The P2-like prophage b0847 promotes the fitness and virulence of ExPEC strain CFT073 during systemic infection of zebrafish embryos

The ExPEC strain CFT073 was isolated from the blood of a human patient with acute pyelonephritis (kidney infection) [26], [39]. This urosepsis isolate is versatile, with the apparent ability to traverse several host microenvironments to reach the bloodstream, and has a relatively large genome of 5,369 protein-coding genes and several GIs. In previous work, we found that CFT073 is exceptionally lethal in an infection model that uses zebrafish embryos as surrogate hosts for the high-throughput analysis of ExPEC virulence [40]. At 48 h post-fertilization (hpf), zebrafish possess an innate immune system composed primarily of phagocytic cells and antimicrobial peptides [41], [42], [43], [44]. These defenses mirror those employed by mammalian hosts to combat ExPEC.

To identify GI-associated virulence factors carried by CFT073, we screened 11 previously described deletion mutants that each lack a specific GI (Table 1 and Figure 1A) [45]. In blinded assays, 48 hpf zebrafish embryos were infected with 1,000 to 2,000 colony-forming units (CFU) of either wild type CFT073 or one of the 11 GI mutants. Bacteria were delivered into one of two injection sites: the fluid-filled sac surrounding the heart referred to as the pericardial cavity (PC), which mimics a localized tissue infection, and the circulation valley (CV), which facilitates rapid dispersal of bacteria into the bloodstream [40]. Each of these sites likely challenges the pathogen with different nutrient limitations, receptor availability, and host defenses.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 1. The φb0847 island is important for CFT073 pathogenicity during systemic infection in zebrafish embryos.

(A) Diagram of GIs and their location within the CFT073 chromosome that were screened in the zebrafish host. Magenta indicates island mutants that had no observable defects, while green denotes island mutants that displayed significant attenuation. (B) The pericardial cavity (PC, top row) and blood (bottom row) of 48 hpf embryos were inoculated with 1,000–2,000 CFU. Fish were scored for death at 0, 24, 48, and 72 h post-inoculation (hpi). Data are presented as Kaplan-Meier survival plots and p values were calculated using a log-rank (Mantel-Cox) test (sample sizes for each curve are listed in Table 1). (C) Equal numbers (1,000–2,000 CFU total) of wild type and CFT073Δφb0847 were inoculated into the bloodstream of embryos. Fish were sacrificed and bacterial loads enumerated at the indicated times by differential plating (n = 10 to 20 embryos). Data are represented as competitive indices, where negative values indicate a reduction in fitness of the mutant strain. (D) Bacteria were prepared as in (C) and inoculated into the PC or yolk. Fish were sacrificed at 18 hpi and bacterial numbers determined (n = 9–10 embryos). Data from blood infections is the same as in (C), provided as a reference. P values were determined using two-tailed Mann-Whitney t tests. Median values are indicated by bars in (C) and (D).

https://doi.org/10.1371/journal.ppat.1003175.g001

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 1. Summary of GI screen in zebrafish infection sites.

https://doi.org/10.1371/journal.ppat.1003175.t001

In this infection model, increased growth of ExPEC is associated with decreased survival of the host [40]. All 11 GI mutants, with the exception of ΔGI-aspV, grew equally well in broth culture at 28.5°C and 37°C (data not shown). Following inoculation into the PC, only deletion of the 123 kb GI PAI-pheV [I] resulted in a significant decrease in virulence relative to wild type CFT073 (Figure 1B, top). This was not surprising as PAI-pheV [I] encodes the notable ExPEC-associated virulence factors α-hemolysin (pore-forming toxin), SAT (vacuolating toxin), P pili (adhesive organelles), aerobactin (iron acquisition system), and K2 capsule (immune evasion). The ability of the ΔPAI-pheV mutant to still kill approximately half of the embryos suggests that additional factors with overlapping roles in virulence within the PC are encoded outside of PAI-pheV and the 10 other GIs tested.

The ΔPAI-pheV mutant was also attenuated following inoculation of the CV to initiate systemic infection, as were the GI mutants ΔGI-selC, ΔGI-cobU, and Δφb0847 (Figure 1B, bottom). In addition to several hypothetical genes, the selC and cobU islands harbor genes that appear to be components of polyamine and iron transport systems, respectively. Both polyamines and iron acquisition systems are known to be important mediators of ExPEC fitness in mouse models of infection [46], [47], [48]. Although the ΔGI-cobU mutant exhibited only a modest reduction in virulence in these assays using inoculation doses of 1,000–2,000 CFU/embryo, with slightly higher doses between 2,000 to 3,000 CFU/embryo this mutant displayed more dramatic and significant (p<0.05) attenuation (Table S1). This observation supports previous findings indicating that the inoculation dose can markedly influence the discernibility of some mutant phenotypes in the zebrafish host [40].

The remaining GI showing a phenotype in our screen is composed of an intact integrated phage genome (prophage) named ‘φb0847’ (Figure 1B) [45]. This prophage is 33 kb in length and contains 48 predicted open reading frames, most of which encode recognizable phage proteins that share homology with genes of tailed phages belonging to the order Caudovirales. More specifically, φb0847 carries genes involved in regulation, replication, and virion assembly that are related to and syntenic with the genes of phage P2 and its relatives (Figure 2). From this analysis, it is clear that the φb0847 prophage is a member of the P2-like phage group and likely represents a fully functional phage genome complete with all the essential genes associated with P2-like phages [49]. Aside from the ΔpheV mutant, with its fairly well characterized assortment of virulence genes, Δφb0847 displayed the most pronounced defect of the island mutants examined. Therefore, the φb0847 GI became the primary focus of our investigation.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 2. Alignment of φb0847 genome to other P2-like bacteriophage.

Related P2-like prophages are aligned relative to their respective integration sites (att). Size is measured in kilobase pairs (Kbp). E. coli phages P2 and 186 and Salmonella phages Fels-2 and SopE5 were previously characterized. HS2 is an uncharacterized prophage contained within the genome of the commensal E. coli strain HS. Our unpublished analysis indicates that P2, 186, and Fels-2 represent three different “sequence types” based on virion proteins, which are typically >85% identical within each of these three groups and 50–70% identical between the groups. Bracketed numbers below φb0847 indicate positions of the deletion mutants generated to assess the functionality of genes within broad regions of φb0847. The neaT gene is distinguished by a red open reading frame in moron position 2.

https://doi.org/10.1371/journal.ppat.1003175.g002

To further define the contribution of φb0847 to the virulence and fitness of CFT073, we carried out competitive assays in which a one-to-one mixture of wild type and mutant bacteria were injected into the CV of zebrafish embryos (Figure 1C). At the indicated time points, the infected embryos were homogenized and bacteria present were enumerated by dilution plating on selective agar. Δφb0847 carries a kanamycin resistance cassette that was used to distinguish wild type and mutant strains. No differences between wild type CFT073 and the Δφb0847 mutant were observed until 6 h post-inoculation (hpi), when Δφb0847 titers began to decline (Figure 1C). These results indicate that the φb0847 island is dispensable during initial stages of a systemic infection, but enhances bacterial fitness at later time points, coordinate with the upregulation of host inflammatory responses engage. The Δφb0847 mutant displayed more modest, though still significant, decreases in fitness during competitive assays against wild type CFT073 within the PC and yolk sac at 18 hpi (Figure 1D). Phagocytes are recruited into the PC en masse in response to infection with ExPEC [40], possibly contributing to the competitive disadvantage of the Δφb0847 mutant within this niche. On the other hand, the yolk is a rich source of nutrients for bacteria and is mostly free of phagocytes and other immunosurveillance mechanisms. However, the yolk does contain maternally inherited antimicrobial compounds that could account for the slight reduction in fitness of Δφb0847 within this host environment [50]. Competitive experiments in broth culture did not reveal appreciable differences between the wild type and mutant strains (data not shown).

φb0847 harbors a multigenic region that contributes to fitness

To identify genes within φb0847 that, when deleted, recapitulate the attenuated phenotypes of Δφb0847, we constructed partial deletion mutants lacking one of three nearly equal-sized regions of the prophage island (designated Δ1–2, Δ2–3, and Δ3–4, as indicated along the φb0847 genome in Figure 2). In competitive assays, the Δ1–2 and Δ2–3 mutants were significantly more fit than the full Δφb0847 mutant at 12 hpi (Figure 3A). Analysis at 12 hpi allowed time for selection to take place, while limiting artifacts due to bacterial replication at later time points in dead and dying hosts where selective pressures are presumably weaker. In these assays, only the Δ3–4 mutant phenocopied the complete φb0847 island deletion mutant (Figure 3A). Lethality of this mutant variant was also significantly reduced in comparison to wild type CFT073 and the Δ1–2 and Δ2–3 mutants in independent challenges (Figure 3B). These results indicate that one or more genes within the terminal 3–4 region of the φb0847 prophage enhances both the fitness and virulence of CFT073 during systemic infections within the zebrafish host.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 3. φb0847 harbors multiple loci that contribute to the fitness of CFT073 during systemic challenge.

(A) Equal numbers (1,000–2,000 CFU total) of wild type CFT073 and each mutant derivative indicated were inoculated into the bloodstream of embryos. Fish were sacrificed and bacterial loads enumerated at 12 hours post inoculation (hpi) by differential plating (n>28). Data are presented as competitive indices with negative values indicating a reduction in fitness of the mutant. P values were determined using two-tailed Mann-Whitney t tests; bars indicate median values. (B), (C), and (D) 1,000–2,000 CFU of wild type CFT073, the indicated mutant, or recombinant derivative were each inoculated into the blood of 48 hpf embryos. Fish were scored for death every 6 h starting at 18 hpi until 48 hpi (n = 40 or more embryos for each curve). pGEN-mcs in (D) serves as an empty vector control for pGEN-neaT^Pnative. Data in (B), (C), and (D) are presented as Kaplan-Meier survival plots. A log-rank (Mantel-Cox) test was used to determine p values; ns = not significant.

https://doi.org/10.1371/journal.ppat.1003175.g003

Temperate prophage genomes like φb0847 can carry ‘lysogenic conversion’ genes that affect the bacterial host but are not essential for lytic phage growth. To avoid disruption of critical phage processes, the integration of this genetic material is generally tolerated only in certain regions of the prophage genome. These added sequences are known as ‘morons’, because bacteriophages with such insertions have more DNA [51], [52]. Moron genes typically contain their own regulatory elements and vary among individual phage genomes. They often alter the surface structure or physiology of the bacterial host and can benefit the phage by making its host refractory to competing parasites or otherwise promoting bacterial survival and growth [52], [53].

The P2-like phages appear to have at least two variable moron positions (Figure 2). Using phage P2 as a reference, the location of moron position 1 is between the DNA replication gene A and head assembly gene Q, and moron position 2 is between the tail fiber gene G and tail sheath gene FI (Figure 2) [49]. In φb0847 within CFT073, the second moron site, which is absent from the Δ3–4 mutant, contains one open reading frame that is oriented in the opposite transcriptional direction to the flanking tail genes. This gene, which we named neaT for reasons described later, encodes a putative acyltransferase (Pfam:PF01757). This gene is not conserved among P2-like phages and is likely not critical for lytic replication of φb0847.

In light of this information, neaT, the immediately proximal gene yfdK (homologous to P2 phage tail gene G), and the collection of distal tail genes (FI through D) were individually deleted from the φb0847 prophage in CFT073. All three mutant derivatives—ΔyfdK, ΔneaT, and ΔFI-D—were attenuated in their ability to kill zebrafish embryos after injection into the blood via the CV (Figure 3C). Despite the significantly reduced virulence of these mutants, no defects in fitness were observed in competitive assays with wild type CFT073 (data not shown). The lack of any discernable fitness defects in competition assays may 1) reflect the ability of wild type CFT073 to trans-complement the mutant strains in vivo and/or 2) indicate that there is cooperative interplay among the yfdK, neaT, and FI-D loci. Of note, disruption of loci flanking neaT did not appreciably alter its expression in broth culture (Figure S1). Furthermore, we found no evidence that the neaT mutant could be complemented in vivo during competition assays by acquiring φb0847 sequences from the wild type strain (Figure S2). Interestingly, a yfdK homologue was recently shown to aid the survival of a K-12 laboratory strain of E. coli in acidic environments [54], but no mechanism for this effect is known, and to the authors' knowledge, yfdK homologues have not been implicated in pathogenesis.

The neaT gene restores virulence to the φb0847 island deletion mutant

The in vivo assays presented in Figure 3C and bioinformatic analyses described below highlight neaT as a gene of potential importance to the fitness and virulence of CFT073. To test this possibility, the neaT locus, including an upstream promoter region of 211 bp, was amplified from the CFT073 chromosome and cloned into the high-retention plasmid pGEN-mcs, yielding pGEN-neaT^Pnative. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) indicated that neaT transcript levels made from the pGEN-neaT^Pnative vector in broth culture were about 1.7-fold higher than those observed in wild type CFT073 (Figure S3). Complementation experiments were performed comparing the lethality of wild type CFT073/pGEN-mcs, Δφb0847/pGEN-mcs, and Δφb0847/pGEN-neaT^Pnative in zebrafish embryos after inoculation of the CV (Figure 3D). The complete prophage deletion mutant Δφb0847 carrying the empty vector pGEN-mcs exhibited a significant delay in killing relative to either the wild type strain CFT073/pGEN-mcs or the complemented mutant Δφb0847/pGEN-neaT^Pnative. In total, these experiments identify neaT as a virulence determinant contained within the φb0847 island of CFT073; therefore, the uncharacterized neaT gene became the focal point for the remainder of our investigation.

neaT is required for fitness during systemic, but not localized infections in a mammalian host

To extend our observations made using zebrafish, we employed a murine model to further define the requirement for neaT during localized and systemic infections. For localized challenges, we took advantage of a well-characterized mouse model of urinary tract infection. Wild type CFT073 and the ΔneaT mutant were mixed at a one-to-one ratio and inoculated via transurethral catheterization into adult female CBA/J mice. After 3 days, animals were sacrificed and bacterial titers within the bladders and kidneys were enumerated, revealing no outright competitive advantage for wild type CFT073 over the ΔneaT mutant in either organ (Figure 4A). To appraise the requirement for neaT during systemic infections, we utilized a recently described sub-lethal bacteremia model in which CBA/J mice were injected with a one-to-one mixture of wild type and mutant bacteria via the tail vein [55]. At 24 hpi the ΔneaT mutant was recovered at significantly reduced levels from the spleen and liver compared to wild type CFT073 (Figure 4B). These results confirm and extend our findings in the zebrafish host, demonstrating that neaT provides niche-specific advantages to CFT073 during systemic infections.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 4. neaT enhances the fitness of CFT073 in a murine model of bacteremia.

(A) Equal numbers (10⁷ CFU total) of wild type CFT073 and CFT073ΔneaT were transurethrally inoculated into the bladder of CBA/J female mice. Mice were sacrificed, organs harvested, and bacterial loads enumerated at 3 d post inoculation. (B) Equal numbers (10⁶ CFU total) of wild type and CFT073ΔneaT were inoculated into the bloodstream of CBA/J female mice via tail vein injection and bacterial titers present in the spleen and liver were enumerated 24 h later. Data are shown as competitive indices, where negative values indicate a reduction in the fitness of CFT073ΔneaT. Bars indicate median values for each group; n≥9 mice. P values determined using Wilcoxon-matched paired signed rank; ns = not significant.

https://doi.org/10.1371/journal.ppat.1003175.g004

Diversity and phage association of NeaT homologues

There are no closely related homologues of NeaT in E. coli. Only four matches were found in the current NCBI collection of 170 RefSeq E. coli genomes (as of June 2012) that produce an alignment E value<10⁻⁶ with similarity over >50% of the NeaT protein length. A PCR-based survey for the presence of neaT within various clinical E. coli isolates corroborated our in silico observation that neaT is rare among this taxon (Figure S4). Out of 21 randomly chosen isolates, none carried the CFT073 neaT allele. Homologues of neaT are also rarely detected in P2-like phage genomes; among 45 randomly chosen P2-like phages and prophages in E. coli, Salmonella, Shigella, and Enterobacter that we examined, only φb0847 carries a neaT-like gene. The closest match to NeaT in the NCBI database is encoded by locus Ent638_2581 of Enterobacter sp. 638, whose protein product is only about 33% identical to NeaT. We note that several more distantly related neaT homologues are present in the genomes of other temperate phages and prophages of the bacterial family Enterobacteriaceae (Table 2). They are found, for example, in the Shigella flexneri phage Sf6 genome and several uncharacterized prophages of S. flexneri, in E. coli phage φV10 and a nearly identical prophage in the Shiga toxin-producing E. coli isolate DEC4D, and in a putative prophage within Citrobacter rodentium strain ICC168. The above Enterobacter homologue Ent638_2581 is also carried within a putative prophage that is similar to Shigella phage SfV. Each of these phage-associated neaT homologues is un-ameliorated with respect to its bacterial host genome (see below), and each lies within a known moron position in its phage genome. Because neaT homologues differ substantially in sequence conservation and are found in a variety of tailed-phages, neaT-like genes may have been laterally acquired by Enterobacteriaceae lineages on several occasions, possibly via phage. Multiple neaT acquisition events would indicate that this gene has an underlying evolutionary importance to either the phages themselves or their hosts. In considering its putative function (see Figures S5, S6, and Text S1), its apparent lateral acquisition, and its allelic variance within the proteobacterial lineage, this gene was named ‘neaT’—nomadically evolved acyltransferase. In the following sections we explore the evolutionary history of this gene by analyzing the diversity and distribution of neaT-like genes in more detail.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 2. neaT homologues associated with bacteriophages.

https://doi.org/10.1371/journal.ppat.1003175.t002

Phyletic distribution of neaT

To investigate the evolutionary source of E. coli neaT genes, we assessed the phyletic distribution of its homologues. BLASTp alignments were performed on the publically available NCBI database using NeaT from CFT073 as a probe for the search sets of Proteobacteria, Firmicutes, Bacteroidetes, Actinobacteria, Spirochaetes, and Fusobacteria [56]. Sequences were declared to be homologous if they had an alignment significance (E value) of <10⁻⁶ over >50% of their lengths [57]. These searches retrieved a total of 317 non-paralogous NeaT-like sequences. The distribution of phyla containing these sequences is depicted in Figure 5A (left), revealing that the majority of neaT homologues are from the Firmicutes and Bacteroidetes rather than Proteobacteria. In an attempt to control for the inherent bias of NCBI databases, we plotted the number of available gene sequences for each phylum represented in Figure 5A (right). This plot demonstrates that the high number of neaT homologues identified among non-proteobacterial phyla is not due to a skew in sequence abundances. To the contrary, total proteobacterial gene sequences overshadow those from other phyla and therefore underscore the relative rarity of neaT alleles in this taxon.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 5. neaT is discordantly conserved.

(A) Left: Phyletic distribution of neaT homologues among genomes retrieved from NCBI (n = 317). Right: Number of gene sequences deposited in NCBI for each phylum as of November 2011. (B) Left: Pie graph showing distribution of neaT homologues among each phylum represented in the custom database (n = 21 non-paralogous neaT genes). Right: Theoretical distribution of neaT homologues within phyla present in the custom database based on random chance. P values for the observed versus theoretical phyletic abundance of neaT homologues were calculated by .score analysis. (C) Upper y-axis: bar graph depicts total number of non-paralogous homologues retrieved from the custom database (DB) for each gene encoded within φb0847 (plotted along the x-axis with respect to its position within the prophage genome). The neaT open reading frame is indicated in red. Lower y-axis: bar graph showing the percent of proteobacterial homologuesfound in the homologue set for each φb0847 gene. Sequences unique to CFT073 were assigned 100% proteobacterial conservation.

https://doi.org/10.1371/journal.ppat.1003175.g005

To quantify the phyletic distribution of NeaT homologues with greater statistical confidence, we performed bi-directional alignments of NeaT using BLASTp with a manually assembled database of open reading frames from a representative, yet broad, assortment of 165 phylogenetically classified bacterial genomes and associated plasmids obtained from NCBI (Table S3). This analysis confirmed that, compared to random chance, neaT homologues are significantly enriched among species belonging to the phyla Firmicutes and Bacteroidetes (Figure 5B). Moreover, many of the neaT homologues were detected in notable plant and animal pathogens, including Erwinia spp., Bacillus spp., Staphylococcus aureus, Streptococcus oralis, Clostridium botulinum, and Porphyromonas spp.

Results from similar alignments of neaT and all other φb0847-encoded genes are presented graphically in Figure 5C. For each prophage gene, the number of non-paralogous matches found in the custom database are represented as bars (upper axis) and the percent of those hits that are harbored within proteobacterial genomes (lower axis) are plotted against the position of the gene within φb0847 (x-axis). Given the host range of known P2-like phages, it is not unexpected that the majority of genes within φb0847 were exclusive to the proteobacterial phylum. Exceptions, in addition to neaT, include homologues of φb0847 genes encoding the phage integrase and a Dam methylase. However, neaT is unique among the φb0847 prophage genes in that over 75% of its matches (16 of 21) were from outside the Proteobacteria (Figure 5C and Table S4). The discordant conservation of neaT highlights its likely extra-phyletic origin.

neaT displays signatures of recent lateral transfer

If a gene has origins outside its immediate genome, it would carry sequence-level vestiges of its previous host until it adopts the characteristics of the current host—a process known as ‘amelioration’ [27], [58]. Commonly used parameters that distinguish laterally transferred and unameliorated genes are atypical codon usage and guanine-cytosine (G+C) content [57], [59], [60]. We analyzed these features of neaT in the context of the CFT073 genome and φb0847 prophage. Using all 5,369 protein-coding genes of CFT073, the frequency with which specific codons are used for each amino acid was calculated (Table S5). Each gene was then assigned a ‘codon deviation score’ representing how often it uses atypical codons (Methods and Table S6). Scoring correlates with conformity; genes scoring low have a more typical codon usage, whereas poorly conformed genes score high. This analysis shows that neaT possesses a significantly abnormal codon usage compared to the rest of the CFT073 genome (p = 0.0260) (Figure 6A, left panel). The neaT gene was also observed to be G+C-poor (29.84%), making it a significant outlier from the CFT073 genome-wide median of 51.5% (p = 0.0001) (Figure 6A, right panel). We also analyzed the codon deviation score (Figure 6B, upper axis) and nucleotide composition (Figure 6B, lower axis) of neaT with respect to the genome of φb0847. Most genes withinφb0847 conform to the codon usage and G+C content of CFT073. This is expected for a parasite that has been co-evolving with proteobacterial hosts over an extensive period of evolutionary time [59]. Thus, the aberrant codon usage and nucleotide composition of neaT is not simply an inherited trait of φb0847. Because of its relatively low G+C content and poorly conformed codon usage, we conclude that neaT is a relatively recent acquisition by both φb0847 and the genome of CFT073.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 6. neaT is maintained in an un-ameliorated state.

(A) Left: distribution of codon deviation scores assigned to the 5,369 protein-coding genes of CFT073. Right: distribution of %GC content of each protein-coding gene of CFT073. Bar and whiskers indicate median and interquartile ranges. Red ‘X’ marks position of neaT within each distribution. (B) Upper y-axis: bar graph depicting codon deviation score for each φb0847 gene plotted with respect to position within the prophage (x-axis cartoon, with the neaT gene highlighted in red.). Lower y-axis: line graph representing fluctuations in %GC content across φb0847 (window = 182 bp, step = 182 bp). (C) %GC content of neaT alleles found across phyla (color-coded) plotted against the %GC content of each respective genome (x-axis). Inset shows the same sort of analysis for the poxB allele as a comparison. Points falling on the dashed lines represent alleles that are completely ameliorated with respect to their host genomes. (D) Graph shows the ratio of %GC content of neaT alleles and total genomic %GC content for individual isolates within the indicated phyla. Bars indicate medians, and p values were determined using two-tailed Mann-Whitney t tests (n = 18 (Proteobacteria), 51 (Firmicutes), 22 (Bacteroidetes)).

https://doi.org/10.1371/journal.ppat.1003175.g006

To determine if the apparently unameliorated state of neaT in CFT073 is unique or if it is hinting at a more widespread phenomenon, we plotted the G+C content of a representative subset of neaT homologues identified in Figure 5 against the G+C content of their respective genomes (Figure 6C). As a control, we also plotted the G+C content of poxB, which encodes the metabolic enzyme pyruvate oxidase and exists in an ameliorated state within several phyla (Figure 6C, inset). Most proteobacterial neaT genes were significantly less ameliorated than those found in the genomes of Bacteroidetes and many Firmicutes (Figure 6C and D). Interestingly, even though Firmicutes genomes generally have a low G+C content, neaT-like genes within this lineage are still relatively G+C-poor, at least in a major fraction of Firmicute species (Figure 6C). Cumulatively, these results indicate that, at least among the three phyla compared here, neaT-like genes have likely been associated with Bacteroidetes the longest, whereas acquisition by the Proteobacteria was a more recent event.

Summary and impact of findings

Presented here are the results from a screen conducted using the ExPEC isolate CFT073 and a high-throughput zebrafish surrogate host model of infection. We screened GIs for novel virulence genes, which were expected to have a history of lateral gene transfer. Three loci within the P2-like prophage φb0847 were found to contribute to the virulence of CFT073 during systemic infection. A previously uncharacterized gene—designated here as neaT—was discovered to augment the virulence capacity of CFT073, independent of other prophage components (Figure 3D). We demonstrated that neaT is conditionally required for maximal fitness during bacteremic infections of both zebrafish and mice, suggesting that CFT073 has potentially co-opted this phage-borne gene for specific virulence behaviors. By tracing the evolutionary history of the neaT gene, we found that it is relatively rare and has sequence-based features suggesting that it was recently absorbed into the proteobacterial supraspecies pan-genome. Signs of its novelty are typified by high allelic variance—possibly a result of multiple entries into the Proteobacteria lineage via phage—and its mostly unameliorated state within proteobacterial genomes.

We also investigated the putative function(s) of NeaT in vitro. The NeaT protein shares homology with several characterized acyltransferases encoded within a variety of non-E. coli genomes. These putative membrane-localized enzymes can modify components of the bacterial cell wall, particularly peptidoglycan [61], [62], [63], [64]. Alteration of this macromolecule can often provide bacterial pathogens with protection from host antimicrobial peptides and enzymes such as lysozyme. However, deletion of neaT had no effect on the sensitivity of CFT073 to lysozyme, the antimicrobial cationic peptide polymyxin B, or antibacterial factors present in human serum (see accompanying supplemental Text S1). Interestingly, expression of neaT did alter the behavior of CFT073 in swarming assays and induced bacterial aggregation on swim plates (Figure S5A–C, Text S1)—phenotypes that may be attributable to NeaT-mediated modification of components within the bacterial envelope. We also found that expression of recombinant NeaT can inhibit production of surface structures like curli and cellulose in some strain backgrounds (Fig. S5D–E, Text S1), supporting the notion that NeaT can affect salient properties of the bacterial surface and thereby alter bacterial group behavior.

The apparent capacity of NeaT to modulate bacterial aggregation (Fig. S5C) is especially intriguing in light of a recent work demonstrating that aggregate formation can promote bacterial survival within the bloodstream of infected mice [65]. Building on these observations, we found that expression of the neaT gene from a low copy number plasmid significantly decreased the capacity of CFT073 to associate with murine macrophages, suggesting that NeaT serves as an immune evasion factor (Fig. S6). The specific mechanism(s) by which NeaT promotes bacterial fitness during systemic infections, as well as the environmental cues that control neaT expression, require further investigation. As it stands, this work contributes to the idea that ExPEC isolates do not all share the same set of virulence factors, which are likely dictated by the distinct evolutionary trajectory and particular niche tropism of each strain.

neaT-based models for evolution of laterally acquired genes

Our analysis defines neaT as a recently acquired locus of the Proteobacteria. Evidence for this is drawn from its discordant conservation, abnormal codon usage, and low G+C content. In large part, the unameliorated state of neaT-like genes in Proteobacteria and Firmicutes genomes suggests that there is a general phenomenon accounting for its relative A+T-rich composition beyond having originated in an A+T-rich genome, as previously suggested [59]. We posit that the observed A+T-richness of laterally transferred genes can be, to some extent, accounted for by an ‘exploratory mechanism’ [16]. Upon introgression of a foreign gene, its retention depends on its adaption to the host's genetic and cellular machinery, a process that can take several millions of years [66]. During this time the gene may fall under relaxed selection whereby mutations accrue until a beneficial allele is ‘discovered’ and acted upon by selection. Connecting relaxed selection to reduced G+C content is the observation that there is a universal mutation bias for G/C to A/T transitions in bacterial genomes [67], [68], [69]. It then follows that immediately after a gene is acquired, it will initially accumulate A+T-rich character until a selectable version is ameliorated. From the findings presented here, we speculate that the neaT variant in CFT073 is an example of a newfound allele that is being used to promote bacterial fitness in pathogenic contexts.

Arguably, neaT may represent an ancient gene that has simply failed to fix within the proteobacterial lineage. Therefore, an alternative hypothesis is that the conditional requirement for neaT by CFT073 within different environments may have driven its current evolved state. We observed in two vertebrate model systems that neaT contributes significantly to pathogen fitness primarily during systemic infections. Considering the ecology of many bacterial pathogens, a question often left unaddressed is: what are the evolutionary forces that act on niche-specific genes in the absence of selective pressure? Particularly for E. coli, which has a complex multi-niche life cycle, the evolutionary consequences resulting from time outside selective environments on genes like neaT are not clear.

Work directly addressing this question is scarce. However, insight into this issue is provided by findings that genes under relaxed constraint have increased variance at the sequence level [17], [70], [71], [72], [73]. In contrast to relaxed selection, which occurs when purifying selection is alleviated, as discussed above, ‘relaxed constraint’ refers to a limitation in the exposure of a particular gene to selection. For example, eukaryotic genes with expression patterns that are sex-restricted are effectively ‘hidden’ from selection in half of the population. This is the case for the Drosophila spp. maternal-effect gene bicoid, which is maternally-restricted and critical for the embryonic development of fruit flies [70]. The bicoid gene was found to have a 2-fold higher heterozygosity compared to zygotically-expressed genes. Similarly, genes with caste-biased expression (i.e., queen versus worker) in the social insects Solenopsis invicta (fire ant) and Apis mellifera (honey bee) were shown to be evolving more rapidly than genes expressed among all castes [71]. For both of these situations, the higher mutation rate observed for contextually expressed genes was concluded to be due to relaxed constraint. Further investigation into the exploratory mechanism and relaxed constraint hypotheses of neaT evolution is required and must be considered in parallel with other processes and factors, including, for example, the susceptibility of laterally transferred genes to endogenous restriction enzymes [59].

ExPEC individuality and virulence

There exists an enormous amount of genetic heterogeneity among Eubacteria lineages. Genome sequencing and bioinformatic analyses have underscored this extensively. Perhaps the most intriguing aspect of this diversity is that even closely related members of the same species can differ greatly with respect to their gene contents. Strikingly, any two E. coli genomes can differ by up to 20–30% of their respective gene contents—in sharp contrast to the relatively minor difference of 1% that exists between, for example, the mouse and human species [32], [74]. Decades worth of epidemiological and experimental studies have focused on the identification of genes that define the pathogenic behavior of ExPEC [7], [32]. However, it appears that a single, ubiquitous genetic identifier of ExPEC, such as a gene encoding a particular toxin or adhesin, does not exist and, rather, what actually binds these pathogens is more qualitative and multigenic in nature [32], [75].

In support of this view, we recently demonstrated that the toxin α-hemolysin, shared among many ExPEC isolates, is differentially required for virulence depending on strain background [40]. Similarly, we found that the pathogenicity of particular ExPEC isolates depends on another toxin, cytotoxic necrotizing factor, while other equally virulent strains naturally lack this gene. Coupled with the work presented here, these observations suggest that there exists a spectrum of only partially overlapping virulence gene requirements among ExPEC, reflecting the idea that these pathogens have emerged from distinct evolutionary trajectories driven by LGT [76], [77]. Accordingly, we found that the expression of NeaT from plasmid pGEN-neaT^Pnative in other E. coli strains, including Nissle 1917 (gut isolate), F11 (cystitis isolate), and S88 (meningitis isolate), did not augment virulence in the zebrafish infection model (data not shown). These findings suggest that the ability of a rare gene like neaT to affect fitness and virulence is dependent upon the genetic background of individual bacterial strains. The beneficial effects of neaT, and its potential to sweep through bacterial populations, is therefore likely linked to the presence, or coordinate acquisition, of other as-yet undefined bacterial factor(s). The identification, characterization, and continued monitoring of rare genes like neaT will be important to our understanding of ExPEC evolution. As a case in point, we note that the sasX gene, originally defined as rare among strains of methicillin resistant Staphylococcus aureus (MRSA), increased in prevalence among MRSA isolates between 2003 and 2011 and is now considered an emerging virulence determinant [78]. Interestingly, like neaT, sasX is also maintained within a prophage and can affect bacterial interactions with phagocytes. At this point, it is difficult to predict if neaT will sweep ExPEC populations in the future, but work presented here along with recent findings concerning sasX underscore how laterally acquired genes can alter the virulence potential of bacterial pathogens, continually challenging the development of broad spectrum therapeutics.

Going forward, as we continue to characterize the composition of pan-genomic elements of ExPEC and other pathogens, it will be important to consider the evolutionary context of their virulence genes. Identifying the spatial and temporal parameters that govern the lateral acquisition of virulence genes from distant lineages will need to be reconciled. Genome compatibility (codon and tRNA usage) and ecology are thought to be influential in the success of LGT events between bacteria [79], [80], [81]. In light of this, several interesting questions arise. How did neaT come to be in the proteobacterial gene pool? How does residence of neaT within a prophage impact its evolution? What conditions fostered the assimilation of neaT into the virulence regulon of its host? Using neaT as a stepping-stone, it will be informative to resolve the amount of strain-specific innovation that goes into producing and fine-tuning pathogen genomes. By understanding the mechanisms of chromosome assembly and the sources of individual genetic components, unrealized patterns may emerge that could prove useful for future diagnostics and disease mitigation.

Ethics statement

Animals used in this study were handled in accordance with IACUC protocols approved at either the University of Utah or the University of Michigan Medical School following standard guidelines as described at www.zfin.org and in the Guide for the Care and Use of Laboratory Animals, 8th Edition [55], [82].

Bacterial strains and plasmids

All bacterial strains and plasmids used in this study are listed in Table 3. Unless specified otherwise, bacteria were cultured statically at 37°C for 24 h in 20 ml of a defined M9 minimal medium (6 g/l Na₂HPO₄, 3 g/l KH₂PO₄, 1 g/l NH₄Cl, 0.5 g/l NaCl, 1 mM MgSO₄, 0.1 mM CaCl₂, 0.1% glucose, 0.0025% nicotinic acid, 0.2% casein amino acids, and 16.5 mg/ml thiamine in H₂O). Antibiotics (kanamycin or ampicillin) were added to the growth medium when necessary to maintain recombinant plasmids or select for mutants.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 3. Bacterial strains and plasmids.

https://doi.org/10.1371/journal.ppat.1003175.t003

Targeted gene knockouts were generated in the ExPEC isolate CFT073 using the lambda Red-mediated linear transformation system [83], [84]. Briefly, a kanamycin resistance cassette was amplified using polymerase chain reaction (PCR) from pKD4 with 40-base pair overhangs specific to the 5′ and 3′ ends of each targeted locus. PCR products were introduced via electroporation into CFT073 carrying pKM208, which encodes an IPTG (isopropyl-β-D-thiogalactopyranoside)-inducible lambda red recombinase. Knockouts were confirmed by PCR. Primer sets used are listed in Table S8.

Cloning and construction of neaT expression constructs were done using standard molecular techniques employing the high-retention plasmid pGEN-mcs [85]. For native regulation, neaT (locus tag: c0970), plus 211 bp of upstream sequences, were amplified from the chromosome of CFT073 and TA-cloned into pCR2.1-TOPO vector per manufacture's protocol (Invitrogen). Subsequently, the cloned fragment was isolated using BamHI and NotI restriction enzymes (New England Biosciences) and ligated into pGEN-mcs using the same sites, yielding pGEN-neaT^Pnative. For construction of pGEN-neaT^Plac, a synthetic ribosome binding sequence was introduced upstream of neaT within the 5′ PCR primer, and the resulting PCR product was ligated via an engineered NdeI restriction site with the lac promoter amplified from pGFPmut3.1 (Clonetech). The ligated Plac-neaT product was amplified and TA-cloned into the pCR2.1-TOPO vector. Using BamHI and NcoI restrictions sites, the Plac controlled neaT variant was then sub-cloned into pGEN-mcs. All experiments involving pGEN-neaT^Plac were performed without IPTG induction. Primer sequences used to generate these plasmids are listed in Table S8.

Zebrafish embryos

*AB wild-type zebrafish embryos were collected from a laboratory-breeding colony that was maintained on a 14-h/10-h light/dark cycle. Embryos were grown at 28.5°C in E3 medium (5 mM NaCl, 0.17 mM KCl, 0.4 mM CaCl₂, 0.16 mM MgSO₄) containing 0.000016% methylene blue as an anti-fungal agent.

Infection of zebrafish embryos

One ml from each 24 h bacterial culture was pelleted, washed once with 1 ml sterile PBS (Hyclone) and re-suspended in 1 ml PBS to obtain appropriate bacterial densities for microinjection. Prior to injection, 48 hpf embryos were manually dechorionated, briefly anesthetized using 0.77 mM ethyl 3- aminobenzoate methanesulfonate salt (tricaine) (Sigma-Aldrich), and embedded in 0.8% low-melt agarose (MO BIO Laboratories) without tricaine. Approximately 1 nl of bacteria was injected directly into the pericardial cavity or the blood via the circulation valley located ventral to the yolk sac using a YOU-1 micromanipulator (Narishige), a Narishige IM-200 microinjector, and a JUN-AIR model 3-compressor setup. For each experiment, average CFU introduced per injection were determined by adding 10 drops of each inoculum into 1 ml 0.7% NaCl, which was then serially diluted and plated on Luria-Bertani (LB) agar plates. For co-challenge experiments, input doses were plated on LB agar+/−kanamycin (50 µg/ml) to determine relative numbers of the wild type and mutant strains present. After injection, embryos were carefully extracted from the agar and placed individually into wells of a 96-well microtiter plate (Nunc) containing E3 medium lacking both tricaine and methylene blue. For lethality assays, fish were examined at indicated times over the course of a 48 or 72 h period and scored for “death”, defined here as the complete absence of heart rhythm and blood flow. Survival graphs depict total pooled results from two or more independent experiments in which groups of 10 to 20 embryos were injected. To quantify bacterial numbers during the course of co-challenge experiments, embryos were homogenized at the indicated time points in 500 µL PBS containing 0.5% Triton X-100 using a mechanical PRO 250 homogenizer (PRO Scientific). Homogenates were then serially diluted and plated on LB agar+/−kanamycin (50 µg/ml) to determine relative numbers of wild type and mutant bacteria.

Mouse infections

For co-challenge during urinary tract infection, seven- to nine-week old female CBA/J mice (Jackson Labs) mice were anesthetized using isoflurane inhalation and inoculated via transurethral catheterization with 50 µl of a 1∶1 wild type to mutant bacterial suspension containing a total of 10⁷ bacteria suspended in PBS. Bladders and kidneys were recovered 3 days later and each was weighed and homogenized in 1 ml containing 0.025% Triton X-100. Homogenates were serially diluted and plated on LB agar+/−kanamycin (50 µg/ml) to determine number of both wild type and mutant bacteria. Mouse experiments were repeated at least twice.

For systemic infections, female CBA/J mice (Jackson Labs) aged 6 to 8 weeks were restrained using a Universal Restrainer (Braintree Scientific, Braintree, MA) and inoculated via the tail vein over a 30 s period with a 100 µl bacterial suspension, delivering 10⁶ CFU/mouse. The inoculum was prepared by re-suspending overnight cultures in PBS and diluting them to 1×10⁷ CFU/ml. For co-challenges, wild type and mutant suspensions were mixed 1∶1 before inoculation. Perfusion was performed on euthanized animals by cutting a small hole in the right cardiac ventricle and infusing the left ventricle slowly with 40 ml 0.9% sterile saline before organ removal. Blanching of the organs occurred with the first 20 ml of sterile saline. Excised spleens and livers were homogenized in 3 ml PBS using a mechanical homogenizer (Omni International, Marietta, GA), and homogenates were plated using an Autoplate 4000 (Spiral Biotech, Norwood, MA) onto LB agar+/−kanamycin (50 µg/ml) to differentiate wild type and mutant strains.

Statistical analysis of zebrafish and mouse infections

Kaplan-Meier survival and scatter plots were generated using GraphPad Prism 5. For Kaplan-Meier survival plots (independent challenges), the log-rank (Mantel-Cox) test was used to determine statistical differences between datasets. For competitive assays (co-challenges), numbers of wild type and mutant bacteria present in the inoculum and recovered from host tissues were determined as described above and a competitive index was calculated using the following equation where wt represents numbers wild type bacteria:Negative values obtained using the competitive index equation indicate a reduction in mutant fitness. To determine statistical significance, the Wilcoxon signed-rank test (with a hypothetical value of 0) on log-transformed competitive index values was used for co-challenges and the two-tailed Mann-Whitney statistical analysis was performed to determine significant differences between samples in non-competitive assays.

Bioinformatic analyses