Genetic diversity of Streptococcus suis isolates as determined by comparative genome hybridization

Abstract

Background: Streptococcus suis is a zoonotic pathogen that causes infections in young piglets. S. suis is a heterogeneous species. Thirty-three different capsular serotypes have been described, that differ in virulence between as well as within serotypes.

Results: In this study, the correlation between gene content, serotype, phenotype and virulence among 55 S. suis strains was studied using Comparative Genome Hybridization (CGH). Clustering of CGH data divided S. suis isolates into two clusters, A and B. Cluster A isolates could be discriminated from cluster B isolates based on the protein expression of extracellular factor (EF). Cluster A contained serotype 1 and 2 isolates that were correlated with virulence. Cluster B mainly contained serotype 7 and 9 isolates. Genetic similarity was observed between serotype 7 and serotype 2 isolates that do not express muramidase released protein (MRP) and EF (MRP⁻EF⁻), suggesting these isolates originated from a common founder. Profiles of 25 putative virulence-associated genes of S. suis were determined among the 55 isolates. Presence of all 25 genes was shown for cluster A isolates, whereas cluster B isolates lacked one or more putative virulence genes. Divergence of S. suis isolates was further studied based on the presence of 39 regions of difference. Conservation of genes was evaluated by the definition of a core genome that contained 78% of all ORFs in P1/7.

Conclusions: In conclusion, we show that CGH is a valuable method to study distribution of genes or gene clusters among isolates in detail, yielding information on genetic similarity, and virulence traits of S. suis isolates.

Figure 1

Dendrogram of normalized CGH results. S. suis strains are listed in the first column, serotype and phenotype (muramidase released protein (MRP) and extracellular factor (EF) expression) in the second column. MLST sequence type (ST) and clonal complex (CC) are listed in the last column. Red color indicates probes that are present in more copies than in P1/7, whereas green color indicates probes that are present in P1/7, and absent in the test strain. Asterisks indicate statistically significant knots. Solid boxed isolates were shown to be virulent or weakly virulent in experimental infections; dotted boxed isolates were shown to be avirulent or very weakly virulent in experimental infections; striped - dotted boxed isolates were isolates from human patients. ^humanindicates an isolate that was shown to be avirulent in experimental infection, but was isolated from a human patient.

Figure 2

Presence/absence of 25 putative virulence genes represented in a dendrogram. Naming (SSU numbering) is derived from the annotated genome sequence of P1/7 [7]. Presence of 25 described putative virulence factors was studied: muramidase released protein (mrp), and extracullar factor (epf) [13], suilysin (sly) [20], sortases (srtA, srtBCD, srtF) [34], surface antigen one (sao) [42], hyaluronidase (hylA) [17,43], opacity factor (ofs) [37], fibronectin binding protein (fbps) [44], arginin deiminase (arcA) [45], glyceraldehyde-3-phosphate dehydrogenase (gapdh) [46], regulator of virulence (revS) [35,47], enolase (eno) [48], glutamine synthetase (glnA) [49], igA1 protease [36], inosine 5-monophosphate dehydrogenase (impdh) [50], dipeptidyl peptidase IV (dppIV) [51], ferrous iron transporter (feoB) [52], subtilisin like serine protease (sspA) [53], amylopullulanase (apuA) [54], ferric uptake regulator (fur), and adhesion competence repressor (adcR) [55]. * hylA is present as pseudogene in P1/7 and does not have a SSU-number. '+' indicates all probes have a ratio > -1.5 (present); light grey shading indicates one or more probes have a ratio between -1.5 and -3 (present with slight variation); dark grey shading indicates one or more probes have a ratio between -3 and 4.5 (present with large variation); '-' indicates one or more probes have a ratio < -4.5 (partly or completely absent).

Figure 3

Dendrogram based on the presence/absence of regions of difference (RD) among S. suis isolates. RDs were defined as at least three consecutive ORFs that were absent from at least 1 strain. Naming of clusters is corresponding to the CGH clustering.

Figure 4

Representation of COG categories among the core genome. Relative representation of COG categories in the whole genome (hatched bars) compared to the core genome (black bars) of S. suis strain P1/7. Representation is calculated as the percentage of genes per COG category compared to the total number of genes in the genome. COG categories: J translation, ribosomal structure and biogenesis; K transcription; L replication, recombination and repair; D cell cycle control, cell division, chromosome partitioning; V defense mechanisms; O posttranslational modification, protein turnover, chaperones; M cell wall/membrane/envelope biogenesis; N cell motility; U intracellular trafficking, secretion, and vesicular transport; T signal transduction mechanisms; C energy production and conversion; P inorganic ion transport and metabolism; G carbohydrate transport and metabolism; E amino acid transport and metabolism; F nucleotide transport and metabolism; H coenzyme transport and metabolism; I lipid transport and metabolism; Q secondary metabolites biosynthesis, transport and catabolism; R general function prediction only; S function unknown; 'other' no COG category attached.