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Journal of Clinical Microbiology, August 2001, p. 2964-2966, Vol. 39, No. 8
Department of Microbiology, The Chinese
University of Hong Kong, Prince of Wales Hospital, Hong Kong, SAR,
People's Republic of China
Received 9 April 2001/Returned for modification 8 May 2001/Accepted 18 May 2001
A total of 281 strains of miscellaneous members of the
family Enterobacteriaceae, Pseudomonas
aeruginosa, and other gram-negative bacteria were evaluated by
use of identification tests with the VITEK 2 system
(bioMérieux) and an API identification system (bioMérieux). A total of 237 (95%) strains were correctly
identified to the species level. Only six (2.1%) strains were
misidentified, and eight (2.8%) strains were not identified. Among 14 strains with discrepant identifications, 8 (57.1%) strains were
nonfermenters. The susceptibilities of 228 strains to 11 antibiotics
including amikacin, netilmicin, tobramycin, gentamicin, ciprofloxacin,
imipenem, meropenem, ceftazidime, cefepime,
piperacillin, and piperacillin in combination with tazobactam were
tested with the VITEK 2 AST-No. 12 card
and by the broth microdilution (MB) method, according to NCCLS
guidelines, as a reference. For the 2,508 organism-antibiotic combinations, the rates at which duplicate MICs correlated within ±1 dilution ranged from 84.2 to 95.6%. Only 13 (0.5%) and
10 (0.4%) of the susceptibility tests gave major errors
(resistant with the VITEK 2 system but sensitive by the MB method)
and very major errors (sensitive with the VITEK 2 system but
resistant by the MB method), respectively. Both VITEK 2 ID-GNB (an
identification system) and VITEK 2 AST-No. 12 (a susceptibility testing
system) card systems gave rapid, reliable, and highly reproducible results.
Automated bacterial identification
and susceptibility testing systems have been developed and
commercialized for more than two decades, but only a few of them are
available on the market (1, 2). The VITEK 2 system
(bioMérieux), which uses a new fluorescence-based technology, was
evaluated for the identification and susceptibility testing of
gram-negative clinical isolates.
Clinical isolates were collected from 1996 to 1999 in the Prince of
Wales Hospital, Hong Kong, People's Republic of China, and were stored
in cryotubes at A total of 281 strains of miscellaneous members of the family
Enterobacteriaceae (n = 173),
Pseudomonas aeruginosa (n = 23), and other
gram-negative bacteria (n = 85) were tested with the VITEK 2 ID-GNB cards. The reference identification was obtained with
the API 20E system (bioMérieux) (3).
The susceptibilities of 228 strains to 11 antibiotics were tested with
the VITEK 2 AST-No. 12 card and by the broth microdilution (MB) method,
according to NCCLS guidelines, as a reference (4). The
reference MIC was determined by the MB method (MIC-2000 System; Dynatech, McLean, Va.) with Muller-Hinton broth (Oxoid,
Baskingstoke, United Kingdom) with an inoculum size of
105 CFU/ml. The following antibiotics were
supplied as powders of stated potency and were obtained from the
indicated companies: amikacin, Bristol-Myers Squibb; netilmicin,
Scherling Plough; tobramycin, Sigma; gentamicin, Sigma;
ciprofloxacin, Bayer; imipenem, Merck Sharp & Dohme;
meropenem, AstraZeneca; ceftazidime, Glaxo Wellcome;
cefepime, Bristol-Myers Squibb; piperacillin, Wyeth; and
piperacillin in combination with tazobactam, Wyeth. Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were incorporated as quality control strains. All strains were allowed
to be tested only if the results were correct for all quality
control strains. All strains with discrepant results were retested in
duplicate by both methods.
Among 281 strains (85 nonfermenters) tested, 267 (95%) strains were
correctly identified to the species level (Table
1). Only six (2.1%) strains were
misidentified, and eight (2.8%) strains were not identified. Among the
strains with discrepant results, eight were nonfermenters (Table
2). The VITEK 2 AST-No. 12 card gave
highly reproducible antibiotic susceptibility testing results. For the
2,508 organism-antibiotic combinations (composed of 228 strains tested
against 11 antibiotics), the rates at which duplicate MICs correlated
within ±1 dilution ranged from 84.2 to 95.6%. Only 13 (0.5%) and 10 (0.4%) tests gave major errors (resistant with the VITEK 2 system but
sensitive by the MB method) and very major errors (sensitive with the
VITEK 2 system but resistant by the MB method), respectively (Table
3).
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.8.2964-2966.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Evaluation of VITEK 2 Rapid
Identification and Susceptibility Testing System against Gram-Negative
Clinical Isolates
ABSTRACT
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70°C. The stock culture strains were then
subcultured onto MacConkey agar plates to check their purity. The
turbidity of the bacterial suspensions was adjusted with a densitometer
to match that of a McFarland 0.5 standard in 0.45% sterile sodium
chloride solution. The time interval between suspension
preparation and card filling was less than 30 min to avoid changes in
turbidity. Afterward, the VITEK 2 ID-GNB cards, AST-No. 12 cards, and bacterial suspension were manually loaded into the VITEK 2 system. Each test card was automatically filled with a bacterial
suspension, sealed, and incubated for 3 h. During this period, the
cards were read by kinetic fluorescence measurement every 15 min. The
VITEK 2 system software first analyzed the data and then
reported the results automatically.
TABLE 1.
Organisms with correct identification with the VITEK
2 system
TABLE 2.
Organisms with discrepant identifications with the
VITEK 2 system
TABLE 3.
Agreement of MB method and VITEK system for 228 clinical isolates
In the present study, 8 of 14 (57.1%) organisms with discrepant identifications with the VITEK 2 system (either misidentified or not identified) were nonenteric bacteria such as Pseudomonas spp. (4), Acinetobacter spp. (3), and Alcaligenes (1). The slower rate of metabolism of nonenteric bacteria could cause weaker fluorescent biochemical reactions in the reaction wells of VITEK 2 ID-GNB cards, and this may cause more discrepant identifications with the VITEK 2 system. This evaluation showed that VITEK 2 system could identify members of the family Enterobacteriaceae and fermenters better. An evaluation performed by Funke et al. (1) also showed that the rate of rapid and reliable identification of gram-negative isolates to the species level with the VITEK 2 system is 84.7%. However, both studies revealed that improvements should be made to identify nonenteric organisms and nonfermenters with slower metabolisms.
After comparison of the susceptibility testing results obtained with the VITEK 2 AST-No. 12 cards with those obtained by the MB method, major errors (resistant with the VITEK 2 system but sensitive by the MB method) and very major errors (sensitive with the VITEK 2 system but resistant by the MB method) were randomly distributed among the tests with the 11 drugs listed. No specific pattern was found. Another study performed by Traczewski et al. (M. M. Traczewski, A. L. Barry, S. D. Brown, J. A. Hingler, D. A. Bruckner, and D. F. Sahm, Abstr. 98th Gen. Meet. Am Soc. Microbiol. 1998, p. 27-28, 1998) showed that the rate of agreement of MICs (±1 dilution) from the susceptibility testing results for all cephalosporins tested against gram-negative isolates was over 90%, which is similar to those obtained in our study, which ranged between 84.7 and 95.6%.
During the evaluation, the overall performance of the VITEK 2 system was satisfactory except for a few minor mechanical defects. Several advantages of the VITEK 2 system can be mentioned. First, it is a closed system that can avoid unwanted cross-contamination or environmental contamination. Second, it has a reliable recheck system that can detect and immediately cease operation of the VITEK 2 system if a specimen card is misplaced on the specimen cartridge. Third, the VITEK 2 system is able to handle dozens of specimens automatically at the same time. It is also easy for laboratory staff to prepare and load bacterial specimens. The decreased turnaround and hand-on times greatly improve the efficiencies of routine clinical laboratories. In conclusion, both the VITEK 2 ID-GNB (an identification system) and VITEK 2 AST-No. 12 (a susceptibility testing system) card systems gave rapid, reliable, and highly reproducible results.
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ACKNOWLEDGMENTS |
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We thank bioMérieux (bioMérieux China Limited) for kindly providing the VITEK 2 system and the ID-GNB and AST-No. 12 cards.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Microbiology, The Chinese University of Hong Kong, Prince of Wales Hospital, Hong Kong, SAR, People's Republic of China. Phone: 852-26322307. Fax: 852-26451256. E-mail: lingt{at}cuhk.edu.hk.
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Journal of Clinical Microbiology, August 2001, p. 2964-2966, Vol. 39, No. 8
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.8.2964-2966.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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