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Journal of Clinical Microbiology, August 2007, p. 2723-2725, Vol. 45, No. 8
0095-1137/07/$08.00+0 doi:10.1128/JCM.00015-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Evaluation of Methods To Identify the Klebsiella pneumoniae Carbapenemase in Enterobacteriaceae
K. F. Anderson,*
D. R. Lonsway,
J. K. Rasheed,
J. Biddle,
B. Jensen,
L. K. McDougal,
R. B. Carey,
A. Thompson,
S. Stocker,
B. Limbago, and
J. B. Patel
Centers for Disease Control and Prevention, Division of Healthcare Quality Promotion, Atlanta, Georgia
Received 3 January 2007/
Returned for modification 13 February 2007/
Accepted 6 June 2007

ABSTRACT
The
Klebsiella pneumoniae carbapenem (KPC) ß-lactamase
occurs in
Enterobacteriaceae and can confer resistance to all
ß-lactam agents including carbapenems. The enzyme
may confer low-level carbapenem resistance, and the failure
of susceptibility methods to identify this resistance has been
reported. Automated and nonautomated methods for carbapenem
susceptibility were evaluated for identification of KPC-mediated
resistance. Ertapenem was a more sensitive indicator of KPC
resistance than meropenem and imipenem independently of the
method used. Carbapenemase production could be confirmed with
the modified Hodge test.

TEXT
Carbapenems are commonly used to treat infections caused by
multidrug-resistant
Enterobacteriaceae. In the United States
and other locations, an increasingly common mechanism of carbapenem
resistance is the
Klebsiella pneumoniae carbapenemase (KPC)
(
2,
10,
15,
18,
19,
24,
26,
27). The KPC ß-lactamase
occurs most commonly in
K. pneumoniae, but it has also been
reported sporadically in other species of
Enterobacteriaceae (
Klebsiella oxytoca,
Enterobacter spp.,
Escherichia coli,
Salmonella spp.,
Citrobacter freundii, and
Serratia spp.) and
Pseudomonas aeruginosa (
4,
10-
12,
17,
23,
28). The KPC enzyme confers resistance
to all ß-lactam agents including penicillins, cephalosporins,
monobactams, and carbapenems (
1,
21,
27,
28). Some isolates
containing KPC demonstrate low-level carbapenem resistance,
but when combined with other cellular changes, such as porin
loss, the carbapenem MIC increases (
21,
26). The gene encoding
the KPC enzyme is usually flanked by transposon-related sequences
and has been identified on conjugative plasmids; therefore,
the potential for dissemination is significant (
17,
26-
28).
Several outbreaks of KPC-producing bacteria have occurred in
the northeast United States (
2,
26). Isolates that acquired
this enzyme are usually resistant to several other classes of
antimicrobial agents used as treatment options. Laboratory identification
of KPC-producing clinical isolates will be critical for limiting
the spread of this resistance mechanism. The failure of automated
susceptibility testing systems to detect KPC-mediated resistance
was previously noted (
5,
22).
We evaluated commonly used susceptibility testing methods to identify the most sensitive conditions for KPC detection with 31 KPC-producing Enterobacteriaceae isolates (25 of K. pneumoniae, 2 of K. oxytoca, 1 of E. coli, 1 of Enterobacter spp., 1 of Citrobacter freundii, and 1 of Salmonella spp.). These were isolated from different patients who were hospitalized in 13 different healthcare institutions from seven different states: Maryland (one), New Jersey (two), New York (four), Pennsylvania (two), Michigan (two), Missouri (one), and North Carolina (one). The presence of blaKPC was determined using previously described oligonucleotide primers and cycling conditions (27). Enzyme activity was demonstrated in all isolates by isoelectric focusing (6, 16).
To measure the specificity of methods to detect KPC-mediated resistance, 45 isolates (26 of K. pneumoniae, 9 of K. oxytoca, and 10 of E. coli) were chosen for testing. All 45 isolates were negative for blaKPC by PCR. These isolates were submitted to the CDC for reference susceptibility testing. Using the reference broth microdilution (BMD) method, all isolates met the CLSI extended-spectrum ß-lactamase (ESBL) screening test criteria; that is, they demonstrated reduced susceptibility to at least one extended-spectrum cephalosporin (7, 8). Twenty-six isolates were positive by the CLSI ESBL broth confirmatory test, and the other isolates were presumed to have another broad-spectrum ß-lactamase or other mechanism of cephalosporin resistance. Five isolates were nonsusceptible to a carbapenem (imipenem, meropenem, or ertapenem) by BMD. Since two of the isolates were ESBL producers by BMD and the other three isolates produced an AmpC-type enzyme as demonstrated by isoelectric focusing and PCR (20), it is likely that the mechanism of reduced carbapenem susceptibility is a combination of a cephaloporinase and porin loss (3, 9, 13).
Meropenem, imipenem, and ertapenem susceptibilities were determined by BMD using cation-adjusted Mueller-Hinton broth in panels that were prepared in-house (7), disk diffusion (Becton Dickinson, Sparks, MD) (8), Etest (AB Biodisk, Piscataway, NJ), Microscan Autoscan using the NM32 panel (Dade Behring, West Sacramento, CA), and the Vitek 2 test using the AST GN14 card (bioMérieux, Durham, NC). Susceptibility testing of meropenem and imipenem was performed with the Phoenix test using the NEG MIC 30 panel or NEG MIC 112 panel (Becton Dickinson, Sparks, MD), the Vitek Legacy test using the GNS-122 and GNS-127 panels (bioMérieux, Durham, NC), and the Sensititre Auto Reader using the GN2F panel (Trek Diagnostics, West Lake, OH). All methods were performed according to the manufacturers' recommendations. Quality control testing of susceptibility testing methods was performed with Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853. Two criteria were evaluated for detection of KPC-mediated resistance: (i) an intermediate or resistant susceptibility to a carbapenem or (ii) a carbapenem MIC of >1 µg/ml (see Table 1).
Overall, reference BMD was the most sensitive method for the
identification of KPC-mediated carbapenem resistance (Table
1). An interpretation of intermediacy or resistance for any
of the three carbapenems tested (meropenem, imipenem, and ertapenem)
demonstrated greater than 90% sensitivity. For disk diffusion,
Etest, Vitek 2, and MicroScan greater than 90% sensitivity was
achieved only when ertapenem was tested. When interpreting the
results for the disk diffusion test and the Etest, it was important
to recognize the presence of small colonies growing inside the
zone of inhibition. Meropenem and imipenem susceptibility demonstrated
poor sensitivity for methods other than BMD. However, the specificity
of meropenem and imipenem susceptibility testing was higher
than that for ertapenem susceptibility testing regardless of
test method. For nearly all methods (i.e., those demonstrating
a specificity of

89%), the specificity errors occurred with
one or more of the five isolates that were nonsusceptible to
carbapenem by a mechanism other than carbapenemase production.
It is important that these are not specificity errors in measuring
carbapenem susceptibility but rather errors in the detection
of carbapenemase production.
Using the criterion of a carbapenem MIC of >1 µg/ml, the sensitivity of detecting KPC-mediated resistance increased for ertapenem, meropenem, and imipenem with only small changes in specificity. Meropenem susceptibility testing demonstrated greater than 90% sensitivity by BMD and MicroScan whereas imipenem susceptibility testing was at least 90% sensitive by BMD, Etest, Vitek 2, and MicroScan.
The modified Hodge test (Fig. 1) was also evaluated for detection of KPC-mediated resistance (14). This is a phenotypic test which could be used to determine if reduced susceptibility to carbapenems is mediated by a carbapenemase. The test was performed as described by Lee et al. (14), and it demonstrated 100% sensitivity and specificity for detection of KPC activity.
We tested a limited number of isolates in this study. The actual
sensitivity and specificity of these methods in any given laboratory
will depend upon the prevalent mechanisms of ß-lactam
resistance in the laboratory's patient population. Considering
the potential clinical impact of carbapenemase-producing isolates,
laboratories should consider testing for ertapenem susceptibility
since it was the most sensitive indicator of KPC activity regardless
of method. Ertapenem is not available on panels from all manufacturers;
laboratories that cannot test ertapenem on their current system
may consider using the ertapenem disk diffusion test, which
was a sensitive indicator for the presence of the KPC enzyme
in this study and a study by Bratu et al. (
5). The KPC enzyme
also hydrolyzes extended-spectrum cephalosporins; therefore,
screening for reduced susceptibility to carbapenems could be
limited to isolates that are resistant to these cephalosporins
(e.g., ceftazidime, ceftriaxone, and cefotaxime). Alternatively,
meropenem and imipenem susceptibility could be used to detect
KPC-mediated resistance if an elevated but susceptible MIC (MIC
of >1 µg/ml) is applied. However, it should be noted
that this MIC cutoff was applied only to
Klebsiella spp. and
E. coli in this study. Other species of
Enterobacteriaceae have
higher wild-type carbapenem MICs (
www.eucast.org), and so this
strategy may not be appropriate.
No method demonstrated 100% specificity for carbapenemase activity. In the United States, where metallo-ß-lactamases are uncommon, the most likely alternative mechanism of carbapenem resistance is the combination of an ESBL or AmpC-type enzyme with porin loss (3, 9, 13). A report by Woodford et al. (25) noted that nonsusceptibility to ertapenem is not specific for carbapenemase production, especially when carbapenemase production is uncommon. Also, the authors noted that for isolates with noncarbapenemase mechanisms, ertapenem resistance does not necessarily predict resistance to other carbapenems. In this study, the modified Hodge test was a useful method for confirming carbapenemase production. Alternatively, a laboratory could confirm the carbapenemase with PCR for the blaKPC gene, which has the added benefit of confirming which enzyme is present.
It is important for laboratories to be vigilant about the identification of emerging KPC resistance in their institution. Strategies for laboratory identification of this resistance will likely have to be reviewed and adjusted as this mechanism is further investigated.

FOOTNOTES
* Corresponding author. Mailing address: Centers for Disease Control and Prevention, Mail Stop G-08, 1600 Clifton Road NE, Atlanta, GA 30333. Phone: (404) 639-2824. Fax: (404) 639-1381. E-mail:
ebi2{at}cdc.gov 
Published ahead of print on 20 June 2007. 

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Journal of Clinical Microbiology, August 2007, p. 2723-2725, Vol. 45, No. 8
0095-1137/07/$08.00+0 doi:10.1128/JCM.00015-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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