Our objective was to compare the ertapenem minimal inhibitory concentrations (MICs) for Enterobacter cloacae isolates categorized intermediate or resistant to ertapenem when measured with the Vitek 2 system, with the MICs for these isolates when measured by two methods performed in agar medium: the Etest and agar plate dilution method (APDM). Overall, 50 E. cloacae isolates were included in the study. The mean MIC of ertapenem was 2.92±1.77μg/ml according to the Vitek 2 system, 0.94±0.84μg/ml according to the Etest strips, and 0.93±0.62μg/ml according to the APDM. Furthermore, the MICs determined by the Vitek 2 system were higher than the MICs determined by the two other methods for 96% of strains. Lastly, according to the Etest strips and APDM, 42% of E. cloacae were susceptible to ertapenem. No carbapenemase was identified by the screening method used. Using the Vitek 2 system to determine ertapenem MICs for E. cloacae can have potential consequences in terms of additional carbapenemase-detecting tests and antimicrobial therapy. It would be interesting to determine if the Vitek 2 system is more effective for the detection of carbapenemase producers with low-level carbapenem resistance than the two methods performed in agar medium

A. Chesnay

C. Lamoureux

C. Lebreton

C. Lemarié

C. Mahaza

H. Pailhories

M. Eveillard

M. Kempf

M.L. Joly-Guillou

V. Cassisa

International Journal of Infectious Diseases

SummaryOur objective was to compare the ertapenem minimal inhibitory concentrations (MICs) for Enterobacter cloacae isolates categorized intermediate or resistant to ertapenem when measured with the Vitek 2 system, with the MICs for these isolates when measured by two methods performed in agar medium: the Etest and agar plate dilution method (APDM). Overall, 50 E. cloacae isolates were included in the study. The mean MIC of ertapenem was 2.92±1.77μg/ml according to the Vitek 2 system, 0.94±0.84μg/ml according to the Etest strips, and 0.93±0.62μg/ml according to the APDM. Furthermore, the MICs determined by the Vitek 2 system were higher than the MICs determined by the two other methods for 96% of strains. Lastly, according to the Etest strips and APDM, 42% of E. cloacae were susceptible to ertapenem. No carbapenemase was identified by the screening method used. Using the Vitek 2 system to determine ertapenem MICs for E. cloacae can have potential consequences in terms of additional carbapenemase-detecting tests and antimicrobial therapy. It would be interesting to determine if the Vitek 2 system is more effective for the detection of carbapenemase producers with low-level carbapenem resistance than the two methods performed in agar medium

Pailhoriès, Hélène

Cassisa, Viviane

Lamoureux, Claudie

Chesnay, Adélaïde

Lebreton, Cyrielle

Lemarié, Carole

Kempf, Marie

Mahaza, Chetaou

Joly-Guillou, Marie-Laure

Eveillard, Matthieu

Elsevier - Publisher Connector 

Discordance in the minimal inhibitory concentrations of ertapenem for Enterobacter cloacae: Vitek 2 system versus Etest and agar dilution methods 

International Journal of Infectious Diseases 18 (2014) 94–96Short Communication
Discordance in the minimal inhibitory concentrations of ertapenem for
Enterobacter cloacae: Vitek 2 system versus Etest and agar dilution
methods
He´le`ne Pailhorie`s a, Viviane Cassisa a,b, Claudie Lamoureux a, Ade´laı¨de Chesnay a,
Cyrielle Lebreton a, Carole Lemarie´ a, Marie Kempf a,b, Chetaou Mahaza a,
Marie-Laure Joly-Guillou a,b, Matthieu Eveillard a,b,*
a Laboratoire de Bacte´riologie, Centre Hospitalier Universitaire, 4 rue Larrey, Angers, France
bGroupe d’Etude des Interactions Hoˆtes Pathoge`nes (GEIHP, EA 3127), UFR Me´decine, Universite´ d’Angers, rue Haute de Recule´e, Angers, France
A R T I C L E I N F O
Article history:
Received 12 June 2013
Received in revised form 5 September 2013
Accepted 6 September 2013
Corresponding Editor: Eskild Petersen,
Aarhus, Denmark
Keywords:
Ertapenem
Minimal inhibitory concentration
Enterobacter
S U M M A R Y
Our objective was to compare the ertapenem minimal inhibitory concentrations (MICs) for Enterobacter
cloacae isolates categorized intermediate or resistant to ertapenem when measured with the Vitek 2
system, with the MICs for these isolates when measured by two methods performed in agar medium: the
Etest and agar plate dilution method (APDM). Overall, 50 E. cloacae isolates were included in the study.
The mean MIC of ertapenem was 2.92  1.77 mg/ml according to the Vitek 2 system, 0.94  0.84 mg/ml
according to the Etest strips, and 0.93  0.62 mg/ml according to the APDM. Furthermore, the MICs
determined by the Vitek 2 system were higher than the MICs determined by the two other methods for 96% of
strains. Lastly, according to the Etest strips and APDM, 42% of E. cloacae were susceptible to ertapenem. No
carbapenemase was identiﬁed by the screening method used. Using the Vitek 2 system to determine
ertapenem MICs for E. cloacae can have potential consequences in terms of additional carbapenemase-
detecting tests and antimicrobial therapy. It would be interesting to determine if the Vitek 2 system is more
effective for the detection of carbapenemase producers with low-level carbapenem resistance than the two
methods performed in agar medium.
 2013 The Authors. Published by Elsevier Ltd on behalf of International Society for Infectious
Diseases. 
Contents lists available at ScienceDirect
International Journal of Infectious Diseases
jou r nal h o mep ag e: w ww .e lsev ier . co m / loc ate / i j id
Open access under CC BY-NC-ND license.1. Introduction
Acquired carbapenemase-producing Enterobacteriaceae pose a
considerable threat to clinical patient care and public health.1
Therefore, it is important to identify them in order to implement
speciﬁc measures to prevent their dissemination. It is now
suggested that carbapenemase-detecting phenotypic tests should
be performed in isolates exhibiting even a small reduction in
susceptibility to carbapenems, including ertapenem.2
Our objective was to compare the ertapenem minimal
inhibitory concentrations (MICs) for Enterobacter cloacae when
measured with the Vitek 2 system (bioMe´rieux, France) and when
measured by methods performed in agar medium.       
* Corresponding author. Tel.: +33 2 41353315; fax: +33 41354164.
E-mail address: MaEveillard@chu-angers.fr (M. Eveillard).
1201-9712       2013 The Authors. Published by Elsevier Ltd on behalf of International S
http://dx.doi.org/10.1016/j.ijid.2013.09.0062. Methods
This study focused on E. cloacae strains that had been isolated in
2011 and 2012 in our hospital. All strains belonging to the
intermediate or resistant categories to ertapenem according to the
recommendations of the European Committee on Antimicrobial
Susceptibility Testing (http://www.eucast.org; 0.5 < ertapenem
MIC  1 mg/ml for the intermediate category, and ertapenem MIC
> 1 mg/ml for the resistant category) were included.
MICs had been measured routinely by the automated dilution
method with the Vitek 2 system. For the current study, the MICs
were also measured in Mueller–Hinton agar by means of gradient
diffusion testing with Etest strips for imipenem and meropenem
(Oxoid Ltd, Basingstoke, UK) and for ertapenem and doripenem
(bioMe´rieux, Marcy l’Etoile, France). Lastly, the ertapenem MICs
were determined by agar plate dilution method (APDM; with
ertapenem concentrations between 0.125 and 8 mg/ml). All agar
plates were incubated at 37 8C for 24 h. The following strains were
used for quality control (ertapenem MICs): Escherichia coli ATCC
25922, three E. cloacae isolates that had been characterized
susceptible to ertapenem by Vitek 2 system and Etest strips in ourociety for Infectious Diseases. Open access under CC BY-NC-ND license.
Table 1
Enterobacter cloacae categorized intermediate or resistant to ertapenem by Vitek 2 system: beta-lactamase screening, ertapenem MICs (mg/ml) determined by the three
different methods, and MICs of three other carbapenems determined by Etest strips
Strains Ertapenem
MIC
(Vitek 2)
Ertapenem
MIC
(E-test)
Ertapenem
MIC
(Dilution in
agar medium)
Imipenem MIC
(E-test)
Meropenem
MIC
(E-test)
Doripenem
MIC
(E-test)
ESBL
screening
Carbapenemase-detecting
phenotypic test
1 2 1 0.5 0.12 0.06 0.032 Negative Negative
2 2 0.75 0.25 0.25 0.12 0.094 Negative Negative
3 1 0.5 0.5 0.5 0.12 0.094 Positive Negative
4 4 1 1 0.5 0.12 0.094 Positive Negative
5 4 1 1 0.25 0.12 0.094 Positive Negative
6 >8 3 1 1 1 0.75 Negative Negative
7 4 2 2 0.5 0.12 0.125 Negative Negative
8 4 1.5 0.5 0.5 0.12 0.125 Positive Negative
9 1 0.25 0.125 0.25 0.03 0.032 Positive Negative
10 4 1.5 0.25 0.5 0.25 0.125 Negative Negative
11 4 3 2 0.25 0.12 0.094 Positive Negative
12 >8 1.5 0.5 0.25 0.12 0.64 Negative Negative
13 2 0.75 0.5 0.5 0.12 0.047 Positive Negative
14 2 1 0.5 0.5 0.12 0.094 Negative Negative
15 2 1.5 2 0.5 0.12 0.094 Positive Negative
16 1 0.75 0.25 0.25 0.12 0.094 Positive Negative
17 1 0.125 0.25 0.25 0.03 0.016 Negative Negative
18 2 0.25 1 0.25 0.12 0.064 Positive Negative
19 2 1 1 0.5 0.12 0.094 Negative Negative
20 4 0.75 0.25 0.25 0.03 0.023 Negative Negative
21 4 0.5 0.25 0.25 0.06 0.047 Positive Negative
22 2 0.75 1 0.25 0.06 0.064 Positive Negative
23 1 0.25 1 0.25 0.06 0.047 Negative Negative
24 2 0.38 1 0.5 0.06 0.094 Negative Negative
25 4 1.5 2 0.25 0.12 0.094 Positive Negative
26 0.75 0.75 2 0.5 0.12 0.125 Negative Negative
27 2 1 1 0.25 0.06 0.047 Positive Negative
28 4 0.38 0.5 0.25 0.12 0.032 Positive Negative
29 8 4 1 0.25 0.12 0.094 Positive Negative
30 1 0.25 0.5 0.5 0.06 0.032 Positive Negative
31 4 2 2 1.5 1 0.5 Negative Negative
32 1 3 1 0.5 0.25 0.19 Negative Negative
33 2 1 1 0.25 0.12 0.125 Negative Negative
34 4 0.25 2 0.5 0.12 0.064 Negative Negative
35 4 1 1 0.12 0.06 0.064 Positive Negative
36 1 0.75 0.25 0.12 0.03 0.016 Positive Negative
37 4 1 2 0.25 0.12 0.125 Positive Negative
38 2 0.25 0.5 0.5 0.06 0.064 Positive Negative
39 2 0.75 1 0.25 0.06 0.047 Positive Negative
40 2 0.38 2 0.5 0.12 0.094 Positive Negative
41 2 0.25 0.5 0.25 0.06 0.047 Negative Negative
42 4 0.5 0.25 0.25 0.06 0.032 Negative Negative
43 4 1 1 0.25 0.12 0.064 Negative Negative
44 4 0.5 1 0.25 0.12 0.094 Negative Negative
45 2 0.19 2 0.5 0.06 0.125 Negative Negative
46 4 0.12 0.125 0.06 0.008 0.004 Positive Negative
47 2 0.38 1 0.25 0.06 0.064 Negative Negative
48 2 0.25 1 0.25 0.06 0.047 Positive Negative
49 4 0.25 1 1 0.12 0.25 Negative Negative
50 1 0.25 0.25 0.25 0.06 0.047 Positive Negative
E. coli ATCC 25922 0.5 0.125
E. cloacae
control 1
0.5 0.25
E. cloacae
control 2
0.5 0.25
E. cloacae
control 3
0.5 0.25
OXA-48-producing E. cloacae 4 0.50
MIC, minimal inhibitory concentration; ESBL, extended-spectrum beta-lactamase.
H. Pailhorie`s et al. / International Journal of Infectious Diseases 18 (2014) 94–96 95laboratory, and an OXA-48-producing Klebsiella pneumoniae for
which the ertapenem MIC was 4 mg/ml with the Vitek 2 system
and 0.50 mg/ml with the Etest strips in our laboratory.
In addition, all strains were tested for the presence of an
extended-spectrum beta-lactamase (ESBL) by the combined disk
method (Rosco Diagnostica A/S, Taastrup, Denmark).
Phenotypic testing for carbapenemases was performed by
means of the double-disk synergy test (DDST) KPC + MBL ConﬁrmID Kit (Rosco Diagnostica A/S) using meropenem alone and in
combination with dipicolinic acid, aminophenylboronic acid, and
cloxacillin, with a disk of temocillin for screening OXA-48-
producing strains.3,4
The comparison of the MICs determined with the
different methods was performed using the Student’s
exact test or the non-parametric Kruskal–Wallis test, as
appropriate.
Table 2
Ertapenem MICs (mg/ml) according to three different methods: comparison
between Enterobacter cloacae producing ESBL and not
MIC Production of an ESBL Absence of ESBL
Vitek 2 2.8  1.6 3.1  1.9
Etest strips 0.91  0.88 0.97  0.82
Dilution in agar medium 0.89  0.63 0.97  0.62
MIC, minimal inhibitory concentration; ESBL, extended-spectrum beta-lactamase.
H. Pailhorie`s et al. / International Journal of Infectious Diseases 18 (2014) 94–96963. Results and discussion
Fifty E. cloacae with a MIC of ertapenem >0.5 mg/ml determined
by Vitek 2 system were isolated from 50 patients hospitalized on
20 different wards of the hospital. These isolates were from clinical
or screening samples. An ESBL was identiﬁed in 52% of the strains.
No carbapenemase was detected by the screening method used.
The mean MIC of ertapenem was 2.92  1.77 mg/ml according to
the Vitek 2 system, 0.94  0.84 mg/ml according to the Etests, and
0.93  0.62 mg/ml according to the APDM. The mean MIC determined
by Vitek 2 system was signiﬁcantly different to those determined by
Etest strips and APDM (p < 0.02 and p < 0.05, respectively). Further-
more, the MICs determined by the Vitek 2 system were higher than
the MICs determined by the two other methods for 96% of strains
(Table 1). The mean ratio MIC Vitek 2 system/MIC Etest and MIC Vitek
2 system/MIC APDM was 5.2. The differences between the MICs were
similar for ESBL-producing and ESBL-non-producing strains (Table 2).
Lastly, according to the Etest strips and APDM, 42% of E. cloacae were
susceptible to ertapenem (no strain was susceptible according to the
Vitek 2 system). The mean MICs were 0.38 mg/ml for imipenem,
0.13 mg/ml for meropenem, and 0.11 mg/ml for doripenem.
Ertapenem has been described as the most appropriate
carbapenem for detecting carbapenemase producers, especially
those with low-level resistance to carbapenems.4–6 In our
experience, no E. cloacae identiﬁed as being intermediate or
resistant to ertapenem with the Vitek 2 system has been a
carbapenemase producer. The proportion of non-susceptible
strains identiﬁed in the same population by the two other methods
was only 58%. Therefore it appears that using the Vitek 2 system
may lead to some excessive additional tests, such as the
determination of MICs for other carbapenems and carbapene-
mase-detecting tests for third-generation cephalosporin-resistant
E. cloacae strains. Moreover, the possibly falsely diagnosed non-
susceptible strains may lead to an excessive use of other
carbapenems that have a broader antimicrobial spectrum.
However, two considerations must be taken into account for the
interpretation of these results. First, carbapenemase production is
not the only mechanism that confers a reduced susceptibility to
certain carbapenems. In addition, the inoculum effect can play a
major role in the determination of the MIC.Finally, it is noticeable that the ertapenem MIC of the OXA-48-
producing K. pneumoniae that was used as a control in our study
was measured as 4 mg/ml (resistant) with the Vitek 2 system, and
as 0.50 mg/ml (susceptible) by Etest. Therefore, it would be
interesting to determine if the Vitek 2 system is more effective for
the detection of carbapenemase producers with low-level carba-
penem resistance than the two methods performed in agar
medium.
Conﬂict of interest: No conﬂict of interest to declare.
Funding source: No funding source.
Ethical approval: Ethical approval was not required.
References
1. Cohen-Stuart J, Leverstein-Van Hall MA, on behalf of members of the Dutch
Working Party on the Detection of Highly Resistant Microorganisms. Guideline
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iaceae. Int J Antimicrob Agents 2010;36:205–10.
2. Miriagou V, Cornaglia G, Edelstein M, Galani I, Giske CG, Gniadkowski M, et al.
Acquired carbapenemases in Gram-negative bacterial pathogens: detection and
surveillance issues. Clin Microbiol Infect 2010;16:112–22.
3. Hartl R, Widhalm S, Kerschner H, Apfalter P. Temocillin and meropenem to
discriminate resistance mechanisms leading to decreased carbapenem suscep-
tibility with focus on OXA-48 Enterobacteriaceae. Clin Microbiol Infect
2013;19:E230–2. http://dx.doi.org/10.1111/1469-0691.12146.
4. Birgy A, Bidet P, Genel N, Doit C, Decre´ D, Arlet G, et al. Phenotypic screening of
carbapenemases and associated b-lactamases in carbapenem-resistant Enter-
obacteriaceae. J Clin Microbiol 2012;50:1295–302.
5. Centers for Disease Control, Prevention. Guidance for control of infections with
carbapenem-resistant or carbapenemase-producing Enterobacteriaceae in acute
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6. Nordmann P, Poirel L, Carrer A, Toleman MA, Walsh TR. How to detect NDM-1
producers. J Clin Microbiol 2011;49:718–21.


International audienceOur objective was to compare the ertapenem minimal inhibitory concentrations (MICs) for Enterobacter cloacae isolates categorized intermediate or resistant to ertapenem when measured with the Vitek 2 system, with the MICs for these isolates when measured by two methods performed in agar medium: the Etest and agar plate dilution method (APDM). Overall, 50 E. cloacae isolates were included in the study. The mean MIC of ertapenem was 2.92±1.77μg/ml according to the Vitek 2 system, 0.94±0.84μg/ml according to the Etest strips, and 0.93±0.62μg/ml according to the APDM. Furthermore, the MICs determined by the Vitek 2 system were higher than the MICs determined by the two other methods for 96% of strains. Lastly, according to the Etest strips and APDM, 42% of E. cloacae were susceptible to ertapenem. No carbapenemase was identified by the screening method used. Using the Vitek 2 system to determine ertapenem MICs for E. cloacae can have potential consequences in terms of additional carbapenemase-detecting tests and antimicrobial therapy. It would be interesting to determine if the Vitek 2 system is more effective for the detection of carbapenemase producers with low-level carbapenem resistance than the two methods performed in agar medium.</p

Pailhories, Hélène

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Discordance in the minimal inhibitory concentrations of ertapenem for Enterobacter cloacae: Vitek 2 system versus Etest and agar dilution methods

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Discordance in the minimal inhibitory concentrations ofertapenem for Enterobacter cloacae: Vitek 2 system versusEtest and agar dilution methodsSubmitted by a.bergoend on Thu, 05/07/2015 - 09:03Titre Discordance in the minimal inhibitory concentrations of ertapenem for Enterobactercloacae: Vitek 2 system versus Etest and agar dilution methodsType depublication Article de revueAuteurPailhories, Hélène [1], Cassisa, Viviane [2], Lamoureux, Claudie [3], Chesnay,Adélaïde [4], Lebreton, Cyrielle [5], Lemarié, Carole [6], Kempf, Marie [7], Mahaza,Chetaou [8], Joly-Guillou, Marie-Laure [9], Eveillard, Matthieu [10]Editeur ElsevierType Article scientifique dans une revue à comité de lectureAnnée 2014Langue AnglaisDate 2014 JanPagination 94-96Volume 18Titre de larevue International Journal of Infectious DiseasesISSN 1878-3511Mots-clésAgar [11], Anti-Bacterial Agents [12], Bacterial Proteins [13], beta-Lactamases [14],beta-Lactams [15], Culture Media [16], Drug Resistance, Bacterial [17],Enterobacter cloacae [18], Escherichia coli [19], Microbial Sensitivity Tests [20]Résumé enanglaisOur objective was to compare the ertapenem minimal inhibitory concentrations(MICs) for Enterobacter cloacae isolates categorized intermediate or resistant toertapenem when measured with the Vitek 2 system, with the MICs for these isolateswhen measured by two methods performed in agar medium: the Etest and agarplate dilution method (APDM). Overall, 50 E. cloacae isolates were included in thestudy. The mean MIC of ertapenem was 2.92±1.77μg/ml according to the Vitek 2system, 0.94±0.84μg/ml according to the Etest strips, and 0.93±0.62μg/mlaccording to the APDM. Furthermore, the MICs determined by the Vitek 2 systemwere higher than the MICs determined by the two other methods for 96% of strains.Lastly, according to the Etest strips and APDM, 42% of E. cloacae were susceptibleto ertapenem. No carbapenemase was identified by the screening method used.Using the Vitek 2 system to determine ertapenem MICs for E. cloacae can havepotential consequences in terms of additional carbapenemase-detecting tests andantimicrobial therapy. It would be interesting to determine if the Vitek 2 system ismore effective for the detection of carbapenemase producers with low-levelcarbapenem resistance than the two methods performed in agar medium.URL de lanotice http://okina.univ-angers.fr/publications/ua11075 [21]DOI 10.1016/j.ijid.2013.09.006 [22]Titre abrégé Int. J. Infect. Dis.Identifiant(ID) PubMed 24183718 [23]Liens[1] http://okina.univ-angers.fr/hpailhor/publications[2] http://okina.univ-angers.fr/publications?f[author]=17323[3] http://okina.univ-angers.fr/publications?f[author]=19630[4] http://okina.univ-angers.fr/publications?f[author]=19631[5] http://okina.univ-angers.fr/publications?f[author]=19632[6] http://okina.univ-angers.fr/publications?f[author]=20159[7] http://okina.univ-angers.fr/marie.kempf/publications[8] http://okina.univ-angers.fr/publications?f[author]=19633[9] http://okina.univ-angers.fr/m.joly/publications[10] http://okina.univ-angers.fr/matthieu.eveillard/publications[11] http://okina.univ-angers.fr/publications?f[keyword]=17267[12] http://okina.univ-angers.fr/publications?f[keyword]=10152[13] http://okina.univ-angers.fr/publications?f[keyword]=11654[14] http://okina.univ-angers.fr/publications?f[keyword]=17270[15] http://okina.univ-angers.fr/publications?f[keyword]=17271[16] http://okina.univ-angers.fr/publications?f[keyword]=17268[17] http://okina.univ-angers.fr/publications?f[keyword]=17203[18] http://okina.univ-angers.fr/publications?f[keyword]=17269[19] http://okina.univ-angers.fr/publications?f[keyword]=10431[20] http://okina.univ-angers.fr/publications?f[keyword]=10157[21] http://okina.univ-angers.fr/publications/ua11075[22] http://dx.doi.org/10.1016/j.ijid.2013.09.006[23] http://www.ncbi.nlm.nih.gov/pubmed/24183718?dopt=AbstractPublié sur Okina (http://okina.univ-angers.fr)

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Discordance in the minimal inhibitory concentrations of ertapenem for Enterobacter cloacae: Vitek 2 system versus Etest and agar dilution methods

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