Journal of Clinical Microbiology, November 1998, p. 3396-3398, Vol. 36, No. 11
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Efficacy of API 20C and ID 32C Systems for
Identification of Common and Rare Clinical Yeast Isolates
Rama
Ramani,1
Sally
Gromadzki,1
David H.
Pincus,2
Ira F.
Salkin,1 and
Vishnu
Chaturvedi1,*
Wadsworth Center, New York State Department
of Health, Albany, New York,1 and
bioMérieux Vitek, Inc., Hazelwood,
Missouri2
Received 9 June 1998/Returned for modification 23 July
1998/Accepted 18 August 1998
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ABSTRACT |
The abilities of the API 20C and ID 32C yeast identification
systems to identify 123 common and 120 rare clinical yeast isolates were compared. API 20C facilitated correct identification of 97% common and 88% rare isolates while ID 32C facilitated correct identification of 92% common and 85% rare isolates.
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TEXT |
The incidence of fungal infections
has increased significantly over the past two decades. The National
Nosocomial Infections Surveillance System (Atlanta, Ga.) has found a
sharp rise in the incidence of yeast fungemias between 1980 and 1989 (1). Since many of the yeasts associated with human
infections have been found to be innately resistant or to develop
resistance to the most common antifungal agents, the introduction of
appropriate therapy for these invasive infections depends largely on
the rapid and accurate identification of the etiologic agents. For
example, Candida lusitaniae has been reported to be
resistant in vitro to amphotericin B, while Candida
krusei and Candida glabrata have been found to be
resistant to fluconazole (2, 3, 13, 15).
The classical methods used in the identification of yeast pathogens,
such as the Wickerham and auxanographic techniques, are time consuming
and technically complex (11). The increased incidence of
yeast infections has stimulated the development of rapid and accurate
manual and automated commercial systems for the identification of these
pathogens. Ideally, these products should have the following features:
(i) rapid and precise identification of yeasts isolated from all types
of clinical specimens; (ii) ease of handling and inoculation to permit
rapid processing of multiple isolates; and (iii) an ability to identify
isolates that are less commonly recovered from specimens. The aim of
this study was to compare the efficacy of the ID 32C system
(bioMérieux, Marcy l'Etoile, France) commonly used in European
countries to that of the API 20C yeast identification system
(bioMérieux Vitek, Inc., Hazelwood, Mo.), one of the most commonly used yeast identification systems in the United States.
Yeasts.
A total of 243 isolates of yeasts and yeast-like
organisms comprising 123 commonly recovered isolates and 120 isolates
of less frequently encountered taxa were used in these studies
(see Tables 1 and 2). The yeasts included in these studies had
been definitively identified by conventional biochemical and
physiological methods (11). Yeasts and yeast-like organisms
that were found in 
1% of all yeast-positive clinical specimens were
considered common clinical isolates (12a). The organisms
were maintained on potato dextrose agar slants at 
20°C in the
culture collection of the Mycology Laboratories, Wadsworth
Center, New York State Department of Health, Albany, N.Y.
Additional clinical isolates came from bioMérieux
Vitek, Inc., and were maintained in tryptic soy broth-10% glycerol at
70°C or in sterile distilled water at room temperature. Isolates to
be studied were serially subcultured at least twice on 100-mm petri
plates containing 25 ml of Sabouraud glucose agar at 30°C for 18 to
24 h prior to the inoculation of the two identification systems.
API 20C.
All yeast identification procedures were conducted in
accordance with the manufacturer's instructions. Portions of growth of
each isolate were aseptically transferred from a freshly inoculated stock culture to an ampule of API 20C basal medium and then emulsified to give a density of 1+ on a Wickerham card. Each well of the API 20C strip was inoculated with the suspension, and the strip was
placed in the incubation tray provided by the manufacturer, covered
loosely with a lid, and incubated at 30°C for 72 h. Reactions were visually examined at 72 h and determined to be positive or negative based on the presence or absence of turbidity in the carbohydrate wells. A seven-digit biocode was generated on the basis of
these observations by assigning a weighted score to positive reactions.
These codes were then compared to those listed in the API 20C
Analytical Profile Index. Identifications listed in the index as
excellent, very good, or acceptable were accepted as correct, and no
supplemental tests were conducted. Additionally, presumptive good
likelihood (low selectivity) identifications that required microscopic
morphology for confirmation were considered correct without the need of
supplemental testing. In contrast, supplemental tests, e.g.,
KNO3 utilization, growth at 42°C, and urease production,
were used to confirm remaining presumptive or questionable
identifications.
ID 32C.
This system consists of a single-use disposable
plastic strip with 32 wells containing substrates for 29 assimilation
tests (carbohydrates, organic acids, and amino acids), one
susceptibility test (cycloheximide), one colorimetric test (esculin),
and a negative control. The yeast identification procedures were
conducted in accordance with the manufacturer's instructions. A
portion of growth from well-isolated colonies of each isolate was
aseptically transferred from a freshly inoculated stock culture to
sterile distilled water to prepare a suspension with a final turbidity equivalent to McFarland standard #2. Five drops of this suspension was
then dispensed to an ampule of C medium provided by the manufacturer and homogenized to prepare an even dispersion of inoculum. After homogenizing, the inoculum suspension was used to inoculate the wells
in the strip, the lid of the strip was replaced, and the system was
incubated at 30°C for 48 h. The strips were then visually examined, and growth was determined to be positive or negative based
upon the presence or absence of turbidity in the wells. The results
were transformed into numerical biocodes, and the isolates were
identified through the use of the ID 32C Analytical Profile Index.
Quality control.
Each system was tested with the
manufacturer's recommended quality control test organisms before
starting the formal evaluation. These included Cryptococcus
laurentii (ATCC 18803) and Blastoschizomyces capitatus (ATCC 10663) for the API 20C system and
Candida guilliermondii (ATCC 6260) for the ID 32C
system.
API 20C results.
Correct identifications were obtained for 114 (93%) of the common yeast isolates with the API 20C without
using supplementary tests. Of the remaining isolates, five (4%) were
appropriately identified with additional tests (Table
1). In contrast, 105 (88%) of the rarely
encountered isolates were identified correctly with no supplemental
tests, and an additional 2 (2%) isolates were correctly identified
when the manufacturer's recommended supplemental tests were used in
conjunction with the assimilation profiles (Table
2). The biocodes of several rarely
recovered isolates (e.g., Candida sake, Pichia
membranaefaciens, and Zygosaccharomyces species)
generated assimilation patterns in the API 20C which were not
listed in its Analytical Profile Index and were recorded as
no identification.
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