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Antimicrobial Agents and Chemotherapy, October 2001, p. 2965-2968, Vol. 45, No. 10
Service de Bactériologie-Virologie,
Hôpital de Bicêtre, Assistance Publique/Hôpitaux de
Paris, Faculté de Médecine Paris-Sud, 94275 Le
Kremlin-Bicêtre Cedex, France
Received 23 January 2001/Returned for modification 15 May
2001/Accepted 13 July 2001
From whole-cell DNA of a clinical isolate of the enterobacterial
species Rahnella aquatilis, a Rahnella
aquatilis is an enterobacterial species (11)
that is widely distributed in water and soils as well as in foods (26). It is an opportunistic pathogen that predominantly
causes bacteremia and septicemia, as well as endocarditis, urinary
tract infections, and wound infections, mostly in immunocompromised patients (7, 14, 17, 26).
A recent study investigated the antibiotic susceptibility patterns of
72 R. aquatilis strains of clinical and
environmental origins (26). It showed that R. aquatilis is resistant to amoxicillin, ticarcillin,
cefuroxime, and cephalothin; is of intermediate susceptibility to
cefotaxime and ceftriaxone; and is susceptible to ceftazidime and
imipenem. R. aquatilis is susceptible to
In this work, we report on characterization of a novel class A
Bacterial strains, plasmid analysis, and conjugation.
R. aquatilis clinical strain
DON-1 was isolated from a central venous catheter of a 48-year-old
patient hospitalized in the neurosurgery unit of the Hôpital
Bicêtre (France) in September 2000. It was identified by the API
20-E system (bioMérieux, Marcy-L'Etoile, France). R. aquatilis reference strain CIP 103.904 was from the Institut Pasteur strain collection (Paris). Plasmid DNA extractions, performed as described previously (24), failed to identify
plasmids in R. aquatilis DON-1. Direct transfer
of an amoxicillin resistance marker from R. aquatilis DON-1 to rifampin-resistant
Escherichia coli JM109 also failed (24).
Cloning and sequence analysis of the
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.10.2965-2968.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Biochemical-Genetic Characterization of the
Chromosomally Encoded Extended-Spectrum Class A
-Lactamase from
Rahnella aquatilis
ABSTRACT
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Abstract
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References
-lactamase gene
was cloned that encoded a chromosomally encoded Ambler class A enzyme,
RAHN-1. RAHN-1, with a pI of 7.2, shares 76, 73, and 71% amino acid
identity with the extended-spectrum -lactamase of chromosomal origin
from Serratia fonticola and with the plasmid-mediated
-lactamases CTX-M-2 and CTX-M-1, respectively. The hydrolysis
spectrum of the clavulanic acid-inhibited RAHN-1 was expanded to
cephalosporins such as cefuroxime, cefotaxime, and ceftriaxone, but not
to ceftazidime. Its expression was not inducible.
TEXT
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Abstract
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References
-lactam combinations such as amoxicillin-clavulanic acid and
piperacillin-tazobactam. This -lactam susceptibility pattern
suggested the presence of a clavulanic acid-inhibited Ambler class A
-lactamase (26).
-lactamase with an extended-spectrum hydrolysis profile from R. aquatilis.
-lactamase gene from
R. aquatilis DON-1.
Whole-cell DNA of
R. aquatilis DON-1 was extracted as described
previously (23). It was partially digested with
Sau3AI and ligated into the BamHI site of the
pBK-CMV phagemid (Stratagene, Amsterdam, The Netherlands) as described
previously (23). The ligation products were electroporated
into electrocompetent E. coli DH10B (Bio-Rad,
Ivry-sur-Seine, France). E. coli DH10B cells harboring
recombinant plasmids were selected on kanamycin (30 µg/ml)- and
amoxicillin (20 µg/ml)-containing Trypticase soy agar plates. The
sizes of the inserts were determined by double restriction analysis
(25).
-lactamases possessing
a serine active site or penicillin-binding proteins (13).
Three other structural elements were also found: serine-aspartic
acid-asparagine (S-D-N) in positions 130 to 132, glutamate-X-X-leucine-asparagine (E-X-X-L-N) in position 166 to 170, which is part of the so-called omega loop of class A -lactamases, and lysine-threonine-glycine (K-T-G) in positions 234 to 236 (Fig. 1).
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FIG. 1.
Alignment of the RAHN-1 amino acid sequence with those
of the most closely related enzymes, SFO-1, CTX-M-1, and CTX-M-2.
Numbering is according to that of Ambler et al. (1). Dots
indicate identical amino acid residues, and dashes indicate gaps
introduced to optimize the alignment. The vertical arrow is the
putative cleavage site of the leader peptide of RAHN-1 -lactamase.
Four structural elements characteristic of class A -lactamases are
boxed in gray.
-lactamases. The highest percentages of
identity were found with the plasmid-mediated extended-spectrum -lactamase (ESBL) SFO-1 (75.9%), which originated from
Serratia fonticola (18, 20), and with the
plasmid-mediated extended-spectrum -lactamases CTX-M-2
(73.2%) and CTX-M-1 (71.5%) (3, 4). CTX-M-2
-lactamase has been recently reported as being related to the
chromosomally encoded class A -lactamase from Kluyvera ascorbata (GenBank accession no. AJ 272538). RAHN-1 shared 69, 65, and 62% amino acid identity with the chromosomally encoded class A
-lactamases from Klebsiella oxytoca (2),
Citrobacter diversus (22), and Proteus
vulgaris (21), respectively.
Upstream of the ORF coding for RAHN-1, no putative LysR-type
regulator gene was identified, while such chromosome-encoded AmpR regulators were divergently transcribed usptream of the
-lactamase genes coding for the class A -lactamases of C. diversus, P. vulgaris, and S. fonticola
(9, 12, 18). However, 126 bp upstream of
blaRAHN-1, a stop codon of a 1,664-bp ORF
was identified that encoded a putative protein sharing 50% identity
with a methyl-accepting chemotaxis protein of E. coli
(5). Further upstream, another 996-bp ORF was identified
that encoded a putative protein that shared 36% amino acid identity
with an ATP-binding protein of E. coli (10).
Downstream of blaRAHN-1, no ORF could have
been identified, since the end of the 5.6-kb insert of pRAHN-1 was located 2 bp downstream of the stop codon of
blaRAHN-1. A Southern transfer
(25) was performed with a PCR-obtained 861-bp internal fragment of blaRAHN-1 as a labeled probe
(23) and whole-cell DNAs of R. aquatilis DON-1 and the CIP 103.904 reference strain. In
both cases, a positive signal was detected at the chromosome migration
position, further indicating the chromosomal origin of
blaRAHN-1 (data not shown).
Susceptibility testing.
MICs of selected -lactams were
determined by an agar dilution technique on Mueller-Hinton plates as
described previously (24) and interpreted according to the
guidelines of the National Committee for Clinical Laboratory Standards
(19). R. aquatilis DON-1 was
resistant to amoxicillin, ticarcillin, cephalothin, and cefuroxime
(Table 1). It was of intermediate
susceptibility to piperacillin, cefotaxime, ceftriaxone, cefepime, and
aztreonam and remained susceptible to ceftazidime and imipenem
(Table 1). This pattern of -lactam susceptibility corresponded to
that described previously for R. aquatilis
strains and R. aquatilis reference strain
CIP 103.904 (Table 1) (26). Once cloned in pBK-CMV
(pRAHN-1) and expressed in E. coli DH10B (Table 1), the same
antibiotic resistance pattern was obtained, thus showing the
contribution of RAHN-1 to the natural -lactam resistance phenotype
of R. aquatilis DON-1. Addition of clavulanic
acid and tazobactam decreased the MICs of -lactams, especially those
of cefotaxime and ceftriaxone, thus showing that RAHN-1 is a
clavulanic acid-inhibited ESBL.
|
-lactam resistance pattern of R. aquatilis was similar to that of S. fonticola
(20). Moreover, R. aquatilis
strains, like C. diversus and P. vulgaris strains, are of intermediate susceptibility or resistance
to cefuroxime. In contrast to K. oxytoca, which
overproduces its chromosomally encoded class A -lactamase
(8), R. aquatilis was not
resistant to aztreonam.
Biochemical properties of -lactamase RAHN-1.
Cultures of
E. coli DH10B(pRAHN-1) were grown overnight at 37°C in 4 liters of Trypticase soy broth containing amoxicillin (30 µg/ml) and
kanamycin (30 µg/ml). The -lactamase extract was obtained after
sonification as described previously (24). It was loaded
onto a preequilibrated Q-Sepharose column (Amersham Pharmacia Biotech).
The enzyme was eluted with a linear NaCl gradient (0 to 1 M) in
Tris-HCl buffer (pH 9.0). The -lactamase was eluted at a
concentration of 100 to 150 mM NaCl. Fractions containing -lactamase
activity were loaded onto the same preequilibrated Q-Sepharose column
in the same buffer and eluted with a linear NaCl gradient (0 to 200 mM)
in the same Tris-HCl buffer at a concentration of 100 mM NaCl.
Fractions containing -lactamase activity were pooled and dialyzed
overnight against 100 mM phosphate buffer (pH 7.0) and then
concentrated 10-fold with Centrisart-C30 spin column (Sartorius,
Goettingen, Germany). The specific activity of the purified RAHN-1
enzyme was 15,100 mU mg of protein
1, determined
with 100 µM cephalothin as a substrate with a 35-fold purification.
Its purity was estimated to be 80% by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)
(25).
-lactamase was used for kinetic measurements
performed at 30°C in 100 mM sodium phosphate (pH 7.0). The initial rates of hydrolysis were determined with an ULTROSPEC 2000 UV spectrophotometer (Amersham Pharmacia Biotech) as described previously (24). The 50% inhibitory concentrations
(IC50s) were determined for clavulanic acid and
tazobactam as reported previously (24).
RAHN-1 had a strong activity against benzylpenicillin, piperacillin,
cephalothin, cefuroxime, and ceftriaxone (Table
2). A significant hydrolytic activity
against cefotaxime was also observed, while no activity against
ceftazidime was detectable. The activity against cefuroxime is shared
by the -lactamases of C. diversus, P. vulgaris, and S. fonticola (9, 15, 16, 20,
21). RAHN-1 affinity for cefepime, cefpirome, and aztreonam was
low (high Km values) (Table 2). Inhibition
studies showed that the IC50s of clavulanic acid
and tazobactam (0.4 and 0.06 µM, respectively) were similar to those
of TEM-1 -lactamase. These results showed that the RAHN-1 enzyme is
a -lactamase that may belong to the group 2be -lactamases of the
Bush-Jacoby-Medeiros classification (6).
|
-lactamase with a pI of 7.2. The expected cleavage site of
the peptide leader was between an alanine residue and a glutamine
residue (between the S-W-A and Q-T motifs [Fig. 1]). The same pI was
found for culture of R. aquatilis reference strain CIP 103.904. The relative molecular mass of RAHN-1, determined with purified enzyme by SDS-PAGE as described previously
(24), was ca. 29 kDa (data not shown). This value
corresponds to those reported for class A -lactamases
(6).
Induction studies with 0.5 µg of cefoxitin per ml as a -lactam
inducer (23) and 100 µM benzylpenicillin as a substrate failed to detect induction of -lactamase with cultures of
R. aquatilis DON-1 and CIP 103.904. These
results, although different from those reported previously
(14), are consistent with the lack of a LysR-type
regulator gene located just upstream of
blaRAHN-1. The chromosomally encoded class
A -lactamase of R. aquatilis is not
inducible, as opposed to the naturally occurring class A enzymes
of P. vulgaris, S. fonticola, and
C. diversus.
Conclusion.
Comparison of the RAHN-1 sequence to those of
other class A ESBLs identified several amino acid residues that may be
involved in its extended spectrum of hydrolysis (15). This
is the case for the serine residue in position 237 found, for example,
in -lactamases of S. fonticola, P. vulgaris,
and CTX-M-type enzymes (20, 21, 27, 28). The hydrolysis
spectrum of RAHN-1 is similar to that of CTX-M-type enzymes that do not
compromise ceftazidime.
-lactams in treating R. aquatilis
infections, since failure of a cefotaxime-containing regimen has been
reported (17). Finally, -lactamases from this
enterobacterial species, R. aquatilis, may be
progenitors of plasmid-encoded ESBLs, as recently described for the
plasmid-encoded -lactamases SFO-1 and CTX-M-2 related to the
chromosomally encoded -lactamases of S. fonticola and
K. ascorbata, respectively.
Nucleotide sequence accession number. The nucleotide sequences of the RAHN-1 gene and the sequence of the 5.6-kb insert of pRAHN-1 have been assigned to the GenBank nucleotide database under accession no. AF338038.
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ACKNOWLEDGMENTS |
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This work was funded by a grant from the Ministères de l'Education Nationale et de la Recherche (UPRES, JE 2227), Université Paris XI, Faculté de Médecine Paris-Sud, Le Kremlin Bicêtre, France.
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FOOTNOTES |
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* Corresponding author. Mailing address: Service de Bactériologie-Virologie, Hôpital de Bicêtre, 78 rue du Général Leclerc, 94275 Le Kremlin-Bicêtre Cedex, France. Phone: 33-1-45-21-36-32. Fax: 33-1-45-21-63-40. E-mail: nordmann.patrice{at}bct.ap-hop-paris.fr.
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Antimicrobial Agents and Chemotherapy, October 2001, p. 2965-2968, Vol. 45, No. 10
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.10.2965-2968.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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