The Early Stage of Bacterial Genome-Reductive Evolution in the Host

Summary

The equine-associated obligate pathogen Burkholderia mallei was developed by reductive evolution involving a substantial portion of the genome from Burkholderia pseudomallei, a free-living opportunistic pathogen. With its short history of divergence (∼3.5 myr), B. mallei provides an excellent resource to study the early steps in bacterial genome reductive evolution in the host. By examining 20 genomes of B. mallei and B. pseudomallei, we found that stepwise massive expansion of IS (insertion sequence) elements ISBma1, ISBma2, and IS407A occurred during the evolution of B. mallei. Each element proliferated through the sites where its target selection preference was met. Then, ISBma1 and ISBma2 contributed to the further spread of IS407A by providing secondary insertion sites. This spread increased genomic deletions and rearrangements, which were predominantly mediated by IS407A. There were also nucleotide-level disruptions in a large number of genes. However, no significant signs of erosion were yet noted in these genes. Intriguingly, all these genomic modifications did not seriously alter the gene expression patterns inherited from B. pseudomallei. This efficient and elaborate genomic transition was enabled largely through the formation of the highly flexible IS-blended genome and the guidance by selective forces in the host. The detailed IS intervention, unveiled for the first time in this study, may represent the key component of a general mechanism for early bacterial evolution in the host.

Introduction

The genomes of host-adapted bacteria, including endosymbionts and obligatory intracellular pathogens, go through reductive evolution [1], [2], [3]. Such changes are partly due to a reduced pressure to maintain genes that are not essential for survival in the host. Similarly, decreased efficiency of purifying selection, resulting from the reduced population size from a restricted life, results in inactivated genes, including beneficial genes, through genetic drift [3]. During the early stage of the genome reduction process, the majority of genes are lost as large chromosomal fragments spanning multiple genes. Such genome reduction has been documented in diverse bacterial groups, including Firmicutes, Chlamydiae, Spirochetes, and γ-Proteobacteria [1], [3], [4], [5], [6], [7]. Most of these bacteria have large expansion of IS elements (insertion sequences), and thus it has been suggested that the IS elements may play an essential role during the genome reduction process [1], [3], [8], [9], [10].

Burkholderia pseudomallei and Burkholderia mallei belong to the ß-Proteobacteria family, and are the causative agents of melioidosis and glanders, respectively [11], [12], [13], [14], [15], [16], [17]. B. mallei has very recently (∼3.5 myr) evolved from a clone of B. pseudomallei through extensive genome reduction [18], [19], accounting for as much as 1.41 Mb or 20% of the genome, as estimated by the size difference between the genomes of B. mallei ATCC 23344 and B. pseudomallei K96243 [18], [20], [21]. Concomitant with this process, B. mallei became constantly associated with mammalian hosts, specifically equines [22], [23], while B. pseudomallei maintains an opportunistic pathogenic lifestyle [17]. Preliminary analyses of the two type strains, B. mallei ATCC 23344 and B. pseudomallei K96243, have suggested that genome reduction and rearrangement in B. mallei were mediated by IS elements that are widely spread throughout the genome [20], [21]. Genes that have been deleted from the B. mallei genome but are maintained in B. pseudomallei include genes that are required for environmental survival. Many of these genes encode metabolic functions for the synthesis of metabolites or the utilization of various sugars and amino acids, without which bacterial propagation in the environment could be significantly hindered [20].

While the genomic reduction during bacterial restriction to their hosts has been well documented [1], [8], [10], most of the stepwise processes have not yet been elucidated. The B. mallei genome has unique significance, as it is much younger than the other genomes in which the genome-reductive evolutionary processes have been most studied to date, including Buchnera (>150 million years) and other much older groups [1], [3], [4], [5], [6], [7]. The studies with these older genomes have been challenging due to the subsequent genomic- and nucleotide-level mutations that accumulated over a long evolutionary history. In this study, we dissected 10 genomes each of B. pseudomallei and B. mallei to understand the early-stage processes that drive genome-reductive evolution in host-associated bacteria.

Results/Discussion

Multiple IS elements with massive proliferation

It is well known that bacteria specialized to a (host) niche, often have a large number of IS elements compared to their free-living relatives [1], [3], [8], [9], [10]. Likewise, by comparing genome sequences, we found that three types of IS elements, ISBma1, ISBma2, and IS407A, were significantly increased in B. mallei compared to B. pseudomallei (Fig. 1A). By contrast, other types of IS, including IS1356, ISBma3, ISBma4, and ISBma5 were found in low copy number in both species of bacteria. These elements appeared to be mostly degenerate evolutionary remnants (i.e., part of the IS disrupted or deleted) of the Burkholderia lineage. ISBma1, ISBma2, and IS407A also had degenerate elements in each species; the ISBma1 elements had the highest levels of degeneration (44%), followed by ISBma2 (20%), and by IS407A (5%) (Fig. 1A).

Intriguingly, up to almost 90% of ISBma1, ISBma2, and IS407A (88.5%, 86.1%, and 89.6%, respectively) were found to be present at the corresponding loci in all 10 B. mallei strains, when examined after the rearranged genomic fragments in each strain were aligned against a reference genome of B. pseudomallei K96243 (Fig. 1B; for a scaled map with the IS insertion sites in all B. mallei and B. pseudomallei strains, see Fig. S1; for the patterns of genomic rearrangements in the strains of each species, see Fig. S2; for the actual comparative blast data, see Tables S1 and S2). In contrast to these “core” elements, those elements that were not present in all (singletons and those found in a few strains), collectively referred to as “accessory” elements, were much less common. That the core elements, expected to be associated with the speciation of B. mallei from B. pseudomallei, accounted for most of the elements clearly reflects the common origin of B. mallei strains from a clone of B. pseudomallei [18], [20]. More importantly, it also suggests that further transpositions were significantly slowed after subsequent geographical segregation of the bacteria. There are 13 core elements in B. mallei that have matching IS elements located at the same sites in B. pseudomallei strains (Table 1). These elements were found to be composed of elements of ISBma1 and ISBma2 but not of IS407A. This finding suggests that ISBma1 and ISBma2 have a longer history of association with B. pseudomallei than IS407A does.

IS-mediated large genomic deletions and rearrangements

Among the three largely expanded elements, we found that IS407A and ISBma2 were associated with almost all of the large genomic deletions and rearrangements in the B. mallei strains (Fig. 2; Figs. S1 and S2; Tables S1 and S2). The only exception to this was a large deletion found in the strain ATCC 23344 and its direct derivatives, FMH, JHU, and GB8 horse 4 [24], between the 43^rd and the 44^th elements in chromosome 2 (Fig. 2; Table S1). No genomic rearrangement was mediated by features other than the two IS elements. ISBma1, which was significantly increased in B. mallei, was not directly involved in any of the genomic deletions or rearrangements, however as many as 35% of it served as secondary entry points for IS407A. The majority of the core elements of IS407A, 71.8% and 63.3% in chromosomes 1 and 2, respectively, mediated rearrangements, deletions, or both (Fig. 3A). By contrast, accessory elements of IS407A contributed less, but were more active in chromosome 2 than in chromosome 1. By contrast, 50.4% and 53.2% of the core elements of ISBma2 in chromosomes 1 and 2, respectively, contributed to rearrangements and/or deletions, and the accessory elements in both chromosomes were very rarely involved (Figs. 2 and 3). We identified 59 and 28 genomic fragments in chromosomes 1 and 2, respectively, which were encompassed by core elements of IS407A or ISBma2; these core elements mediated genomic rearrangements in at least one strain (Figs. 2; Table S1). We referred to these genomic fragments as BRUs (basic rearrangement units), a set of basic units for genomic reduction and rearrangement in B. mallei. The BRUs formed various rearrangement patterns in the B. mallei strains (Fig. S2A). By contrast, B. pseudomallei strains had little variation in genome arrangement among one another due to low levels of IS elements- a few rearrangements were found but were around non-IS repeat sequences (Fig. S2B).

When the pattern of the IS insertions and their involvement in genome-reductive and rearrangement processes in strains were used to construct a phylogenetic tree, strains sharing a recent common ancestry (e.g., ATCC 23344 and its immediate derivative isolates, FMH, JHU, and GB8 horse 4) or common recent geographical origins (e.g., strains NCTC 10257, NCTC 10229, and 2002721280 from European countries) were grouped together (Fig. 3B). This phylogenic relationship supports the hypothesis that the accessory IS elements, which provided the major determinants for the tree rather than the common core elements, occurred following the speciation and geographical segregation of the B. mallei strains. By contrast, such patterns were not obvious among the B. pseudomallei strains which did not go through IS element expansions; Australian strains 1655 and 668 did not branch separately from the South Asian strains.

The deletions and rearrangements that were mediated by accessory elements were most frequently noted in strains SAVP1 and 2002721280, which lost virulence after successive passages in laboratory cultures [25] (Figs. 2 and S1). Most of the extra deletions in these strains were more prominent in chromosome 2 than in chromosome 1. In SAVP1, an IS407A-mediated deletion removed a major group of virulence genes encoding the animal-type type III secretion system in the BRU B22 (Figs. 2 and S1); this deletion may be a major cause of the avirulence of that strain. By contrast, there is no obvious deletion that may be responsible for the loss of virulence in strain 2002721280. That the strains SAVP1 and 2002721280 obtained deleterious mutations from in vitro culturing suggests that maintenance of the genomic contents in B. mallei requires selective pressure for survival in the host environment. By contrast, the fully virulent strain PRL-20 showed more frequent deletions and rearrangements mediated by accessory elements than other virulent strains. This strain may represent one of the more evolved (more genome-reduced) strains of B. mallei.

Although extra deletions and rearrangements were noted, the actual number of the accessory IS elements was not significantly increased in PRL-20, SAVP1 or 2002721280. Furthermore, none of the direct derivatives of the strain ATCC 23344 (i.e. FMH, JHU, and GB8 horse 4) had new IS insertions (Fig. 2 and Table S1). These ATCC 23344 derivatives also did not have genomic rearrangements; the only change found was a single IS407A-mediated deletion located within the BRU B17 in the strain JHU (Fig. 2 and Table S1). These lines of evidence suggest that B. mallei genomes are structurally flexible with regard to deletions, however perhaps not as much anymore for additional IS transpositions or genomic rearrangements.

Different insertion target preferences

IS407A elements are known to generate 4-bp target region duplications as direct repeats around them when they transpose [26]. We found that ISBma1 generates 8-bp target region duplications, and that ISBma2 generates longer repeats of various lengths (18–26 bp) (Table 2; for the entire data, see Table S3). In addition to the various lengths of duplications, these target regions of the three types of IS had different nucleotide compositions and patterns. Most notably, the sequences of ISBma1 contained homopolymers of A and/or T in up to 8-bp stretches of nucleotides (Fig. 4A). The target sequences of ISBma2 had a loose pattern in which the GC-rich central region was encompassed by strands of As and Ts on either side. Target sequences of IS407A had the least characteristic composition. It is intriguing to note that each IS element showed different levels of copy number expansion, ISBma1 with the lowest (3.3×), ISBma2 with an increased level (9.5×), and IS407A with the highest (16.7×) (Fig. 1A). Perhaps this difference, at least in part, resulted from the availability of genomic sites suited for insertion targets.

There were concordant patterns of disruption of the core elements of one type by another, in that ISBma1 and ISBma2 were intersected by transposed IS407A (Fig. 4B), while the reverse (IS407A disrupted by ISBma1 or ISBma2) was not found. A possible explanation for these insertion patterns may be that ISBma1 and ISBma2 could not transpose into IS407A due to the lack of sites suited for their rather uncommon target preferences, while IS407A did not have this problem. Consistent with this hypothesis, ISBma1 and ISBma2 also did not have self-disrupted elements, while there were several self-disrupted IS407A elements. The involvement of the three IS elements with different target sites increased the total number of IS insertions in the genome. Furthermore, this increase led to further spread of IS407A, because ISBma1 and ISBma2 provided neutral insertion points for the element. This in turn directly improved the efficiency of IS407A-mediated recombination in the genome, resulting in more sophisticated deletions and rearrangements.

We estimated that 83.7% of IS407A and 65.6% of ISBma2 elements in the B. mallei genomes lost their matching target duplicates, while all of the elements from intact ISBma1 elements were maintained (Table S3). Almost all of the IS407A (see Table S3 for details) and all of the ISBma2 elements that contained matching repeats were not involved in genomic rearrangements in B. mallei. This indicates that recombination among the elements were the major cause of the loss of the matching target duplicates.

Nucleotide-level mutations in B. mallei

B. mallei still has a high nucleotide-level identity (99%) to B. pseudomallei. Consistent with this, there was no AT-biased genome deviation in B. mallei, unlike that seen in many old symbionts or obligatory host-associated pathogens [1], [3]. Although the overall identity is still very high, significant nucleotide-level divergence exists, especially at the SSRs (simple sequence repeats), where there are intrinsically high mutation rates [27]. These SSRs were abundant in both B. mallei and B. pseudomallei at corresponding sites in the genomes. However, there were more genes that were disrupted by frameshift mutations in B. mallei compared to B. pseudomallei (Table S4). Most of these disrupted genes were commonly present in all B. mallei strains, reflecting the clonal origin of the strains. Some of these gene disruptions may have contributed to better adaptation of the bacteria (increased persistence) in the host environment or simply became obsolete [28]. One of the most characteristic loss of function or of surface structure in B. mallei is the loss of flagella [20]. A gene essential for flagellum biogenesis, fliP, [29] in the strain ATCC23344 was disrupted by a 65-kb fragment flanked by IS407A elements, and this mutation completely turned B. mallei flagella-less. This disruption in fliP is present in all B. mallei strains (Table S1; Fig. 2, between BRUs A2 and A3), implying the significance of losing flagella in the evolution of host-restricted B. mallei. The loss of flagella has been noted in other bacteria, including Bordetella pertussis and Bordetella parapertussis during their host specialization, derived from the strains of Bordetella bronchiseptica, [9] and Yersinia pestis during its conversion from a gut to a systemic pathogen [30]. Additional disrupted genes not present in all strains were found at approximately the same levels as in B. pseudomallei, suggesting that there were no significant increases in mutation rates in B. mallei after geographical segregation. There also was no significant level of erosion of these, so called, pseudogenes by purifying selection at levels high enough to contribute to the actual genome size reduction (data not shown).

Genomic potential for gene expression divergence

The extensiveness of the genome-wide reduction and rearrangements as well as additional nucleotide-level mutations may suggest that there is a potential for altered gene expression patterns in B. mallei. A total of 341 potential regulatory genes survived the general IS-mediated genomic reduction in B. mallei (not taking into account the diverse strain-specific deletions that occurred after speciation). Among these genes, only a small fraction (about 10) in each strain had deleterious (e.g. frameshift, null, or IS-insertion) mutations (for the list of the genes, see Table S5). In addition, none of the predicted operons in B. mallei, which correspond to the putative operons previously found in B. pseudomallei K96243 [31], were disrupted by IS elements (data not shown). We also estimated the potential for changes in promoters. There were 2,473 upstream sequences of genes, many of which may overlap or contain promoters, in the reference genome of B. pseudomallei K96243 that have homologous sequences (with at least 95% identity over at least 95% of their lengths) in all other strains of B. pseudomallei. We found that up to 99% of these sequences also matched the corresponding regions in B. mallei ATTC23344 at the same homology levels (see Table S6 for the list of the 2,473 upstream sequences, associated gene information, and the blast data). Together, all these data from the analyses of the conserved genomic regions suggest that there is only a low potential for the genes in B. mallei to have significantly divergent gene expression patterns from B. pseudomallei.

By contrast, there were 56 genes with putative regulatory functions that were lost along with the commonly deleted genomic fragments of the B. mallei genome. These genes include potential global regulatory genes, such as those encoding a quorum-sensing system (genes BPSS1176 and BPSS1180 in the reference genome of B. pseudomallei K96243), a two-component regulatory system (the pair BPSS1994 and BpSS1995 in B. pseudomallei K96243), and a number of regulators of various families (Table S7). Whether the loss of any of these 56 regulatory genes affects the expression of the remaining genes in the B. mallei genome was yet to be examined.

Similar gene expression profiles in B. mallei and B. pseudomallei

To experimentally estimate the possible transcriptomic divergence between B. pseudomallei and B. mallei, we infected female BALB/c mice with B. mallei ATCC 23344 or B. pseudomallei K96243, employing the previously established aerosol models of acute glanders and melioidosis [32]. Gene expression was compared in the bacteria that colonized the lungs and the spleens of the mice. Both B. mallei- and B. pseudomallei-challenged animals showed increases in the bacterial loads within these organs over time, with B. pseudomallei having slightly faster growth rates (Fig. 5). In our experience, B. pseudomallei also grew faster than B. mallei in vitro (data not shown). Unlike the mice infected by B. mallei, sampling the B. pseudomallei-challenged animals after 72 hr was not possible due to animal mortality from the more rapid disease progression. When gene expression profiles in the spleens and lungs were compared between B. mallei and B. pseudomallei at middle- (i.e., 24 hr for both bacteria) and late stages (i.e., 48 hr for B. pseudomallei and 72 hr for B. mallei) of infection (a total of four comparison pairs), conserved B. mallei and B. pseudomallei orthologs showed nearly identical patterns with high Pearson correlation coefficient (R) values ranging from 0.94 to 0.97, regardless of the host tissue type (Fig. 5). Therefore, there was no indication of significant modifications of the expression schemes in the genes required by B. mallei to thrive in BALB/c mice compared to those in B. pseudomallei. This is consistent with the findings of our previous gene expression studies in culture and in vivo, which also showed similar gene expression patterns in B. mallei and B. pseudomallei [20], [33], [34], [35]. These data suggest that, during the early stage, genomic reduction proceeds conservatively, not seriously affecting the indigenous gene expression patterns. In contrast to B. mallei, most of the transcription units in the insect symbiont Buchnera were altered, most likely due to complex genomic alterations accumulated over a long period of time [2].

Conclusions

In this study, we unveiled the mechanics of genomic deletions and rearrangements that occur in the early stage of bacterial specialization in the host, by conducting comparative analyses of B. mallei and its parental species, B. pseudomallei. It became clear that stepwise IS intervention was the main driving force mediating a large genomic reduction in B. mallei. Expansion of ISBma1 and ISBma2 in a clone of B. pseudomallei set the stage for the wide spread of IS407A, allowing its proliferation to sites, to which the element itself may rarely target. Actual genomic deletions and rearrangements occurred through recombination reactions mainly among IS407A and also among ISBma2 (Fig. 2). These processes achieved highly efficient deletions of dispensable genomic regions, causing only small disruptions to the portions of the genome that were maintained. This was possible due to the guidance by selective forces in the host and via the intrinsic flexibility of the compactly IS-blended genome. The B. mallei genome currently appears to still be structurally flexible with regard to deletions but is now less flexible with regard to genomic rearrangements and additional transpositions. This may indicate that the genomic evolution in B. mallei has been moving into a second stage, in which large-scale genomic alterations are reduced and nucleotide-level erosion has become more important. On the other hand, a large number of genes disrupted by frameshift mutations in SSRs were found in the B. mallei genome. The loss of function encoded by these genes and of flagella via disruption in fliP by IS407A (Table S1), could be part of the adaptive evolution for survival in the host environment, which will eventually lead to genome size reduction by erosion over time. Widespread relics of IS elements found in diverse symbionts and obligate pathogens [1], [3], [8] clearly suggest that a similar sequential IS intervention, modeled in Figure 6, may illustrate a general mechanism, by which elaborate genome transition occurs during early bacterial evolution after establishing constant association with the host.

Materials and Methods

Ethics statement

All research involving live animals was conducted in compliance with the Animal Welfare Act and other federal statutes and regulations relating to animals and experiments involving animals and adhered to the principles stated in the Guide for the Care and Use of Laboratory Animals, National Research Council, 1996. All mouse experiments conducted in the USAMRIID (US Army Medical Research Institute of Infectious Diseases) were approved by the Association for Assessment and Accreditation of Laboratory Animal Care International.

Sequencing and annotation

The type strains for B. mallei (ATCC23344) [20] and B. pseudomallei (K96243) [21] were previously sequenced. Strains FMH, JHU, and GB8 horse 4 were direct derivatives of strain ATCC 23344 after passages in the human or horse, and these strains were also sequenced previously [24]. B. mallei strains NCTC10229, NCTC10247, and SAVP1 were sequenced with full closure and manually annotated as previously described [20]. The remaining three strains (2002721280, ATCC10399, and PRL-20) were sequenced to 8× Sanger sequence coverage by the whole genome shotgun method [36] without closure, and assembled using the Celera Assembler [37], and contigs were oriented by alignment to the reference strain ATCC23344 using PROMER [38]. ORFs were predicted and annotated automatically using GLIMMER [39], [40]. Pseudo-chromosomes were constructed from the ordered scaffolds, using manual examination where necessary. Similarly, B. pseudomallei strains 1106a, 1710b, and 668 were sequenced with full closure and manual annotation, while 1655, 406e, S13, and Pasteur 6068 were sequenced without closure and annotated automatically.

Comparative genomic analyses with B. mallei and B. pseudomallei

For the analyses of genomic deletions and rearrangements in B. mallei and B. pseudomallei, 5,799 predicted protein sequences from the B. pseudomallei type strain K96243 were compared with the nucleotide sequences of the genomes of B. mallei (ATCC 23344, 2002721280, ATCC 10399, FMH, JHU, GB8 horse 4, PRL-20, NCTC 10229, NCTC 10247, SAVP1) and the other strains of B. pseudomallei (1106a, 1106b, 1655, 1710a, 1710b, 406e, 668, Pasteur, S13) using tblastn (http://blast.wustl.edu). For the mapping of the insertions of ISBma1, ISBma2, and IS407A in the genomes of B. mallei and B. pseudomallei, the entire sequences of the IS elements were searched against the 20 genomes using blastn (http://blast.wustl.edu). For the analysis of association of the IS elements with genomic deletions and rearrangements in B. mallei and of the target sequences in the genomes, strain ATCC 23344 represented all of its immediate derivatives, FMH, JHU, and GB8 horse 4, to avoid redundancy in the data, because the three strains showed identical patterns. To compare the patterns of genome rearrangements in the B. mallei strains, the positions of the BRUs in each strain of B. mallei relative to B. pseudomallei K96243 were visualized using a genome-comparative software tool ACT ([41]; http://www.sanger.ac.uk/Software/ACT), and the displays were compared in parallel among the strains.

We also examined B. mallei and B. pseudomallei for intergenic regions that potentially containing promoters, putative regulatory genes, and disruptions of putative operons to estimate the possibility of causing gene expression divergence. For intergenic region comparisons, up to 100 bp upstream of the start codon, or up to as much as available if the neighboring gene was closer, of the genes that contain at least 50 bp of an untranslated upstream region were retrieved from the genome of B. pseudomallei K96243. Then, these sequences (2,268 and 1,566 from chromosomes 1 and 2, respectively) were searched against the genomes of B. mallei and B. pseudomallei using blastn (http://blast.wustl.edu), and the length-match as well as the identity values of the orthologous regions were calculated. Putative operons reported by Rodrigues et al. from the genome of B. pseudomallei K6243 [31] were used to match the orthologous gene clusters in the genome of B. mallei ATCC 23344, and these gene clusters were examined for any disruptions caused by IS elements. All the genome sequences of B. mallei and B. pseudomallei used in this study are available through the Pathema web site (http://pathema.jcvi.org/cgi-bin/Burkholderia/PathemaHomePage.cgi) at the J. Craig Venter Institute (http://www.jcvi.org/).

Construction of the phylogenetic tree

A phylogenetic tree was constructed with the strains of B. mallei and B. pseudomallei based on the insertion patterns of and the role played in the genomic deletions and rearrangements by the three major IS elements, ISBma1, ISBma2, and IS407A. All the data used are shown in Tables S1 and S2 and Figure 2. Bootstrapped maximum parsimony trees were calculated using the PAUP package with default parameters, and a consensus tree was produced from the bootstrap replicates. Branches with bootstrap scores of less than 50 were collapsed in the tree.

Determination of the target sequence patterns

Among the duplicated target regions encompassing the IS elements ISBma1, ISBma2, and IS407A, those regions that had perfectly matching sequences were first collected. Then, among the sequences from unmatched pairs, those that occurred in more than two strains were assumed to be un-mutated valid sequences and, therefore, were added to the data pool for the analysis. Strain ATCC 23344 represented all its direct derivatives (FMH, JHU, and GB8 horse 4) in this analysis to avoid redundancy in the data. The collected sequences were aligned with Clustal X, and the alignments were graphically visualized using Sequence logos [42].

Mouse infection, bacterial load estimation, and RNA preparation

Exposure of mice to bacterial aerosol was performed as described by Roy et al. [43]. Fresh overnight cultures of B. pseudomallei DD503 [44] and B. mallei ATCC 23344 were prepared in LB or in LBG (LB supplemented with 4% glycerol), respectively, at 37°C with aeration (250 rpm). Thirty female BALB/c mice six to eight weeks old (National Cancer Institute, Frederick, MD, USA) were infected with these bacteria: nine mice each with B. pseudomallei and B. mallei for the gene expression studies, and six mice each for the bacterial load assays. The mice infected with B. mallei received an inhaled dose of 7.2×10³ cfu (7.2×LD₅₀), and those mice infected with B. pseudomallei received 1.8×10⁴ cfu (18×LD₅₀), as estimated by colony counting on agar plates. The infected mice were provided with rodent feed and water ad libitum and maintained on a 12-hr light cycle. After 24 and 48 hr (for both B. mallei and B. pseudomallei) or 72 hr (for B. mallei) of infection, five mice from each point in time were euthanized in a CO2 chamber, and their spleens and lungs were removed. Due to animal mortality, a 72 hr point in time was not possible for B. pseudomallei. The organs from two randomly picked mice were saved for bacterial load estimations, and the rest were homogenized in 1 ml of Trizol (Invitrogen Corp., Carlsbad, CA, USA) using a Tissue-Tearor (BioSpec Products, Bartlesville, OK, USA). Total RNA was purified according to the manufacturer's recommendations (Invitrogen Corp., Carlsbad, CA, USA). The bacterial load in the mouse organs was estimated as described by Ulrich and DeShazer [32].

RNA labeling and microarray analysis

Total RNA, both bacterial and mouse, from the same organ types from three mice was pooled to compensate for potential individual variation. These pooled RNA samples were used for the experiments without further purification of the bacterial RNA because RNA from mice does not cross-hybridize to the B. mallei microarray at a level affecting the legitimate interactions between the B. mallei array and the Burkholderia transcriptome [35]. The B. mallei whole genome array used in this study for both B. mallei and the closely related B. pseudomallei (average gene identity at the nucleotide level of 99%) was described in detail previously [33]. The B. mallei- and B. pseudomallei-infected organ samples were paired for the hybridization reactions based on early and late pathological states. A total of eight hybridization reactions or four different comparisons were performed, each of which was replicated in flip-dye pairs and the final ratios were calculated as log₂ (B. pseudomallei gene expression intensity/B. mallei gene expression intensity). Labeling of the probes, slide hybridization, and slide scanning were carried out as previously described [35]. The independent TIFF slide images from each channel were analyzed using TIGR Spotfinder to assess the relative expression levels, and the data were normalized using a local regression technique LOWESS (LOcally WEighted Scatterplot Smoothing) with the MIDAS software (<http://www.jcvi.org/cms/research/software>, The J. Craig Venter Institute, Rockville, MD, USA). The resulting data were averaged from triplicate genes on each microarray and from duplicate flip-dye arrays for each experiment.

Supporting Information

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