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Journal of Clinical Microbiology, January 2000, p. 40-43, Vol. 38, No. 1
0095-1137/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Convenient Test for Screening
Metallo-
-Lactamase-Producing Gram-Negative Bacteria by Using
Thiol Compounds
Yoshichika
Arakawa,1,*
Naohiro
Shibata,1
Keigo
Shibayama,1
Hiroshi
Kurokawa,1
Tetsuya
Yagi,1
Hiroshi
Fujiwara,1 and
Masafumi
Goto2
Department of Bacterial and Blood Products,
National Institute of Infectious Diseases,
Tokyo,1 and Faculty of Pharmaceutical
Sciences, Kumamoto University, Kumamoto,2 Japan
Received 28 June 1999/Returned for modification 21 August
1999/Accepted 22 September 1999
 |
ABSTRACT |
A simple disk diffusion test was constructed for detection of
IMP-1-type metallo-
-lactamase-producing gram-negative bacteria. Two
Kirby-Bauer disks containing ceftazidime (CAZ) and a filter disk
containing a metallo-
-lactamase inhibitor were used in this test.
Several IMP-1 inhibitors such as thiol compounds including 2-mercaptopropionic acid, heavy metal salts, and EDTA were evaluated for this test. Two CAZ disks were placed on a Mueller-Hinton agar plate
on which a bacterial suspension was spread according to the method
recommended by the National Committee for Clinical Laboratory
Standards. The distance between the disks was kept to about 4 to 5 cm,
and a filter disk containing a metallo-
-lactamase inhibitor was
placed near one of the CAZ disks within a center-to-center distance of
1.0 to 2.5 cm. For IMP-1-producing strains, the growth-inhibitory zone
between the two disks expanded, while no evident change in the shape of
the growth-inhibitory zone was observed for CAZ-resistant strains
producing serine
-lactamases such as AmpC or SHV-12. As a result, 2 to 3 µl of undiluted 2-mercaptopropionic acid or mercaptoacetic acid
able to block IMP-1 activity gave the most reproducible and clearest
results, and CAZ-resistant strains producing AmpC or extended-spectrum
-lactamases were distinguishable from IMP-1 producers by this test.
A similar observation was made with IMP-1-producing clinical isolates
such as Serratia marcescens, Klebsiella
pneumoniae, Escherichia coli, Enterobacter
cloacae, Enterobacter aerogenes, Citrobacter
freundii, Proteus vulgaris, Pseudomonas
aeruginosa, Pseudomonas putida,
Acinetobacter spp., and Alcaligenes
xylosoxidans. The specificity and sensitivity of this test were
comparable to those of PCR analysis using
blaIMP-specific primers. Therefore, this
convenient test would be valuable for daily use in clinical laboratories.
 |
INTRODUCTION |
Carbapenem-resistant gram-negative
bacterial species such as Serratia marcescens and
Pseudomonas aeruginosa have emerged in Japan, and these
isolates usually produce IMP-1 metallo-
-lactamase (7, 9, 13,
15, 17). The blaIMP genes responsible for the IMP-1 production are usually mediated by integrons carried by
transferable large plasmids (1). About 4.4% of S. marcescens strains and 1.3% of P. aeruginosa strains
have already acquired IMP-1 productivity in Japan (manuscript in
preparation), and transmissions of the blaIMP
gene cassette have been observed among various gram-negative rods
(18). Since IMP-1 producers tend to demonstrate a wide range
of resistance to various broad-spectrum
-lactams including the
oxyimino cephalosporins, cephamycins, and carbapenems, early recognition of IMP-1 producers is very important for rigorous infection
control (3). The worldwide spread of this kind of organism
is becoming a general concern, since several
metallo-
-lactamase-producing gram-negative bacteria have recently
been reported outside Japan (5, 11, 19). Indeed, PCR
analyses usually give reliable and satisfactory results
(18), but this method is of limited practical use for daily
application in clinical laboratories because of the cost. Thus, the
development of a simple and inexpensive testing method for screening of
IMP-1 producers has become necessary.
 |
MATERIALS AND METHODS |
Bacterial strains.
Clinically isolated ceftazidime
(CAZ)-resistant (MIC, >64 µg/ml) gram-negative bacterial strains
were used in the test. Several of these isolates were later found to
carry the blaIMP gene by PCR. A
well-characterized extended-spectrum
-lactamase (ESBL) (SHV-12)
producer and AmpC hyperproducers were also used as the control strains.
A list of the bacterial strains tested in this study is shown in Table
1.
Evaluation of metallo-
-lactamase inhibitors.
CuCl2, FeCl2, EDTA, and thiol compounds
including mercaptoacetic acid, 2-mercaptopropionic acid, and
mercaptoethanol were used and evaluated for IMP-1 inhibition, because
these agents have been reported to block metallo-
-lactamase (2,
6, 12, 16). A colony of each bacterial strain was suspended and
diluted with Mueller-Hinton (MH) broth to 106 CFU/ml and
spread on an MH agar plate with a cotton swab according to the protocol
recommended by the National Committee for Clinical Laboratory Standards
(14). Two commercially supplied Kirby-Bauer (KB) disks, each
containing 30 µg of CAZ (Eiken Co. Ltd., Tokyo, Japan), were then
placed on the plates. The distance between the two CAZ disks was kept
at about 4 to 5 cm, and a filter disk was placed near one of the CAZ
disks within a center-to-center distance of 1.0 to 2.5 cm. Two to five
microliters of each inhibitor solution was added to the filter disk on
the agar, and each agar plate was incubated at 37°C overnight. The
concentration and amount of each inhibitor solution added to the filter
disk were as follows: for CuCl2, 100 mM (5 µl); for
FeCl2, 100 mM (5 µl); for EDTA, 100 mM (5 µl); and for
thiol compounds, an undiluted solution (2 to 3 µl).
PCR analysis.
CAZ-resistant strains used in this study were
tested by PCR analysis to confirm the presence of the
blaIMP gene according to the method of Senda et
al. (18) by using a new set of PCR primers
(5'-ACCGCAGCAGAGTCTTTGCC-3' and
5'-ACAACCAGTTTTGCCTTACC-3').
 |
RESULTS AND DISCUSSION |
Among the metallo-
-lactamase inhibitors used in this study,
2-mercaptopropionic acid gave the clearest results, because this chemical agent blocked IMP-1 activity very strongly even at a low
concentration (6). Mercaptoacetic acid also gave a clear result, but its inhibitory effect was slightly weaker than that of
2-mercaptopropionic acid. By using 2-mercaptopropionic acid, apparent
growth-inhibitory zones were observed with all IMP-1-producing strains
tested, including S. marcescens MKDM17, Klebsiella
pneumoniae MKD115, and P. aeruginosa MKAM12, while no
distinct change in the appearance of the growth-inhibitory zone was
observed for CAZ-resistant strains producing AmpC or SHV-12 (Fig.
1). Of 3,222 S. marcescens
isolates and 2,533 P. aeruginosa isolates, 141 and 88 isolates carrying blaIMP, respectively,
demonstrated an expansion of the growth-inhibitory zone by
2-mercaptoacetic acid, and Escherichia coli strains
producing Toho-1 or MEN-1 that show resistance to cefotaxime were
distinguishable from IMP-1 producers by this method (data not shown).
The other IMP-1 producers belonging to the gram-negative bacterial
species also showed results similar to those observed with
IMP-1-producing S. marcescens and P. aeruginosa
strains, as shown in Fig. 2. However,
relatively weak and ambiguous growth-inhibitory zones appeared for
IMP-1-producing Citrobacter freundii and Enterobacter cloacae, even when two disks containing CAZ and 2-mercaptoacetic acid, respectively, were placed as close together as 1 cm (from center
to center) (Fig. 2). This may be due to the hyperproduction of AmpC
and/or to a change in membrane permeability in these bacteria. Further
study is needed to improve the method for these strains, though
IMP-1-producing strains of C. freundii and E. cloacae are still very rare.

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FIG. 1.
Inhibitory effects of 2-mercaptopropionic acid (2-MPA)
on IMP-1 producers and non-IMP-1 producers. Three CAZ-resistant
strains belonging to the gram-negative bacterial species P. aeruginosa, S. marcescens, and K. pneumoniae
and producing IMP-1 metallo- -lactamase or serine- -lactamases
(SHV-12 or AmpC) were tested. For each IMP-1 producer, a distinct
growth-inhibitory zone appeared between the KB disk containing CAZ and
the filter disk containing 2-MPA (left column). No change is evident
around the two KB disks containing CAZ with or without 2-MPA for each
serine -lactamase producer (right column).
|
|

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FIG. 2.
Appearance of growth-inhibitory zone in IMP-1-producing
strains by use of CAZ and 2-mercaptopropionic acid (2-MPA). Various
levels of growth inhibition were observed in the IMP-1-producing
gram-negative bacterial species tested. Marked growth inhibitions were
observed in Acinetobacter sp., Alcaligenes
xylosoxidans, Enterobacter aerogenes, E. coli, Proteus vulgaris, and Pseudomonas
putida, whereas weak and ambiguous growth inhibitions were
observed in C. freundii and E. cloacae.
|
|
Heavy metal salts such as CuCl2 and FeCl2
usually formed ring-shaped areas of precipitation around the filter
disk and demonstrated their own bactericidal activity, while the
growth-inhibitory zone expanded to the disk containing CAZ, as shown in
Fig. 3A. The inhibitory effects of both
heavy metal salts were similar, but the results were ambiguous in
several strains. HgCl2 itself has rather strong
bactericidal activity and yielded better results than CuCl2
and FeCl2 in the preliminary tests. However, the use of
Hg2+ salt is not recommended from the viewpoint of human
health and environmental conservation.


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FIG. 3.
(A) Inhibitory effects of FeCl2 on IMP-1
producers and non-IMP-1 producers. A slight expansion of
growth-inhibitory zones between two disks was observed for all three
IMP-1 producers (arrowheads). No change in the shape of the
growth-inhibitory zone was evident for any serine- -lactamase
producer. (B) Inhibitory effects of EDTA on IMP-1 producers.
Growth-inhibitory zones between two disks appeared for all three IMP-1
producers (arrowheads) when 5 µl of 500 mM EDTA solution was added to
the filter.
|
|
EDTA also created a growth-inhibitory zone between the two disks, but
its appearance and reproducibility were relatively poor in several
strains, even when a thick EDTA solution (500 mM) was added to the
filter disk (Fig. 3B).
CAZ seemed to be the most suitable substrate for this test, because
IMP-1 producers usually demonstrated high-level resistance to CAZ (MIC,
>64 µg/ml) in our previous study (17, 18), and a marked
inhibitory effect of thiol compounds was usually observed, as shown in
Fig. 1 and 2. Indeed, any kind of broad-spectrum
-lactam disk can be
used in this test, but IMP-1 producers usually demonstrate various
levels of resistance to imipenem (IPM) (MIC, 4 to >128 µg/ml).
However, the inhibitory effect of thiol compounds tends to be
ambiguous, especially in strains that demonstrate reduced susceptibility to IPM (MIC, 4 to 8 µg/ml) when the KB disk (IPM) is
used (data not shown).
The emergence of gram-negative bacterial species with acquired
resistance to various broad-spectrum
-lactams is becoming a
worldwide clinical problem. Strains producing TEM- or SHV-derived ESBLs
(4, 10) usually demonstrate high-level resistance to broad-spectrum oxyimino
-lactams such as CAZ and cefotaxime. Moreover, several K. pneumoniae strains that showed
resistance to cephamycins as well as oxyimino cephalosporins were also
found to produce AmpC-type
-lactamases such as MOX-1 (8).
In Japan, furthermore, the emergence of carbapenem-resistant
gram-negative bacterial strains in species such as S. marcescens or P. aeruginosa is becoming a clinical
threat. Some of these isolates produce IMP-1 metallo-
-lactamase, and
these strains tend to demonstrate a wide range of resistance to various
broad-spectrum cephalosporins, cephamycins, and carbapenems (7, 9,
15, 17). Recently, gram-negative bacterial strains that were
speculated to produce metallo-
-lactamases very similar to IMP-1 were
also isolated in the United Kingdom, Italy, and Singapore (5, 11,
19). Thus, there is a need to develop a simple and specific
method to distinguish IMP-1 producers from other bacteria showing a
similar antibiotic resistance profile through the production of AmpC, ESBLs, or Toho-1-type
-lactamases. Indeed, PCR analysis usually gives satisfactory results in the detection of IMP-1 producers (7,
17), but it is not suitable for daily testing in clinical laboratories due to the cost. Therefore, the method described in this
study is very helpful for screening IMP-1-producing strains in daily
clinical laboratory testing.
 |
ACKNOWLEDGMENT |
This work was supported by a grant (The Research Project for
Emerging and Re-Emerging Infectious Diseases: Molecular Analyses of
Drug-Resistant Bacteria and Establishment of Rapid Identification Methods, 1997-1999) from the Ministry of Health and Welfare of Japan.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Bacterial and Blood Products, National Institute of Infectious
Diseases, 4-7-1 Gakuen, Musashi-Murayama, Tokyo 208-0011, Japan. Phone: 81-42-561-0771, ext. 500. Fax: 81-42-561-7173. E-mail:
yarakawa{at}nih.go.jp.
 |
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Journal of Clinical Microbiology, January 2000, p. 40-43, Vol. 38, No. 1
0095-1137/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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