clinical isolates was tested against amphotericin B, itraconazole, voriconazole, and terbinafine. Inoculum size (104 CFU/ml versus 105 CFU/ml) and glucose supplementation (0.2% versus 2%) did not have significant effects on antifungal susceptibility testing results and higher inoculum size and glucose concentration did not falsely elevate MICs. In addition, antifungal susceptibility testing procedure with an inoculum size of 105 CFU/ml distinctly differentiated amphotericin B or itraconazole-resistant Aspergillus strains in vivo from the susceptible ones. Time of incubation significantly affected the final values of MICs, showing major increases (two to six twofold dilutions, P < 0.01 by analysis of variance) between MIC readings at 24 and 48 h, but no differences were observed between antifungal susceptibility testing results obtained at 48 h and at 72 h. Significantly higher MICs were uniformly associated with higher concentrations of Tween (P < 0.01), used as a dispersing agent in the preparation of inoculum suspensions. The geometric mean MICs showed increases of between 1.5- and 10-fold when the Tween concentration varied from 0.1% (the geometric means for amphotericin B, itraconazole, voriconazole, and terbinafine were 1.29, 0.69, 1.06, and 0.64 µg/ml, respectively) to 5% (the geometric means for amphotericin B, itraconazole, voriconazole, and terbinafine were 1.97, 5.79, 1.60, and 4.66 µg/ml, respectively). The inhibitory effect of Tween was clearly increased with inoculum sizes of 105 CFU/ml and was particularly dramatic for itraconazole, terbinafine, and Aspergillus terreus. The inoculum effect was not observed when the Tween concentration was below 0.5% (P > 0.01).

* 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, March 2005, p. 1251-1255, Vol. 43, No. 3
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.3.1251-1255.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Cuenca-Estrella, M., Alastruey-Izquierdo, A., Alcazar-Fuoli, L., Bernal-Martinez, L., Gomez-Lopez, A., Buitrago, M. J., Mellado, E., Rodriguez-Tudela, J. L. (2008). In Vitro Activities of 35 Double Combinations of Antifungal Agents against Scedosporium apiospermum and Scedosporium prolificans. Antimicrob. Agents Chemother. 52: 1136-1139 [Abstract] [Full Text]  
  • Rodriguez-Tudela, J. L., Alcazar-Fuoli, L., Alastruey-Izquierdo, A., Monzon, A., Mellado, E., Cuenca-Estrella, M. (2007). Time of Incubation for Antifungal Susceptibility Testing of Aspergillus fumigatus: Can MIC Values Be Obtained at 24 Hours?. Antimicrob. Agents Chemother. 51: 4502-4504 [Abstract] [Full Text]  
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