Dispatch
Cefepime MIC as a Predictor
of the Extended-Spectrum ß-Lactamase Type in Klebsiella
pneumoniae, Taiwan
Wen Liang Yu,*† Michael A. Pfaller,† Patricia L. Winokur,† and
Ronald N. Jones†‡§
*China Medical College Hospital, Taichung, Taiwan; †University
of Iowa College of Medicine, Iowa City, Iowa, USA; ‡JONES Group/JMI
Laboratories, North Liberty, Iowa, USA; and §Tufts University School
of Medicine, Boston, Massachusetts, USA
To guide selection
of carbapenems or fourth-generation cephalosporins as therapy,
110 Klebsiella pneumoniae isolates with extended-spectrum
ß-lactamases from Taiwan were characterized by phenotypic
(MICs), molecular, and chemical methods. MIC patterns of ceftazidime
and cefepime clearly differentiate strains treatable by cefepime
and those capable of efficiently hydrolyzing available cephalosporins
(CTX-M series and SHV-types). Continued use of cefepime appears
to be a treatment option in cases for which MIC results are available
and interpreted by the criteria presented.
In recent years, extended-spectrum ß-lactamase-producing
Klebsiella pneumoniae (ESBL-KP) strains of the TEM, SHV,
and CTX-M types have been discovered worldwide. Reference broth
microdilution susceptibility rates (MIC <8 mg/L) for cefepime
among ESBL-KP in various geographic regions show a wide range: Canada
94.4%, United States 87.6%, Western Pacific 76.1%, Europe 63.6%,
and Latin America 49.6% (1). In Taiwan, the in
vitro cefepime susceptibilities of ESBL-KP ranged from 37% to 100%
(2,3). The gene encoding SHV-5 (pI 8.2) has been
reported to be the most common ESBL in Klebsiellae in Taiwan (2,4).
The CTX-M-3 (pI 8.4) enzyme has also been discovered in Escherichia
coli isolates in southern Taiwan (5). For our
study, we focused on the mechanisms of cefepime resistance among
ESBL-KP isolates in Taiwan and attempted to predict cephalosporin
therapeutic potentials by simple phenotypic patterns.
The
Study
We initially conducted reference broth microdilution tests (6,7)
for 211 isolates of endemic and epidemic ESBL-KP from Taiwan; 53%
of isolates had a cefepime MIC of <8 mg/L (susceptible).
Isoelectric focusing (IEF) was then performed by the method of Matthew
et al. (8). Approximately 40% of isolates had an
enzyme with a pI of 8.2 (SHV-5); 40% of isolates produced enzymes
with a pI of 7.9, 8.4, or 8.8 (CTX-M-type); an additional 20% of
isolates contained both an SHV-5 plus a CTX-M enzyme.
The IEF results of 110 geographically representative isolates of
ESBL-KP were categorized by cefepime MIC level (Table).
The enzymes with pIs of 7.6 and 5.4 were SHV-1 and TEM-1, respectively,
which have been reported previously in Taiwan hospitals (2,4).
All the enzymes with pIs of 5.4, 7.6, and 8.2 were evenly distributed
among the isolates regardless of cefepime MIC values, indicating
no association with resistance to this fourth-generation cephalosporin.
All 23 isolates with pI 8.2 enzymes and a nonsusceptible cefepime
MIC (>16 mg/L) contained enzymes with pIs of 7.9, 8.4,
or 8.8. In the absence of these CTX-M enzymes, isolates with pI
8.2 enzymes remained susceptible to cefepime. Thus, the high MIC
level for cefepime was attributed to enzymes with pIs of 7.9, 8.4,
and 8.8. This finding is supported by the fact that those isolates
with a single CTX-M enzyme (10 with pI 7.9 enzymes [CTX-M-14]
and 8 with pI 8.4 enzymes [CTX-M-3]) had very elevated cefepime
MIC results in the absence of a pI 8.2 enzyme (9).
Two isolates with pI 8.4 enzymes remained susceptible to cefepime
(MIC 2 µg/mL) and probably produced low levels of CTX-M-3.
These data indicate that cefepime resistance in ESBL-KP isolates
from Taiwan may result from either the cumulative effect of pI 7.9,
8.4, 8.8, or 8.2 enzymes or hyperproduction of any of the enzymes
with the CTX-M phenotype (pI 7.9, 8.4, or 8.8). The enzyme with
a pI of 8.8 is a novel CTX-M ß-lactamase most similar to CTX-M-3
(unpub. data).
Several CTX-M enzymes have been shown to confer high MIC levels
for cefepime (10-12). Bauernfeind
et al. reported an isolate of Salmonella typhimurium that
had a CTX-M-2 enzyme (pI 7.9) and a cefepime MIC of 64 mg/L (10).
Outbreaks have also been reported of isolates producing CTX-M enzymes
(pI 8.4), including K. pneumoniae (cefepime MIC 4-8 mg/L),
E. coli (cefepime MIC 8-32 mg/L), and Serratia marcescens
(cefepime MIC 16-64 mg/L) (11).
Szabo et al. reported an outbreak of 14 ESBL-KP strains (pI 8.2,
probably SHV-5) that had high-level resistance to cefepime (MIC90
>256 mg/L) (12). Tzouvelekis et al. also noticed
seven isolates of ESBL-KP (SHV-5) with cefepime MICs ranging from
32 mg/L to 64 mg/L. These researchers described the elevated cefepime
MIC as being due to the combined effect of SHV-5 hyperproduction
and decreased outer membrane permeability (loss of 36-kDa outer
membrane protein [OMP]) (13). The cefepime MIC
for isolates hyperproducing SHV-5 without loss of the 36-kDa OMP
remained <16 mg/L, a susceptible level (13).
Loss of the 36-kDa OMP also conferred cefoxitin resistance, and
introduction of a plasmid carrying the 36-kDa OMP gene markedly
reduced the MIC of cefoxitin, from 128 mg/L to 16 mg/L (13).
Whether the isolates reported by Szabo et al. also had concomitant
outer membrane defects is unknown, but these authors later recommended
that cefepime not be considered the treatment of choice against
SHV-5-producing ESBL-KP (14). Whether our cefepime-resistant
isolates had a concomitant OMP defect is similarly unknown. However,
in 44 isolates with cefepime MICs >16 mg/L, only 7 were
resistant to cefoxitin. Furthermore, for isolates with high cefepime
MIC values resulting from single CTX-M enzymes (10 with pI 7.9 and
8 with pIs of 8.4), only two (one each with pIs of 7.9 and 8.4)
were resistant to cefoxitin. The relatively low rates of cefoxitin
coresistance provide indirect evidence that 36-kDa OMP loss may
not play an important role in the expression of cefepime resistance
in ESBL-KP strains in Taiwan.
Conclusions
Alternative therapy using cefepime against ESBL-KP strains in Taiwan
could be reliable if appropriately guided by cefepime and ceftazidime
MIC results. The cefepime MIC is useful for predicting the presence
of CTX-M enzymes, which usually confer resistance to this fourth-generation
cephalosporin. Cefepime cannot be used if the MIC exceeds 8 mg/L,
which predicts the presence of CTX-M ß-lactamases. Cefepime
may reasonably be used clinically if the MIC is consistently <1
mg/L, which indicates the absence of a CTX-M enzyme. For isolates
with cefepime MICs >2 to <8 mg/L, use of cefepime
should be further guided by the ceftazidime MIC. If the ceftazidime
MIC remains in the susceptible range (<8 mg/L, predicting
enzymes of pI 7.9, 8.4, or 8.8), cefepime should not be used. If
the ceftazidime MIC is >8 mg/L (predicting enzymes of pI 8.2),
cefepime at appropriate doses has a potential therapeutic role because
most pI 8.2 enzymes rarely elevate the cefepime MIC to >8 mg/L.
In conclusion, outer membrane defects and the inoculum effects
(13) that may adversely elevate MIC values must
still be considered if cefepime is chosen as an alternative therapy
against ESBL-KP strains. This strategy of focused utilization of
a newer cephalosporin could reduce some selective pressures of carbapenem
use among ESBL-KP and thus minimize the development of carbapenem-resistant
strains. In addition, phenotypic characteristics appear to accurately
differentiate two important endemic and epidemic groups of ESBL
types (CTX-M series and SHV-like) in K. pneumoniae strains
in Taiwan.
Acknowledgment
We thank Monto Ho, Microbial Infections Reference Laboratory, National
Health Research Institutes, for providing subcultures of the strains
from the Taiwan Surveillance of Antimicrobial Resistance collection.
Dr. Yu is the SENTRY Antimicrobial Surveillance Program Fellow
for 2000-01 at the University of Iowa College of Medicine (Iowa
City, Iowa). His research focuses on the detection (phenotypic and
genotypic) and characterization of ß-lactamases in gram-negative
bacilli endemic and epidemic in Taiwan, where he is a member of
the medical faculty at the China Medical College, Taichung.
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Table.
Distribution of pIa values in 110 isolates of extended-spectrum
ß-lactamase-producing Klebsiella pneumoniae, stratified
by cefepime MIC level,b Taiwan |
|
Cefepime MIC (mg/L) (n=110)
|
pI 5.4
(n=89)
|
pI 6.3
(n=7)
|
pI 7.6
(n=92)
|
pI 7.8
(n=4)
|
pI 7.9
(n=40)
|
pI 8.2
(n=62)
|
pI 8.4
(n=43)
|
pI 8.8
(n=3)
|
|
>32 (n=38)
|
38
|
0
|
31
|
0
|
29c
|
20d
|
18e
|
2
|
16 (n=6)
|
6
|
0
|
5
|
0
|
5c
|
3d
|
5e
|
0
|
8 (n=26)
|
20
|
2f
|
22
|
0
|
6g
|
8h
|
12g
|
0
|
4 (n=19)
|
9
|
4f
|
17
|
1f
|
0
|
12h
|
6g
|
1g
|
2 (n=10)
|
7
|
1f
|
8
|
3f
|
0
|
8h
|
2g
|
0
|
<1 (n=11)
|
9
|
0
|
9
|
0
|
0
|
11h
|
0
|
0
|
|
apI, Isoelectric point. Each strain
may have multiple pIs.
bMIC test, reference broth microdilution method
according to National Committee for Clinical Laboratory Standards.
cTen of 34 isolates having a cefepime MIC >16
mg/L did not have enzymes with pI values of 8.2 or 8.4.
dAll 23 isolates were simultaneously coexistent
with pI 7.9, 8.4, or 8.8 (12 with pI 7.9 plus 8.4; 9 with
pI 7.9; and 2 with pI 8.8).
eEight of 23 isolates (cefepime MIC >16
mg/L) were not coexistent with pI 8.2 or 7.9.
fAll the 11 isolates (pI 7.8 or 6.3) were coexistent
with pI 8.2.
gCeftazidime MIC <8 mg/L; ceftriaxone
MIC >32 mg/L.
hCeftazidime MIC >16 mg/L.
|
|