Background

Spirochetes belonging to the genus Borrelia are obligate, tick- or louse-borne parasites of vertebrates and cause zoonotic infections in humans worldwide [1]. Phylogeny based on multiple genomic loci has confirmed the presence of two major Borrelia lineages, one represented by B. burgdorferi, the agent of Lyme borreliosis (LB), and the other by B. hermsii, the agent of tick-borne relapsing fever (RF) [2]. Members of the Borrelia appear to have the most complex genomic architecture among known prokaryotes. Sequencing of the first Borrelia genome revealed a 910-kilobase linear chromosome and twenty-one linear and circular plasmids with a total length of 610 kilobases in B. burgdorferi B31, a tick isolate from New York, USA [3, 4]. Subsequent sequencing efforts showed that the linear chromosome is conserved and co-linear between the LB and RF groups [5–13]. A group of 30-kilobase circular plasmids appear to be conserved and syntenic across the entire genus [14]. A large ~160-kilobase linear plasmid specific to the RF group shares considerable syntenic regions with the lp54 plasmids of the LB group due to lateral transfer [15]. The linear plasmid lp23 in two RF group genomes is syntenic to the cp26 plasmids in LB group genomes [6]. Within the B. burgdorferi sensu lato species group (the LB group), a linear plasmid lp54 and a circular plasmid cp26 are universally present and largely syntenic among all species while the presence and the gene order of other plasmids are much less conserved [16, 17]. Nevertheless, the total gene complement is quite stable within the LB group Borrelia[16, 18].

The number of sequenced Borrelia genomes is rising rapidly. At the time of writing, the Bacterial Bioinformatics Resource Center (the PATRIC database, http://wwww.patricbrc.org) lists thirty-two completed and draft Borrelia genomes [19]. These genomic sequences represent twenty-eight LB Borrelia strains and six RF Borrelia strains, covering eight recognized or proposed species of the LB Group including B. burgdorferi sensu stricto, B. afzelii, B. garinii, B. bavariensis, B. spielmanii, B. valaisiana, B. bissettii, and B. finlandensis, and five species of the RF group including B. recurrentis, B. duttonii, B. crocidurae, B. hermsii, and B. turicatae. The chromosome of B. miyamotoi, an RF species, has also recently been sequenced [7]. A search on the GOLD genome-project registry (http://genomesonline.org/) yielded 47 completed, in-progress, and targeted Borrelia genome projects [20].

The rapid increase of sequenced Borrelia genomes allows for understanding Borrelia genome functions through comparative analysis, such as identification of host-interacting genes, regulatory elements, and pathways that are conserved among all species as well as species- and strain-specific genomic variations [21]. The comparative analysis of Borrelia genomes is presently hindered by a lack of dedicated online portal for their archival, dissemination, and comparative analysis. The Spirochete Genome Browser (SGD, http://sgb.fli-leibniz.de) currently provides web-based tools for analyzing the genomes of four strains of LB-group Borrelia (PKo, ZS7, B31, and PBi), five strains of RF-group Borrelia (DAH, A1, 91E135, and Ly), three species of Leptospiro (L. biflexa, L. borgpetersenii, and L. interrogans), and two species of Treponema (T. deticola and T. pallidum). Web portals devoted to the comparative analysis of a single bacterial genus or species have been developed. A notable example is the Pseudomonas Genome Database, which facilitates population and evolutionary analysis of Pseudomonas genomes by providing up-to-date, computer-predicted orthologous gene sequences at both within- and between-species levels [22]. GenoList, a comprehensive database of over 700 completed bacterial genomes, provides tools and information for comparative proteomics, such as identification of proteins specific to a bacterial group [23]. At present, GenoList hosts genomic and proteomic information of three major Borrelia species causing Lyme disease (B. burgdorferi sensu stricto, B. garinii, and B. afzelii) and three Borrelia species causing relapsing fever (B. recurrentis, B. duttonii, and B. hermsii). The PATRIC database hosts a more comprehensive list of genomes of pathogenic bacterial genera including 34 genomes of Borrelia[19]. Comparative genomic information is also available from OrthologueDB, another comprehensive database of orthologous genes, which uses a rigorous, phylogeny-based algorithm to improve predictions of orthology in completed bacterial and archaeal genomes [24]. To the best of our knowledge, at present there is no genome database dedicated specifically to Borrelia or to providing comparative information such as sequences of orthologous ORFs in this genus.

We have recently released complete and draft genome sequences of twenty-one additional LB Group strains [10–13]. Population genomic analysis revealed genome-wide recombination among co-existing strains and adaptive genome diversification driven by antigenic variations within local Borrelia populations [17, 25]. Despite large variations in the plasmid composition and the gene order on the majority of plasmids, the total gene content is remarkably stable among LB-group genomes, suggesting that species divergence is driven mainly by copy-number and adaptive sequence variations of surface lipoprotein genes [16]. Here we describe the design, content, and usage of BorreliaBase, a database developed during the course of the aforementioned studies. BorreliaBase is not only the first publicly available database dedicated to Borrelia genomes, it also implements a novel graphic user-interface design that encourages comparisons of bacterial genomes under a framework of their shared phylogenetic history.

Construction and content

Borreliagenomes

BorreliaBase is currently populated with sequences of 35 genomes representing eight LB-group Borrelia species, including B. burgdorferi sensu stricto, B. garinii, B. afzelii, B. bavariensis, B. spielmanii, B. valaisiana, B. bissettii, and B. finlandensis, and seven RF-group Borrelia species, including B. miyamotoi, B. hermsii, B. parkeri, B. turicatae, B. crocidurae, B. recurrentis, and B. duttonii. B. burgdorferi sensu stricto, a major pathogenic species causing Lyme disease in North America and Europe, is represented by genomes of eleven North American and two European strains. Table 1 lists sequenced Borrelia genomes that are presently in the BorreliaBase.

Table 1 Complete and draft Borrelia genomes a
Full size table

While BorreliaBase includes basic genome annotations such as plasmid identity, ORF descriptions, and graphic maps of ORF locations, single-genome information is not the focus of BorreliaBase, considering that such information is available in primary or aggregate databases such as the NCBI Microbial Genomes website (http://www.ncbi.nlm.nih.gov/genomes/MICROBES/microbial_taxtree.html), the UCSC Microbial Genome Browser (http://microbes.ucsc.edu/), and the PATRIC database (http://patricbrc.org). Instead, BorreliaBase focuses on providing high-quality comparative information for sequenced Borrelia genomes including genome phylogeny, genome synteny, and orthologous ORFs and intergenic spacers (IGSs) (Table 2).

Table 2 Main features of BorreliaBase
Full size table

Orthologous ORFs and IGSs

At its current state, the Borreliabase contains sequences of 750, 26, and 61 orthologous ORF families on the main chromosome, the cp26 plasmid, and the lp54 plasmid, respectively [17], as well as sequences of 541, 17, and 29 sets of unpublished orthologous IGSs on the main chromosome, the cp26 plasmid, and the lp54 plasmid, respectively. Methods on the identification of orthologous ORFs on the main chromosome, lp54, and cp26 have been described previously [17]. Briefly, the ORF orthology was determined with a combination of sequence-similarity search using BLAST [30], clustering of homologous proteins using MCL [31], and manual validation based on genome synteny. Subsequently, we identified orthologous IGS sequences as those sharing a pair of flanking orthologous ORFs. To ensure the quality of IGS sequences in individual genomes, a consensus start-codon position was identified for each orthologous ORF family with the assumption that start-codon position and N-terminus sequences are conserved within as well as between species. As a result, many ORF sequences have been manually adjusted and their N-terminus sequences may differ from those in NCBI and other genome repositories. Plans have been made to reconcile ORF coordinates between BorreliaBase and the NCBI genome repository.

Genome-based phylogeny

Currently, comparative data on BorreliaBase include a statistically robust genome phylogeny. First, 266 sets of single-copy ortholog ORF families present in all 35 Borrelia genomes were identified from the ortholog database (Figure 1). Subsequently, each ORF family was translated into protein sequences, which were then aligned with MUSCLE (v3.7) [32]. Codon alignments corresponding to aligned protein sequences were obtained and then concatenated using customized PERL scripts based on BioPerl [33]. A maximum likelihood tree was obtained by using RAxML with three partitions of nucleotide substitution rates corresponding to the three codon positions [34]. Most of these orthologs are chromosome-borne. The phylogenetic position of the strain 297, the main chromosome of which remains to be sequenced, was determined and manually added to the RAxML tree based on phylogenies previously derived using plasmid-borne SNPs [18].

Figure 1
figure 1

Database schema. Five main relational entities and their relations are shown. Each row of the “genome” table describes a sequenced genome. Each row of the “contig” table contains the nucleotide sequence of a plasmid (for completed genomes) or contig (for draft genomes). Each row of the “orf” table lists coordinates and coding direction of an ORF. Each row of the “orth_orf” the table assigns an ortholog family identifier (“orth_orf_id”) to an ORF. Each row of the “orth_igs” table defines a unique intergenic spacer (IGS).

Full size image

Sequence alignments

In addition to serving the raw orthologous sequences, BorreliaBase provides alignments of these sequences to allow direct interactive exploration of sequence variability (see Utility and Discussion). For orthologous IGS sequences, we provide online visualization of nucleotide alignments produced by MUSCLE (v3.7) with individual sequences ordered according to the genome phylogeny (see Use Case 3 below). For each set of orthologous ORF sequences, we provide MUSCLE-aligned amino-acid sequences as well as the corresponding codon alignment based on the amino-acid alignment (see Use Case 2 below).

Relational database and web interface

BorreliaBase is supported by a database in the back end with five main relational entities (Figure 1). The database is implemented with the PostgreSQL database management system (server version 9.1.9, http://www.postgresql.org). A dynamic web front was developed using the Asynchronous HTTP Request (AJAX) technology. On the server side, customized Perl-based Common Gateway Interface (CGI) scripts receive user inputs, fetch sequences from the back-end database, perform alignment if necessary, and output requested sequences and alignments as JavaScript Object Notation (JSON) objects. On the client side, customized JavaScript functions built with the jQuery (http://jquery.com) and D3 (http://d3js.org) libraries render these JSON objects into dynamic HTML contents.

Utility and discussion

With BorreliaBase, we aim to facilitate comparative analysis of Borrelia genomes in a phylogenetic framework. For example, users can download manually curated orthologous ORF and IGS sequences or directly visualize sequence variations online. We do not duplicate existing single-genome features already provided by general-purpose microbial genome browsers such as the NCBI Microbial Genomes and the Integrated Microbial Genomes (IMG) databases. These databases provide comprehensive genome annotations including genome maps, plasmid identities, and ORF descriptions. In the following, we describe four examples to illustrate how BorreliaBase may be productively used for comparative exploration of Borrelia genomes.

Use case 1: synteny analysis- identify gene gains and losses

Synteny analysis compares co-linearity of genes on a replicon to identify evolutionary gains and losses of genes in genomes. A reliable synteny analysis requires the identification of a set of orthologous genes across multiple genomes as well as a phylogeny of these genomes. In this example, it has been identified that orthologs of ospB is absent in the genomes of two B. garinii strains PBr and Far04 (Figure 2). By displaying gene synteny in the context of the genome phylogeny, BorreliaBase allows us to conclude based on the principle of maximal parsimony that there is a single, lineage-specific deletion of ospB on the branch leading to B. garinii. This evolutionary history is consistent with the function of ospB being a gene expressed primarily in a mammalian environment [35] and with the fact that B. garinii uses birds as primary reservoir hosts [36]. Since the synteny view on BorreliaBase serves dynamic rather than static images, one can obtained customized synteny views centered on a specific gene (ospA in this case, Figure 2). A further customization is that one may select or hide genomes by clicking on the branches or tips of the tree.

Figure 2
figure 2

Use case 1. Identify gene gains and losses. In this synteny view of BorreliaBase, orthologous ORFs are shaded with the same colors (right) and arranged according to the genome phylogeny (left). Such a display helps to identify evolutionary gains and losses of genes. In this example, the gene ospB (shared vertically in gray) is apparently lost during the evolution of B. garinii (represented here by two strains PBr and Far04). Uncolored ORFs contain frameshift mutations, which may also be sequencing errors. The synteny view of BorreliaBase is dynamic rather than static, allowing users to obtain customized synteny views centered on any specific gene (ospA in this case, marked by a vertical red line on the ruler, which is based on the scale of the B31 genome).

Full size image

Use case 2: phylogenomics- visualize synonymous and nonsynonymous variations

Lipoproteins, especially those localized to the outer cell membrane, are primary candidates for the development of diagnostics for and vaccines against Borrelia infection, since they play key roles in establishing infections in arthropod vectors and vertebrate hosts [37, 38]. For example, B. burgdorferi undergoes phase changes in its outer surface protein composition through the expression of ospA in the tick phase, the expression of ospC during host invasion, and the expression of vls genes in the mammalian phase [39]. Genes encoding outer surface lipoproteins tend to show elevated sequence variability within species, between species, or both within and between species [16, 40]. As such, sequence variability is a reliable guide for identifying Borrelia proteins and protein regions that interact directly with the host. A frequent request Borrelia researchers make to us is the identification of sequences orthologous to a gene of interest in all sequenced genomes. BorreliaBase fulfills this need by providing online access to orthologous ORF sequences.

A user begins by searching for a gene of interest using either a locus name (e.g., a15, the locus name for ospA in strain B31, the reference genome for LB group Borrelia) or a gene name (e.g., ospA). If the gene exists, a synteny view centered on the requested ORF appears showing its genomic context (Figure 2). At the same time, both the amino-acid alignment and the corresponding codon alignment of its orthologs are displayed (Figure 3). The integration of codon and amino-acid alignments allows easy determination of whether a nucleotide substitution is synonymous or non-synonymous (Figure 3). With the codon-alignment view, a user is able to visually scan for genes or regions that are potentially host-interacting based on a high density of non-synonymous substitutions before conducting a formal nonsynonymous-to-synonymous (K A /K S ) ratio test [41]. Conversely, a user may identify gene regions with both low rates of synonymous and nonsynonymous substitutions between the Borrelia species. For example, using the codon-alignment view of BorreliaBase, one can visualize that the 5’ leader sequences of ospC is highly conserved across the eight LB-group Borrelia species in both synonymous and nonsynonymous nucleotide sites (Figure 3). Conservation of synonymous sites at the 5’ regions of a gene is common in bacteria and is typically associated with a preference for rare codons and a high level of gene expression [42].

Figure 3
figure 3

Use case 2. Identify synonymous and nonsynonymous variations. Integrated visualization of codon (top panel) and amino-acid (bottom panel, colored by chemical groups) alignments, with individual sequences arranged according to the genome phylogeny, allows identification of synonymous and nonsynonymous variations. This view also facilitates exploration of selective mechanisms underlining sequence variations. Here, three notable patterns of sequence variability are highlighted for the first 44 codons of ospC. Some sites vary synonymously within and between species (high-lighted in blue), indicating strong purifying selection. Two sites vary non-synonymously within a single species (in orange), suggesting balancing selection. The first 21 sites (in red) are nearly constant not only in amino acids but also in codons, hinting strong selection for codon usage in this gene region.

Full size image

A user may download aligned and unaligned DNA and protein sequences of a set of orthologous ORFs. Further, users may select or de-select strains using the genome phylogeny.

Use case 3: phylogenetic footprinting- identify regulatory sequences

For each set of orthologous IGS sequences, a user can browse a nucleotide alignment to identify potential cis-regulatory sequences by looking for nucleotide sites conserved among the Borrelia species. Functional elements embedded in IGSs, e.g., cis-regulatory sequences and sequences encoding small RNAs, tend to be conserved among closely related bacterial species, allowing identification of such elements through comparative genomics with an approach called phylogenetic footprinting [43–45]. Using BorreliaBase, for example, we were able to show a high level of cross-species conservation of cis-regulatory sequences upstream of the ospAB operon, including the ribosomal-binding site, the transcription start position, the −35 and −10 RpoD (σ70)-binding sites, and a T-rich region required for ospA/B repression in mammalian hosts [46] (Figure 4).

Figure 4
figure 4

Use case 3. Identify conserved regulatory sequence. In this alignment view of the promoter region of the ospA/ospB operon, a dot (“.”) represents a nucleotide or a gap that is the same as in strain SV1, the closest outgroup of B. burgdorferi sensu stricto and used as the reference. Known cis-regulatory elements within this promoter region are highly conserved across LB-group Borrelia species, including, from left to right, a direct repeat (DR, in purple) and the T-rich region (in yellow) associated with RpoS-mediated repression in mammalian environment, the putative RpoD-binding sites (−35 region in green and −10 region in blue), the transcription start site (“+1”, in pink), and the ribosome-binding site (RBS, in orange).

Full size image

Use case 4: population genomics- identify recombination

Homologous recombination in bacteria, which typically takes the form of gene conversion, facilitates adaptive evolution in bacteria [47, 48]. Indeed, gene conversion underlines much of the hypervariability observed at loci encoding dominant antigens such as ospC in LB-group Borrelia and the rfb operon (coding for the O-antigen) and the fim locus (coding for an adhesion) in Escherichia coli[17, 49]. We have previously established that geographically coexisting Borrelia strains recombine at a rate approximately three times the mutation rate [17, 25]. Identification of homoplasy, a genetic variation that is inconsistent with the overall genome phylogeny, forms the basis of many tests of recombination such as the four-gamete test and the homoplasy test [50–52]. With sequence alignments displayed in the context of a genome phylogeny, a user of BorreliaBase can visually identify genomic regions affected by recombination and even estimate local recombination rates based on identification of homoplasies. In the first example, among strains of the same Borrelia species (B. burgdorferi sensu stricto), the presence of all four haplotypes at a pair of SNP sites is indicative of at least one past recombination event involving one of the SNP sites (Figure 5A). The second example shows that a portion of the b22 gene of the B. burgdorferi sensu stricto strain BOL26 is most similar to its homologs in B. afzelii, a different but sympatric species, than to its homologs in con-specific strains, indicating an unambiguous case of cross-species recombination (Figure 5B). Breakage points of this specific recombination event have been identified by recognizing where such phylogenetic inconsistency ends [17]. In the last example, one estimates recombination rates relative to mutation rates by scanning for sequence differences between a pair of sister-group strains (Figure 5C). A SNP segregating between two phylogenetic sister-group strains may be caused either by mutation or by recombination. For such a SNP site, if one of the two SNP states does not occur in any other strains in a large population sample it is likely to be a recent point mutation, while it is more likely to be introduced by recombination if it appears in other strains as well [53]. Since these visual methods for identifying footprints of recombination do not require performing any statistical analysis, in our experience BorreliaBase makes an effective pedagogical tool as well.

Figure 5
figure 5

Use case 4. Identify recombination. Recombination among bacterial genomes leaves genomic footprints that are identifiable by tests of homoplasy (i.e., phylogenetic inconsistency). (A) The “four-gametes” test. Two SNP sites within the b18-b19 IGS region are shown. All four possible haplotypes (labeled “1” through “4” and highlighted in yellow) are present in this region among the 14 B. burgdorferi sensu stricto genomes, suggesting at least one recombination event between these two SNP sites. (B) Cross-species gene conversion. A codon alignment of a region of b22 is shown. The B. burgdorferi sensu stricto strain BOL26 has a haplotype in this region that closely resembles its homologs in two B. afzelii strains (PKo and ACA-1), caused apparently by a replacement of the allele in BOL26 by an allele originated from a B. afzelii strain. (C) The “sister-group” test. Two SNP sites in a18 segregating between 156a and 297, two phylogenetic sister-group strains, are shown. The “C” at the first SNP site is a singleton while both alleles at the second SNP site have multiple copies. Since sister-group genomes are most closely related to each other, the first SNP suggests a recent point mutation (non-synonymous in this case) while the second SNP is likely to be introduced by recombination. The latter two analyses require a phylogenetic framework.

Full size image

Conclusions

The phylogeny-centered design of BorreliaBase encourages comparing Borrelia genomes in the framework of their shared phylogenetic history. More than simply serving as a graphic guide for selecting and de-selecting strains, the genome phylogeny provides visual, evolution-based expectations on comparative genomic information such as an average level of sequence variability and the expected phylogenetic consistency of SNPs. When, and only when, an observed pattern deviates significantly from these evolutionary expectations should functional importance be conjectured. For example, the observed high levels of within-species sequence variability of lipoprotein genes such as ospC, dbpA, and vls deviate greatly from the expected sequence variability, suggesting strong diversifying selection at these loci driven by, e.g., immune escape from the host. Conversely, the observed high levels of sequence conservation of cis-regulatory elements deviate from the expected level of sequence variations between Borrelia species, consistent with gene-regulatory roles these IGS elements play (Figure 4). A phylogenetic expectation is especially valuable for identifying genomic footprints of recombination (Figure 5). Nexplorer, an online tool for analyzing evolution of characters (e.g., presence and absence of introns) associated with a protein family, similarly employs a phylogeny-centered user-interface design [54].

While allowing interactive comparative exploration of Borrelia genomes, the current version of BorreliaBase (public release 1.0) is admittedly limited in genomic scope. For example, comparative features, including gene synteny and orthologs, are only available for the three universally present replicons. We plan to include results of other replicons in future releases of BorreliaBase.

Availability and requirements

The BorreliaBase is publicly accessible online at http://BorreliaBase.org. The source code of the website is publicly available at http://sourceforge.net/projects/phylobrowse/.