scholarly journals Complete Nucleotide Sequences ofblaKPC-4- andblaKPC-5-Harboring IncN and IncX Plasmids from Klebsiella pneumoniae Strains Isolated in New Jersey

2012 ◽  
Vol 57 (1) ◽  
pp. 269-276 ◽  
Author(s):  
Liang Chen ◽  
Kalyan D. Chavda ◽  
Henry S. Fraimow ◽  
José R. Mediavilla ◽  
Roberto G. Melano ◽  
...  
Keyword(s):  
New Jersey ◽  
Klebsiella Pneumoniae ◽  
Sequence Similarity ◽  
Genomic Analysis ◽  
Multidrug Resistant ◽  
Comparative Genomic ◽  
Bacterial Strains ◽  
Gene Encoding ◽  
Content Type ◽  
Plasmid Backbone

ABSTRACTKlebsiella pneumoniaecarbapenemase (KPC)-producingEnterobacteriaceaehave emerged as major nosocomial pathogens.blaKPC, commonly located on Tn4401, is found in Gram-negative bacterial strains, with the two most common variants,blaKPC-2andblaKPC-3, identified in plasmids with diverse genetic backgrounds. In this study, we examinedblaKPC-4- andblaKPC-5-bearing plasmids recovered from twoK. pneumoniaestrains, which were isolated from a single New Jersey hospital in 2005 and 2006, respectively. IncN plasmid pBK31551 is 84 kb in length and harborsblaKPC-4,blaTEM-1,qnrB2,aac(3)-Ib,aph(3′)-I,qacF,qacEΔ1,sul1, anddfrA14, 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 Tn4401sequence revealed a large IS110- and Tn6901-carrying element (8.3 kb) inserted into theistAgene, encoding glyoxalase/bleomycin resistance, alcohol dehydrogenase, andS-formylglutathione hydrolase. Plasmid pBK31567 is 47 kb in length and harborsblaKPC-5,dfrA5,qacEΔ1, andsul1. 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. TheblaKPC-5gene is carried on a Tn4401element and differs from the genetic environment ofblaKPC-5described inPseudomonas aeruginosastrain P28 from Puerto Rico. This study underscores the genetic diversity of multidrug-resistant plasmids involved in the spread ofblaKPCgenes 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.

scholarly journals Emergence and Evolution of Multidrug-Resistant Klebsiella pneumoniae with both blaKPC and blaCTX-M Integrated in the Chromosome

2017 ◽  
Vol 61 (7) ◽  
Author(s):  
Weihua Huang ◽  
Guiqing Wang ◽  
Robert Sebra ◽  
Jian Zhuge ◽  
Changhong Yin ◽  
...  
Keyword(s):  
Antimicrobial Resistance ◽  
Klebsiella Pneumoniae ◽  
Resistance Genes ◽  
De Novo ◽  
Genomic Analysis ◽  
Multidrug Resistant ◽  
Comparative Genomic ◽  
Type I ◽  
Content Type ◽  
Antimicrobial Resistance Genes

ABSTRACT The extended-spectrum-β-lactamase (ESBL)- and Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae represent serious and urgent threats to public health. In a retrospective study of multidrug-resistant K. pneumoniae, we identified three clinical isolates, CN1, CR14, and NY9, carrying both bla CTX-M and bla KPC genes. The complete genomes of these three K. pneumoniae isolates were de novo assembled by using both short- and long-read whole-genome sequencing. In CR14 and NY9, bla CTX-M and bla KPC were carried on two different plasmids. In contrast, CN1 had one copy of bla KPC-2 and three copies of bla CTX-M-15 integrated in the chromosome, for which the bla CTX-M-15 genes were linked to an insertion sequence, ISEcp1, whereas the bla KPC-2 gene was in the context of a Tn4401a transposition unit conjugated with a PsP3-like prophage. Intriguingly, downstream of the Tn4401a-bla KPC-2-prophage genomic island, CN1 also carried a clustered regularly interspaced short palindromic repeat (CRISPR)-cas array with four spacers targeting a variety of K. pneumoniae plasmids harboring antimicrobial resistance genes. Comparative genomic analysis revealed that there were two subtypes of type I-E CRISPR-cas in K. pneumoniae strains and suggested that the evolving CRISPR-cas, with its acquired novel spacer, induced the mobilization of antimicrobial resistance genes from plasmids into the chromosome. The integration and dissemination of multiple copies of bla CTX-M and bla KPC from plasmids to chromosome depicts the complex pandemic scenario of multidrug-resistant K. pneumoniae. Additionally, the implications from this study also raise concerns for the application of a CRISPR-cas strategy against antimicrobial resistance.

scholarly journals Genomic Analysis of Multiresistant Staphylococcus capitis Associated with Neonatal Sepsis

2018 ◽  
Vol 62 (11) ◽  
Author(s):  
Glen P. Carter ◽  
James E. Ussher ◽  
Anders Gonçalves Da Silva ◽  
Sarah L. Baines ◽  
Helen Heffernan ◽  
...  
Keyword(s):  
Neonatal Sepsis ◽  
Neonatal Intensive Care ◽  
Genomic Analysis ◽  
Bloodstream Infections ◽  
Multidrug Resistant ◽  
Comparative Genomic ◽  
Coagulase Negative Staphylococci ◽  
Content Type ◽  
Staphylococcus Capitis ◽  
Hospital Infection Control

ABSTRACT Coagulase-negative staphylococci (CoNS), such as Staphylococcus capitis, are major causes of bloodstream infections in neonatal intensive care units (NICUs). Recently, a distinct clone of S. capitis (designated S. capitis NRCS-A) has emerged as an important pathogen in NICUs internationally. Here, 122 S. capitis isolates from New Zealand (NZ) underwent whole-genome sequencing (WGS), and these data were supplemented with publicly available S. capitis sequence reads. Phylogenetic and comparative genomic analyses were performed, as were phenotypic assessments of antimicrobial resistance, biofilm formation, and plasmid segregational stability on representative isolates. A distinct lineage of S. capitis was identified in NZ associated with neonates and the NICU environment. Isolates from this lineage produced increased levels of biofilm, displayed higher levels of tolerance to chlorhexidine, and were multidrug resistant. Although similar to globally circulating NICU-associated S. capitis strains at a core-genome level, NZ NICU S. capitis isolates carried a novel stably maintained multidrug-resistant plasmid that was not present in non-NICU isolates. Neonatal blood culture isolates were indistinguishable from environmental S. capitis isolates found on fomites, such as stethoscopes and neonatal incubators, but were generally distinct from those isolates carried by NICU staff. This work implicates the NICU environment as a potential reservoir for neonatal sepsis caused by S. capitis and highlights the capacity of genomics-based tracking and surveillance to inform future hospital infection control practices aimed at containing the spread of this important neonatal pathogen.

scholarly journals Characterization of an IncR Plasmid with Two Copies of ISCR-Linked qnrB6 from ST968 Klebsiella pneumoniae

2020 ◽  
Vol 2020 ◽  
pp. 1-8
Author(s):  
Changrui Qian ◽  
Xinyi Zhu ◽  
Junwan Lu ◽  
Kai Shen ◽  
Qianqian Chen ◽  
...  
Keyword(s):  
Klebsiella Pneumoniae ◽  
Resistance Genes ◽  
Genomic Analysis ◽  
The Other ◽  
Comparative Genomic ◽  
Clear Understanding ◽  
Class 1 Integron ◽  
Bacterial Chromosomes ◽  
Class 1 ◽  
Plasmid Backbone

To characterize the molecular structure of IncR plasmid-related sequences, comparative genomic analysis was conducted using 261 IncR plasmid backbone-related sequences. Among the sequences, 257 were IncR plasmids including the multidrug-resistance IncR plasmid pR50-74 from Klebsiella pneumoniae strain R50 of this work, and the other four were from bacterial chromosomes. The IncR plasmids were derived from different bacterial genera or species, mainly Klebsiella pneumoniae (70.82%, 182/257), Escherichia coli (11.28%, 29/257), Enterobacter cloacae (7.00%, 18/257), and Citrobacter freundii (3.50%, 9/257). The bacterial chromosomes carrying IncR plasmid backbone sequences were derived from Proteus mirabilis AOUC-001 and Klebsiella pneumoniae KPN1344, among others. The IncR backbone sequence of P. mirabilis AOUC-001 chromosome shows the highest identity with that of pR50-74. Complex class 1 integrons carrying various copies of ISCR1-sdr-qnrB6-△qacE/sul1 (ISCR1-linked qnrB6 unit) were identified in IncR plasmids. In addition to two consecutive copies of qnrB6-qacE-sul1, the other resistance genes encoded on pR50-74 are all related to mobile genetic elements, such as IS1006, IS26, and the class 1 integron. This study provides a clear understanding of the mobility and plasticity of the IncR plasmid backbone sequence and emphasizes the important role of ISCR in the recruitment of multicopy resistance genes.

scholarly journals Epidemic Klebsiella pneumoniae ST258 Is a Hybrid Strain

mBio ◽  
2014 ◽  
Vol 5 (3) ◽  
Author(s):  
Liang Chen ◽  
Barun Mathema ◽  
Johann D. D. Pitout ◽  
Frank R. DeLeo ◽  
Barry N. Kreiswirth
Keyword(s):  
Molecular Evolution ◽  
Klebsiella Pneumoniae ◽  
Treatment Strategies ◽  
The United States ◽  
Multidrug Resistant ◽  
Effective Control ◽  
Comparative Genomic ◽  
Hybrid Strain ◽  
Content Type ◽  
Polysaccharide Biosynthesis

ABSTRACT Carbapenem-resistant Enterobacteriaceae (CRE), especially Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae, pose an urgent threat in health facilities in the United States and worldwide. K. pneumoniae isolates classified as sequence type 258 (ST258) by multilocus sequence typing are largely responsible for the global spread of KPC. A recent comparative genome study revealed that ST258 K. pneumoniae strains are two distinct genetic clades; however, the molecular origin of ST258 largely remains unknown, and our understanding of the evolution of the two genetic clades is incomplete. Here we compared the genetic structures and single-nucleotide polymorphism (SNP) distributions in the core genomes of strains from two ST258 clades and other STs (ST11, ST442, and ST42). We identified an ~1.1-Mbp region on ST258 genomes that is homogeneous to that of ST442, while the rest of the ST258 genome resembles that of ST11. Our results suggest ST258 is a hybrid clone—80% of the genome originated from ST11-like strains and 20% from ST442-like strains. Meanwhile, we sequenced an ST42 strain that carries the same K-antigen-encoding capsule polysaccharide biosynthesis gene (cps) region as ST258 clade I strains. Comparison of the cps-harboring regions between the ST42 and ST258 strains (clades I and II) suggests the ST258 clade I strains evolved from a clade II strain as a result of cps region replacement. Our findings unravel the molecular evolution history of ST258 strains, an important first step toward the development of diagnostic, therapeutic, and vaccine strategies to combat infections caused by multidrug-resistant K. pneumoniae. IMPORTANCE Recombination events and replacement of chromosomal regions have been documented in various bacteria, and these events have given rise to successful pathogenic clones. Here we used comparative genomic analyses to discover that the ST258 K. pneumoniae genome is a hybrid—80% of the chromosome is homologous to ST11 strains, while the remaining 20% is homologous to that of ST442. Meanwhile, a recent study indicated that ST258 strains can be segregated into two ST258 clades, with distinct capsule polysaccharide gene (cps) regions. Our analysis suggests ST258 clade I strains evolved from clade II through homologous recombination of cps region. Horizontal transfer of the cps region appears to be a key element driving the molecular diversification in K. pneumoniae strains. These findings not only extend our understanding of the molecular evolution of ST258 but are an important step toward the development of effective control and treatment strategies for multidrug-resistant K. pneumoniae.

scholarly journals Pseudonocardia abyssalis sp. nov. and Pseudonocardia oceani sp. nov., two novel actinomycetes isolated from the deep Southern Ocean

2021 ◽  
Vol 71 (9) ◽  
Author(s):  
Jonathan Parra ◽  
Sylvia Soldatou ◽  
Liam M. Rooney ◽  
Katherine R. Duncan
Keyword(s):  
Fatty Acid ◽  
Southern Ocean ◽  
Type Strain ◽  
Sequence Similarity ◽  
Genomic Analysis ◽  
Distinct Species ◽  
Diaminopimelic Acid ◽  
Comparative Genomic ◽  
Rrna Gene ◽  
Content Type

The actinomycetes strains KRD168T and KRD185T were isolated from sediments collected from the deep Southern Ocean and, in this work, they are described as representing two novel species of the genus Pseudonocardia through a polyphasic approach. Despite sharing >99 % 16S rRNA gene sequence similarity with other members of the genus, comparative genomic analysis allowed species delimitation based on average nucleotide identity and digital DNA–DNA hybridization. The KRD168T genome is characterized by a size of 6.31 Mbp and a G+C content of 73.44 mol%, while the KRD185T genome has a size of 6.82 Mbp and a G+C content of 73.98 mol%. Both strains contain meso-diaminopimelic acid as the diagnostic diamino acid, glucose as the major whole-cell sugar, MK-8(H4) as a major menaquinone and iso-branched hexadecanoic acid as a major fatty acid. Biochemical and fatty acid analyses also revealed differences between these strains and their phylogenetic neighbours, supporting their status as distinct species. The names Pseudonocardia abyssalis sp. nov. (type strain KRD168T=DSM 111918T=NCIMB 15270T) and Pseudonocardia oceani (type strain KRD185T=DSM 111919T=NCIMB 15269T) are proposed.

scholarly journals Lactobacillus salitolerans sp. nov., a novel lactic acid bacterium isolated from spent mushroom substrates

2019 ◽  
Vol 69 (4) ◽  
pp. 964-969 ◽  
Author(s):  
Masanori Tohno ◽  
Yasuhiro Tanizawa ◽  
Yoichiro Kojima ◽  
Mitsuo Sakamoto ◽  
Yasukazu Nakamura ◽  
...  
Keyword(s):  
Sequence Analysis ◽  
Type Species ◽  
Sequence Similarity ◽  
Genomic Analysis ◽  
Comparative Genomic ◽  
Rrna Gene ◽  
Content Type ◽  
Link Type ◽  
Gene Sequence Analysis ◽  
Bacterium Strain

A taxonomic study of a Gram-stain-positive, rod-shaped, non-motile, non-spore-forming, catalase-negative bacterium, strain YK43T, isolated from spent mushroom substrates stored in Nagano, Japan was performed. Growth was detected at 15–45 °C, pH 5.0–8.5, and 0–10 % (w/v) NaCl. The genomic DNA G+C content of strain YK43T was 43.6 mol%. The predominant fatty acids were C16 : 0, C18 : 1 ω9c and summed feature 8. Based on 16S rRNA gene sequence analysis, the type strains of Lactobacillus acidipiscis (sequence similarity, 97.6 %) and Lactobacillus pobuzihii (97.4 %) were most closely related to YK43T. The average nucleotide identities were 74.1 % between strain YK43T and L. acidipiscis DSM 15836T and 74.0 % between YK43T and L. pobuzihii E100301T. Based on a multilocus sequence analysis, comparative genomic analysis and a range of phenotypic and chemotaxonomic characteristics, strain YK43T represents a novel species of the genus Lactobacillus , for which the name Lactobacillus salitolerans sp. nov. is proposed. The type strain is YK43T (=JCM 31331T = DSM 103433T).

scholarly journals In VivoEvolution to Colistin Resistance by PmrB Sensor Kinase Mutation in KPC-Producing Klebsiella pneumoniae Is Associated with Low-Dosage Colistin Treatment

2014 ◽  
Vol 58 (8) ◽  
pp. 4399-4403 ◽  
Author(s):  
Antonio Cannatelli ◽  
Vincenzo Di Pilato ◽  
Tommaso Giani ◽  
Fabio Arena ◽  
Simone Ambretti ◽  
...  
Keyword(s):  
Klebsiella Pneumoniae ◽  
Genomic Analysis ◽  
Amino Acid Position ◽  
Comparative Genomic ◽  
Sensor Kinase ◽  
Wild Type ◽  
Colistin Resistance ◽  
Content Type ◽  
Before And After ◽  
Low Dosage

ABSTRACTColistin is a key drug for the treatment of infections caused by extensively drug-resistant strains ofEnterobacteriaceaeproducing carbapenemases. However, the emergence of colistin resistance is being increasingly reported, especially amongKlebsiella pneumoniaestrains producing KPC-type carbapenemases (KPC-KP). In this work, we investigated colistin-susceptible (KPB-1) and colistin-resistant (KPB-2) sequential isolates obtained from a patient with a KPC-KP infection before and after low-dosage colistin treatment, respectively. By using a next-generation sequencing approach and comparative genomic analysis of the two isolates, we detected in KPB-2 a nonsynonymous nucleotide substitution in the gene encoding the PmrB sensor kinase, resulting in a leucine-to-arginine substitution at amino acid position 82. Compared with KPB-1, KPB-2 exhibited upregulated transcription ofpmrAand ofpmrK, which is part of thepmrHFIJKLMoperon responsible for modification of the colistin lipopolysaccharide target. Complementation with wild-typepmrBin KPB-2 restored colistin susceptibility and reduced the transcription ofpmrAandpmrKto basal levels, while expression of PmrBL82Rin KPB-1 did not alter colistin susceptibility or upregulatepmrAandpmrKexpression, confirming the dominance of wild-type PmrB versus the PmrBL82Rmutant. The present results indicated that PmrB mutations mediating colistin resistance may be selected during low-dosage colistin treatment. The colistin-resistant phenotype of KPB-2 was stable for up to 50 generations in the absence of selective pressure and was not associated with a significant fitness cost in a competition experiment.

scholarly journals Genome Sequencing of an Escherichia coli Sequence Type 617 Strain Isolated from Beach Ghost Shrimp (Callichirus major) from a Heavily Polluted Ecosystem Reveals a Wider Resistome against Heavy Metals and Antibiotics

2019 ◽  
Vol 8 (3) ◽  
Author(s):  
Daniel F. Monte ◽  
Fábio P. Sellera ◽  
Miriam R. Fernandes ◽  
Quézia Moura ◽  
Mariza Landgraf ◽  
...  
Keyword(s):  
Escherichia Coli ◽  
Coastal Waters ◽  
Draft Genome ◽  
Genomic Analysis ◽  
Multidrug Resistant ◽  
Sequence Type ◽  
Comparative Genomic ◽  
Content Type ◽  
E Coli ◽  
Ghost Shrimp

Here, we present the draft genome sequence of a multidrug-resistant (MDR) Escherichia coli strain belonging to sequence type 617 (ST617), isolated from beach ghost shrimp from polluted coastal waters in Brazil. These data provide valuable information for comparative genomic analysis, related to the dissemination of MDR E. coli in marine ecosystems.

scholarly journals In VivoEmergence of Colistin Resistance in Klebsiella pneumoniae Producing KPC-Type Carbapenemases Mediated by Insertional Inactivation of the PhoQ/PhoPmgrBRegulator

2013 ◽  
Vol 57 (11) ◽  
pp. 5521-5526 ◽  
Author(s):  
Antonio Cannatelli ◽  
Marco Maria D'Andrea ◽  
Tommaso Giani ◽  
Vincenzo Di Pilato ◽  
Fabio Arena ◽  
...  
Keyword(s):  
Klebsiella Pneumoniae ◽  
Genomic Analysis ◽  
Negative Regulator ◽  
Resistant Isolate ◽  
Comparative Genomic ◽  
Colistin Resistance ◽  
Content Type ◽  
Insertional Inactivation ◽  
Resistant Mutants

ABSTRACTColistin is one of the few agents that retain activity against extensively drug-resistant strains ofKlebsiella pneumoniaeproducing KPC-type carbapenemases (KPC-KP). However, resistance to colistin is increasingly reported among KPC-KP. Comparative genomic analysis of a pair of sequential KPC-KP isolates from the same patient including a colistin-susceptible isolate (KKBO-1) and a colistin-resistant isolate (KKBO-4) selected after colistin exposure revealed that insertional inactivation of themgrBgene, encoding a negative regulator of the PhoQ/PhoP signaling system, is a genetic mechanism for acquired colistin resistance. The role ofmgrBinactivation in acquired colistin resistance was confirmed by complementation experiments with wild-typemgrB, which restored colistin susceptibility in KKBO-4, and by construction of anmgrBdeletion mutant from KKBO-1, which exhibited a colistin-resistant phenotype. InsertionalmgrBinactivation was also detected in 60% of colistin-resistant mutants selected from KKBO-1in vitro, following plating on colistin-containing medium, confirming the role (although not unique) of this mechanism in the emergence of acquired colistin resistance. In colistin-resistant mutants carrying insertional inactivation or deletion of themgrBgene, upregulated transcription ofphoP,phoQ, andpmrK(which is part of thepmrHFIJKLMoperon) was detected. These findings confirmed the MgrB regulatory role inK. pneumoniaeand were in agreement with the known association between upregulation of the PhoQ/PhoP system and activation of thepmrHFIJKLMoperon, which eventually leads to resistance to polymyxins by modification of the lipopolysaccharide target.

scholarly journals Genome Dynamics and Molecular Infection Epidemiology of Multidrug-Resistant Helicobacter pullorum Isolates Obtained from Broiler and Free-Range Chickens in India

2016 ◽  
Vol 83 (1) ◽  
Author(s):  
Shamsul Qumar ◽  
Mohammad Majid ◽  
Narender Kumar ◽  
Sumeet K. Tiwari ◽  
Torsten Semmler ◽  
...  
Keyword(s):  
Broiler Chicken ◽  
Genomic Analysis ◽  
Multidrug Resistant ◽  
Comparative Genomic Analysis ◽  
Resistant Bacteria ◽  
Comparative Genomic ◽  
Whole Genome ◽  
Multidrug Resistant Bacteria ◽  
Content Type ◽  
Free Range

ABSTRACT Some life-threatening, foodborne, and zoonotic infections are transmitted through poultry birds. Inappropriate and indiscriminate use of antimicrobials in the livestock industry has led to an increased prevalence of multidrug-resistant bacteria with epidemic potential. Here, we present a functional molecular epidemiological analysis entailing the phenotypic and whole-genome sequence-based characterization of 11 H. pullorum isolates from broiler and free-range chickens sampled from retail wet markets in Hyderabad City, India. Antimicrobial susceptibility tests revealed all of the isolates to be resistant to multiple antibiotic classes such as fluoroquinolones, cephalosporins, sulfonamides, and macrolides. The isolates were also found to be extended-spectrum β-lactamase producers and were even resistant to clavulanic acid. Whole-genome sequencing and comparative genomic analysis of these isolates revealed the presence of five or six well-characterized antimicrobial resistance genes, including those encoding a resistance-nodulation-division efflux pump(s). Phylogenetic analysis combined with pan-genome analysis revealed a remarkable degree of genetic diversity among the isolates from free-range chickens; in contrast, a high degree of genetic similarity was observed among broiler chicken isolates. Comparative genomic analysis of all publicly available H. pullorum genomes, including our isolates (n = 16), together with the genomes of 17 other Helicobacter species, revealed a high number (8,560) of H. pullorum-specific protein-encoding genes, with an average of 535 such genes per isolate. In silico virulence screening identified 182 important virulence genes and also revealed high strain-specific gene content in isolates from free-range chickens (average, 34) compared to broiler chicken isolates. A significant prevalence of prophages (ranging from 1 to 9) and a significant presence of genomic islands (0 to 4) were observed in free-range and broiler chicken isolates. Taken together, these observations provide significant baseline data for functional molecular infection epidemiology of nonpyloric Helicobacter species such as H. pullorum by unraveling their evolution in chickens and their possible zoonotic transmission to humans. IMPORTANCE Globally, the poultry industry is expanding with an ever-growing consumer base for chicken meat. Given this, food-associated transmission of multidrug-resistant bacteria represents an important health care issue. Our study involves a critical baseline approach directed at genome sequence-based epidemiology and transmission dynamics of H. pullorum, a poultry pathogen having established zoonotic potential. We believe our studies would facilitate the development of surveillance systems that ensure the safety of food for humans and guide public health policies related to the use of antibiotics in animal feed in countries such as India. We sequenced 11 new genomes of H. pullorum as a part of this study. These genomes would provide much value in addition to the ongoing comparative genomic studies of helicobacters.

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