Recombination and population structure in Salmonella enterica

CRISPR typing and subtyping for improved laboratory surveillance of Salmonella infections

Laboratory surveillance systems for salmonellosis should ideally be based on the rapid serotyping and subtyping of isolates. However, current typing methods are limited in both speed and precision. Using 783 strains and isolates belonging to 130 serotypes, we show here that a new family of DNA repeats named CRISPR (clustered regularly interspaced short palindromic repeats) is highly polymorphic in Salmonella. We found that CRISPR polymorphism was strongly correlated with both serotype and multilocus sequence type. Furthermore, spacer microevolution discriminated between subtypes within prevalent serotypes, making it possible to carry out typing and subtyping in a single step. We developed a high-throughput subtyping assay for the most prevalent serotype, Typhimurium. An open web-accessible database was set up, providing a serotype/spacer dictionary and an international tool for strain tracking based on this innovative, powerful typing and subtyping tool.

Genome-wide survey of mutual homologous recombination in a highly sexual bacterial species

The nature of a species remains a fundamental and controversial question. The era of genome/metagenome sequencing has intensified the debate in prokaryotes because of extensive horizontal gene transfer. In this study, we conducted a genome-wide survey of outcrossing homologous recombination in the highly sexual bacterial species Helicobacter pylori. We conducted multiple genome alignment and analyzed the entire data set of one-to-one orthologous genes for its global strains. We detected mosaic structures due to repeated recombination events and discordant phylogenies throughout the genomes of this species. Most of these genes including the "core" set of genes and horizontally transferred genes showed at least one recombination event. Taking into account the relationship between the nucleotide diversity and the minimum number of recombination events per nucleotide, we evaluated the recombination rate in every gene. The rate appears constant across the genome, but genes with a particularly high or low recombination rate were detected. Interestingly, genes with high recombination included those for DNA transformation and for basic cellular functions, such as biosynthesis and metabolism. Several highly divergent genes with a high recombination rate included those for host interaction, such as outer membrane proteins and lipopolysaccharide synthesis. These results provide a global picture of genome-wide distribution of outcrossing homologous recombination in a bacterial species for the first time, to our knowledge, and illustrate how a species can be shaped by mutual homologous recombination.

Phylogenetic incongruence in E. coli O104: understanding the evolutionary relationships of emerging pathogens in the face of homologous recombination

Escherichia coli O104:H4 was identified as an emerging pathogen during the spring and summer of 2011 and was responsible for a widespread outbreak that resulted in the deaths of 50 people and sickened over 4075. Traditional phenotypic and genotypic assays, such as serotyping, pulsed field gel electrophoresis (PFGE), and multilocus sequence typing (MLST), permit identification and classification of bacterial pathogens, but cannot accurately resolve relationships among genotypically similar but pathotypically different isolates. To understand the evolutionary origins of E. coli O104:H4, we sequenced two strains isolated in Ontario, Canada. One was epidemiologically linked to the 2011 outbreak, and the second, unrelated isolate, was obtained in 2010. MLST analysis indicated that both isolates are of the same sequence type (ST678), but whole-genome sequencing revealed differences in chromosomal and plasmid content. Through comprehensive phylogenetic analysis of five O104:H4 ST678 genomes, we identified 167 genes in three gene clusters that have undergone homologous recombination with distantly related E. coli strains. These recombination events have resulted in unexpectedly high sequence diversity within the same sequence type. Failure to recognize or adjust for homologous recombination can result in phylogenetic incongruence. Understanding the extent of homologous recombination among different strains of the same sequence type may explain the pathotypic differences between the ON2010 and ON2011 strains and help shed new light on the emergence of this new pathogen.

Ecological and temporal constraints in the evolution of bacterial genomes

Studies on the experimental evolution of microorganisms, on their in vivo evolution (mainly in the case of bacteria producing chronic infections), as well as the availability of multiple full genomic sequences, are placing bacteria in the playground of evolutionary studies. In the present article we review the differential contribution to the evolution of bacterial genomes that processes such as gene modification, gene acquisition and gene loss may have when bacteria colonize different habitats that present characteristic ecological features. In particular, we review how the different processes contribute to evolution in microbial communities, in free-living bacteria or in bacteria living in isolation. In addition, we discuss the temporal constraints in the evolution of bacterial genomes, considering bacterial evolution from the perspective of processes of short-sighted evolution and punctual acquisition of evolutionary novelties followed by long stasis periods.

Identification of Salmonella enterica species- and subgroup-specific genomic regions using Panseq 2.0

The pan-genome of a taxonomic group consists of evolutionarily conserved core genes shared by all members and accessory genes that are present only in some members of the group. Group- and subgroup-specific core genes are thought to contribute to shared phenotypes such as virulence and niche specificity. In this study we analyzed 39 Salmonella enterica genomes (16 closed, 23 draft), a species that contains two human-specific serovars that cause typhoid fever, as well as a large number of zoonotic serovars that cause gastroenteritis in humans. Panseq 2.0 was used to define the pan-genome by adjusting the threshold at which group-specific "core" loci are defined. We found the pan-genome to be 9.03 Mbp in size, and that the core genome size decreased, while the number of SNPs/100 bp increased, as the number of strains used to define the core genome increased, suggesting substantial divergence among S. enterica subgroups. Subgroup-specific "core" genes, in contrast, had fewer SNPs/100 bp, likely reflecting their more recent acquisition. Phylogenetic trees were created from the concatenated and aligned pan-genome, the core genome, and multi-locus-sequence typing (MLST) loci. Branch support increased among the trees, and strains of the same serovar grouped closer together as the number of loci used to create the tree increased. Further, high levels of discrimination were achieved even amongst the most closely related strains of S. enterica Typhi, suggesting that the data generated by Panseq may also be of value in short-term epidemiological studies. Panseq provides an easy and fast way of performing pan-genomic analyses, which can include the identification of group-dominant as well as group-specific loci and is available as a web-server and a standalone version at http://lfz.corefacility.ca/panseq/.

Genome sequencing reveals diversification of virulence factor content and possible host adaptation in distinct subpopulations of Salmonella enterica

Background

Divergence of bacterial populations into distinct subpopulations is often the result of ecological isolation. While some studies have suggested the existence of Salmonella enterica subsp. enterica subclades, evidence for these subdivisions has been ambiguous. Here we used a comparative genomics approach to define the population structure of Salmonella enterica subsp. enterica, and identify clade-specific genes that may be the result of ecological specialization.

Results

Multi-locus sequence analysis (MLSA) and single nucleotide polymorphisms (SNPs) data for 16 newly sequenced and 30 publicly available genomes showed an unambiguous subdivision of S. enterica subsp. enterica into at least two subpopulations, which we refer to as clade A and clade B. Clade B strains contain several clade-specific genes or operons, including a β-glucuronidase operon, a S-fimbrial operon, and cell surface related genes, which strongly suggests niche specialization of this subpopulation. An additional set of 123 isolates was assigned to clades A and B by using qPCR assays targeting subpopulation-specific SNPs and genes of interest. Among 98 serovars examined, approximately 20% belonged to clade B. All clade B isolates contained two pathogenicity related genomic islands, SPI-18 and a cytolethal distending toxin islet; a combination of these two islands was previously thought to be exclusive to serovars Typhi and Paratyphi A. Presence of β-glucuronidase in clade B isolates specifically suggests an adaptation of this clade to the vertebrate gastrointestinal environment.

Conclusions

S. enterica subsp. enterica consists of at least two subpopulations that differ specifically in genes involved in host and tissue tropism, utilization of host specific carbon and nitrogen sources and are therefore likely to differ in ecology and transmission characteristics.