Laboratory-based evaluation of the colorimetric VITEK-2 Compact ...

Antimicrobial Susceptibility Studies 

Laboratory-based evaluation of the colorimetric VITEK-2 Compact 

system for species identification and of the Advanced Expert System 

for detection of antimicrobial resistances: 

VITEK-2 Compact system identification and 

antimicrobial susceptibility testing 

Isamu Nakasone a, 4, Tohru Kinjo a , Nobuhisa Yamane b , Kyoko Kisanuki a , Chika M. Shiohira b 

Abstract 

a 

Clinical Laboratories, University Hospital of the Ryukyus, University of the Ryukyus, Nishihara-Nakagami, Okinawa 903-0215, Japan 

b 

Department of Laboratory Medicine, Graduate School and Faculty of Medicine, University of the Ryukyus, Okinawa 903-0215, Japan 

Received 4 October 2006; accepted 8 December 2006 

The newly redesigned colorimetric VITEK-2 Compact system with updated Advanced Expert System (AES) (bioMerieux, Marcy 

l’Etoile, France) was evaluated for its accuracy and rapidity to identify clinical isolates and to detect several antimicrobial resistances. 

Overall, the VITEK-2 gave 95.8% of compatibility with the reference API strips (bioMerieux) in the identifications (IDs) of Gram-positive 

cocci (GPC), Gram-negative rods (GNR), and yeasts. The accuracy was finally estimated to 98.3% through additional confirmatory tests. 

Also, N90% of IDs of GPC and GNR were obtained within 7 h of incubations. The VITEK AES correctly detected 97.7% of antimicrobial 

resistances, including extended-spectrum h-lactamases, oxacillin and inducible clindamycin resistances in staphylococci, vancomycin 

resistance in enterococci, and penicillin and erythromycin resistances in Streptococcus pneumoniae. The most resistant isolates were 

identified within 12 h of incubations. In conclusion, the new colorimetric VITEK-2 Compact system with AES greatly improved its accuracy 

in species ID and detection of antimicrobial resistances, and it will be highly acceptable to clinical microbiology laboratory function. 

D 2007 Elsevier Inc. All rights reserved. 

Keywords: VITEK-2 Compact system; Advanced Expert System; Antimicrobial resistance 

1. Introduction 

A series of the VITEK systems (bioMerieux, Marcy 

l’Etoile, France) has been a fully automated instrument that 

provides species identification (ID) and antimicrobial 

susceptibility testing (AST) for a variety of clinical isolates, 

and are presently used in many clinical microbiology 

laboratories worldwide. During the past 3 decades, several 

revisions have been introduced to the system, resulting in a 

Presented in part at the 106th General Meeting of American Society for 

Microbiology, Orlando, FL, May 2006, Abstract Current #C-008. 

4 Corresponding author. Tel.: +81-98-895-3331x3332; fax: +81-98- 

895-463. 

E-mail address: isamu@jim.u-ryukyu.ac.jp (I. Nakasone). 

0732-8893/$ – see front matter D 2007 Elsevier Inc. All rights reserved. 

doi:10.1016/j.diagmicrobio.2006.12.008 

Diagnostic Microbiology and Infectious Disease 58 (2007) 191–198 

www.elsevier.com/locate/diagmicrobio 

stepwise improvement of the system performance. Recently, 

extensive revisions, including reintroduction of colorimetric 

reading in lieu of fluorescence technology, and addition of 

several biochemical substrates and taxa covered by the 

broadened database comparable with the well-established 

API series (bioMerieux) are created (Funke and Funke- 

Kissling, 2004; Funke and Funke-Kissling, 2005; Aubertine 

et al., 2006). The efforts have been focused upon the 

accurate ID, in particular, to solve its inherent weakness in 

the IDs of glucose-nonfermentative Gram-negative rods 

(GNR) and members of the family Streptococcaceae 

(Joyanes et al., 2001; Gavin et al., 2002). In this communication, 

we describe the results to evaluate the accuracy of 

ID by the respective VITEK test cards on the VITEK-2 

Compact system and to detect several antimicrobial

192 

Table 1 

Accuracy in species ID by the VITEK-2 Compact system 

Species ID 

by API strip 

(no. of isolates 

tested) 

No. of 

tests 

agreed 

I. Nakasone et al. / Diagnostic Microbiology and Infectious Disease 58 (2007) 191–198 

Discrepant 

ID by the 

VITEK-2 

(no. of cases) 

Final ID 


was correct 

or incorrect 


S. aureus (21) 19 S. caprae (2) a 

S. caprae (2) correct b 

S. capitis (14) 14 

S. caprae (7) 7 

Staphylococcus 

cohnii (2) 

2 


epidermidis (18) 

18 


haemolyticus (9) 

9 


hominis (1) 

1 


lugdunensis (10) 

10 


saprophyticus (1) 

1 


schleiferi (3) 

3 


sciuri (2) 

2 

S. simulans (1) 0 S. capitis (1) S. capitis (1) correct 


warneri (1) 

1 

Enterococcus avium (8) 8 

E. casseliflavus (8) 8 

Enterococcus durans (1) 1 

E. faecalis (14) 14 

E. faecium (10) 10 

E. gallinarum (13) 12 E. faecium (1) E. faecium (1) correct 

Streptococcus 15 S. dysgalactiae (1) S. dysgalactiae 

agalactiae (16) 

(1) correct 

Streptococcus 14 S. parasanguis (1) S. parasanguis 

anginosus (15) 

(1) correct 

S. constellatus (4) 3 Streptococcus S. constellatus 

gordonii (1) (1) incorrect 

S. dysgalactiae (5) 5 

Streptococcus 20 Streptococcus S. oralis (1) incorrect 

mitis/oralis (21) 

sanguis (1) 

Streptococcus 

mutans (1) 

1 

S. parasanguis (2) 2 

S. pneumoniae (11) 11 

Streptococcus 

pyogenes (11) 

11 

Streptococcus 

salivarius (3) 

3 

S. sanguis (2) 1 S. parasanguis (1) S. parasanguis 

(1) correct 

Citrobacter freundii (7) 7 

Citrobacter koseri (5) 5 

Enterobacter 

aerogenes (5) 

5 

Enterobacter 

cloacae (6) 

6 

E. coli (12) 12 

Klebsiella 

oxytoca (5) 

5 

K. pneumoniae (7) 7 

Morganella 

morganii (5) 

5 

Table 1 (continued) 

Species ID 

by API strip 

(no. of isolates 

tested) 

No. of 

tests 

agreed 

Discrepant 

ID by the 



Final ID 


was correct 

or incorrect 


Proteus 

mirabilis (5) 

5 

Proteus 

vulgaris (5) 

5 

Providencia 

rettgeri (1) 

1 

Providencia 

stuartii (5) 

5 

Salmonella 

spp. (13) 

13 

Serratia 

marcescens (12) 

12 

Acinetobacter 

baumannii (11) 

11 

Acinetobacter 

junii (4) 

4 

A. lwoffii (2) 0 Alcaligenes A. lwoffii 

faecalis (2) (2) incorrect 

Aeromonas 

hydrophila (8) 

8 

Aeromonas 

sobria (2) 

2 

A. faecalis (5) 5 

Alcaligenes 

xylosoxidans (16) 

16 

Burkholderia 

cepacia (2) 

2 

Chryseobacterium 

indologenes (4) 

4 

Chryseobacterium 

meningosepticum (4) 

4 

Ochrobactrum 2 R. radiobacter (2) R. radiobacter 

anthropi (4) 

(2) correct 

P. aeruginosa (15) 15 

P. fluorescens (2) 0 P. aeruginosa (1), P. aeruginosa 

A. baumannii (1) (1) correct, 

P. fluorescens 

(1) incorrect 

Pseudomonas 

1 P. aeruginosa (1), P. aeruginosa 

putida (4) 

P. fluorescens (2) (1) correct, 

P. fluorescens 

(2) incorrect 

P. stutzeri (1) 0 S. paucimobilis (1) P. stutzeri 

(1) incorrect 

R. radiobacter (3) 3 

Sphingobacterium 0 S. paucimobilis (1) S. paucimobilis 

multivorum (1) 

(1) correct 

Candida albicans (24) 24 

Candida glabrata (9) 9 

Candida intermedia (1) 1 

Candida krusei (1) 1 

Candida 

parapsilosis (15) 

15 

Candida tropicalis (6) 6 

Trichosporon asahii (2) 2 

a Indicates the species identification (no. of cases) by the VITEK-2 but 

disagreed with the reference API identification. 

b Indicates the final identification with additional phenotypic testing 

and whether the ID results by the VITEK-2 was correct or incorrect.

esistances by the updated VITEK Advanced Expert System 

(AES) (Sanders et al., 2000; Barry et al., 2003). 

2. Materials and methods 

2.1. Isolates and testing by Vitek-2 Compact system 

I. Nakasone et al. / Diagnostic Microbiology and Infectious Disease 58 (2007) 191–198 193 

A total of 474 clinical isolates comprising 235 of Grampositive 

cocci (GPC), 181 of GNR, and 58 yeasts were 

included in the ID study. In addition, a total of 321 clinical 

isolates including 96 strains of Enterobacteriaceae, 107 of 

staphylococci, 61 of enterococci, and 57 of S. pneumoniae 

were used for the detection of specific antimicrobial 

resistances. All the isolates were the collection of clinical 

isolates stored 80 8C and were subcultured twice onto the 

Columbia agar plates supplemented with 5% sheep blood 

before testing. The inoculum suspension was prepared in 

0.45% saline, giving the equivalent of a 0.5-McFarland 

turbidity. The following respective VITEK test cards were 

filled with cell suspension according to the manufacturer’s 

instruction: GP for ID of GPC, GN for ID of GNR, YE for 

ID of yeasts, AST-N034 for AST of Enterobacteriaceae, 

AST-P546 for AST of staphylococci and enterococci, and 

AST-P518 for AST of S. pneumoniae. In this study, VITEK- 

2 Compact system with the software version V2C 1.01 

was used. 

2.2. Reference methods 

All the isolates included in ID study were identified by 

the respective API test strips as follows: ID 32 STAPH for 

staphylococci, RAPID ID 32 STREP for streptococci and 

enterococci, RAPID ID 32 E for Enterobacteriaceae, ID 

32 GN for glucose-nonfermentative GNR and members of 

the genus Aeromonas, and ID 32 C for yeasts. The API test 

strips were read by the autoreader, mini API (bioMerieux), 

and its database version 1.3.1. was used. When the VITEK- 

2 gave the discrepant ID result compared with the respective 

API strip, additional phenotypic tests were performed 

according to the algorithm established (Ruoff, 2003; 

Schreckenberger and Wong; 2003). The flowcharts, based 

on Gram stain characteristics and a selected number of 

additional easily performed enzymatic, biochemical, and 

biologic tests, determined which ID results were correct. 

The specific antimicrobial resistances evaluated include 

extended-spectrum h-lactamase (ESBL)–producing isolates 

of Escherichia coli and Klebsiella pneumoniae, oxacillin 

resistance and inducible clindamycin resistance among 

staphylococci, vancomycin resistance among enterococci, 

and penicillin resistance and erythromycin resistance among 

S. pneumoniae. As the reference, ESBL-producing isolates 

were first screened according to the initial disk screen test 

described (Clinical and Laboratory Standards Institute 

[CLSI], 2005, formerly National Committee for Clinical 

Laboratory Standards), then confirmed and classified on the 

basis of DNA amplification of the respective target genes by 

polymerase chain reaction (PCR) and agarose gel electro- 

phoresis of the PCR products. Detection of the bla gene 

sequences coding the TEM, SHV, and CTX-M enzymes were 

performed as previously described using the specific primer 

pairs (Yagi et al., 2000). For oxacillin-resistant staphylococci, 

detection of penicillin-binding protein 2a (PBP2a) by the 

latex agglutination, MRSA-Screen test (Denka-Seiken, 

Tokyo, Japan), was used (Cavassini et al., 1999; Horstkotte 

et al., 2001). Inducible clindamycin resistance was first 

phenotypically determined by an erythromycin (15-Ag disk)– 

clindamycin (2-Ag disk) double-disk test (D-zone test) and 

then was genetically confirmed for the presence of specific 

genes coding ermA and ermC by PCR as described (Khan 

et al., 1999; Volokhov et al., 2003). Vancomycin resistances 

among enterococci were determined by significant growth 

on vancomycin resistance enterococci screening agar plates 

(CLSI, 2003) and by PCR to detect vanA and vanB genes 

(Woodford et al., 1995). Penicillin resistance and erythromycin 

resistance of S. pneumoniae were determined by PCR 

for the respective genes of pbp1a, pbp2x, pbp2b, mefA, and 

ermB using the commercially available test reagents, 

penicillin-resistant S. pneumoniae gene detection version 

2.0 (Wakunaga Pharmaceutical, Hiroshima, Japan) (Ubukata 

et al., 1996; Ubukata et al., 2003). 

3. Results 

3.1. ID of clinical isolates by the VITEK-2 Compact system 

Table 1 shows the results when the VITEK-2 and the 

respective API strips comparatively identified a total of 

474 clinical isolates. Of 474 isolates tested, 454 (95.8%) 

were comparable IDs, resulting in 20 discrepant results 

comprising 9 of GPC and 11 of GNR. For the isolates 

belonging to Enterobacteriaceae and yeasts, all the ID 

results were completely identical to each other. For the 

Fig. 1. Cumulative distribution of time to require for final ID by the 

VITEK-2 Compact system. ! – ! = staphylococci; E–E = enterococci; 

z–z = streptococci; o–o = Enterobacteriaceae; 5–5 = glucosenonfermentative 

GNR. The isolates of Aeromonas spp. were included in 

glucose-nonfermentative GNR.

194 

Table 2 

Accuracy to detect specific resistant isolates by the VITEK AES 

Antimicrobial resistance and 

clinical isolates tested 

Nos. of 

isolates 

tested 

Interpretation by VITEK AES 

Enterobacteriaceae Staphylococci 

ESBL Non-ESBL MLSB 

inducible 

MLSB 

inducible 

negative 

Modification 

of PBP 

ESBLs 

E. coli (TEM type) 21 20 1 

E. coli (SHV type) 2 2 

E. coli (CTX-M type) 3 3 

K. pneumoniae (TEM type) 21 21 

K. pneumoniae (SHV type) 1 1 

K. pneumoniae 

(TEM and SHV type) 

Non-ESBL 

3 3 

E. coli 24 24 

K. pneumoniae 

D-zone test–positive and ermA 

and/or ermC-positive 

staphylococci 

21 21 

S. aureus 33 32 1 

S. epidermidis 8 8 

S. haemolyticus 3 3 

S. hominis 2 2 

D-zone test–negative and ermA- and 

ermC-negative S. aureus 

Oxacillin-resistant and 

PBP2a-positive staphylococci 

16 16 

S. aureus 27 27 

S. capitis 4 4 

S. epidermidis 30 30 

S. haemolyticus 3 3 


S. lugdunensis 1 1 

S. simulans 1 1 

S. warneri 

Oxacillin-susceptible and 

PBP2a-negative 

staphylococci 

1 1 

S. aureus 22 22 

S. capitis 8 8 


S. lugdunensis 5 5 

S. warneri 1 1 

Antimicrobial 

resistance and 

clinical isolates 

tested 


Nos. of 

isolates 

tested 

Interpretation by VITEK AES 

Enterococci S. pneumoniae 

Vancomycin resistance Modification of PBP 

VanA like VanB like Wild (VanC) High-level 

resistance 

vanA-positive enterococci 

E. casseliflavus 1 1 

E. faecalis 1 1 

E. faecium 18 18 

E. gallinarum 

vanB-positive enterococci 

1 1 


E. faecalis 14 14 


E. gallinarum 

vanA- and vanB-positive 

enterococci 

1 1 

Low-level 

resistance 

Nonmodification 

of PBP 

Resistant 

(MLSB) 

Wild


Table 2 (continued) 

Antimicrobial 

Nos. of Interpretation by VITEK AES 

resistance and 

clinical isolates 

tested 

isolates 

tested 

Enterococci 

Vancomycin resistance 

S. pneumoniae 

Modification of PBP 

VanA like VanB like Wild (VanC) High-level Low-level Resistant Wild 

resistance resistance (MLSB) 


E. gallinarum 

vanA- and vanB-negative 

enterococci 

3 3 


E. gallinarum 

Penicillin-resistant 

mutant 

6 6 

S. pneumoniae 

Penicillin-susceptible, wild 

40 28 12 

S. pneumoniae 

Erythromycin-resistant 

mutant 

17 2 15 

S. pneumoniae 

Erythromycin-susceptible, 

wild 

44 42 2 

S. pneumoniae 13 13 

discrepant results, additional phenotypic characterizations 

were performed and determined, in which ID result was 

correct. Of the 9 discrepant results for GPC, 7 ID results by 

VITEK-2 were correct, but the API ID 32 STAPH and 

RAPID ID 32 STREP resulted in misidentifications. Two 

isolates of Staphylococcus aureus, which were identified by 

ID 32 STAPH, resulted in the ID as Staphylococcus caprae 

by VITEK-2, and the IDs of VITEK-2 consisted of negative 

coagulase, negative clumping factor, and positive urease 

results. Also, one isolate was identified as Staphylococcus 

simulans by ID 32 STAPH, but VITEK-2 identified it as 

Staphylococcus capitis. Additional biochemical testing, acid 

from lactose, positive urease, and negative h-galactosidase 

supported the VITEK-2 ID result. One isolate of Enterococcus 

gallinarum, which was identified by RAPID ID 

32 STREP, resulted in Enterococcus faecium by VITEK-2, 

and nonmotility and not producing yellow pigment simply 

supported the VITEK-2 ID result. There were 5 discrepant 

results for streptococci. Of these, 3 results by VITEK-2, 

1 of Streptococcus dysgalactiae and 2 of Streptococcus 

parasanguis, were regarded as being correct IDs by the 

additional biochemical testing: negative CAMP test and 

nonfermentation of sorbitol, and positive h-glucosidase and 

fermentation of trehalose for S. dysgalactiae, and negative 

Voges–Proskauer test, and positive h-glucosidase and 

h-galactosidase for S. parasanguis. Finally, the VITEK-2 

correctly identified 233 (99.1%) isolates of GPC, compared 

with 228 (97.0%) isolates by the reference API strips, ID 32 

STAPH and RAPID ID 32 STREP. 

For GNR, a total of 11 discrepant results were obtained 

by the VITEK-2, and all the discrepant results came from 

the isolates of glucose-nonfermentative bacteria. Of these, 

5 ID results by VITEK-2 were finally concluded as being 

correct: 2 isolates of Rhizobium radiobacter with negative 

gas production from nitrate, positive nitrate reduction, and 

positive O-nitrophenyl-h-d-galactopyranoside; 2 isolates of 

Pseudomonas aeruginosa with significant growth at 428C, 

positive hydrolysis of acetamide, and gas production from 

nitrate; and 1 isolate of Sphingomonas paucimobilis with 

positive motility, positive urease, and susceptibility to 

polymyxin B. However, there remained 6 misidentifications 

confirmed, and they were 2 isolates of Acinetobacter lwoffii, 

3 isolates of Pseudomonas fluorescens, and 1 isolate of 

Pseudomonas stutzeri. Overall, the VITEK-2 correctly 

identified 175 (96.7%) isolates of GNR, compared with 

176 (97.2%) isolates by the reference API, RAPID ID 32 E 

and ID 32 GN. 

Fig. 1 indicated the cumulative distribution of time to 

require for final ID by the VITEK-2 Compact system for the 

respective bacterial groups. For all the groups of the isolates 

tested, N50% of ID results were obtained within 4 to 6 h, 

and all the final ID results were completed after 7- to 10-h 

incubation cycles. 

3.2. Detection of specific antimicrobial resistances by 

the AES 

The interpretation results by the VITEK AES to detect 

specific antimicrobial resistances were summarized in 

Table 2. Of 51 ESBL-producing isolates of E. coli and 

K. pneumoniae, the VITEK AES correctly identified 

50 isolates (98.0%) and missed 1 isolate of TEM-type 

ESBL-positive E. coli, which was interpreted as an acquired 

penicillinase plus cephalosporinase-producing isolate. However, 

all the ESBL-nonproducing isolates were correctly 

interpreted as being non-ESBL isolates, results indicating 

98.0% sensitivity and 100% specificity. Also, the VITEK 

AES produced highly correlative interpretations to detect 

positive D-zone test associated with macrolide, lincosamide,

196 


Fig. 2. Cumulative distribution of time to detect the respective antimicrobial 

resistance by the VITEK AES. ! – ! = ESBL-producing isolate; n–n = 

inducible clindamycin-resistant staphylococci positive for D-zone test and 

ermA and/or ermC genes; E–E = oxacillin-resistant staphylococci 

positive for PBP2a; z–z = vancomycin-resistant enterococci; o–o = 

penicillin-resistant S. pneumoniae; 5–5 = erythromycin-resistant 

S. pneumoniae. 

and type B streptogramin (MLSB) resistance coded by 

ermA and/or ermC genes and oxacillin resistance positive 

for PBP2a among staphylococci. Of the 46 isolates of 

staphylococci positive for D-zone test and also positive for 

ermA and/or ermC genes, 45 (97.8%) were correctly 

interpreted as bMLSB inducibleQ by VITEK AES, whereas 

all the 16 isolates, which were negative in the respective 

reference tests, were reported as bMLSB inducible negativeQ. 

One remaining isolate positive for both D-zone test 

and ermA gene was reported as being constitutively resistant 

to macrolide and streptogramin. A total of 69 oxacillinresistant 

staphylococcal isolates positive for PBP2a and 

38 oxacillin-susceptible isolates negative for PBP2a were 

tested by VITEK-2, the AES interpretations giving none of 

discrepant result with the reference tests. 

Some significant discrepant results by the VITEK AES 

were demonstrated in detecting vancomycin resistance 

among enterococci. All the 25 isolates positive for vanA 

gene, including 4 isolates positive for both vanA and vanB, 

were correctly identified as bVanA-likeQ. Also, 25 vanBpositive 

isolates of Enterococcus faecalis and E. faecium 

gave consistent interpretations. However, 3 isolates of 

Enterococcus casseliflavus and 1 of E. gallinarum positive 

for vanB gene were incorrectly identified as bwild (VanC)Q 

by the VITEK AES. The MICs determined by the VITEK-2 

were 32 Ag/mL (2 strains) and 8.0 Ag/mL (1 strain) for 

E. casseliflavus and 32 Ag/mL for E. gallinarum. For 

the penicillin resistance among S. pneumoniae, all the 

isolates with mutations on penicillin-binding protein genes, 

pbp1a, pbp2x, and/or pbp2b, were correctly identified as 

bmodification of PBPQ with either high-level or low-level 

resistances. However, 2 of 17 wild susceptible isolates were 

incorrectly interpreted as low-level resistance with modification 

of PBP. Also, all the 13 wild susceptible isolates of 

S. pneumoniae for the erythromycin resistance determinant 

genes were correctly identified as bwildQ, but 2 isolates 

positive for ermB gene resulted in incorrect interpretations, 

bwildQ. 

Fig. 2 indicated the cumulative distribution of time to 

detect antimicrobial resistances by the VITEK AES. More 

than 50% of all the resistant isolates were interpreted within 

7 to 9 h of incubation cycles. The testing of GPC, in general, 

required longer incubation period; however, most resistant 

isolates, including vancomycin-resistant enterococci and 

oxacillin-resistant staphylococci, were correctly determined 

within 12 h of incubations. 

4. Discussion 

Overall, the evaluation results of the newly redesigned 

colorimetric VITEK-2 ID impressed us by the performance 

because more than 98% of the isolates were correctly 

identified to the species level without any further additional 

tests. Also, our obtained results indicated that the current 

VITEK-2 has overcome its inherent weakness in IDs of 

streptococci and glucose-nonfermentative GNR. Until 

present, API test strips has been long considered as the 

bgold standardQ in ID test (Fortin et al., 2003; Aubertine 

et al., 2006), but the accuracy of the VITEK-2 was finally 

estimated to be 98.3%, compared with 97.5% by the 

respective API test strips. Our obtained results were highly 

consistent with a series of evaluation results recently 

published for GPC (Funke and Funke-Kissling, 2005), 

GNR (Funke and Funke-Kissling, 2004), and yeast 

(Aubertine et al., 2006). When compared with the previous 

VITEK ID based on fluorescent technology, the current 

VITEK-2 ID broadened the database concerning the 

relativity to reaction tests and taxa identified; the expanded 

database corresponds to GP containing 43 tests for 115 taxa, 

GN containing 47 tests for 159 taxa, and YE containing 

46 tests for 53 species and 14 genera. Although our 

evaluation study did not cover all the taxa included in the 

database and very small numbers of the clinical isolates for 

some species were included, it became apparent that the 

extension of the database led significantly improved ID 

accuracy rather than poorer ID results. However, there are 

still several misidentifications in the results of isolates 

belonging to the family Streptococcaceae and glucosenonfermentative 

GNR: 1 isolate each of Streptococcus 

constellatus and Streptococcus oralis in GP, 2 isolates of 

A. lwoffii, 3ofP. fluorescens, and 1 of P. stutzeri in GN. 

Both bacterial groups are taxonomically diverse and are still 

problematic in phenotypic ID tests. Gene sequence analysis 

of 16S ribosomal DNA or RNA demonstrates considerable 

heterogeneity, and the original classification of the genera 

has undergone extensive revision (Kawamura et al., 1995; 

Kersterns et al., 1996). Although the current VITEK-2 also

has inherent limitations in the ID of the abovementioned 

groups, addition of biochemical reactions and broadened 

database enable us to provide more accurate ID results 

comparable with the up-to-date taxonomy. 

The VITEK AES has been created to analyze the AST 

results using the well-established knowledge base of 

approximately 100 species and 20000 ranges of MIC to 

detect more than 2300 phenotypic antimicrobial resistances. 

VITEK-2 and AES have been evaluated in several countries 

(Sanders et al., 2000; Barry et al., 2003), the results 

described indicating that the AES detected and interpreted 

resistance mechanisms appropriately and would provide aids 

for therapeutic choice and accurate epidemiologic analysis. 

In our evaluation, we included 6 clinically important 

antimicrobial resistances well characterized by genetic and 

phenotypic reference methods. The accuracy of AES was 

estimated to be 97.7%, and 10 isolates, comprising 1 isolate 

each of TEM-type ESBL-producing E. coli and inducible 

MLSB-resistant S. aureus, 2 isolates each of penicillinsusceptible 

wild S. pneumoniae and erythromycin-resistant 

S. pneumoniae, and 4 isolates of VRE, were misidentified. 

In particular, all the 4 isolates of E. casseliflavus and 

E. gallinarum positive for vanB gene were interpreted as 

wild (VanC). AES uses enterococcal ID and MICs against 

vancomycin and teicoplanin to characterize VRE isolates. 

The MIC results for the above 4 isolates were interpreted to 

be resistant to vancomycin and susceptible to teicoplanin 

similar to the other vanB-positive E. faecalis and E. faecium. 

However, when the VITEK identifies the test isolate as being 

E. casseliflavus or E. gallinarum, the VITEK AES automatically 

reports the message bwildQ (VanC). Both VanA and 

VanB phenotypes are most commonly detected in E. faecalis 

and E. faecium but have been found in other species (Clark 

et al., 1993). Also, discrepancy between VanB phenotype 

and vanA genotype was recently reported (Song et al., 2006). 

Thus, refinements in AES algorithm should be urgent to 

improve accuracy to detect a variety of VRE phenotypes. 

5. Conclusions 


The colorimetric VITEK-2 Compact system achieved 

an excellent performance to provide accurate species ID 

results. In the evaluation, 466 (98.3%) of 474 clinical 

isolates, including a variety of species of staphylococci, 

enterococci, streptococci, Enterobacteriaceae, glucose-nonfermentative 

GNR, and yeast, were correctly identified. Also, 

the VITEK AES provided interpretations comparable with 

phenotypic and genotypic characterizations to determine 

specific antimicrobial resistances such as ESBL, inducible 

MLSB- and oxacillin-resistant staphylococci, VRE, and 

penicillin- and erythromycin-resistant S. pneumoniae. Overall, 

the accuracy to detect antimicrobial resistances evaluated 

was estimated to be 431 of 440 (98.0%). Although our study 

has limitations on the taxa and on the numbers of the isolates 

included, it can be concluded that the current colorimetric 

VITEK-2 combined with AES will greatly contribute to 

laboratory function in the field of clinical microbiology. 

Acknowledgments 

The authors thank Miyako Higa and Fusako Furugen for 

their valuable technical assistance and Yukiko Izumi for her 

help in the preparation of the manuscript. 

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Laboratory-based evaluation of the colorimetric VITEK-2 Compact ...

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