Journal of Clinical Microbiology, September 2008, p. 3184-3185, Vol. 46, No. 9
0095-1137/08/$08.00+0 doi:10.1128/JCM.01107-08
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In Vitro Susceptibility of Invasive Isolates of Candida spp. to Anidulafungin, Caspofungin, and Micafungin: Six Years of Global Surveillance
M. A. Pfaller, L. Boyken, R. J. Hollis, J. Kroeger, S. A. Messer, S. Tendolkar, and D. J. DiekemaDepartments of Pathology and Medicine, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, Iowa
Volume 46, no. 1, p. 150-156, 2008. In our recent report describing the in vitro susceptibility of 5,346 Candida spp. to the echinocandins, we noted the excellent activity of the echinocandins and the lack of evidence of emerging echinocandin resistance over 6 years of global surveillance. We also described 12 (out of a total of 2,869) isolates of Candida albicans that demonstrated a high-anidulafungin-MIC phenotype, which we defined as an anidulafungin MIC of 2 µg/ml. We noted that these isolates, while still susceptible according to the recently approved Clinical and Laboratory Standards Institute (CLSI) breakpoint of 
2 µg/ml (minutes of the June 2007 meeting of the CLSI Antifungal Subcommittee), were nonetheless of interest because their echinocandin MICs were distinctly higher than the normal wild-type distribution of echinocandin MICs for C. albicans (page 152, column 2, lines 1-18).
While pulling these 12 isolates from our bank for further evaluation, we noted that none of them demonstrated the characteristic green color of C. albicans on CHROMagar Candida (Becton Dickinson and Company, Sparks, MD), yet the Vitek Yeast Biochemical Card (YBC; bioMerieux, Inc., Hazelwood, MO) identified each isolate as C. albicans. To further evaluate these isolates, we performed randomly amplified polymorphic DNA (RAPD) analysis (A. Tavanti, A. D. Davidson, N. A. Gow, M. C. Maiden, and F. C. Odds, J. Clin. Microbiol. 43:284-292, 2005). All 12 isolates were confirmed to be C. parapsilosis by RAPD analysis.
Further evaluation of these isolates (with the assistance of David Pincus and Nancy Moss in the research and development group at bioMerieux, Inc.) revealed that all demonstrated the same atypical pattern for the Vitek YBC database (for C. parapsilosis), specifically for three reactions, those involving alpha methyl-D-glucoside (AMG), melezitose (MLZ), and xylitol (XLT). All 12 isolates were AMG negative, MLZ negative, and XLT positive in our hands. Based upon these three atypical reactions, the YBC algorithm generated C. albicans as the best match for each isolate. These findings were confirmed by sending the isolates to bioMerieux, Inc., for testing, which revealed highly variable AMG statuses (with only 3 of the 12 isolates being strongly negative), negative MLZ results for 11 of the 12 isolates, and positive XLT results for all 12 isolates. When tested with the API 20-C system, 11 of the 12 isolates were correctly identified as C. parapsilosis, and when tested on the Vitek 2 YST card, all 12 were correctly identified, with 3 of the 12 giving a "low discrimination" call with C. famata.
We regret that we did not detect this species identification error prior to publication. At the time of the initial receipt and characterization of the isolates, all were identified as C. albicans based upon the Vitek YBC result. We are in the process of systematically assessing the error rate of our Vitek YBC system for identification of C. parapsilosis, in particular the rare misidentification of C. parapsilosis as C. albicans. Preliminary findings suggest that this is an uncommon event, occurring for fewer than 2% of all C. parapsilosis isolates tested on the YBC (data not shown). In each case of misidentification we have detected thus far, the CHROMagar appearance of the organism is not consistent with that of C. albicans. We no longer accept a Vitek YBC result of C. albicans for an isolate without a characteristic CHROMagar appearance. In addition, we confirm the identification of all Candida species isolates with any echinocandin MIC of >1 µg/ml by means including the use of molecular methods as required (S. R. Lockhart, S. A. Messer, M. A. Pfaller, and D. J. Diekema, J. Clin. Microbiol., 46:2659-2664, 2008).
The misidentification of the 12 C. parapsilosis isolates does not change any of the aggregate susceptibility data presented in our article, since all of the isolates in question were susceptible to the echinocandins, nor does it alter the MIC50 and MIC90 data reported or the conclusions of the study, which were that all three echinocandins have excellent in vitro activity against invasive isolates of Candida from centers worldwide and that our prospective sentinel surveillance reveals no evidence of emerging echinocandin resistance among invasive clinical isolates of Candida. These findings further emphasize the potent in vitro activity of the echinocandins against C. albicans.
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Page 151, Table 1: The entries for C. albicans and C. parapsilosis should appear as shown below.
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Page 152, Table 2: The entries for C. albicans and C. parapsilosis should appear as shown below.
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Pages 153-154, Table 3: The entries for C. albicans and C. parapsilosis should appear as shown below.
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Journal of Clinical Microbiology, September 2008, p. 3184-3185, Vol. 46, No. 9
0095-1137/08/$08.00+0 doi:10.1128/JCM.01107-08
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