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3.  Population Structure of KPC-Producing Klebsiella pneumoniae Isolates from Midwestern U.S. Hospitals 
Genome sequencing of carbapenem-resistant Klebsiella pneumoniae isolates from regional U.S. hospitals was used to characterize strain diversity and the blaKPC genetic context. A phylogeny based on core single-nucleotide variants (SNVs) supports a division of sequence type 258 (ST258) into two distinct groups. The primary differences between the groups are in the capsular polysaccharide locus (cps) and their plasmid contents. A strict association between clade and KPC variant was found. The blaKPC gene was found on variants of two plasmid backbones. This study indicates that highly similar K. pneumoniae subpopulations coexist within the same hospitals over time.
doi:10.1128/AAC.00125-14
PMCID: PMC4136011  PMID: 24913165
4.  Reclaiming the Efficacy of β-Lactam–β-Lactamase Inhibitor Combinations: Avibactam Restores the Susceptibility of CMY-2-Producing Escherichia coli to Ceftazidime 
CMY-2 is a plasmid-encoded Ambler class C cephalosporinase that is widely disseminated in Enterobacteriaceae and is responsible for expanded-spectrum cephalosporin resistance. As a result of resistance to both ceftazidime and β-lactamase inhibitors in strains carrying blaCMY, novel β-lactam–β-lactamase inhibitor combinations are sought to combat this significant threat to β-lactam therapy. Avibactam is a bridged diazabicyclo [3.2.1]octanone non-β-lactam β-lactamase inhibitor in clinical development that reversibly inactivates serine β-lactamases. To define the spectrum of activity of ceftazidime-avibactam, we tested the susceptibilities of Escherichia coli clinical isolates that carry blaCMY-2 or blaCMY-69 and investigated the inactivation kinetics of CMY-2. Our analysis showed that CMY-2-containing clinical isolates of E. coli were highly susceptible to ceftazidime-avibactam (MIC90, ≤0.5 mg/liter); in comparison, ceftazidime had a MIC90 of >128 mg/liter. More importantly, avibactam was an extremely potent inhibitor of CMY-2 β-lactamase, as demonstrated by a second-order onset of acylation rate constant (k2/K) of (4.9 ± 0.5) × 104 M−1 s−1 and the off-rate constant (koff) of (3.7 ± 0.4) ×10−4 s−1. Analysis of the reaction of avibactam with CMY-2 using mass spectrometry to capture reaction intermediates revealed that the CMY-2–avibactam acyl-enzyme complex was stable for as long as 24 h. Molecular modeling studies raise the hypothesis that a series of successive hydrogen-bonding interactions occur as avibactam proceeds through the reaction coordinate with CMY-2 (e.g., T316, G317, S318, T319, S343, N346, and R349). Our findings support the microbiological and biochemical efficacy of ceftazidime-avibactam against E. coli containing plasmid-borne CMY-2 and CMY-69.
doi:10.1128/AAC.02625-14
PMCID: PMC4136032  PMID: 24820081
5.  Surveillance of Carbapenem-Resistant Klebsiella pneumoniae: Tracking Molecular Epidemiology and Outcomes through a Regional Network 
Carbapenem resistance in Gram-negative bacteria is on the rise in the United States. A regional network was established to study microbiological and genetic determinants of clinical outcomes in hospitalized patients with carbapenem-resistant (CR) Klebsiella pneumoniae in a prospective, multicenter, observational study. To this end, predefined clinical characteristics and outcomes were recorded and K. pneumoniae isolates were analyzed for strain typing and resistance mechanism determination. In a 14-month period, 251 patients were included. While most of the patients were admitted from long-term care settings, 28% of them were admitted from home. Hospitalizations were prolonged and complicated. Nonsusceptibility to colistin and tigecycline occurred in isolates from 7 and 45% of the patients, respectively. Most of the CR K. pneumoniae isolates belonged to repetitive extragenic palindromic PCR (rep-PCR) types A and B (both sequence type 258) and carried either blaKPC-2 (48%) or blaKPC-3 (51%). One isolate tested positive for blaNDM-1, a sentinel discovery in this region. Important differences between strain types were noted; rep-PCR type B strains were associated with blaKPC-3 (odds ratio [OR], 294; 95% confidence interval [CI], 58 to 2,552; P < 0.001), gentamicin nonsusceptibility (OR, 24; 95% CI, 8.39 to 79.38; P < 0.001), amikacin susceptibility (OR, 11.0; 95% CI, 3.21 to 42.42; P < 0.001), tigecycline nonsusceptibility (OR, 5.34; 95% CI, 1.30 to 36.41; P = 0.018), a shorter length of stay (OR, 0.98; 95% CI, 0.95 to 1.00; P = 0.043), and admission from a skilled-nursing facility (OR, 3.09; 95% CI, 1.26 to 8.08; P = 0.013). Our analysis shows that (i) CR K. pneumoniae is seen primarily in the elderly long-term care population and that (ii) regional monitoring of CR K. pneumoniae reveals insights into molecular characteristics. This work highlights the crucial role of ongoing surveillance of carbapenem resistance determinants.
doi:10.1128/AAC.02636-14
PMCID: PMC4068524  PMID: 24798270
6.  Multiplex PCR for Identification of Two Capsular Types in Epidemic KPC-Producing Klebsiella pneumoniae Sequence Type 258 Strains 
We developed a multiplex PCR assay capable of identifying two capsular polysaccharide synthesis sequence types (sequence type 258 [ST258] cps-1 and cps-2) in epidemic Klebsiella pneumoniae ST258 strains. The assay performed with excellent sensitivity (100%) and specificity (100%) for identifying cps types in 60 ST258 K. pneumoniae sequenced isolates. The screening of 419 ST258 clonal isolates revealed a significant association between cps type and K. pneumoniae carbapenemase (KPC) variant: cps-1 is largely associated with KPC-2, while cps-2 is primarily associated with KPC-3.
doi:10.1128/AAC.02673-14
PMCID: PMC4068549  PMID: 24733470
7.  Crystal Structure of Mox-1, a Unique Plasmid-Mediated Class C β-Lactamase with Hydrolytic Activity towards Moxalactam 
Mox-1 is a unique plasmid-mediated class C β-lactamase that hydrolyzes penicillins, cephalothin, and the expanded-spectrum cephalosporins cefepime and moxalactam. In order to understand the unique substrate profile of this enzyme, we determined the X-ray crystallographic structure of Mox-1 β-lactamase at a 1.5-Å resolution. The overall structure of Mox-1 β-lactamase resembles that of other AmpC enzymes, with some notable exceptions. First, comparison with other enzymes whose structures have been solved reveals significant differences in the composition of amino acids that make up the hydrogen-bonding network and the position of structural elements in the substrate-binding cavity. Second, the main-chain electron density is not observed in two regions, one containing amino acid residues 214 to 216 positioned in the Ω loop and the other in the N terminus of the B3 β-strand corresponding to amino acid residues 303 to 306. The last two observations suggest that there is significant structural flexibility of these regions, a property which may impact the recognition and binding of substrates in Mox-1. These important differences allow us to propose that the binding of moxalactam in Mox-1 is facilitated by the avoidance of steric clashes, indicating that a substrate-induced conformational change underlies the basis of the hydrolytic profile of Mox-1 β-lactamase.
doi:10.1128/AAC.02363-13
PMCID: PMC4068568  PMID: 24777102
8.  Comparative Genomic Analysis of KPC-Encoding pKpQIL-Like Plasmids and Their Distribution in New Jersey and New York Hospitals 
The global spread of Klebsiella pneumoniae carbapenemase (KPC) is predominately associated with K. pneumoniae strains genotyped as sequence type 258 (ST258). The first ST258-associated plasmid, pKpQIL, was described in Israel in 2006, but its history in the northeastern United States remains unknown. Six pKpQIL-like plasmids from four K. pneumoniae isolates (three ST258 and one ST234), one Escherichia coli isolate, and one Enterobacter aerogenes isolate, collected from 2003 to 2010 in New York (NY) and New Jersey (NJ) hospitals, were completely sequenced. The sequences and overall sizes of the six plasmids are highly similar to those of pKpQIL; the major difference is that five of six NJ/NY strains harbor blaKPC-2, while pKpQIL contains blaKPC-3. Moreover, a 26.7-kb fragment was inverted in pKpQIL-234 (from ST234 K. pneumoniae), while a 14.5-kb region was deleted in pKpQIL-Ec (from ST131 E. coli). PCR screening of 284 other clinical K. pneumoniae isolates identified 101 (35.6%) harboring pKpQIL-like plasmids from 9 of 10 surveyed hospitals, demonstrating the wide dissemination of pKpQIL in this region of endemicity. Among the positive isolates, 87.1% were typed as ST258 and 88.1% carried blaKPC-2. The finding of pKpQIL-like plasmid in this study from strains that predate the initial report of KPC in Israel provides evidence that pKpQIL may have originated in the United States. Our findings demonstrate that pKpQIL plasmids are both spreading clonally in ST258 strains and spreading horizontally to different sequence types and species, further highlighting the clinical and public health concerns associated with carbapenem resistance.
doi:10.1128/AAC.00120-14
PMCID: PMC3993205  PMID: 24614371
9.  Molecular Survey of the Dissemination of Two blaKPC-Harboring IncFIA Plasmids in New Jersey and New York Hospitals 
Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae strains have spread worldwide and become a major threat in health care facilities. Transmission of blaKPC, the plasmid-borne KPC gene, can be mediated by clonal spread and horizontal transfer. Here, we report the complete nucleotide sequences of two novel blaKPC-3-harboring IncFIA plasmids, pBK30661 and pBK30683. pBK30661 is 74 kb in length, with a mosaic plasmid structure; it exhibits homologies to several other plasmids but lacks the plasmid transfer operon (tra) and the origin of transfer (oriT) that are required for plasmid transfer. pBK30683 is a conjugative plasmid with a cointegrated plasmid structure, comprising a 72-kb element that highly resembles pBK30661 (>99.9% nucleotide identities) and an extra 68-kb element that harbors tra and oriT. A PCR scheme was designed to detect the distribution of blaKPC-harboring IncFIA (pBK30661-like and pBK30683-like) plasmids in a collection of clinical Enterobacteriaceae isolates from 10 hospitals in New Jersey and New York. KPC-harboring IncFIA plasmids were found in 20% of 491 K. pneumoniae isolates, and all carried blaKPC-3. pBK30661-like plasmids were identified mainly in the epidemic sequence type 258 (ST258) K. pneumoniae clone, while pBK30683-like plasmids were widely distributed in ST258 and other K. pneumoniae sequence types and among non-K. pneumoniae Enterobacteriaceae species. This suggests that both clonal spread and horizontal plasmid transfer contributed to the dissemination of blaKPC-harboring IncFIA plasmids in our area. Further studies are needed to understand the distribution of this plasmid group in other health care regions and to decipher the origins of pBK30661-like and pBK30683-like plasmids.
doi:10.1128/AAC.02749-13
PMCID: PMC4023724  PMID: 24492370
10.  New β-Lactamase Inhibitors: a Therapeutic Renaissance in an MDR World 
As the incidence of Gram-negative bacterial infections for which few effective treatments remain increases, so does the contribution of drug-hydrolyzing β-lactamase enzymes to this serious clinical problem. This review highlights recent advances in β-lactamase inhibitors and focuses on agents with novel mechanisms of action against a wide range of enzymes. To this end, we review the β-lactamase inhibitors currently in clinical trials, select agents still in preclinical development, and older therapeutic approaches that are being revisited. Particular emphasis is placed on the activity of compounds at the forefront of the developmental pipeline, including the diazabicyclooctane inhibitors (avibactam and MK-7655) and the boronate RPX7009. With its novel reversible mechanism, avibactam stands to be the first new β-lactamase inhibitor brought into clinical use in the past 2 decades. Our discussion includes the importance of selecting the appropriate partner β-lactam and dosing regimens for these promising agents. This “renaissance” of β-lactamase inhibitors offers new hope in a world plagued by multidrug-resistant (MDR) Gram-negative bacteria.
doi:10.1128/AAC.00826-13
PMCID: PMC4023773  PMID: 24379206
11.  Complete Sequence of a KPC-Producing IncN Multidrug-Resistant Plasmid from an Epidemic Escherichia coli Sequence Type 131 Strain in China 
We report here the nucleotide sequence of a novel blaKPC-2-harboring incompatibility group N (IncN) plasmid, pECN580, from a multidrug-resistant Escherichia coli sequence type 131 (ST131) isolate recovered from Beijing, China. pECN580 harbors β-lactam resistance genes blaKPC-2, blaCTX-M-3, and blaTEM-1; aminoglycoside acetyltransferase gene aac(6′)-Ib-cr; quinolone resistance gene qnrS1; rifampin resistance gene arr-3; and trimethoprim resistance gene dfrA14. The emergence of a blaKPC-2-harboring multidrug-resistant plasmid in an epidemic E. coli ST131 clone poses a significant potential threat in community and hospital settings.
doi:10.1128/AAC.02587-13
PMCID: PMC4023777  PMID: 24395232
12.  Structural Origins of Oxacillinase Specificity in Class D β-Lactamases 
Since the discovery and use of penicillin, the increase of antibiotic resistance among bacterial pathogens has become a major health concern. The most prevalent resistance mechanism in Gram-negative bacteria is due to β-lactamase expression. Class D β-lactamases are of particular importance due to their presence in multidrug-resistant Acinetobacter baumannii and Pseudomonas aeruginosa. The class D enzymes were initially characterized by their ability to efficiently hydrolyze isoxazolyl-type β-lactams like oxacillin. Due to this substrate preference, these enzymes are traditionally referred to as oxacillinases or OXAs. However, this class is comprised of subfamilies characterized by diverse activities that include oxacillinase, carbapenemase, or cephalosporinase substrate specificity. OXA-1 represents one subtype of class D enzyme that efficiently hydrolyzes oxacillin, and OXA-24/40 represents another with weak oxacillinase, but increased carbapenemase, activity. To examine the structural basis for the substrate selectivity differences between OXA-1 and OXA-24/40, the X-ray crystal structures of deacylation-deficient mutants of these enzymes (Lys70Asp for OXA-1; Lys84Asp for OXA-24) in complexes with oxacillin were determined to 1.4 Å and 2.4 Å, respectively. In the OXA-24/40/oxacillin structure, the hydrophobic R1 side chain of oxacillin disrupts the bridge between Tyr112 and Met223 present in the apo OXA-24/40 structure, causing the main chain of the Met223-containing loop to adopt a completely different conformation. In contrast, in the OXA-1/oxacillin structure, a hydrophobic pocket consisting of Trp102, Met99, Phe217, Leu161, and Leu255 nicely complements oxacillin's nonpolar R1 side chain. Comparison of the OXA-1/oxacillin and OXA-24/40/oxacillin complexes provides novel insight on how substrate selectivity is achieved among subtypes of class D β-lactamases. By elucidating important active site interactions, these findings can also inform the design of novel antibiotics and inhibitors.
doi:10.1128/AAC.01483-13
PMCID: PMC3910802  PMID: 24165180
13.  Can Inhibitor-Resistant Substitutions in the Mycobacterium tuberculosis β-Lactamase BlaC Lead to Clavulanate Resistance?: a Biochemical Rationale for the Use of β-Lactam–β-Lactamase Inhibitor Combinations 
Antimicrobial Agents and Chemotherapy  2013;57(12):6085-6096.
The current emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis calls for novel treatment strategies. Recently, BlaC, the principal β-lactamase of Mycobacterium tuberculosis, was recognized as a potential therapeutic target. The combination of meropenem and clavulanic acid, which inhibits BlaC, was found to be effective against even extensively drug-resistant M. tuberculosis strains when tested in vitro. Yet there is significant concern that drug resistance against this combination will also emerge. To investigate the potential of BlaC to evolve variants resistant to clavulanic acid, we introduced substitutions at important amino acid residues of M. tuberculosis BlaC (R220, A244, S130, and T237). Whereas the substitutions clearly led to in vitro clavulanic acid resistance in enzymatic assays but at the expense of catalytic activity, transformation of variant BlaCs into an M. tuberculosis H37Rv background revealed that impaired inhibition of BlaC did not affect inhibition of growth in the presence of ampicillin and clavulanate. From these data we propose that resistance to β-lactam–β-lactamase inhibitor combinations will likely not arise from structural alteration of BlaC, therefore establishing confidence that this therapeutic modality can be part of a successful treatment regimen against M. tuberculosis.
doi:10.1128/AAC.01253-13
PMCID: PMC3837893  PMID: 24060876
14.  Complete Nucleotide Sequence of a blaKPC-Harboring IncI2 Plasmid and Its Dissemination in New Jersey and New York Hospitals 
Antimicrobial Agents and Chemotherapy  2013;57(10):5019-5025.
Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae strains have spread worldwide and become a significant public health threat. blaKPC, the plasmid-borne KPC gene, was frequently identified on numerous transferable plasmids in different incompatibility replicon groups. Here we report the complete nucleotide sequence of a novel blaKPC-3-harboring IncI2 plasmid, pBK15692, isolated from a multidrug-resistant K. pneumoniae ST258 strain isolated from a New Jersey hospital in 2005. pBK15692 is 78 kb in length and carries a backbone that is similar to those of other IncI2 plasmids (pR721, pChi7122-3, pHN1122-1, and pSH146-65), including the genes encoding type IV pili and shufflon regions. Comparative genomics analysis of IncI2 plasmids reveals that they possess a conserved plasmid backbone but are divergent with respect to the integration sites of resistance genes. In pBK15692, the blaKPC-3-harboring Tn4401 was inserted into a Tn1331 element and formed a nested transposon. A PCR scheme was designed to detect the prevalence of IncI2 and pBK15692-like plasmids from a collection of clinical strains from six New Jersey and New York hospitals isolated between 2007 and 2011. IncI2 plasmids were found in 46.2% isolates from 318 clinical K. pneumoniae strains. Notably, 59 pBK15692-like plasmids (23%) have been identified in 256 KPC-bearing K. pneumoniae strains, and all carried KPC-3 and belong to the epidemic ST258 clone. Our study revealed that the prevalence of IncI2 plasmids has been considerably underestimated. Further studies are needed to understand the distribution of this plasmid group in other health care regions and decipher the association between IncI2 plasmids and blaKPC-3-bearing ST258 strains.
doi:10.1128/AAC.01397-13
PMCID: PMC3811408  PMID: 23896467
15.  Structures of the Class D Carbapenemases OXA-23 and OXA-146: Mechanistic Basis of Activity against Carbapenems, Extended-Spectrum Cephalosporins, and Aztreonam 
Antimicrobial Agents and Chemotherapy  2013;57(10):4848-4855.
Class D β-lactamases that hydrolyze carbapenems such as imipenem and doripenem are a recognized danger to the efficacy of these “last-resort” β-lactam antibiotics. Like all known class D carbapenemases, OXA-23 cannot hydrolyze the expanded-spectrum cephalosporin ceftazidime. OXA-146 is an OXA-23 subfamily clinical variant that differs from the parent enzyme by a single alanine (A220) inserted in the loop connecting β-strands β5 and β6. We discovered that this insertion enables OXA-146 to bind and hydrolyze ceftazidime with an efficiency comparable to those of other extended-spectrum class D β-lactamases. OXA-146 also binds and hydrolyzes aztreonam, cefotaxime, ceftriaxone, and ampicillin with higher efficiency than OXA-23 and preserves activity against doripenem. In this study, we report the X-ray crystal structures of both the OXA-23 and OXA-146 enzymes at 1.6-Å and 1.2-Å resolution. A comparison of the two structures shows that the extra alanine moves a methionine (M221) out of its normal position, where it forms a bridge over the top of the active site. This single amino acid insertion also lengthens the β5-β6 loop, moving the entire backbone of this region further away from the active site. A model of ceftazidime bound in the active site reveals that these two structural alterations are both likely to relieve steric clashes between the bulky R1 side chain of ceftazidime and OXA-23. With activity against all four classes of β-lactam antibiotics, OXA-146 represents an alarming new threat to the treatment of infections caused by Acinetobacter spp.
doi:10.1128/AAC.00762-13
PMCID: PMC3811470  PMID: 23877677
17.  Complete Sequence of a blaKPC-2-Harboring IncFIIK1 Plasmid from a Klebsiella pneumoniae Sequence Type 258 Strain 
We report the nucleotide sequence of a novel blaKPC-2-harboring IncFIIK1 plasmid, pBK32179, isolated from a carbapenem-resistant Klebsiella pneumoniae ST258 strain from a New York City patient. pBK32179 is 165 kb long, consists of a large backbone of pKPN3-like plasmid, and carries an 18.5-kb blaKPC-2-containing element that is highly similar to plasmid pKpQIL. pBK32179-like plasmids were identified in 8.3% of strains in a collection of 96 K. pneumoniae isolates from hospitals in the New York City area.
doi:10.1128/AAC.02332-12
PMCID: PMC3591897  PMID: 23295924
18.  Complete Nucleotide Sequences of blaKPC-4- and blaKPC-5-Harboring IncN and IncX Plasmids from Klebsiella pneumoniae Strains Isolated in New Jersey 
Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae have emerged as major nosocomial pathogens. blaKPC, commonly located on Tn4401, is found in Gram-negative bacterial strains, with the two most common variants, blaKPC-2 and blaKPC-3, identified in plasmids with diverse genetic backgrounds. In this study, we examined blaKPC-4- and blaKPC-5-bearing plasmids recovered from two K. pneumoniae strains, which were isolated from a single New Jersey hospital in 2005 and 2006, respectively. IncN plasmid pBK31551 is 84 kb in length and harbors blaKPC-4, blaTEM-1, qnrB2, aac(3)-Ib, aph(3′)-I, qacF, qacEΔ1, sul1, and dfrA14, which confer resistance to β-lactams, quinolones, aminoglycosides, quaternary ammonium compounds, and co-trimoxazole. The conserved regions within pBK31551 are similar to those of other IncN plasmids. Surprisingly, analysis of the Tn4401 sequence revealed a large IS110- and Tn6901-carrying element (8.3 kb) inserted into the istA gene, encoding glyoxalase/bleomycin resistance, alcohol dehydrogenase, and S-formylglutathione hydrolase. Plasmid pBK31567 is 47 kb in length and harbors blaKPC-5, dfrA5, qacEΔ1, and sul1. pBK31567 belongs to a novel IncX subgroup (IncX5) and possesses a highly syntenic plasmid backbone like other IncX plasmids; however, sequence similarity at the nucleotide level is divergent. The blaKPC-5 gene is carried on a Tn4401 element and differs from the genetic environment of blaKPC-5 described in Pseudomonas aeruginosa strain P28 from Puerto Rico. This study underscores the genetic diversity of multidrug-resistant plasmids involved in the spread of blaKPC genes and highlights the mobility and plasticity of Tn4401. Comparative genomic analysis provides new insights into the evolution and dissemination of KPC plasmids belonging to different incompatibility groups.
doi:10.1128/AAC.01648-12
PMCID: PMC3535970  PMID: 23114770
19.  Early Insights into the Interactions of Different β-Lactam Antibiotics and β-Lactamase Inhibitors against Soluble Forms of Acinetobacter baumannii PBP1a and Acinetobacter sp. PBP3 
Antimicrobial Agents and Chemotherapy  2012;56(11):5687-5692.
Acinetobacter baumannii is an increasingly problematic pathogen in United States hospitals. Antibiotics that can treat A. baumannii are becoming more limited. Little is known about the contributions of penicillin binding proteins (PBPs), the target of β-lactam antibiotics, to β-lactam–sulbactam susceptibility and β-lactam resistance in A. baumannii. Decreased expression of PBPs as well as loss of binding of β-lactams to PBPs was previously shown to promote β-lactam resistance in A. baumannii. Using an in vitro assay with a reporter β-lactam, Bocillin, we determined that the 50% inhibitory concentrations (IC50s) for PBP1a from A. baumannii and PBP3 from Acinetobacter sp. ranged from 1 to 5 μM for a series of β-lactams. In contrast, PBP3 demonstrated a narrower range of IC50s against β-lactamase inhibitors than PBP1a (ranges, 4 to 5 versus 8 to 144 μM, respectively). A molecular model with ampicillin and sulbactam positioned in the active site of PBP3 reveals that both compounds interact similarly with residues Thr526, Thr528, and Ser390. Accepting that many interactions with cell wall targets are possible with the ampicillin-sulbactam combination, the low IC50s of ampicillin and sulbactam for PBP3 may contribute to understanding why this combination is effective against A. baumannii. Unraveling the contribution of PBPs to β-lactam susceptibility and resistance brings us one step closer to identifying which PBPs are the best targets for novel β-lactams.
doi:10.1128/AAC.01027-12
PMCID: PMC3486531  PMID: 22908165
20.  Novel Mechanism for Fluoroquinolone Resistance in Acinetobacter baumannii 
An EZ::TNTnp transposon insertion in an open reading frame of unknown function (ncr) in Acinetobacter baumannii resulted in an 8-fold increase in ciprofloxacin resistance (Cipr). Transposon insertions in an ncr mutant that reduced Cipr back to wild type mapped to three genes encoding subunits of the RecCBD exonuclease. The ncr mutation increased transcription of the recCBD genes, and overexpression of the recCBD genes in a wild-type background resulted in a 4-fold increase in Cipr.
doi:10.1128/AAC.00739-12
PMCID: PMC3421888  PMID: 22733072
21.  Understanding the Molecular Determinants of Substrate and Inhibitor Specificities in the Carbapenemase KPC-2: Exploring the Roles of Arg220 and Glu276 
β-Lactamases are important antibiotic resistance determinants expressed by bacteria. By studying the mechanistic properties of β-lactamases, we can identify opportunities to circumvent resistance through the design of novel inhibitors. Comparative amino acid sequence analysis of class A β-lactamases reveals that many enzymes possess a localized positively charged residue (e.g., R220, R244, or R276) that is critical for interactions with β-lactams and β-lactamase inhibitors. To better understand the contribution of these residues to the catalytic process, we explored the roles of R220 and E276 in KPC-2, a class A β-lactamase that inactivates carbapenems and β-lactamase inhibitors. Our study reveals that substitutions at R220 of KPC-2 selectively impact catalytic activity toward substrates (50% or greater reduction in kcat/Km). In addition, we find that residue 220 is central to the mechanism of β-lactamase inhibition/inactivation. Among the variants tested at Ambler position 220, the R220K enzyme is relatively “inhibitor susceptible” (Ki of 14 ± 1 μM for clavulanic acid versus Ki of 25 ± 2 μM for KPC-2). Specifically, the R220K enzyme is impaired in its ability to hydrolyze clavulanic acid compared to KPC-2. In contrast, the R220M substitution enzyme demonstrates increased Km values for β-lactamase inhibitors (>100 μM for clavulanic acid versus 25 ± 3 μM for the wild type [WT]), which results in inhibitor resistance. Unlike other class A β-lactamases (i.e., SHV-1 and TEM-1), the amino acid present at residue 276 plays a structural rather than kinetic role with substrates or inhibitors. To rationalize these findings, we constructed molecular models of clavulanic acid docked into the active sites of KPC-2 and the “relatively” clavulanic acid-susceptible R220K variant. These models suggest that a major 3.5-Å shift occurs of residue E276 in the R220K variant toward the active S70 site. We anticipate that this shift alters the shape of the active site and the positions of two key water molecules. Modeling also suggests that residue 276 may assist with the positioning of the substrate and inhibitor in the active site. These biochemical and molecular modeling insights bring us one step closer to understanding important structure-activity relationships that define the catalytic and inhibitor-resistant profile of KPC-2 and can assist the design of novel compounds.
doi:10.1128/AAC.05769-11
PMCID: PMC3421566  PMID: 22687511
22.  Multiplex Real-Time PCR for Detection of an Epidemic KPC-Producing Klebsiella pneumoniae ST258 Clone 
We describe a multiplex real-time PCR assay capable of identifying both the epidemic Klebsiella pneumoniae ST258 clone and blaKPC carbapenemase genes in a single reaction. The assay displayed excellent sensitivity (100%) and specificity (100%) for identification of ST258 clone and blaKPC in a collection of 75 K. pneumoniae isolates comprising 41 sequence types. Our results suggest that this assay is an effective tool for surveillance of this clone among carbapenem-resistant K. pneumoniae clinical isolates.
doi:10.1128/AAC.00316-12
PMCID: PMC3370784  PMID: 22450983
23.  Crystal Structures of KPC-2 β-Lactamase in Complex with 3-Nitrophenyl Boronic Acid and the Penam Sulfone PSR-3-226 
Class A carbapenemases are a major threat to the potency of carbapenem antibiotics. A widespread carbapenemase, KPC-2, is not easily inhibited by β-lactamase inhibitors (i.e., clavulanic acid, sulbactam, and tazobactam). To explore different mechanisms of inhibition of KPC-2, we determined the crystal structures of KPC-2 with two β-lactamase inhibitors that follow different inactivation pathways and kinetics. The first complex is that of a small boronic acid compound, 3-nitrophenyl boronic acid (3-NPBA), bound to KPC-2 with 1.62-Å resolution. 3-NPBA demonstrated a Km value of 1.0 ± 0.1 μM (mean ± standard error) for KPC-2 and blocks the active site by making a reversible covalent interaction with the catalytic S70 residue. The two boron hydroxyl atoms of 3-NPBA are positioned in the oxyanion hole and the deacylation water pocket, respectively. In addition, the aromatic ring of 3-NPBA provides an edge-to-face interaction with W105 in the active site. The structure of KPC-2 with the penam sulfone PSR-3-226 was determined at 1.26-Å resolution. PSR-3-226 displayed a Km value of 3.8 ± 0.4 μM for KPC-2, and the inactivation rate constant (kinact) was 0.034 ± 0.003 s−1. When covalently bound to S70, PSR-3-226 forms a trans-enamine intermediate in the KPC-2 active site. The predominant active site interactions are generated via the carbonyl oxygen, which resides in the oxyanion hole, and the carboxyl moiety of PSR-3-226, which interacts with N132, N170, and E166. 3-NPBA and PSR-3-226 are the first β-lactamase inhibitors to be trapped as an acyl-enzyme complex with KPC-2. The structural and inhibitory insights gained here could aid in the design of potent KPC-2 inhibitors.
doi:10.1128/AAC.06099-11
PMCID: PMC3346646  PMID: 22330909
25.  High-Level Expression of Chromosomally Encoded SHV-1 β-Lactamase and an Outer Membrane Protein Change Confer Resistance to Ceftazidime and Piperacillin- Tazobactam in a Clinical Isolate of Klebsiella pneumoniae 
We describe Klebsiella pneumoniae 15571, a clinical isolate resistant to ceftazidime MIC = 32 μg/ml) and piperacillin-tazobactam (MICs = 1,024 and 128 μg/ml). K. pneumoniae 15571 expresses a single β-lactamase with a pI of 7.6. However, when cloned in a high-copy-number vector in Escherichia coli, this blaSHV-1 gene did not confer resistance to ceftazidime, a spectrum consistent with the nucleotide sequence, which was nearly identical to those of previously described blaSHV-1 genes. Outer membrane protein (OMP) analysis of K. pneumoniae 15571 revealed a decrease in the quantity of a minor 45-kDa OMP in comparison to that in K. pneumoniae 44NR, a low-level ampicillin-resistant strain that also expresses a chromosomally determined blaSHV-1. Crude β-lactamase enzyme extracts from K. pneumoniae 15571 produced roughly 200-fold more β-lactamase activity than K. pneumoniae 44NR. Northern hybridization analysis revealed that this difference was explainable by quantifiable differences in transcription of the blaSHV-1 gene in the two strains. Primer extension analysis of blaSHV-1 mRNA from K. pneumoniae 15571 and 44NR indicated that the transcriptional start sites were identical in the two strains. DNA sequencing of the promoter regions upstream of the of blaSHV-1 open reading frames in the two K. pneumoniae strains revealed an A→C change in the second position of the −10 region in K. pneumoniae 44NR compared to that in 15571. Site-directed mutagenesis of the cloned K. pneumoniae 15571 blaSHV-1, in which the A in the second position of the 15571 −10 region was changed to a C, resulted in a substantial lowering of the MIC of ampicillin. When the levels of β-lactamase enzyme expression in E. coli were compared, the blaSHV-1 downstream of the altered −10 region produced 17-fold less β-lactamase enzyme. These results indicate that elevated levels of ceftazidime resistance can result from a combination of increased enzyme production and minor OMP changes and that levels of chromosomally encoded SHV-1 β-lactamase production can vary substantially with a single-base-pair change in promoter sequence.
PMCID: PMC89684  PMID: 10639363

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