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Antimicrobial Agents and Chemotherapy, July 2005, p. 2941-2948, Vol. 49, No. 7
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.7.2941-2948.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Identification of a New Allelic Variant of the Acinetobacter baumannii Cephalosporinase, ADC-7 ß-Lactamase: Defining a Unique Family of Class C Enzymes

Kristine M. Hujer,^1, Nashaat S. Hamza,^2, Andrea M. Hujer,¹ Federico Perez,¹ Marion S. Helfand,¹ Christopher R. Bethel,¹ Jodi M. Thomson,³ Vernon E. Anderson,⁴ Miriam Barlow,⁵ Louis B. Rice,¹ Fred C. Tenover,⁶ and Robert A. Bonomo^1,3*

Research Service, Louis Stokes Veterans Affairs Medical Center,¹ Division of Infectious Diseases, University Hospitals of Cleveland,² Department of Pharmacology,³ Department of Biochemistry, Case School of Medicine, Cleveland, Ohio,⁴ Rollins School of Public Health, Emory University, Atlanta, Georgia,⁵ Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia⁶

Received 17 June 2004/ Returned for modification 19 August 2004/ Accepted 18 March 2005

Top Abstract Introduction Materials and Methods Results and Discussion References

 
Acinetobacter spp. are emerging as opportunistic hospital pathogens that demonstrate resistance to many classes of antibiotics. In a metropolitan hospital in Cleveland, a clinical isolate of Acinetobacter baumannii that tested resistant to cefepime and ceftazidime (MIC = 32 µg/ml) was identified. Herein, we sought to determine the molecular basis for the extended-spectrum-cephalosporin resistance. Using analytical isoelectric focusing, a ß-lactamase with a pI of 9.2 was detected. PCR amplification with specific A. baumannii cephalosporinase primers yielded a 1,152-bp product which, when sequenced, identified a novel 383-amino-acid class C enzyme. Expressed in Escherichia coli DH10B, this ß-lactamase demonstrated greater resistance against ceftazidime and cefotaxime than cefepime (4.0 µg/ml versus 0.06 µg/ml). The kinetic characteristics of this ß-lactamase were similar to other cephalosporinases found in Acinetobacter spp. In addition, this cephalosporinase was inhibited by meropenem, imipenem, ertapenem, and sulopenem (K_i < 40 µM). The amino acid compositions of this novel enzyme and other class C ß-lactamases thus far described for A. baumannii, Acinetobacter genomic species 3, and Oligella urethralis in Europe and South Africa suggest that this cephalosporinase defines a unique family of class C enzymes. We propose a uniform designation for this family of cephalosporinases (Acinetobacter-derived cephalosporinases [ADC]) found in Acinetobacter spp. and identify this enzyme as ADC-7 ß-lactamase. The coalescence of Acinetobacter ampC ß-lactamases into a single common ancestor and the substantial phylogenetic distance separating them from other ampC genes support the logical value of developing a system of nomenclature for these Acinetobacter cephalosporinase genes.

Top Abstract Introduction Materials and Methods Results and Discussion References

 
Acinetobacter spp. are commonly associated with serious nosocomial infections (10, 14, 23, 37, 40, 41). Most recently, American troops wounded in Iraq (Operation Iraqi Freedom) and Afghanistan (Operation Enduring Freedom) have suffered severe infections from antibiotic-resistant Acinetobacter baumannii, making this organism an important pathogen in military health (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5345a1.htm). Unfortunately, antibiotic treatment of Acinetobacter infections has been challenging (11, 33, 46). Resistance against potent ß-lactam antibiotics (extended-spectrum cephalosporins and carbapenems) is reported from many centers, and infections caused by antibiotic-resistant Acinetobacter spp. lead to significant morbidity and mortality (6, 23, 31, 32). Ampicillin-sulbactam, cefoperazone-sulbactam, carbapenems, and antibiotic combinations (e.g., polymyxin B, rifampin, and doxycycline) are among the only therapeutic options effective against multidrug-resistant Acinetobacter infections (12, 13, 22, 33, 46, 48). Randomized control trials to define the best regimen are still forthcoming (22).

A growing number of ß-lactamases that confer resistance to extended-spectrum cephalosporins have been found in Acinetobacter spp. Recently, a group of AmpC-type cephalosporinases with highly alkaline isoelectric points (pI 9.0) have been described (4, 7, 24, 31, 32, 47). Here, we describe a novel class C ß-lactamase of A. baumannii found in a clinical isolate recovered from a hospital in Cleveland, Ohio. Based upon phylogenetic analysis, we propose a uniform designation for this family of ß-lactamases: ADC, for Acinetobacter-derived cephalosporinases. Since six related cephalosporinases have thus far been described, five from Acinetobacter spp. and one from Oligella urethralis, this enzyme is identified as ADC-7 ß-lactamase.

Top Abstract Introduction Materials and Methods Results and Discussion References

 
Bacterial isolate identification and susceptibility testing. The isolate originated from a patient hospitalized at MetroHealth Medical Center, an 800-bed Case Western Reserve University School of Medicine-affiliated hospital. API strips (BioMerieux, Durham, NC) were used to identify A. baumannii, and initial susceptibility testing was performed using Kirby Bauer disks (Becton Dickinson, Cockeysville, MD) according to criteria established by the National Committee for Clinical Laboratory Standards (NCCLS) (27). ß-Lactam agar dilution susceptibility tests (MICs) were performed using a 10⁴ inoculum delivered by a Steers replicator. The agar dilution MIC determinations were done a minimum of three times against each ß-lactam. The following ß-lactams were tested: ampicillin (Sigma Chemical Co., St. Louis, MO), sulbactam (a kind gift of Pfizer Research, Groton, CT), ampicillin-sulbactam (in a 2/1 ratio), tazobactam (a kind gift of Wyeth Pharmaceuticals, Pearl River, NY), piperacillin (Sigma Chemical Co.), piperacillin-tazobactam (in an 8/1 ratio, which is an exception to the NCCLS guidelines for testing piperacillin-tazobactam at a fixed concentration of 4 µg/ml), cephalothin (Sigma Chemical Co.), cefoxitin, imipenem-cilistatin and ertapenem (Merck Research Laboratories, Rahway, NJ), meropenem (AstraZeneca, Wilmington, DE), sulopenem (a kind gift of Pfizer Research), ceftazidime (GlaxoSmithKline, Research Triangle Park, NC), and cefepime (Bristol-Myers Squibb, Princeton, NJ). Escherichia coli DH10B was used as a control and host strain in these experiments (20).

Analytical isoelectric focusing. The pI of the ß-lactamase was estimated by liberating the enzyme from the periplasmic space using lysozyme and EDTA, according to a method developed in our laboratory (29). The extracted ß-lactamase was resolved on Ampholine PAG plates, pI 3.5 to 9.5 (Amersham Biosciences, Piscataway, NJ), with analytical isoelectric focusing standards as controls (Bio-Rad, Hercules, CA). To visualize the ß-lactamase, a 1 mM solution of nitrocefin (Becton Dickinson) was used in an overlay method as previously described (29).

PCR amplification, cloning, and DNA sequencing. Two sets of oligonucleotides for PCR amplification and identification of the cephalosporinase were used (Table 1). The first set (class C-1 and class C-2) amplifies AmpC ß-lactamases with significant homology to CMY-2 ß-lactamases (e.g., ACT-1, P99, and the CMY ß-lactamases). This class C primer set produces a 549-bp product (19-21). The second set of primers (ABAMPC-1 and ABAMPC-2) is specific for Acinetobacter sp. ß-lactamases and produces a 1,152-bp product (6, 7).

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TABLE 1. PCR amplification and sequencing primers used in these experiments

PCR amplification of bla_ADC-7 was performed in the following manner. A 1:10 dilution of an overnight culture was boiled for 10 min. Amplification was then performed with 10 µl of this dilution as the DNA template. PCR conditions included 28 cycles of amplification under the following conditions: denaturation at 94°C for 30 s, annealing at 50°C for 1 min, and extension at 72°C for 1 min. Cycling was followed by a final extension at 72°C for 10 min. PCR products were resolved on 1% agarose gels, stained with ethidium bromide, and photographed with UV illumination. X174 replicative-form DNA HaeIII fragments (Gibco BRL Life Technologies, Rockville, MD) were used to assess PCR product size.

To accurately determine the bla_ADC-7 sequence, high-fidelity PCR amplification (GeneAmp XL PCR kit; Applied Biosystems, Foster City, CA) on the A. baumannii isolate was performed as described above, using the ABAMPC-1 and ABAMPC-2 primers.

The PCR product generated was cloned into the pCR 2.1-TOPO vector (Invitrogen, Carlsbad, CA). Using an ALF Express automated DNA sequencer (Amersham Biosciences) with a Thermo Sequenase fluorescence-labeled primer cycle sequencing kit (Amersham Biosciences), we determined the sequence of the bla_ADC-7 gene from three independent TOPO clones that were created using the high-fidelity PCR amplification product. bla_ADC-7 in the TOPO clones was cycle sequenced under the following conditions: DNA was heated to 94°C for 1 min, followed by 30 cycles of 94°C for 30 s, 55°C for 1 min, and 60°C for 2 min. The Cy5-labeled sequencing primers are listed in Table 1.

Cloning of bla_ADC-7 into pET24a (+) plasmid for ß-lactamase characterization. For large-scale protein expression and ß-lactamase characterization, the bla_ADC-7 gene was cloned into the pET24a (+) vector (kanamycin resistance) (Novagen, Madison, WI) according to the following method. Using the GeneAmp XL PCR kit (Applied Biosystems), high-fidelity amplification of bla_ADC-7 in the TOPO clone was performed with primers EBF bla_ADC-7 and EBR bla_ADC-7, listed in Table 1. The cycling conditions used were 95^oC for 30 s, 55^oC for 1 min, and 72^oC for 1 min for 3 cycles and then 95^oC for 30 s, 60^oC for 1 min, and 72^oC for 1 min for 25 cycles, after which there was final extension at 72^oC for 10 min. A restriction digest of the pET24a (+) vector was done using NdeI, after which it was treated with Klenow to blunt the ends and then digested with BamHI. The amplification product was purified using the Promega Wizard kit (Madison, WI) and digested using EcoRV and BamHI. The resulting digest was gel purified. This product was ligated to the digested pET24a (+) vector and electroporated into E. coli DH10B. The resulting construct was sequenced with the Cy5-labeled primers ABAC 481F and ABAC 628R. After sequencing verification, the correct construct was transformed into E. coli BL21(DE3) cells for protein expression. This approach permitted us to express ADC-7 ß-lactamase without the accompanying C-terminal His tag.

Cloning of bla_ADC-7 into pBC SK (+) phagemid for MIC determinations. For MIC determinations, bla_ADC-7 was cloned into the pBC SK (+) phagemid vector (Stratagene, La Jolla, CA). First, bla_ADC-7 from the pET24a (+) bla_ADC-7 construct was digested with XbaI and BamHI in Multi-Core buffer (Promega) so as to maintain the stop codon. The digest mixture was supplemented with BamHI halfway through the reaction to further enhance cutting. This allowed us to maintain the 5' upstream flanking region from the pET24a (+) vector in front of the insert when ligated into pBC SK (+). The digests were gel purified and ligated to the XbaI- and BamHI-cut pBC SK (+) vector. This plasmid construct was transformed into electrocompetent E. coli DH10B cells (Invitrogen) and selected on plates containing 100 µg/ml of ampicillin and 20 µg/ml of chloramphenicol. Select colonies from ampicillin-chloramphenicol plates were isolated. Plasmids were extracted from these isolates, and bla_ADC-7 was sequenced using M13 Universal, M13 Reverse, and the two internal primers listed above, which were all Cy5 labeled.

ß-Lactamase induction. The induction of A. baumannii cephalosporinase by cefoxitin was tested using the disk approximation method (35). A single colony of the A. baumannii isolate was inoculated into 1 ml of fresh antibiotic-free Mueller-Hinton broth until turbidity approximating a 0.5 McFarland standard was obtained. This suspension was then used to create a lawn culture on Mueller-Hinton agar using a sterile cotton swab. Antibiotic disks containing cefoxitin and ceftazidime were placed onto the lawn culture at distances of 10, 15, and 20 mm apart. The plates were incubated for 18 to 24 h at 37°C. Induction of ß-lactamase by cefoxitin in A. baumannii was assessed by visual inspection for a flattening of the zone of inhibition between the disks containing cefoxitin and ceftazidime. This was done in triplicate.

ß-Lactamase purification. After successful cloning of bla_ADC-7 into pET24a (+), ADC-7 cephalosporinase was expressed and purified to homogeneity. Induction of a log-phase culture of E. coli BL21(DE3) possessing pET24a (+) bla_ADC-7 with 0.2 mM isopropyl-ß-D-thiogalactopyranoside (IPTG) was performed at 37°C for 4 h in 500 ml Luria-Bertani broth. These cells were pelleted and resuspended in 50 ml of 50 mM Tris (pH 7.4), and ß-lactamase was liberated with lysozyme and EDTA according to the established method (see above).

Preparative isoelectric focusing was next performed using a Sephadex G100 gel matrix (Amersham) and commercially prepared ampholines, pH 3.5 to 10.0 (Amersham Biosciences). The run conditions used for preparative isoelectric focusing have been described previously (20). After overnight isoelectric focusing, areas of the gel demonstrating ß-lactamase activity by nitrocefin overlay were cut from the gel, eluted with phosphate-buffered saline, pH 7.4, on polyethylene glycol columns (Amersham Biosciences), and concentrated using an Amicon concentrator.

Protein concentrations were determined by a Bio-Rad protein assay using bovine serum albumin as a standard. Sodium dodecyl sulfate-polyacrylamide gels were stained using Coomassie blue R-250, and protein molecular weight was estimated using prestained low-molecular-weight standards supplied by Bio-Rad.

MALDI-TOF mass spectrometry. ADC-7 ß-lactamase was desalted and eluted using C₁₈ Millipore Ziptips according to the manufacturer's recommended protocol (Millipore Corp., Billerica, MA). Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) spectra were acquired on a BiFlex III MALDI-TOF spectrometer (Bruker Daltonics, Billerica, MA) equipped with a pulsed N₂ laser (3-ns pulse at 337 nm) and XTOF acquisition software. Aliquots of the loaded samples were eluted onto a stainless steel MALDI target plate with the wetting solution saturated with sinapinic acid, the matrix compound. The ion source was operated with an accelerating voltage of –25 kV and had an ion deflector to deflect interfering low-mass ions that reduce sensitivity. Linear mode was used with a laser power attenuation ranging from 70 to 85 and an average of 300 scans acquired. Data were analyzed using Bruker software (42).

Kinetics. Steady-state kinetic parameters (K_m, k_cat, V_max, and k_cat/K_m) were measured by continuous assays at room temperature and determined using nonlinear least squares employing the program Enzfitter (Sigma) (20). Each rate determination was performed in triplicate in 20 mM phosphate-buffered saline at pH 7.4 in an Agilent 8453 diode array spectrophotometer with a 1-cm path length. Direct velocity measurements were obtained using cephaloridine (₂₆₀ = –10,200 M^–1 cm^–1), cephalothin (₂₆₂ = –7,660 M^–1 cm^–1), and nitrocefin (₄₈₂ = 17,400 M^–1 cm^–1).

A competition assay was performed to determine the dissociation constant for the preacylation complex, K_i, of the inhibitors (clavulanic acid, sulbactam, tazobactam, imipenem, meropenem, ertapenem, and sulopenem), cefoxitin, extended-spectrum cephalosporins (ceftazidime, cefotaxime, and cefepime), and aztreonam. We used a final concentration of 100 µM nitrocefin as the indicator substrate and 1.2 nM ADC-7 ß-lactamase. The apparent K_m or K_i was calculated using the following formula: true K_m or K_i = apparent K_m or K_i/(l + [S]/K_{m ncf}), where [S] is the substrate concentration and ncf is nitrocefin.

BLAST search. The ampC and ampC homolog DNA sequences were identified with a pBLASTn (1, 2) search of the nonredundant National Center for Biotechnology Information sequence database and the completed microbial genomes database using characterized ampC genes as query sequences.

Alignment. The protein sequences of the identified AmpC ß-lactamases and their homologs were aligned with ClustalX 1.8 (45) using the Gonet 250 similarity matrix with a gap opening penalty of 35 and a gap extension penalty of 0.75 for the pairwise alignment stage and a gap opening penalty of 15 and a gap extension penalty of 0.3 for the multiple-alignment stage.

The corresponding DNA coding sequences were aligned by introducing triplet gaps between codons corresponding to gaps in the aligned protein sequences by using the program CodonAlign (16). CodonAlign for Macintosh and for PC (Windows) computers, and source code that can be compiled for other platforms, is available at no charge from Sinauer.

Phylogenetic reconstruction. Phylogenies were constructed by the Bayesian method (25, 26, 34) as implemented by the program MrBayes (18). MrBayes is available at no charge from www.mrbayes.net. The evolutionary model used was the General Time Reversible model (44). Because evolutionary rates are not homogeneous for every site in a gene, site variation in evolutionary rate was estimated separately for first, second, and third positions of sites within codons. Four chains, with a "temperature" of 0.2 for the heated chains, were run for each tree. Trees were sampled every 100 generations. A total of 2,500,000 generations were run with a burn-in of 500 trees. The length of burn-in was set at a value that exceeded twice the number of trees required for convergence of the ln likelihood. Because the consensus trees calculated by MrBayes do not include the posterior probabilities of the clades, each entire set of trees was imported into PAUP* (43), and the same trees used by MrBayes to calculate a consensus were used to calculate a 50% majority rule consensus in PAUP* (43). The resulting tree shows the posterior probabilities of the clades, i.e., the percentage of time that those taxa are included in the clade.

The consensus tree calculated by MrBayes was imported into PAUP* for the purposes of displaying and printing the tree. The tree was rooted using experimentally determined penicillin binding proteins as the outgroup.

Nucleotide sequence accession number. The nucleotide sequence generated for bla_ADC-7 was deposited in GenBank (AY648950).

Top Abstract Introduction Materials and Methods Results and Discussion References

 
Antimicrobial susceptibility profile of A. baumannii. Agar dilution antimicrobial susceptibility testing revealed that the representative A. baumannii isolate (clone 9) was resistant to ampicillin, piperacillin, and all cephalosporins tested (Table 2). Most notably, resistance was demonstrated against cephalothin, ceftazidime, cefepime, cefoxitin, and cefotaxime (MIC 32 µg/ml). This isolate was fully susceptible to sulbactam (4 µg/ml), ampicillin-sulbactam (4/2 µg/ml), imipenem-cilistatin (0.25 µg/ml), and meropenem (0.25 µg/ml). As expected, the MIC of ertapenem (2 µg/ml) was elevated for clone 9 of A. baumannii compared to the MICs of other carbapenems. The MIC for this isolate of sulopenem, a penem antibiotic formerly known as CP 65,207, was 0.5 µg/ml (NCCLS susceptibility standards are not yet available for this penem). To our surprise, MICs were 64/8 µg/ml for piperacillin-tazobactam and 16 µg/ml for tazobactam alone.

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TABLE 2. MICs of ß-lactams for the A. baumannii clinical isolate and cloned blaADC-7 compared to E. coli DH10B control

Susceptibility profile of E. coli DH10B expressing the ADC-7 ß-lactamase. To evaluate the ß-lactamase activity of bla_ADC-7 in a uniform and defined genetic background (E. coli DH10B), we cloned bla_ADC-7 from pET24a (+) into the pBC SK (+) vector. This construct possessed the promoter used in pET24a (+) (see above).

It was evident that ADC-7 ß-lactamase expressed in E. coli DH10B contributed to ampicillin, piperacillin, cephalothin, and cefoxitin resistance (MICs 32 µg/ml) (Table 2). The MICs of ceftazidime and cefotaxime were both 4 µg/ml. Unlike with the parent A. baumannii isolate, cefepime resistance was not observed (MIC = 0.06 µg/ml). Furthermore, for E. coli DH10B, sulbactam and tazobactam MICs were >16 µg/ml. Their activities improved in ß-lactam-ß-lactamase inhibitor combinations with ampicillin or piperacillin (ampicillin-sulbactam MIC = 8/4 µg/ml and piperacillin-tazobactam MIC = 32/4 µg/ml). Meropenem, ertapenem, imipenem, and sulopenem remained active against E. coli DH10B possessing pBC SK (+) bla_ADC-7 (MIC = 0.06 µg/ml). It is clear that even behind a strong promoter, resistance to cefepime, the penems, and carbapenems was not due to the ß-lactamase.

bla_ADC-7 identification and analysis. PCR primers used for detecting bla_AmpC enzymes related to CMY-2 (class C-1 and class C-2 primers) were unable to amplify AmpC-like ß-lactamases. However, primers based upon the nucleotide sequence of bla_AmpC of A. baumannii described by Bou et al. (ABAMPC-1 and ABAMPC-2 primers) successfully amplified the bla_ADC-7 gene (Tables 1 and 3) (6, 7).

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TABLE 3. Comparison of DNA sequences of class C cephalosporinases described for Acinetobacter spp. and O. urethralis

DNA sequencing using primers specific for the A. baumannii cephalosporinase gene that we cloned into the pCR 2.1-TOPO vector revealed a ß-lactamase with 8 amino acid differences compared to ABAC-1 (ADC-3) (Table 3) and Acinetobacter genomic species 3 (ADC-5) AmpC and 7 amino acid differences compared to ABAC-2 (ADC-4) (Table 3) (Fig. 1). The importance of these changes in amino acid sequence and their impact on the hydrolytic profile of ADC-7 are unknown. Given the high degree of similarity of the bla gene detected in Cleveland to the sequences of six bla genes present in A. baumannii, Acinetobacter genomic species 3, and O. urethralis and the resultant phylogenetic analysis, we propose a uniform designation for this family of cephalosporinases (Acinetobacter-derived cephalosporinases [ADC]) and identify this enzyme as ADC-7 ß-lactamase (see below) (Table 3) (4, 7, 24, 38).

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FIG. 1. Nucleotide sequence of the 1,152-bp amplification product of the ADC-7 ß-lactamase. The deduced amino acid sequence of the ADC-7 ß-lactamase is shown in single letter code below the nucleotide triplets. ATG and TAA represent the initiation and termination codons, respectively. The positions of the primers used to amplify blaADC-7 are indicated by arrowheads. We have represented the ADC-7 ß-lactamase active site as S-V-S-K in bold, the conserved triad K-T-G in bold, and the class C typical motif Y-X-N in bold. The predicted signal peptide cleavage site is between amino acids 23 and 24 (www.expasy.org). The GenBank accession numbers for each of the comparison cephalosporinase genes found in Acinetobacter spp. and O. urethralis are listed.

ß-Lactamase induction. The induction of A. baumannii AmpC ADC-7 by cefoxitin was evaluated using a disk approximation method. As others have found, we were not able to show that the ADC-7 ß-lactamase from this isolate was inducible (7).

ß-Lactamase purification and characterization. The ß-lactamase possessed a pI of 9.2. Purification of ADC-7 ß-lactamase was possible from E. coli BL21(DE3) cells containing pET24a (+) bla_ADC-7 grown in 500 ml Luria-Bertani broth. The estimated molecular mass based upon comparison to prestained protein standards was 41,000 Da. The calculated molecular mass of A. baumannii AmpC ADC-7 ß-lactamase is 40,631 Da, and the predicted pI is 9.22 (www.expasy.org). The MALDI-TOF mass spectrum indicated a molecular mass of 40,540 Da (±0.2% error).

Substrate profile of ADC-7 ß-lactamase. The substrate profile of ADC-7 ß-lactamase closely resembled other AmpC ß-enzymes found in Acinetobacter spp. (30) (Table 4). Narrow-spectrum cephalosporins were the most efficiently hydrolyzed substrates. It is remarkable that the k_cat/K_m ratio for the hydrolysis of nitrocefin, cephaloridine, and cephalothin approaches the upper limits of catalytic efficiency for substrates by class C ß-lactamases (30 x 10⁶ M^–1 s^–1) (8). These kinetic data are consistent with the determined MICs showing high-level cephalothin resistance.

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TABLE 4. Substrate and inhibitor profile of the ADC-7 ß-lactamase

Using the Agilent 8453 diode array spectrophotometer with a 1-cm path length, the hydrolysis of aztreonam, ceftazidime, cefepime, and cefotaxime was not measurable compared to the narrow-spectrum cephalosporins assayed. To assess the apparent affinity of these agents for ADC-7 ß-lactamase, we employed a competition assay using nitrocefin as the indicator substrate. In this manner, we found that cefotaxime and aztreonam possessed the greatest affinities for ADC-7 ß-lactamase (0.1 and 0.7 µM, respectively), while the affinity for ceftazidime was less (11.6 µM). Cefepime, a zwitterionic cephalosporin, demonstrated the most elevated K_i (>500 µM).

Since imipenem, meropenem, and ertapenem are ß-lactams that contain large, ethoxy substituents in the 6 position, we reasoned that these substrates would serve as effective inhibitors of ADC-7 ß-lactamase (3). As expected, our experiments revealed that imipenem, ertapenem, and meropenem were superior inhibitors of ADC-7 ß-lactamase when compared to tazobactam, clavulanate, and sulbactam (Table 4). It is noteworthy that tazobactam and sulbactam demonstrated a modest ability to inhibit ADC-7 ß-lactamase activity (5, 9). Sulopenem, a penem antibiotic that combines features of penicillin and cephalosporins, also has a low K_i against ADC-7 ß-lactamase (15, 17). The singular property of carbapenems, sulfones, and penems to serve as inhibitors of penicillin binding proteins and class C enzymes merits consideration.

Phylogenetic reconstruction. Phylogenetic reconstruction of the AmpC ß-lactamases shows the relationship of the Acinetobacter ampC genes to other ampC genes (Fig. 2). Because this tree is represented as a phylogram, the lengths of the branches represent the number of DNA mutations that have occurred since the divergence of the genes represented in this phylogeny. The number of mutations separating any two genes in this phylogeny can be estimated by summing the lengths of the horizontal lines necessary to draw a path from one gene to the other. Vertical lines do not represent the occurrence of mutations within genes; rather, vertical lines indicate that a divergence has occurred, and they serve as a marker for the relative point during ampC evolution at which that divergence occurred. The number of mutations separating any two genes is greater than the number of sites that differ between genes because multiple mutations can occur at any given site. Because of the way in which phylograms represent the evolutionary history of a gene, the vertical proximity of taxon labels to each other is not an accurate measure of the relatedness of taxa.

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FIG. 2. Phylogenetic analysis of the ADC-7 ß-lactamase. This phylogram, inferred through Bayesian analysis, represents an estimate of the relationships that exist among the class C ß-lactamases and their homologs. Branch lengths are representative of the number of nucleotide mutations that have occurred since the divergence of the genes represented in this tree. Posterior probabilities for groupings that occurred in less than 80% of the trees sampled are indicated by a circle that contains the percentage of time during which that grouping did occur. Plasmidic resistance genes are indicated in boldface italics. A single asterisk represents a known penicillin binding protein, and a double asterisk represents putative penicillin binding proteins. The phylogram was based upon sequence data generated from AmpC ß-lactamases in the supplemental material.

As illustrated by the phylogeny, the Acinetobacter ampC genes are descended from a common ancestor and are more closely related to each other than ampC genes found in other species of bacteria. Furthermore, the Acinetobacter ampC genes are separated from their closest relative, PimmA5, by nearly 2,000 mutations. The coalescence of Acinetobacter ampC genes to a single common ancestor and the substantial distance separating them from other ampC genes support the logical value of developing a system of nomenclature for these genes (see the supplemental material).

In conclusion, we have found a new Acinetobacter spp. cephalosporinase, ADC-7 ß-lactamase. Among Acinetobacter genospecies, it is likely that this cephalosporinase is widespread and that many separate alleles exist. Based upon the kinetic behavior of this enzyme, it is becoming clear that extended-spectrum-cephalosporin resistance seems to be a consequence of the interplay of this enzyme and other inherent permeability properties related to the Acinetobacter sp. outer cell membrane (7, 30, 36). The resistance to cefepime in A. baumannii compared to what was seen in E. coli DH10B containing bla_ADC-7 supports this notion. Investigations are under way to examine for the presence of integrons and insertion sequence elements (28, 39). Insertion sequence elements can cause increased expression of this class C ß-lactamase and result in very high levels of cephalosporin resistance (24, 38).

 
The Veterans Affairs Medical Career Development and Merit Review Program supported M.S.H., R.A.B., and L.B.R. Research grants from Pfizer Corporation, AstraZeneca, Ortho-McNeil, and Merck Research Laboratories supported the efforts of K.M.H. At the time of this research, M.S.H. was supported by a training grant awarded from the National Institutes of Health; she is now a recipient of a Department of Veterans Affairs Career Development Award. J.M.T. was supported in part by NIH T32 GM07250 and the Case Medical Scientist Training Program. M.B.'s work was supported through funding from Project ICARE. Phase 5 of Project ICARE is supported in part by unrestricted grants to the Rollins School of Public Health of Emory University by AstraZeneca Pharmaceuticals; BioMerieux, Incorporated; Elan Pharmaceuticals, San Diego, CA; and Pfizer Incorporated, New York, NY.

We thank Michael Jacobs and Anne Morrissey for assistance with reidentification of Acinetobacter isolates at the Clinical Microbiology Laboratory of University Hospitals, Cleveland, Ohio.

 
* Corresponding author. Mailing address: Research Service, Louis Stokes Cleveland Veterans Affairs Medical Center, 10701 East Blvd., Cleveland, OH 44016. Phone: (216) 791-3800, ext. 4399. Fax: (216) 231-3482. E-mail: robert.bonomo{at}med.va.gov.

Supplemental material for this article may be found at http://aac.asm.org/.

Contributed equally to this project.

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Antimicrobial Agents and Chemotherapy, July 2005, p. 2941-2948, Vol. 49, No. 7
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.7.2941-2948.2005
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