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Crowley et al .: Journal of AOAC International Vol. 95, No. 5, 2012 1425
Evaluation of the VITEK® 2 Gram Positive (GP) Microbial
Identification Test Card: Collaborative Study
Erin C rowley, Patrick Bird, Kiel Fisher, Katherine G oetz, Megan Boyle, M. Joseph Benzinger, Jr, Marc Juenger,
James Agin, and David G oins
Q Laboratories, Inc., 1400 Harrison Ave, Cincinnati, OH 45214
Ronald L. Johnson1
bioMérieux, Inc., 595 Anglum Rd, Hazelwood, MO 63042
Collaborators: J. Canale, D. Cherney, M. Coakley, J. Colón-Reveles, J. Crowe, R. Daugherty, P. Dombroski, H.P. Dwivedi, J. Dyer,
C. Elems, J. Finnigan, G. Fraser, A. Garza, J. Glover, P. Hanson, M. Hayman, E. High, L. Johnson, M. Kelly, S. Kim, S.G. Kim,
M. Kingsley, M. McDonough, A. Miller, J. Mills, S. Montez, M.A. Murphy, G. Parra, U. Patel, L. Robertson, S. Robeson, J. Ruebl,
A. Saldana, Y. Salfinger, M. Segarra, B. Shaw, T. Sidebottom, D. Toney, H. Wang, P. Wikoff, K. Wilson
Submitted for publication March 1, 2012.
The recommendation was approved by the Methods Committee on
Microbiology as First Action. See “Standards News,” (2012) Inside
Laboratory Management, March/April issue.
1Corresponding author’s e-mail:
DOI: 10.5740/jaoacint.CS2012_02
FOOD BIOLOGICAL CONTAMINANTS
A collaborative study was conducted to evaluate
the performance of the VITEK® 2 Gram Positive
(GP) identification card for use with the VITEK 2
automated microbial identification system. The GP
test card is used in the identification of selected
Gram positive organisms, including Listeria and
Staphylococcus species. The VITEK 2 GP card is
based on 43 biochemical tests measuring carbon
source utilization, inhibition and resistance, and
enzymatic activities. A total of 20 laboratories
representing government, industry, and private
testing laboratories throughout the United States
participated. In this study, 720 Gram-positive
inclusivity isolates were analyzed by the GP
Identification method. Of the 720 well-characterized
isolates, 714 were identified correctly, zero were
misidentified, zero were unidentified, and six were
not characterized as a Gram-positive organism by
the VITEK 2 GP method. Additionally, 120 strains
exclusive of Gram-positive organisms were screened
by Gram stain. A total of 106 isolates were correctly
excluded. Fourteen organisms were incorrectly
characterized by Gram stain procedures, thus
resulting in improper analysis and misidentification
by VITEK GP. The VITEK 2 GP identification method
is an acceptable automated method for the rapid
identification of selected Gram-positive bacteria.
The rapid and accurate identification of foodborne
organisms is a critical component for ensuring the safety
of consumers. Traditional methods to detect foodborne
bacteria often rely on time-consuming growth in culture
media, followed by isolation, biochemical identification, and
sometimes serology (1). The VITEK 2® system addresses this
need by providing automated testing to identify microorganisms
present in raw materials, production environments, and
finished products. The technology of the VITEK 2 system
enables fast identification, within hours rather than the days
required for classical methods (2). Gram-positive organism
identification is achieved through the use of the Gram Positive
(GP) Identification Card. This card is intended to be used with
the VITEK 2 system for the automated identification of most
significant Gram-positive organisms. The GP card is a single-
use disposable and is based on established biochemical methods
and newly developed substrates measuring carbon source
utilization, resistance, and enzymatic activities. There are 43
biochemical tests and one negative control well within each
GP card. Identification results are available in approximately
8 h or less (3). The identification of the test organism is based
on the data and knowledge about the organism and reactions
being analyzed. As part of the identification process, the VITEK
2 software compares the test set of reactions to the expected
set of reactions of each organism. A qualitative value, referred
to as the percent probability of the identification result, is
calculated and reported alongside the identification result. This
value relates to how well the observed reactions compare to the
typical reactions of each organism (4).
The VITEK 2 GP test method was validated according to
AOAC guidelines for Performance Tested MethodSM (PTM)
and Official MethodSM (OMA) precollaborative studies for the
identification of Listeria and Staphylococcus species (5). In the
combined contract laboratory andAOAC independent laboratory
studies, 63 inclusivity strains representing seven Listeria species
and four Staphylococcal species claims (Listeria grayi, Listeria
innocua, Listeria ivanovii ssp. Ivanovii, Listeria ivanovii ssp.
Londoniensis, Listeria monocytogenes, Listeria seeligeri,
Listeria welshimeri, Staphylococcus hyicus, Staphylococcus
intermedius, Staphylococcus aureus, and Staphylococcus
epidermidis) from the VITEK 2 GP database and 40 exclusivity
strains were challenged for correct identification. All test strains
were tested from three recommended culture media: Trypticase
soy agar (TSA), Columbia agar with 5% sheep blood (CBA), and
Trypticase soy agar plates with 5% sheep blood (TSAB) at 12
and 48 h of incubation. Overall percent correct identifications in
the combined internal and independent studies from 12 h TSA,
CBA, and TSAB cultures were 98, 97, and 98%, respectively.
Overall correct identifications in the combined internal and
independent studies for all test strains from 48 h TSA, CBA,
Candidates for 2016 Method of the Year
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