The core genome of the anaerobic oral pathogenic bacterium Porphyromonas gingivalis
© Brunner et al; licensee BioMed Central Ltd. 2010
Received: 10 June 2010
Accepted: 29 September 2010
Published: 29 September 2010
The Gram negative anaerobic bacterium Porphyromonas gingivalis has long been recognized as a causative agent of periodontitis. Periodontitis is a chronic infectious disease of the tooth supporting tissues eventually leading to tooth-loss. Capsular polysaccharide (CPS) of P. gingivalis has been shown to be an important virulence determinant. Seven capsular serotypes have been described. Here, we used micro-array based comparative genomic hybridization analysis (CGH) to analyze a representative of each of the capsular serotypes and a non-encapsulated strain against the highly virulent and sequenced W83 strain. We defined absent calls using Arabidopsis thaliana negative control probes, with the aim to distinguish between aberrations due to mutations and gene gain/loss.
Our analyses allowed us to call aberrant genes, absent genes and divergent regions in each of the test strains. A conserved core P. gingivalis genome was described, which consists of 80% of the analyzed genes from the sequenced W83 strain. The percentage of aberrant genes between the test strains and control strain W83 was 8.2% to 13.7%. Among the aberrant genes many CPS biosynthesis genes were found. Most other virulence related genes could be found in the conserved core genome. Comparing highly virulent strains with less virulent strains indicates that hmuS, a putative CobN/Mg chelatase involved in heme uptake, may be a more relevant virulence determinant than previously expected. Furthermore, the description of the 39 W83-specific genes could give more insight in why this strain is more virulent than others.
Analyses of the genetic content of the P. gingivalis capsular serotypes allowed the description of a P. gingivalis core genome. The high resolution data from three types of analysis of triplicate hybridization experiments may explain the higher divergence between P. gingivalis strains than previously recognized.
Periodontitis is a chronic destructive infectious disease of the tooth-supporting tissues. It is one of the most prevalent infectious diseases in the world. With percentages of moderate disease ranging from just below 20% in an age group of 30 to 40 year-olds in Swedish and Norwegian studies to even up to 38% of severe cases in the United States in an on average 75 year-old male population [1–3]. Besides high prevalence also links to systemic diseases have been described. Periodontitis has been associated with, amongst others, cardiovascular diseases, diabetes mellitus and rheumatoid arthritis [4–7].
Periodontitis leads to loss of sound teeth as supporting bone and connective tissue are slowly degraded as a result of an exaggerated host immune response triggered against a polymicrobial biofilm .
In the oral cavity around 7000 species can be detected, in subgingival and supragingival biofilm/plaque over 400 bacterial species are present [9–11]. Many disease-related bacterial species in the subgingival plaque have been shown to be Gram-negative anaerobes. Among them, Porphyromonas gingivalis a black-pigmented bacterium from the phylum Bacteroidetes is a major causative agent in periodontal disease .
Interaction with other bacteria residing in the periodontal pocket is important to sustain the infectious biofilm. One of the structures involved in the inter-species adherence is the capsular polysaccharide (CPS) of P. gingivalis . CPS has been described as a virulence factor of various pathogenic bacteria, mainly as being involved in evasion of the host immune system [14–16]. In P. gingivalis encapsulated strains have been shown to be more resistant to serum killing and phagocytosis. The explanation for this increased resistance compared to the non-encapsulated strains may be the increased hydrophilicity and the lower induction of the alternative complement pathway .
Encapsulated P. gingivalis strains have also been shown to be more virulent than non-encapsulated strains in the mouse infection model . To date, six capsular serotypes (K1-K6) have been described [19, 20] and a seventh serotype (K7) has been suggested by R. E. Schifferle (personal communication). In a mouse subcutaneous infection model several strains of each of the serotypes have been shown to be highly virulent . The variation of virulence within serotypes shows that besides CPS there have to be more virulence factors of importance in P. gingivalis. Many of its virulence factors have been studied in the last decades including fimbriae, hemagglutinins, lipopolysaccharide (LPS), outer membrane proteins (OMPs) and an extremely wide variety of proteinases. High quality reviews have been published on the wide variety of P. gingivalis virulence factors [21–23].
Using comparative whole-genome hybridization analysis of the encapsulated W83 strain and the non-encapsulated ATCC33277 a CPS biosynthesis locus had been found, after which a knock-out study has proven that the CPS locus was functional [24, 25]. K1 CPS from W83 has been shown to induce a stronger chemokine response than CPS from the other serotypes in murine macrophages . Recent work in our group, however, has shown that an isogenic W83 mutant lacking CPS triggers a higher pro-inflammatory immune response in human gingival fibroblasts than strain W83 carrying K1 CPS . The exact roles of CPS in P. gingivalis are still unclear, but reducing the host immune response is certainly one of them.
In the latest years an increasing number of genomes have been sequenced paving the path for genomics-based approaches. For P. gingivalis genome sequences of the virulent strain W83 and the less-virulent strain ATCC33277 have become available [28, 29]. Comparative genomic hybridization (CGH) analysis using microarrays of these well-described bacterial strains could yield new insights in the virulence mechanisms of P. gingivalis. A recent study reported on the CGH analysis of several P. gingivalis strains to describe the genetic variety among them .
P. gingivalis strains used in this study
Periodontitis patient with diabetes mellitus
Advanced adult periodontitis patient
37-year-old male periodontitis patient
28-year-old female periodontitis patient
Severe periodontitis patient
Adult periodontitis patient
Triplicate hybridization experiments and three types of analysis, 1) aberrant gene calling, 2) breakpoint analysis and 3) absent gene calling, have been performed for optimal use of the new genetic information. The careful design of the experiment and the thorough analysis of the data lead to a high resolution data set, yielding more detailed information on the genetic differences between strains than has been shown before. In this study we initiate the description of a core-gene set of P. gingivalis allowing a more focused search for potential important virulence factors.
Results and discussion
Microarray performance and data interpretation
Probes excluded from analysis due to redundancy
DNA-binding protein, histone-like family
ISPg5, transposase Orf2
ISPg5, transposase Orf2
DNA-binding protein, histone-like family
conserved hypothetical protein
ISPg5, transposase Orf2
conserved hypothetical protein
MATE efflux family protein
ISPg5, transposase Orf1
NAD(P) transhydrogenase, beta subunit
ISPg1, transposase, authentic frameshift
transcriptional regulator, AraC family
conserved hypothetical protein
rteC protein, truncation
ISPg1, transposase, authentic frameshift
Dead probes excluded from the results due to low hybridization signals
DNA-binding protein, histone-like family
ribosomal protein L13
autoinducer-2 production protein LuxS
bacterioferritin comigratory protein
membrane protein, putative
ISPg2, transposase, truncation
RNA polymerase sigma-70 factor, ECF subfamily
CRISPR-associated protein, TM1794 family
conserved hypothetical protein
In order to maximize the mining of the genomic information, we subjected the data to three complementary analyses: 1) analysis for aberrations as detected by individual probes, 2) analysis for breakpoints, and 3) analysis for genomic loss. The rationale behind the three analyses is as follows. The probed genomic sites are on average 1250 bp apart from each other (median was 1018), which was not considered to be a high interrogation density. We therefore decided to analyze each probe individually for indication that the genomic site interrogated is aberrant from W83. Deviations from W83 that were detected with a false discovery rate corrected p-value (FDR) < 0.05 were considered significant. This aberrance could have occurred due to mutations or loss (or due to W83 gain), and this was regarded as point-variability between the strains. Nevertheless, if several neighboring probes indicate aberrations, then this may indicate highly variable regions due to mutations or loss. Hence, a breakpoint analysis was executed to quantitatively specify such regions. Finally, we used the negative controls to define absent calls with the aim to distinguish whether an aberration was found more likely due to mutation or loss. If the probes that indicated aberrations in the first analysis also showed the same intensities as the negative controls with FDR corrected p-value < 0.01 (see M&M), the genomic site was considered as mutated, and otherwise it was considered as lost. This last analysis enhanced our interpretation of the data and the definition of the core genome.
P. gingivalis core genome
Research on microbial pathogens is mostly performed to unravel mechanisms of virulence in order to design effective treatments. Virulence mechanisms present in all strains of a species are especially attractive. The description of a core set of genes present in a species is thus a key step for better understanding. From an analysis of eight P. gingivalis strains we found that 1476 genes were non-aberrantly present in each of the strains and 2 hypothetical genes were called absent but non-aberrant (Additional file 1). The conserved core genes make up 80% of all genes included in this study. Hence, 20% (374) of all genes of W83 were aberrant in at least one of the strains. Core genomes from several bacterial species have been described [39–45]. The fraction of a bacterial genome that consists of core genes depends highly on the amount of strains included to describe the core genome . The more strains are used, the smaller the core genome will be. As such, the very well studied Escherichia coli core genome makes up only 46% of the average E. coli genome. Other bacterial species, including Gram positives and Gram negatives, have been found to have a core genome which covers 52% to 85% of a genome [39–45]. The 80% of W83 genes which are part of the conserved core genome can therefore be understood. It must be clear though that the core genome of P. gingivalis as described here must be seen as a first step. The core genome will be found to be smaller as more genetic information on different P. gingivalis strains will become available.
Combining our findings on the core genome with a study describing 1490 conserved CDSs when comparing the genome sequences of W83 and ATCC33277 , makes it tempting to speculate that the core genome as described here may already be close to its final size. An analysis combining the conserved CDSs from that study with our 1476 conserved core genes showed that when strain ATCC33277 is included the core genome size decreased to 1384 genes.
Another virulence gene set was also tested for presence in the conserved core gene set of P. gingivalis. The set was composed of genes shown to be up-regulated in infection experiments [46, 47]. Genes up-regulated in an in vitro human epithelial cell infection experiment were combined with a gene set in vivo up-regulated on protein level in a mouse subcutanuous chamber experiment to make a set of 39 virulence genes. The former experiment was chosen as an early response gene set, whereas the latter includes genes involved in sustaining an infection in vivo. 37 of the 39 virulence genes were present among the core gene set (Figure 3B). The two genes that were not in the core gene set were a thiol protease (PG1055)  and tetR a transcription regulator (PG1240). The thiol protease is aberrant in each strain except for strain ATCC49417, from the 16S-23S ISR heteroduplex type that together with the type of strain W83 has the highest association with disease . This is another indication that this thiol protease may be an important determinant in virulence of P. gingivalis. Transcription regulator tetR was only found to be aberrant in strain FDC381, which is the least virulent and the only non-encapsulated strain [18, 32].
The analysis of the core gene set shows the presence of almost all virulence related genes. The genes that are not present in the core genome may be determinants of the differences in virulence found between the strains.
Aberrant and absent CDSs of P. gingivali s strains
Non-core protease genes of P. gingivalis
peptidase, M49 family
Aberrant only in 34-4
Non-aberrant only in W83 and ATCC49417 (absent in FDC381)
Aberrant in HG1691, 34-4 and FDC381
hemagglutinin protein HagE
Aberrant and absent only in HG1025
protease PrtT, degenerate
Non-aberrant only in W83
Aberrant in HG184, HG1025 and FDC381
The capsular polysaccharide biosynthesis locus
Highly variable regions
Highly variable P. gingivalis genomic regions
Gene content of the region
Potential pathogenicity island . Many DNA mobilization proteins
Potential pathogenicity island . Many transposon related genes.
Virulence associated ragA-ragB locus  highly variable in strains other than W83 and ATCC49417
PHP domain protein, transposases
transcriptional regulator, type 1 restriction modification gene
PgaA and hypothetical protein
Integrase/mobilization, hypothetical proteins
Transcriptional regulator, DNA binding protein, hypothetical proteins
Lipoproteins, hypothetical proteins
Another region that was found to be interesting in this analysis is region PG1981-PG1986 which is comprised of clustered regularly interspaced short palindromic repeat (CRISPR) associated genes (CAS) . Together with CRISPRs, located directly downstream of PG1981, these types of genes have been described as the immune system of bacteria against foreign DNA, e.g. plasmids and viruses. Recently they also have been described as a useful tool in epidemiology . Variation is expected to be high in these regions as they encompass exogenous DNA sequence fragments from infection events that happened to the strain or its ancestors. Here variation within the CAS genes is evident, but not as high as the other regions mentioned in this section.
Strain W83 has been described as a highly virulent strain. What makes this strain special is however not specifically known. The purified CPS of W83 has been shown to induce a higher immune response than other types of CPS . Removal of the capsular structure, by genetic interruption of CPS-biosynthesis, however resulted in a much higher immune response when infecting fibroblasts with viable P. gingivalis . What this means for virulence in a mouse model has not yet been addressed. With the data presented here a more detailed study is possible to find specific traits that make W83 different. A list of all genes that are aberrant in each of the test strains and absent in each of the test strains is presented (see Additional file 2). Among the 65 genes that were aberrant in all test strains there were 10 DNA mobilization genes, 5 DNA modification genes, 3 CPS biosynthesis genes, 2 carbohydrate metabolism genes, 2 transcriptional regulator genes, 2 lipoprotein genes and 36 (conserved) hypothetical protein genes. From this gene set 39 genes were W83-specific as they were absent in each of the test strains. In this way the prtT protease gene and a fimbrillin gene (fimA) were found to be aberrant in all test strains, but not W83-specific as they were present in one or more test strains. The results for fimA support the findings that the gene is widely distributed, but variable at the probe locus among P. gingivalis strains. Many of the genes found in this analysis are located within the highly variable regions described in earlier publications using whole-genome analysis. The existence of those regions were supported by data comparing the genome sequences of P. gingivalis strains W83 and ATCC33277 . Also in this study we found these regions back in the analysis as described above
Genes only aberrant in FDC381
Genes only aberrant in strain FDC381
TPR domain protein
methlytransferase, UbiE-COQ5 family
conjugative transposon protein TraQ
alpha-1,2-mannosidase family protein
conserved hypothetical protein
hemagglutinin protein HagB
Although these data do not directly show any CPS biosynthesis specific genes aberrant only in the non-encapsulated FDC381 it does give hints towards other virulence associated traits that are missing in FDC381.
High versus lower virulence strains
When comparing the core gene set of only the highly virulent strains W83, HG1025, ATCC49417 and HG1690 with the genes aberrant in each of the less virulent strains HG184, HG1691, 34-4 and FDC381 an interesting result was seen. There is only a single gene, hmuS, that is present in all highly virulent strains but aberrant in each of the less virulent strains. HmuS is part of the hmuYRSTUV haemin uptake system . Haemin acquisition is vital for P. gingivalis which makes the haemin uptake and storage system relevant study objects. Lacking part of an important uptake mechanism could have consequences for infection and survival. However, in these experiments no functional differences have been shown.
In this study we analyzed the genetic contents of representative strains of each of the seven capsular serotypes. Comparative genomic hybridization shows that gene aberrance among P. gingivalis strains can be up to 13.7%, which is higher than previously reported. The P. gingivalis genome is variable with 20% of the W83 gene content being aberrant in at least one of the seven test strains. Analysis of virulence-related genes conservation was performed; only a few virulence-related genes were shown to be aberrant among test strains. As could be expected due to the choice of strains it was found that among the most aberrant virulence genes were the CPS biosynthesis genes.
In this study we initiated the description of a core genome of the anaerobic bacterium P. gingivalis, one of the most important causative agents of periodontitis allowing a more focused search for potential important virulence factors of which several were identified
Bacterial strains and maintenance
P. gingivalis strains used in this study are listed in Table 1, including serotype, origin and virulence level. P. gingivalis strains were first grown on 5% horse blood agar plates (Oxoid no. 2, Basingstoke, UK) supplemented with hemin (5 μg/ml) and menadione (1 μg/ml) (BA+H/M plates) at 37°C in an anaerobic atmosphere of 80% N2, 10% H2, and 10% CO2. From these plates 10 ml of liquid brain heart infusion broth supplemented with hemin (5 μg/ml) and menadione (1 μg/ml) (BHI+H/M) was inoculated and grown overnight as a pre-culture at 37°C in an anaerobic atmosphere. From the pre-culture a 300 ml 1:100 dilution in BHI+H/M was made, which was grown overnight at 37°C in an anaerobic atmosphere. The bacteria were washed 3 times in phosphate-buffered saline (PBS) and then pelleted and stored at -80°C until DNA isolation was performed.
Whole-genome microarrays made for P. gingivalis strain W83 kindly provided by the Pathogen Functional Genomics Resource Center (The Institute for Genomic Research (TIGR), Rockville, MD) were used in this study. The aminosilane-coated microarrays contain 1,907 70-mer oligonucleotide probes designed on the 1,990 annotated W83 ORFs as found by TIGR. Each probe was designed to be unique for an ORF, so ORFs that were not unique were excluded. The arrays also included 500 Arabidopsis thaliana control probes. Each probe was printed four times on an array. Specific information about the microarrays can be found at http://pfgrc.jcvi.org/index.php/microarray/array_description/porphyromonas_gingivalis/version1.html
P. gingivalis pellets were frozen at -80°C until DNA isolation. Frozen pellets were ground in liquid nitrogen with a mortar and pestle until a fine powder was obtained. 500 μl of this powder was transferred to a liquid nitrogen pre-chilled 15 ml tube. DNA was extracted by addition of 1500 μl 65°C CTAB extraction buffer made to 2% (v/v) 2-mercaptoethanol before use (100 mM Tris-Cl (pH 8.0), 2.0 M NaCl, 20 mM EDTA, 3% (w/v) CTAB (H6269, Sigma-Aldrich), 2% (w/v) PVP-40 (PVP40, Sigma-Aldrich); Filter sterilized and stored at room temperature). After incubation for 30 min at 65°C with occasional mixing, DNA was extracted with 1500 μl phenol/chloroform/isoamylalcohol (25:24:1) (pH 7.9) (AM9730, Ambion). After centrifugation at 6,000 × g for 15 min, the aqueous phase was transferred to a clean 15 ml tube and DNA was precipitated with an equal volume of ice-cold isopropanol. DNA was pelleted at 6,000 × g for 15 min. The DNA pellet was washed twice with ice-cold 70% ethanol and centrifugation at 6,000 × g for 5 min. The remaining liquid was removed by decanting and the pellet was air dried. This pellet was resuspended in 600 μl TE and 1 μl RNAse A (10 mg/ml, R6513, Sigma-Aldrich) was added. Residual RNA was removed by overnight incubation at 37°C and DNA was re-extracted with an equal volume of phenol/chloroform/isoamylalcohol (25:24:1) pH 7.9. The aqueous phase was recovered by centrifugation at 6,000 × g for 15 min. The aqueous layer was treated with an equal volume of chloroform/IAA (96:4) and centrifuged at 6,000 × g for 10 min at room temperature. The final aqueous phase was treated with an equal volume of 100% ethanol and 1/10 volume of 3 M sodium acetate (pH 5.2) and incubated for 30 min @ -20°C. DNA was pelleted for 15 min at 6,000 × g. Residual liquid was removed and the pellet was washed once with ice-cold 70% ethanol. DNA was pelleted for 5 min at 6,000 × g and the pellet was air-dried. The DNA pellet was resuspended in an appropriate volume of TE. DNA quality was verified with gel electrophoresis (0.5% agarose in TAE).
Genomic DNA labelling, microarray hybridization, scanning and data extraction
1 μg of genomic DNA was labeled with Cy3 or Cy5 using the CGH labeling kit for oligo arrays (ENZO Life Sciences). Labeled genomic DNA was purified with the QiaQuick PCR purification kit (Qiagen). P. gingivalis (W83) version 1 arrays were obtained from the Pathogen Functional Genomics Resource Center (PFGRC). Individual arrays were hybridized with 5 μg Cy3- and 5 μg Cy5-labeled material (test strains versus FDC381, which served as common reference), without dye swap, according to the Oligonucleotide Array-Based CGH for Genomic DNA Analysis manual (Agilent Technologies version 5.0). Briefly, labeled DNA was combined with 52 μl 10 × Blocking Agent and 260 μl 2 × Gex Hybridization Buffer Hi-RPM (Gene Expression Hybridization Kit, Agilent Technologies) in a total volume of 520 μl. Hybridization samples were incubated at 95°C for 3 min, spun down and hybridized at 37°C for 30 min. Samples were spun down and 490 μl of each sample was loaded onto a 1 × 244 k backing in a SureHyb hybridization chamber (Agilent Technologies) and a P. gingivalis version 1 array was placed on top. Hybridization was performed at 65°C for 24 h and 10 RPM in a hybridization oven (G2545A, Agilent Technologies). After the hybridization the backings were removed in LSW (2 × SSC, 0.1% Sarkosyl (L9150, Sigma-Aldrich) at room temperature, washed for 5 min at 42°C in LSW, washed for 10 min at room temperature in HSW (0.1 × SSC, 0.1% Sarkosyl) and finally washed for 1 min at room temperature in FW (0.1 × SSC). Each array was dipped 5 times in H2O and quickly submerged in isopropanol. Microarrays were spun dry for 1 min at 232 × g and scanned on an Agilent G2505B scanner at 5 μm resolution and data was extracted with Feature Extraction version 184.108.40.206. (Protocol GE2-NonAT_95_Feb07).
Experimental design and Microarray data analysis
Each strain was cultured in triplicate, in three experimental batches. DNA isolations and hybridizations were therefore performed three times for each strain, each being a biological replicate analyzed in one experimental block. On each array four technical replicate spots were spotted.
y ijklmn represents log2 expression intensities, μ is the gene specific mean, τ represents fixed strain effects (i = 1, ..., 8), ρ is an indicator variable indicating the common reference, S represents random spot effects (j = 1, ..., 96), A represents random array effects (i = 1, ..., 24), and B represents experimental batch effects (m = 1, ..., 3). Normalized average (Cy5) intensities for each strain were calculated as y i * = μ + τ i and normalized average log2-ratio's with respect to W83 were calculated as Y i * = τ i - τ 1 , for each i ≠ 1 (which represents W83).
Hence, each strain was compared with W83, and deviations in log2-ratio's were interpreted as aberrations. Given j genes divergence from zero were modelled as posterior probabilities of change under a mixture model, where non-divergent Y ij * ~ N(0,s i 2) and divergent Y ij * follows a uniform distribution .
Highly variable regions due to mutations or loss were quantified according to , using their GLAD (Gain and Loss Analysis of DNA) package with default parameter settings. Finally, we used the negative control probes from Arabidopsis thaliana to define absent calls with the aim to quantify whether an aberration was found more likely due to mutation or loss. The distributions of intensities suggested a distinguishable mixed distribution of intensities from probes interrogating present genes (high) and probes interrogating absent genes (low; Figure 1). Given j probes, probe intensities were modelled using a standard Gaussian mixture model where probes interrogating present genes y ij * ~ N1(μ1 i ,s1 i 2) and probes interrogating absent genes y ij * ~ N2(μ2 i ,s2 i 2). The probe specific membership probabilities of N1(μ1 i ,s1 i 2) represents the null-hypothesis of "not absent", which is the hypothesis under test. False discovery rate correction as described by  was applied to both the test for quantifying aberrations as well as to the test for quantifying genomic losses. The data was visualized using the Integrated Genome Browser . The final data set including dead probes and conserved, aberrant and absent genes is shown in additional file 3.
amplified fragment length polymorphism
Horse blood agar plates supplemented with hemin and menadione
Comparative genomic hybridization
Outer membrane protein
Open reading frame
Saline-sodium citrate buffer
We acknowledge Arie Jan van Winkelhoff for help with the study design and useful discussions. We furthermore gratefully acknowledge the National Institute of Dental and Craniofacial Research and the Pathogen Functional Genomics Research Centre of the J. Craig Venter Institute (formerly The Institute for Genomic Research) for providing the microarrays.
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