A North American Yersinia pestis draft genome sequence: SNPs and phylogenetic analysis

Ancient pathogen genomics as an emerging tool for infectious disease research

Over the past decade, a genomics revolution, made possible through the development of high-throughput sequencing, has triggered considerable progress in the study of ancient DNA, enabling complete genomes of past organisms to be reconstructed. A newly established branch of this field, ancient pathogen genomics, affords an in-depth view of microbial evolution by providing a molecular fossil record for a number of human-associated pathogens. Recent accomplishments include the confident identification of causative agents from past pandemics, the discovery of microbial lineages that are now extinct, the extrapolation of past emergence events on a chronological scale and the characterization of long-term evolutionary history of microorganisms that remain relevant to public health today. In this Review, we discuss methodological advancements, persistent challenges and novel revelations gained through the study of ancient pathogen genomes.

Whole genome multilocus sequence typing as an epidemiologic tool for Yersinia pestis

Human plague is a severe and often fatal zoonotic disease caused by Yersinia pestis. For public health investigations of human cases, nonintensive whole genome molecular typing tools, capable of defining epidemiologic relationships, are advantageous. Whole genome multilocus sequence typing (wgMLST) is a recently developed methodology that simplifies genomic analyses by transforming millions of base pairs of sequence into character data for each gene. We sequenced 13 US Y. pestis isolates with known epidemiologic relationships. Sequences were assembled de novo, and multilocus sequence typing alleles were assigned by comparison against 3979 open reading frames from the reference strain CO92. Allele-based cluster analysis accurately grouped the 13 isolates, as well as 9 publicly available Y. pestis isolates, by their epidemiologic relationships. Our findings indicate wgMLST is a simplified, sensitive, and scalable tool for epidemiologic analysis of Y. pestis strains.

A review of methods for subtyping Yersinia pestis: From phenotypes to whole genome sequencing

Numerous subtyping methods have been applied to Yersinia pestis with varying success. Here, we review the various subtyping methods that have been applied to Y. pestis and their capacity for answering questions regarding the population genetics, phylogeography, and molecular epidemiology of this important human pathogen. Methods are evaluated in terms of expense, difficulty, transferability among laboratories, discriminatory power, usefulness for different study questions, and current applicability in light of the advent of whole genome sequencing.

Single-Nucleotide Polymorphisms Reveal Spatial Diversity Among Clones of Yersinia pestis During Plague Outbreaks in Colorado and the Western United States

BACKGROUND

In western North America, plague epizootics caused by Yersinia pestis appear to sweep across landscapes, primarily infecting and killing rodents, especially ground squirrels and prairie dogs. During these epizootics, the risk of Y. pestis transmission to humans is highest. While empirical models that include climatic conditions and densities of rodent hosts and fleas can predict when epizootics are triggered, bacterial transmission patterns across landscapes, and the scale at which Y. pestis is maintained in nature during inter-epizootic periods, are poorly defined. Elucidating the spatial extent of Y. pestis clones during epizootics can determine whether bacteria are propagated across landscapes or arise independently from local inter-epizootic maintenance reservoirs.

MATERIAL AND METHODS

We used DNA microarray technology to identify single-nucleotide polymorphisms (SNPs) in 34 Y. pestis isolates collected in the western United States from 1980 to 2006, 21 of which were collected during plague epizootics in Colorado. Phylogenetic comparisons were used to elucidate the hypothesized spread of Y. pestis between the mountainous Front Range and the eastern plains of northern Colorado during epizootics. Isolates collected from across the western United States were included for regional comparisons.

RESULTS

By identifying SNPs that mark individual clones, our results strongly suggest that Y. pestis is maintained locally and that widespread epizootic activity is caused by multiple clones arising independently at small geographic scales. This is in contrast to propagation of individual clones being transported widely across landscapes. Regionally, our data are consistent with the notion that Y. pestis diversifies at relatively local scales following long-range translocation events. We recommend that surveillance and prediction by public health and wildlife management professionals focus more on models of local or regional weather patterns and ecological factors that may increase risk of widespread epizootics, rather than predicting or attempting to explain epizootics on the basis of movement of host species that may transport plague.

Adhesive properties of YapV and paralogous autotransporter proteins of Yersinia pestis

Yersinia pestis is the causative agent of plague. This bacterium evolved from an ancestral enteroinvasive Yersinia pseudotuberculosis strain by gene loss and acquisition of new genes, allowing it to use fleas as transmission vectors. Infection frequently leads to a rapidly lethal outcome in humans, a variety of rodents, and cats. This study focuses on the Y. pestis KIM yapV gene and its product, recognized as an autotransporter protein by its typical sequence, outer membrane localization, and amino-terminal surface exposure. Comparison of Yersinia genomes revealed that DNA encoding YapV or each of three individual paralogous proteins (YapK, YapJ, and YapX) was present as a gene or pseudogene in a strain-specific manner and only in Y. pestis and Y. pseudotuberculosis. YapV acted as an adhesin for alveolar epithelial cells and specific extracellular matrix (ECM) proteins, as shown with recombinant Escherichia coli, Y. pestis, or purified passenger domains. Like YapV, YapK and YapJ demonstrated adhesive properties, suggesting that their previously related in vivo activity is due to their capacity to modulate binding properties of Y. pestis in its hosts, in conjunction with other adhesins. A differential host-specific type of binding to ECM proteins by YapV, YapK, and YapJ suggested that these proteins participate in broadening the host range of Y. pestis. A phylogenic tree including 36 Y. pestis strains highlighted an association between the gene profile for the four paralogous proteins and the geographic location of the corresponding isolated strains, suggesting an evolutionary adaption of Y. pestis to specific local animal hosts or reservoirs.

Sylvatic plague in a Canadian black-tailed prairie dog (Cynomys ludovicianus)

In 2010, a black-tailed prairie dog (Cynomys ludovicianus) was found dead in Grasslands National Park, Saskatchewan, Canada. Postmortem gross and histologic findings indicated bacterial septicemia, likely due to Yersinia pestis, which was confirmed by molecular analysis. This is the first report of Y. pestis in the prairie dog population within Canada.

Whole genome sequencing of bacteria in cystic fibrosis as a model for bacterial genome adaptation and evolution

Cystic fibrosis (CF) airways harbor a wide variety of new and/or emerging multidrug resistant bacteria which impose a heavy burden on patients. These bacteria live in close proximity with one another, which increases the frequency of lateral gene transfer. The exchange and movement of mobile genetic elements and genomic islands facilitate the spread of genes between genetically diverse bacteria, which seem to be advantageous to the bacterium as it allows adaptation to the new niches of the CF lungs. Niche adaptation is one of the major evolutionary forces shaping bacterial genome composition and in CF the chronic strains adapt and become less virulent. The purpose of this review is to shed light on CF bacterial genome alterations. Next-generation sequencing technology is an exciting tool that may help us to decipher the genome architecture and the evolution of bacteria colonizing CF lungs.

Plague in the genomic area

With plague being not only a subject of interest for historians, but still a disease of public health concern in several countries, mainly in Africa, there were hopes that analyses of the Yersinia pestis genomes would put an end to this deadly epidemic pathogen. Genomics revealed that Y. pestis isolates evolved from Yersinia pseudotuberculosis in Central Asia some millennia ago, after the acquisition of two Y. pestis-specific plasmids balanced genomic reduction parallel with the expansion of insertion sequences, illustrating the modern concept that, except for the acquisition of plasmid-borne toxin-encoding genes, the increased virulence of Y. pestis resulted from gene loss rather than gene acquisition. The telluric persistence of Y. pestis reminds us of this close relationship, and matters in terms of plague epidemiology. Whereas biotype Orientalis isolates spread worldwide, the Antiqua and Medievalis isolates showed more limited expansion. In addition to animal ectoparasites, human ectoparasites such as the body louse may have participated in this expansion and in devastating historical epidemics. The recent analysis of a Black Death genome indicated that it was more closely related to the Orientalis branch than to the Medievalis branch. Modern Y. pestis isolates grossly exhibit the same gene content, but still undergo micro-evolution in geographically limited areas by differing in the genome architecture, owing to inversions near insertion sequences and the stabilization of the YpfPhi prophage in Orientalis biotype isolates. Genomics have provided several new molecular tools for the genotyping and phylogeographical tracing of isolates and description of plague foci. However, genomics and post-genomics approaches have not yet provided new tools for the prevention, diagnosis and management of plague patients and the plague epidemics still raging in some sub-Saharan countries.

Comparative genomics of 2009 seasonal plague (Yersinia pestis) in New Mexico

Plague disease caused by the Gram-negative bacterium Yersinia pestis routinely affects animals and occasionally humans, in the western United States. The strains native to the North American continent are thought to be derived from a single introduction in the late 19^th century. The degree to which these isolates have diverged genetically since their introduction is not clear, and new genomic markers to assay the diversity of North American plague are highly desired. To assay genetic diversity of plague isolates within confined geographic areas, draft genome sequences were generated by 454 pyrosequencing from nine environmental and clinical plague isolates. In silico assemblies of Variable Number Tandem Repeat (VNTR) loci were compared to laboratory-generated profiles for seven markers. High-confidence SNPs and small Insertion/Deletions (Indels) were compared to previously sequenced Y. pestis isolates. The resulting panel of mutations allowed clustering of the strains and tracing of the most likely evolutionary trajectory of the plague strains. The sequences also allowed the identification of new putative SNPs that differentiate the 2009 isolates from previously sequenced plague strains and from each other. In addition, new insertion points for the abundant insertion sequences (IS) of Y. pestis are present that allow additional discrimination of strains; several of these new insertions potentially inactivate genes implicated in virulence. These sequences enable whole-genome phylogenetic analysis and allow the unbiased comparison of closely related isolates of a genetically monomorphic pathogen.

Phylogeography and molecular epidemiology of Yersinia pestis in Madagascar

Background

Plague was introduced to Madagascar in 1898 and continues to be a significant human health problem. It exists mainly in the central highlands, but in the 1990s was reintroduced to the port city of Mahajanga, where it caused extensive human outbreaks. Despite its prevalence, the phylogeography and molecular epidemiology of Y. pestis in Madagascar has been difficult to study due to the great genetic similarity among isolates. We examine island-wide geographic-genetic patterns based upon whole-genome discovery of SNPs, SNP genotyping and hypervariable variable-number tandem repeat (VNTR) loci to gain insight into the maintenance and spread of Y. pestis in Madagascar.

Methodology/Principal Findings

We analyzed a set of 262 Malagasy isolates using a set of 56 SNPs and a 43-locus multi-locus VNTR analysis (MLVA) system. We then analyzed the geographic distribution of the subclades and identified patterns related to the maintenance and spread of plague in Madagascar. We find relatively high levels of VNTR diversity in addition to several SNP differences. We identify two major groups, Groups I and II, which are subsequently divided into 11 and 4 subclades, respectively. Y. pestis appears to be maintained in several geographically separate subpopulations. There is also evidence for multiple long distance transfers of Y. pestis, likely human mediated. Such transfers have resulted in the reintroduction and establishment of plague in the port city of Mahajanga, where there is evidence for multiple transfers both from and to the central highlands.

Conclusions/Significance

The maintenance and spread of Y. pestis in Madagascar is a dynamic and highly active process that relies on the natural cycle between the primary host, the black rat, and its flea vectors as well as human activity.

Plague, caused by the bacterium Yersinia pestis, has been a problem in Madagascar since it was introduced in 1898. It mainly affects the central highlands, but also has caused several large outbreaks in the port city of Mahajanga, after it was reintroduced there in the 1990s. Despite its prevalence, the genetic diversity and related geographic distribution of different genetic groups of Y. pestis in Madagascar has been difficult to study due to the great genetic similarity among isolates. We subtyped a set of Malagasy isolates and identified two major genetic groups that were subsequently divided into 11 and 4 subgroups, respectively. Y. pestis appears to be maintained in several geographically separate subpopulations. There is also evidence for multiple long distance transfers of Y. pestis, likely human mediated. Such transfers have resulted in the reintroduction and establishment of plague in the port city of Mahajanga where there is evidence for multiple transfers both from and to the central highlands. The maintenance and spread of Y. pestis in Madagascar is a dynamic and highly active process that relies on the natural cycle between the primary host, the black rat, and its flea vectors as well as human activity.

Yersinia pestis genome sequencing identifies patterns of global phylogenetic diversity

Pandemic infectious diseases have accompanied humans since their origins1, and have shaped the form of civilizations2. Of these, plague is possibly historically the most dramatic. We reconstructed historical patterns of plague transmission through sequence variation in 17 complete genome sequences and 933 single nucleotide polymorphisms (SNPs) within a global collection of 286 Yersinia pestis isolates. Y. pestis evolved in or near China, and has been transmitted via multiple epidemics that followed various routes, probably including transmissions to West Asia via the Silk Road and to Africa by Chinese marine voyages. In 1894, Y. pestis spread to India and radiated to diverse parts of the globe, leading to country-specific lineages that can be traced by lineage-specific SNPs. All 626 current isolates from the U.S.A. reflect one radiation and 82 isolates from Madagascar represent a second. Subsequent local microevolution of Y. pestis is marked by sequential, geographically-specific SNPs.

Accurate, rapid and high-throughput detection of strain-specific polymorphisms in Bacillus anthracis and Yersinia pestis by next-generation sequencing

Background

In the event of biocrimes or infectious disease outbreaks, high-resolution genetic characterization for identifying the agent and attributing it to a specific source can be crucial for an effective response. Until recently, in-depth genetic characterization required expensive and time-consuming Sanger sequencing of a few strains, followed by genotyping of a small number of marker loci in a panel of isolates at or by gel-based approaches such as pulsed field gel electrophoresis, which by necessity ignores most of the genome. Next-generation, massively parallel sequencing (MPS) technology (specifically the Applied Biosystems sequencing by oligonucleotide ligation and detection (SOLiD™) system) is a powerful investigative tool for rapid, cost-effective and parallel microbial whole-genome characterization.

Results

To demonstrate the utility of MPS for whole-genome typing of monomorphic pathogens, four Bacillus anthracis and four Yersinia pestis strains were sequenced in parallel. Reads were aligned to complete reference genomes, and genomic variations were identified. Resequencing of the B. anthracis Ames ancestor strain detected no false-positive single-nucleotide polymorphisms (SNPs), and mapping of reads to the Sterne strain correctly identified 98% of the 133 SNPs that are not clustered or associated with repeats. Three geographically distinct B. anthracis strains from the A branch lineage were found to have between 352 and 471 SNPs each, relative to the Ames genome, and one strain harbored a genomic amplification. Sequencing of four Y. pestis strains from the Orientalis lineage identified between 20 and 54 SNPs per strain relative to the CO92 genome, with the single Bolivian isolate having approximately twice as many SNPs as the three more closely related North American strains. Coverage plotting also revealed a common deletion in two strains and an amplification in the Bolivian strain that appear to be due to insertion element-mediated recombination events. Most private SNPs (that is, a, variant found in only one strain in this set) selected for validation by Sanger sequencing were confirmed, although rare false-positive SNPs were associated with variable nucleotide tandem repeats.

Conclusions

The high-throughput, multiplexing capability, and accuracy of this system make it suitable for rapid whole-genome typing of microbial pathogens during a forensic or epidemiological investigation. By interrogating nearly every base of the genome, rare polymorphisms can be reliably discovered, thus facilitating high-resolution strain tracking and strengthening forensic attribution.

Delineation and analysis of chromosomal regions specifying Yersinia pestis

Yersinia pestis, the causative agent of plague, has recently diverged from the less virulent enteropathogen Yersinia pseudotuberculosis. Its emergence has been characterized by massive genetic loss and inactivation and limited gene acquisition. The acquired genes include two plasmids, a filamentous phage, and a few chromosomal loci. The aim of this study was to characterize the chromosomal regions acquired by Y. pestis. Following in silico comparative analysis and PCR screening of 98 strains of Y. pseudotuberculosis and Y. pestis, we found that eight chromosomal loci (six regions [R1pe to R6pe] and two coding sequences [CDS1pe and CDS2pe]) specified Y. pestis. Signatures of integration by site specific or homologous recombination were identified for most of them. These acquisitions and the loss of ancestral DNA sequences were concentrated in a chromosomal region opposite to the origin of replication. The specific regions were acquired very early during Y. pestis evolution and were retained during its microevolution, suggesting that they might bring some selective advantages. Only one region (R3pe), predicted to carry a lambdoid prophage, is most likely no longer functional because of mutations. With the exception of R1pe and R2pe, which have the potential to encode a restriction/modification and a sugar transport system, respectively, no functions could be predicted for the other Y. pestis-specific loci. To determine the role of the eight chromosomal loci in the physiology and pathogenicity of the plague bacillus, each of them was individually deleted from the bacterial chromosome. None of the deletants exhibited defects during growth in vitro. Using the Xenopsylla cheopis flea model, all deletants retained the capacity to produce a stable and persistent infection and to block fleas. Similarly, none of the deletants caused any acute flea toxicity. In the mouse model of infection, all deletants were fully virulent upon subcutaneous or aerosol infections. Therefore, our results suggest that acquisition of new chromosomal materials has not been of major importance in the dramatic change of life cycle that has accompanied the emergence of Y. pestis.

Complete genome sequences of Yersinia pestis from natural foci in China

Yersinia pestis, the causative agent of plague, is a deadly bacterium that affects humans. Strain D106004 was isolated from a new plague focus in Yulong County, China, in 2006. To gain insights into the epidemic origin, we have sequenced the genomes of D106004 and strains Z176003 and D182038, isolated from neighboring regions.

Genome sequence of the deep-rooted Yersinia pestis strain Angola reveals new insights into the evolution and pangenome of the plague bacterium

To gain insights into the origin and genome evolution of the plague bacterium Yersinia pestis, we have sequenced the deep-rooted strain Angola, a virulent Pestoides isolate. Its ancient nature makes this atypical isolate of particular importance in understanding the evolution of plague pathogenicity. Its chromosome features a unique genetic make-up intermediate between modern Y. pestis isolates and its evolutionary ancestor, Y. pseudotuberculosis. Our genotypic and phenotypic analyses led us to conclude that Angola belongs to one of the most ancient Y. pestis lineages thus far sequenced. The mobilome carries the first reported chimeric plasmid combining the two species-specific virulence plasmids. Genomic findings were validated in virulence assays demonstrating that its pathogenic potential is distinct from modern Y. pestis isolates. Human infection with this particular isolate would not be diagnosed by the standard clinical tests, as Angola lacks the plasmid-borne capsule, and a possible emergence of this genotype raises major public health concerns. To assess the genomic plasticity in Y. pestis, we investigated the global gene reservoir and estimated the pangenome at 4,844 unique protein-coding genes. As shown by the genomic analysis of this evolutionary key isolate, we found that the genomic plasticity within Y. pestis clearly was not as limited as previously thought, which is strengthened by the detection of the largest number of isolate-specific single-nucleotide polymorphisms (SNPs) currently reported in the species. This study identified numerous novel genetic signatures, some of which seem to be intimately associated with plague virulence. These markers are valuable in the development of a robust typing system critical for forensic, diagnostic, and epidemiological studies.

Reordering contigs of draft genomes using the Mauve aligner

Summary: Mauve Contig Mover provides a new method for proposing the relative order of contigs that make up a draft genome based on comparison to a complete or draft reference genome. A novel application of the Mauve aligner and viewer provides an automated reordering algorithm coupled with a powerful drill-down display allowing detailed exploration of results.

Availability: The software is available for download at http://gel.ahabs.wisc.edu/mauve.

Contact:rissman@wisc.edu

Supplementary information:Supplementary data are available at Bioinformatics online and http://gel.ahabs.wisc.edu

Different region analysis for genotyping Yersinia pestis isolates from China

Background

DFR (different region) analysis has been developed for typing Yesinia pestis in our previous study, and in this study, we extended this method by using 23 DFRs to investigate 909 Chinese Y. pestis strains for validating DFR-based genotyping method and better understanding adaptive microevolution of Y. pestis.

Methodology/Principal Findings

On the basis of PCR and Bionumerics data analysis, 909 Y. pestis strains were genotyped into 32 genomovars according to their DFR profiles. New terms, Major genomovar and Minor genomovar, were coined for illustrating evolutionary relationship between Y. pestis strains from different plague foci and different hosts. In silico DFR profiling of the completed or draft genomes shed lights on the evolutionary scenario of Y. pestis from Y. pseudotuberculosis. Notably, several sequenced Y. pestis strains share the same DFR profiles with Chinese strains, providing data for revealing the global plague foci expansion.

Conclusions/significance

Distribution of Y. pestis genomovars is plague focus-specific. Microevolution of biovar Orientalis was deduced according to DFR profiles. DFR analysis turns to be an efficient and inexpensive method to portrait the genome plasticity of Y. pestis based on horizontal gene transfer (HGT). DFR analysis can also be used as a tool in comparative and evolutionary genomic research for other bacteria with similar genome plasticity.

Assays for the rapid and specific identification of North American Yersinia pestis and the common laboratory strain CO92

We present TaqMan-minor groove binding (MGB) assays for an SNP that separates the Yersinia pestis strain CO92 from all other strains and for another SNP that separates North American strains from all other global strains.

Bacterial pathogenomics

Genomes from all of the crucial bacterial pathogens of humans, plants and animals have now been sequenced, as have genomes from many of the important commensal, symbiotic and environmental microorganisms. Analysis of these sequences has revealed the forces that shape pathogen evolution and has brought to light unexpected aspects of pathogen biology. The finding that horizontal gene transfer and genome decay have key roles in the evolution of bacterial pathogens was particularly surprising. It has also become evident that even the definitions for 'pathogen' and 'virulence factor' need to be re-evaluated.

Yersinia pestis evolution on a small timescale: comparison of whole genome sequences from North America

Background

Yersinia pestis, the etiologic agent of plague, was responsible for several devastating epidemics throughout history and is currently of global importance to current public heath and biodefense efforts. Y. pestis is widespread in the Western United States. Because Y. pestis was first introduced to this region just over 100 years ago, there has been little time for genetic diversity to accumulate. Recent studies based upon single nucleotide polymorphisms have begun to quantify the genetic diversity of Y. pestis in North America.

Methodology/Principal Findings

To examine the evolution of Y. pestis in North America, a gapped genome sequence of CA88-4125 was generated. Sequence comparison with another North American Y. pestis strain, CO92, identified seven regions of difference (six inversions, one rearrangement), differing IS element copy numbers, and several SNPs.

Conclusions/Significance

The relatively large number of inverted/rearranged segments suggests that North American Y. pestis strains may be undergoing inversion fixation at high rates over a short time span, contributing to higher-than-expected diversity in this region. These findings will hopefully encourage the scientific community to sequence additional Y. pestis strains from North America and abroad, leading to a greater understanding of the evolutionary history of this pathogen.