Revisiting the Molecular Evolutionary History of Shigella spp

Study of Antimicrobial Resistance (AMR) in Shigella spp. in India

Antimicrobial agents are essential in reducing illness and mortality brought on by infectious diseases in both humans and animals. However, the therapeutic effect of antibiotics has diminished due to an increase in antimicrobial drug resistance (AMR). This article provides a retrospective analysis of AMR in Shigella infections in India, showing a rise in resistance that has contributed to a global burden. Shigella spp. are widespread and the second-leading cause of diarrheal death in people of all ages. The frequency and mortality rates of Shigella infections are decreased by antibiotic treatment. However, the growth of broad-spectrum antibiotic resistance is making it more difficult to treat many illnesses. Reduced cell permeability, efflux pumps, and the presence of enzymes that break down antibiotics are the causes of resistance. AMR is a multifaceted and cross-sectoral problem that affects humans, animals, food, and the environment. As a result, there is a growing need for new therapeutic approaches, and ongoing surveillance of Shigella spp. infections which should definitely be improved for disease prevention and management. This review emphasizes on the epidemiological data of India, and antimicrobial resistance in Shigella spp.

Population structure analysis and laboratory monitoring of Shigella by core-genome multilocus sequence typing

The laboratory surveillance of bacillary dysentery is based on a standardised Shigella typing scheme that classifies Shigella strains into four serogroups and more than 50 serotypes on the basis of biochemical tests and lipopolysaccharide O-antigen serotyping. Real-time genomic surveillance of Shigella infections has been implemented in several countries, but without the use of a standardised typing scheme. Here, we study over 4000 reference strains and clinical isolates of Shigella, covering all serotypes, with both the current serotyping scheme and the standardised EnteroBase core-genome multilocus sequence typing scheme (cgMLST). The Shigella genomes are grouped into eight phylogenetically distinct clusters, within the E. coli species. The cgMLST hierarchical clustering (HC) analysis at different levels of resolution (HC2000 to HC400) recognises the natural population structure of Shigella. By contrast, the serotyping scheme is affected by horizontal gene transfer, leading to a conflation of genetically unrelated Shigella strains and a separation of genetically related strains. The use of this cgMLST scheme will facilitate the transition from traditional phenotypic typing to routine whole-genome sequencing for the laboratory surveillance of Shigella infections.

Lab-based surveillance of Shigella has traditionally been based on serotyping but increasing availability of whole genome sequencing could enable higher resolution typing. Here, the authors apply a core genome multilocus sequence typing scheme to Shigella sequence data and describe its population structure.

The evolutionary history of Shigella flexneri serotype 6 in Asia

Shigella flexneri serotype 6 is an understudied cause of diarrhoeal diseases in developing countries, and has been proposed as one of the major targets for vaccine development against shigellosis. Despite being named as S. flexneri, Shigella flexneri serotype 6 is phylogenetically distinct from other S. flexneri serotypes and more closely related to S. boydii. This unique phylogenetic relationship and its low sampling frequency have hampered genomic research on this pathogen. Herein, by utilizing whole genome sequencing (WGS) and analyses of Shigella flexneri serotype 6 collected from epidemiological studies (1987–2013) in four Asian countries, we revealed its population structure and evolutionary history in the region. Phylogenetic analyses supported the delineation of Asian Shigella flexneri serotype 6 into two phylogenetic groups (PG-1 and −2). Notably, temporal phylogenetic approaches showed that extant Asian S. flexneri serotype 6 could be traced back to an inferred common ancestor arising in the 18^th century. The dominant lineage PG-1 likely emerged in the 1970s, which coincided with the times to most recent common ancestors (tMRCAs) inferred from other major Southeast Asian S. flexneri serotypes. Similar to other S. flexneri serotypes in the same period in Asia, genomic analyses showed that resistance to first-generation antimicrobials was widespread, while resistance to more recent first-line antimicrobials was rare. These data also showed a number of gene inactivation and gene loss events, particularly on genes related to metabolism and synthesis of cellular appendages, emphasizing the continuing role of reductive evolution in the adaptation of the pathogen to an intracellular lifestyle. Together, our findings reveal insights into the genomic evolution of the understudied Shigella flexneri serotype 6, providing a new piece in the puzzle of Shigella epidemiology and evolution.

Cluster-specific gene markers enhance Shigella and enteroinvasive Escherichia coli in silico serotyping

Shigella and enteroinvasive Escherichia coli (EIEC) cause human bacillary dysentery with similar invasion mechanisms and share similar physiological, biochemical and genetic characteristics. Differentiation of Shigella from EIEC is important for clinical diagnostic and epidemiological investigations. However, phylogenetically, Shigella and EIEC strains are composed of multiple clusters and are different forms of E. coli, making it difficult to find genetic markers to discriminate between Shigella and EIEC. In this study, we identified 10 Shigella clusters, seven EIEC clusters and 53 sporadic types of EIEC by examining over 17000 publicly available Shigella and EIEC genomes. We compared Shigella and EIEC accessory genomes to identify cluster-specific gene markers for the 17 clusters and 53 sporadic types. The cluster-specific gene markers showed 99.64% accuracy and more than 97.02% specificity. In addition, we developed a freely available in silico serotyping pipeline named Shigella EIEC Cluster Enhanced Serotype Finder (ShigEiFinder) by incorporating the cluster-specific gene markers and established Shigella and EIEC serotype-specific O antigen genes and modification genes into typing. ShigEiFinder can process either paired-end Illumina sequencing reads or assembled genomes and almost perfectly differentiated Shigella from EIEC with 99.70 and 99.74% cluster assignment accuracy for the assembled genomes and read mapping respectively. ShigEiFinder was able to serotype over 59 Shigella serotypes and 22 EIEC serotypes and provided a high specificity of 99.40% for assembled genomes and 99.38% for read mapping for serotyping. The cluster-specific gene markers and our new serotyping tool, ShigEiFinder (installable package: https://github.com/LanLab/ShigEiFinder, online tool: https://mgtdb.unsw.edu.au/ShigEiFinder/), will be useful for epidemiological and diagnostic investigations.

High Rates of Genome Rearrangements and Pathogenicity of Shigella spp

Shigella are pathogens originating within the Escherichia lineage but frequently classified as a separate genus. Shigella genomes contain numerous insertion sequences (ISs) that lead to pseudogenisation of affected genes and an increase of non-homologous recombination. Here, we study 414 genomes of E. coli and Shigella strains to assess the contribution of genomic rearrangements to Shigella evolution. We found that Shigella experienced exceptionally high rates of intragenomic rearrangements and had a decreased rate of homologous recombination compared to pathogenic and non-pathogenic E. coli. The high rearrangement rate resulted in independent disruption of syntenic regions and parallel rearrangements in different Shigella lineages. Specifically, we identified two types of chromosomally encoded E3 ubiquitin-protein ligases acquired independently by all Shigella strains that also showed a high level of sequence conservation in the promoter and further in the 5′-intergenic region. In the only available enteroinvasive E. coli (EIEC) strain, which is a pathogenic E. coli with a phenotype intermediate between Shigella and non-pathogenic E. coli, we found a rate of genome rearrangements comparable to those in other E. coli and no functional copies of the two Shigella-specific E3 ubiquitin ligases. These data indicate that the accumulation of ISs influenced many aspects of genome evolution and played an important role in the evolution of intracellular pathogens. Our research demonstrates the power of comparative genomics-based on synteny block composition and an important role of non-coding regions in the evolution of genomic islands.

Newly Emerged Serotype 1c of Shigella flexneri: Multiple Origins and Changing Drug Resistance Landscape

Bacillary dysentery caused by Shigella flexneri is a major cause of under-five mortality in developing countries, where a novel S. flexneri serotype 1c has become very common since the 1980s. However, the origin and diversification of serotype 1c remain poorly understood. To understand the evolution of serotype 1c and their antimicrobial resistance, we sequenced and analyzed the whole-genome of 85 clinical isolates from the United Kingdom, Egypt, Bangladesh, Vietnam, and Japan belonging to serotype 1c and related serotypes of 1a, 1b and Y/Yv. We identified up to three distinct O-antigen modifying genes in S. flexneri 1c strains, which were acquired from three different bacteriophages. Our analysis shows that S. flexneri 1c strains have originated from serotype 1a and serotype 1b strains after the acquisition of bacteriophage-encoding gtrIc operon. The maximum-likelihood phylogenetic analysis using core genes suggests two distinct S. flexneri 1c lineages, one specific to Bangladesh, which originated from ancestral serotype 1a strains and the other from the United Kingdom, Egypt, and Vietnam originated from ancestral serotype 1b strains. We also identified 63 isolates containing multiple drug-resistant genes in them conferring resistance against streptomycin, sulfonamide, quinolone, trimethoprim, tetracycline, chloramphenicol, and beta-lactamase. Furthermore, antibiotic susceptibility assays showed 83 (97.6%) isolates as either complete or intermediate resistance to the WHO-recommended first- and second-line drugs. This changing drug resistance pattern demonstrates the urgent need for drug resistance surveillance and renewed treatment guidelines.

Impact of insertion sequences on convergent evolution of Shigella species

Shigella species are specialised lineages of Escherichia coli that have converged to become human-adapted and cause dysentery by invading human gut epithelial cells. Most studies of Shigella evolution have been restricted to comparisons of single representatives of each species; and population genomic studies of individual Shigella species have focused on genomic variation caused by single nucleotide variants and ignored the contribution of insertion sequences (IS) which are highly prevalent in Shigella genomes. Here, we investigate the distribution and evolutionary dynamics of IS within populations of Shigella dysenteriae Sd1, Shigella sonnei and Shigella flexneri. We find that five IS (IS1, IS2, IS4, IS600 and IS911) have undergone expansion in all Shigella species, creating substantial strain-to-strain variation within each population and contributing to convergent patterns of functional gene loss within and between species. We find that IS expansion and genome degradation are most advanced in S. dysenteriae and least advanced in S. sonnei; and using genome-scale models of metabolism we show that Shigella species display convergent loss of core E. coli metabolic capabilities, with S. sonnei and S. flexneri following a similar trajectory of metabolic streamlining to that of S. dysenteriae. This study highlights the importance of IS to the evolution of Shigella and provides a framework for the investigation of IS dynamics and metabolic reduction in other bacterial species.

Salmonella Pathogenicity Island 1 (SPI-1): The Evolution and Stabilization of a Core Genomic Type Three Secretion System

Salmonella Pathogenicity Island 1 (SPI-1) encodes a type three secretion system (T3SS), effector proteins, and associated transcription factors that together enable invasion of epithelial cells in animal intestines. The horizontal acquisition of SPI-1 by the common ancestor of all Salmonella is considered a prime example of how gene islands potentiate the emergence of new pathogens with expanded niche ranges. However, the evolutionary history of SPI-1 has attracted little attention. Here, we apply phylogenetic comparisons across the family Enterobacteriaceae to examine the history of SPI-1, improving the resolution of its boundaries and unique architecture by identifying its composite gene modules. SPI-1 is located between the core genes fhlA and mutS, a hotspot for the gain and loss of horizontally acquired genes. Despite the plasticity of this locus, SPI-1 demonstrates stable residency of many tens of millions of years in a host genome, unlike short-lived homologous T3SS and effector islands including Escherichia ETT2, Yersinia YSA, Pantoea PSI-2, Sodalis SSR2, and Chromobacterium CPI-1. SPI-1 employs a unique series of regulatory switches, starting with the dedicated transcription factors HilC and HilD, and flowing through the central SPI-1 regulator HilA. HilA is shared with other T3SS, but HilC and HilD may have their evolutionary origins in Salmonella. The hilA, hilC, and hilD gene promoters are the most AT-rich DNA in SPI-1, placing them under tight control by the transcriptional repressor H-NS. In all Salmonella lineages, these three promoters resist amelioration towards the genomic average, ensuring strong repression by H-NS. Hence, early development of a robust and well-integrated regulatory network may explain the evolutionary stability of SPI-1 compared to T3SS gene islands in other species.

Complete genome sequence and annotation of the laboratory reference strain Shigella flexneri serotype 5a M90T and genome-wide transcriptional start site determination

Background

Shigella is a Gram-negative facultative intracellular bacterium that causes bacillary dysentery in humans. Shigella invades cells of the colonic mucosa owing to its virulence plasmid-encoded Type 3 Secretion System (T3SS), and multiplies in the target cell cytosol. Although the laboratory reference strain S. flexneri serotype 5a M90T has been extensively used to understand the molecular mechanisms of pathogenesis, its complete genome sequence is not available, thereby greatly limiting studies employing high-throughput sequencing and systems biology approaches.

Results

We have sequenced, assembled, annotated and manually curated the full genome of S. flexneri 5a M90T. This yielded two complete circular contigs, the chromosome and the virulence plasmid (pWR100). To obtain the genome sequence, we have employed long-read PacBio DNA sequencing followed by polishing with Illumina RNA-seq data. This provides a new hybrid strategy to prepare gapless, highly accurate genome sequences, which also cover AT-rich tracks or repetitive sequences that are transcribed. Furthermore, we have performed genome-wide analysis of transcriptional start sites (TSS) and determined the length of 5′ untranslated regions (5′-UTRs) at typical culture conditions for the inoculum of in vitro infection experiments. We identified 6723 primary TSS (pTSS) and 7328 secondary TSS (sTSS). The S. flexneri 5a M90T annotated genome sequence and the transcriptional start sites are integrated into RegulonDB (http://regulondb.ccg.unam.mx) and RSAT (http://embnet.ccg.unam.mx/rsat/) databases to use their analysis tools in the S. flexneri 5a M90T genome.

Conclusions

We provide the first complete genome for S. flexneri serotype 5a, specifically the laboratory reference strain M90T. Our work opens the possibility of employing S. flexneri M90T in high-quality systems biology studies such as transcriptomic and differential expression analyses or in genome evolution studies. Moreover, the catalogue of TSS that we report here can be used in molecular pathogenesis studies as a resource to know which genes are transcribed before infection of host cells. The genome sequence, together with the analysis of transcriptional start sites, is also a valuable tool for precise genetic manipulation of S. flexneri 5a M90T. Further, we present a new hybrid strategy to prepare gapless, highly accurate genome sequences. Unlike currently used hybrid strategies combining long- and short-read DNA sequencing technologies to maximize accuracy, our workflow using long-read DNA sequencing and short-read RNA sequencing provides the added value of using non-redundant technologies, which yield distinct, exploitable datasets.

Plasmids of Shigella flexneri serotype 1c strain Y394 provide advantages to bacteria in the host

Background

Shigella flexneri has an extremely complex genome with a significant number of virulence traits acquired by mobile genetic elements including bacteriophages and plasmids. S. flexneri serotype 1c is an emerging etiological agent of bacillary dysentery in developing countries. In this study, the complete nucleotide sequence of two plasmids of S. flexneri serotype 1c strain Y394 was determined and analysed.

Results

The plasmid pINV-Y394 is an invasive or virulence plasmid of size 221,293 bp composed of a large number of insertion sequences (IS), virulence genes, regulatory and maintenance genes. Three hundred and twenty-eight open reading frames (ORFs) were identified in pINV-Y394, of which about a half (159 ORFs) were identified as IS elements. Ninety-seven ORFs were related to characterized genes (majority of which are associated with virulence and their regulons), and 72 ORFs were uncharacterized or hypothetical genes. The second plasmid pNV-Y394 is of size 10,866 bp and encodes genes conferring resistance against multiple antibiotics of clinical importance. The multidrug resistance gene cassette consists of tetracycline resistance gene tetA, streptomycin resistance gene strA-strB and sulfonamide-resistant dihydropteroate synthase gene sul2.

Conclusions

These two plasmids together play a key role in the fitness of Y394 in the host environment. The findings from this study indicate that the pathogenic S. flexneri is a highly niche adaptive pathogen which is able to co-evolve with its host and respond to the selection pressure in its environment.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1455-1) contains supplementary material, which is available to authorized users.

Geraniol as a novel antivirulence agent against bacillary dysentery-causing Shigella sonnei

Antimicrobial resistance has emerged as a major challenge to modern medicine and it has become urgent to seek alternative approaches to treat infections caused by fast-evolving multi-resistant clones of bacillary dysentery-causing Shigella sonnei. Here, we show that geraniol, a natural substance present in the essential oils of plants such as rose and lemongrass, can reduce S. sonnei proliferation inside host cells and protect Galleria mellonella larvae from killing by S. sonnei infection. We present evidence that geraniol competitively inhibits the catalytic activity of the master virulence regulator, DsbA, a periplasmic disulphide bond oxidoreductase required for Shigella survival in the host cell cytosol. Our observations suggest that geraniol holds a great therapeutic potential that should be further exploited.

Microbial sequence typing in the genomic era

Next-generation sequencing (NGS), also known as high-throughput sequencing, is changing the field of microbial genomics research. NGS allows for a more comprehensive analysis of the diversity, structure and composition of microbial genes and genomes compared to the traditional automated Sanger capillary sequencing at a lower cost. NGS strategies have expanded the versatility of standard and widely used typing approaches based on nucleotide variation in several hundred DNA sequences and a few gene fragments (MLST, MLVA, rMLST and cgMLST). NGS can now accommodate variation in thousands or millions of sequences from selected amplicons to full genomes (WGS, NGMLST and HiMLST). To extract signals from high-dimensional NGS data and make valid statistical inferences, novel analytic and statistical techniques are needed. In this review, we describe standard and new approaches for microbial sequence typing at gene and genome levels and guidelines for subsequent analysis, including methods and computational frameworks. We also present several applications of these approaches to some disciplines, namely genotyping, phylogenetics and molecular epidemiology.

Phylogenetic comparison of enteroinvasive Escherichia coli isolated from cases of diarrhoeal disease in England, 2005-2016

We compared the genomes of 60 isolates of enteroinvasive Escherichia coli (EIEC) in order to better understand the step-wise evolutionary process from non-pathogenic to host-adapted pathogenic E. coli. All isolates belonged to either sequence type (ST) 6, ST99 or ST270. Each ST was located on different branches of the E. coli phylogeny and had invasion plasmids (pINVs) belonging to FII-21 (ST99, ST270), FII-27 (ST270) or FII-28 (ST6, ST270) incompatibility groups. A higher number of insertion sequence (IS) elements were identified in ST6 and ST270 than in ST99, and appeared to be driving the loss of functional genes. Comparison of the pINV from each ST revealed different degrees of gene loss, with pINV from ST270 being most similar to that found in Shigella species. We captured three EIEC STs at different stages of patho-adaptation, with ST270 being the most 'shigella-like' and the most divergent from non-pathogenic E. coli, and ST99 being the least divergent.

Genomic Analysis and Resistance Mechanisms in Shigella flexneri 2a Strain 301

Shigella flexneri is one of the most prominent pathogenic bacteria in developing countries. In the battle against shigellosis and other bacterial diseases, antibiotic resistance has become an increasing global public health threat. Although the serious phenomenon of multidrug resistance (MDR) has been identified as one of the top three burdens on human health, resistance mechanisms are still poorly understood at the molecular level. In this study, we analyzed genomic data and the evolution of resistance in Shigella flexneri under sequential selection stress from three separate antibiotics: ciprofloxacin (CIP), ceftriaxone (CRO), and tetracycline. Through whole-genome sequencing, 82 chromosomal antibiotic resistance genes were identified. Re-sequencing of the evolved populations identified single nucleotide polymorphisms (SNPs) that contributed to MDR and SNPs that were specific to a single drug. A total of 40 SNPs in 8 genes and 3 intergenic regions, including mutations in metG (L582R) and 1538924, 1538924, and 1538924, appeared under each antibiotic. Several nonsynonymous mutations in gyrB (S464Y), ydgA (E378A), rob (R156H), and narX (K75E) were observed under selective pressure from CIP or CRO. Based on a bioinformatic analysis and previous reports, we discuss the contribution of these mutated genes to resistance. Therefore, more circumspect selection and use of antimicrobial drugs for treating shigellosis is necessary.

Identification of Escherichia coli and Shigella Species from Whole-Genome Sequences

Escherichia coli and Shigella species are closely related and genetically constitute the same species. Differentiating between these two pathogens and accurately identifying the four species of Shigella are therefore challenging. The organism-specific bioinformatics whole-genome sequencing (WGS) typing pipelines at Public Health England are dependent on the initial identification of the bacterial species by use of a kmer-based approach. Of the 1,982 Escherichia coli and Shigella sp. isolates analyzed in this study, 1,957 (98.4%) had concordant results by both traditional biochemistry and serology (TB&S) and the kmer identification (ID) derived from the WGS data. Of the 25 mismatches identified, 10 were enteroinvasive E. coli isolates that were misidentified as Shigella flexneri or S. boydii by the kmer ID, and 8 were S. flexneri isolates misidentified by TB&S as S. boydii due to nonfunctional S. flexneri O antigen biosynthesis genes. Analysis of the population structure based on multilocus sequence typing (MLST) data derived from the WGS data showed that the remaining discrepant results belonged to clonal complex 288 (CC288), comprising both S. boydii and S. dysenteriae strains. Mismatches between the TB&S and kmer ID results were explained by the close phylogenetic relationship between the two species and were resolved with reference to the MLST data. Shigella can be differentiated from E. coli and accurately identified to the species level by use of kmer comparisons and MLST. Analysis of the WGS data provided explanations for the discordant results between TB&S and WGS data, revealed the true phylogenetic relationships between different species of Shigella, and identified emerging pathoadapted lineages.

Investigating the Relatedness of Enteroinvasive Escherichia coli to Other E. coli and Shigella Isolates by Using Comparative Genomics

Enteroinvasive Escherichia coli (EIEC) is a unique pathovar that has a pathogenic mechanism nearly indistinguishable from that of Shigella species. In contrast to isolates of the four Shigella species, which are widespread and can be frequent causes of human illness, EIEC causes far fewer reported illnesses each year. In this study, we analyzed the genome sequences of 20 EIEC isolates, including 14 first described in this study. Phylogenomic analysis of the EIEC genomes demonstrated that 17 of the isolates are present in three distinct lineages that contained only EIEC genomes, compared to reference genomes from each of the E. coli pathovars and Shigella species. Comparative genomic analysis identified genes that were unique to each of the three identified EIEC lineages. While many of the EIEC lineage-specific genes have unknown functions, those with predicted functions included a colicin and putative proteins involved in transcriptional regulation or carbohydrate metabolism. In silico detection of the Shigella virulence plasmid (pINV), which is essential for the invasion of host cells, demonstrated that a form of pINV was present in nearly all EIEC genomes, but the Mxi-Spa-Ipa region of the plasmid that encodes the invasion-associated proteins was absent from several of the EIEC isolates. The comparative genomic findings in this study support the hypothesis that multiple EIEC lineages have evolved independently from multiple distinct lineages of E. coli via the acquisition of the Shigella virulence plasmid and, in some cases, the Shigella pathogenicity islands.

The genomic signatures of Shigella evolution, adaptation and geographical spread

Shigella spp. are some of the key pathogens responsible for the global burden of diarrhoeal disease. These facultative intracellular bacteria belong to the family Enterobacteriaceae, together with other intestinal pathogens, such as Escherichia coli and Salmonella spp. The genus Shigella comprises four different species, each consisting of several serogroups, all of which show phenotypic similarity, including invasive pathogenicity. DNA sequencing suggests that this similarity results from the convergent evolution of different Shigella spp. founders. Here, we review the evolutionary relationships between Shigella spp. and E . coli, and we highlight how the genomic plasticity of these bacteria and their acquisition of a distinctive virulence plasmid have enabled the development of such highly specialized pathogens. Furthermore, we discuss the insights that genotyping and whole-genome sequencing have provided into the phylogenetics and intercontinental spread of Shigella spp.

Draft genomes of Shigella strains used by the STOPENTERICS consortium

Background

Despite a significant global burden of disease, there is still no vaccine against shigellosis widely available. One aim of the European Union funded STOPENTERICS consortium is to develop vaccine candidates against Shigella. Given the importance of translational vaccine coverage, here we aimed to characterise the Shigella strains being used by the consortium by whole genome sequencing, and report on the stability of strains cultured in different laboratories or through serial passage.

Methods

We sequenced, de novo assembled and annotated 20 Shigella strains being used by the consortium. These comprised 16 different isolates belonging to 7 serotypes, and 4 derivative strains. Derivative strains from common isolates were manipulated in different laboratories or had undergone multiple passages in the same laboratory. Strains were mapped against reference genomes to detect SNP variation and phylogenetic analysis was performed.

Results

The genomes assembled into similar total lengths (range 4.14–4.83 Mbp) and had similar numbers of predicted coding sequences (average of 4,400). Mapping analysis showed the genetic stability of strains through serial passages and culturing in different laboratories, as well as varying levels of similarity to published reference genomes. Phylogenetic analysis revealed the presence of three main clades among the strains and published references, one containing the Shigella flexneri serotype 6 strains, a second containing the remaining S. flexneri serotypes and a third comprised of Shigella sonnei strains.

Conclusions

This work increases the number of the publically available Shigella genomes available and specifically provides information on strains being used for vaccine development by STOPENTERICS. It also provides information on the variability among strains maintained in different laboratories and through serial passage. This work will guide the selection of strains for further vaccine development.

Distinct mutations led to inactivation of type 1 fimbriae expression in Shigella spp

Shigella spp. are responsible for bacillary dysentery in humans. The acquisition or the modification of the virulence plasmid encoding factors promoting entry of bacteria into and dissemination within epithelial cells was a critical step in the evolution of these bacteria from their Escherichia coli ancestor(s). Incorporation of genomic islands (GI) and gene inactivation also shaped interactions between these pathogens and their human host. Sequence analysis of the GI inserted next to the leuX tRNA gene in S. boydii, S. dysenteriae, S. flexneri, S. sonnei and enteroinvasive E. coli (EIEC) suggests that this region initially carried the fec, yjhATS and fim gene clusters. The fim cluster encoding type I fimbriae is systematically inactivated in both reference strains and clinical isolates and distinct mutations are responsible for this inactivation in at least three phylogenetic groups. To investigate consequences of the presence of fimbriae on the outcome of the interaction of Shigella with host cells, we used a S. flexneri strain harboring a plasmid encoding the E. coli fim operon. Production of fimbriae by this recombinant strain increased the ability of bacteria to adhere to and enter into epithelial cells and had no effect on their ability to disseminate from cell to cell. The observations that production of type I fimbriae increases invasion of epithelial cells and that independent mutations abolish fimbriae production in Shigella suggest that these mutations correspond to pathoadaptive events.

Relationship among Shigella spp. and enteroinvasive Escherichia coli (EIEC) and their differentiation

Shigellosis produces inflammatory reactions and ulceration on the intestinal epithelium followed by bloody or mucoid diarrhea. It is caused by enteroinvasive E. coli (EIEC) as well as any species of the genus Shigella, namely, S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. This current species designation of Shigella does not specify genetic similarity. Shigella spp. could be easily differentiated from E. coli, but difficulties observed for the EIEC-Shigella differentiation as both show similar biochemical traits and can cause dysentery using the same mode of invasion. Sequencing of multiple housekeeping genes indicates that Shigella has derived on several different occasions via acquisition of the transferable forms of ancestral virulence plasmids within commensal E. coli and form a Shigella-EIEC pathovar. EIEC showed lower expression of virulence genes compared to Shigella, hence EIEC produce less severe disease than Shigella spp. Conventional microbiological techniques often lead to confusing results concerning the discrimination between EIEC and Shigella spp. The lactose permease gene (lacY) is present in all E. coli strains but absent in Shigella spp., whereas β-glucuronidase gene (uidA) is present in both E. coli and Shigella spp. Thus uidA gene and lacY gene based duplex real-time PCR assay could be used for easy identification and differentiation of Shigella spp. from E. coli and in particular EIEC.

Defining the phylogenomics of Shigella species: a pathway to diagnostics

Shigellae cause significant diarrheal disease and mortality in humans, as there are approximately 163 million episodes of shigellosis and 1.1 million deaths annually. While significant strides have been made in the understanding of the pathogenesis, few studies on the genomic content of the Shigella species have been completed. The goal of this study was to characterize the genomic diversity of Shigella species through sequencing of 55 isolates representing members of each of the four Shigella species: S. flexneri, S. sonnei, S. boydii, and S. dysenteriae. Phylogeny inferred from 336 available Shigella and Escherichia coli genomes defined exclusive clades of Shigella; conserved genomic markers that can identify each clade were then identified. PCR assays were developed for each clade-specific marker, which was combined with an amplicon for the conserved Shigella invasion antigen, IpaH3, into a multiplex PCR assay. This assay demonstrated high specificity, correctly identifying 218 of 221 presumptive Shigella isolates, and sensitivity, by not identifying any of 151 diverse E. coli isolates incorrectly as Shigella. This new phylogenomics-based PCR assay represents a valuable tool for rapid typing of uncharacterized Shigella isolates and provides a framework that can be utilized for the identification of novel genomic markers from genomic data.

The extant World War 1 dysentery bacillus NCTC1: a genomic analysis

BACKGROUND

Shigellosis (previously bacillary dysentery) was the primary diarrhoeal disease of World War 1, but outbreaks still occur in military operations, and shigellosis causes hundreds of thousands of deaths per year in developing nations. We aimed to generate a high-quality reference genome of the historical Shigella flexneri isolate NCTC1 and to examine the isolate for resistance to antimicrobials.

METHODS

In this genomic analysis, we sequenced the oldest extant Shigella flexneri serotype 2a isolate using single-molecule real-time (SMRT) sequencing technology. Isolated from a soldier with dysentery from the British forces fighting on the Western Front in World War 1, this bacterium, NCTC1, was the first isolate accessioned into the National Collection of Type Cultures. We created a reference sequence for NCTC1, investigated the isolate for antimicrobial resistance, and undertook comparative genetics with S flexneri reference strains isolated during the 100 years since World War 1.

FINDINGS

We discovered that NCTC1 belonged to a 2a lineage of S flexneri, with which it shares common characteristics and a large core genome. NCTC1 was resistant to penicillin and erythromycin, and contained a complement of chromosomal antimicrobial resistance genes similar to that of more recent isolates. Genomic islands gained in the S flexneri 2a lineage over time were predominately associated with additional antimicrobial resistances, virulence, and serotype conversion.

INTERPRETATION

This S flexneri 2a lineage is a well adapted pathogen that has continued to respond to selective pressures. We have created a valuable historical benchmark for shigellae in the form of a high-quality reference sequence for a publicly available isolate.

FUNDING

The Wellcome Trust.

Escherichia fergusonii

Escherichia fergusonii was introduced in the genus Escherichia almost 65 years later than Escherichia coli after which the genus was named. From then (1985) onwards mainly case reports on E. fergusonii associated with disease in individuals of veterinary or human origin have been reported and only very few more extensive studies became available. This has resulted in very fragmented knowledge on this organism. The aim of this manuscript is to give an overview of what is known on E. fergusonii today and to stimulate more research on this organism so that better insight can be obtained in the role that E. fergusonii plays in human and animal infections.

Draft genome sequences of the type strains of Shigella flexneri held at Public Health England: comparison of classical phenotypic and novel molecular assays with whole genome sequence

Background

Public Health England (PHE) holds a collection of Shigella flexneri Type strains isolated between 1949 and 1972 representing 15 established serotypes and one provisional type, E1037. In this study, the genomes of all 16 PHE Type strains were sequenced using the Illumina HiSeq platform. The relationship between core genome phylogeny and serotype was examined.

Results

The most common target gene for the detection of Shigella species in clinical PCR assays, ipaH, was detected in all genomes. The type-specific target genes were correctly identified in each genome sequence. In contrast to the S. flexneri in serotype 5 strain described by Sun et al. (2012), the two PHE serotype 5 Type strains possessed an additional oac gene and were differentiated by the presence (serotype 5b) or absence (serotype 5a) of gtrX. The somatic antigen structure and phylogenetic relationship were broadly congruent for strains expressing serotype specific antigens III, IV and V, but not for those expressing I and II. The whole genome phylogenies of the 15 isolates sequenced showed that the serotype 6 Type Strain was phylogenetically distinct from the other S. flexneri serotypes sequenced. The provisional serotype E1037 fell within the serotype 4 clade, being most closely related to the Serotype 4a Type Strain.

Conclusions

The S. flexneri genome sequences were used to evaluate phylogenetic relationships between Type strains and validate genotypic and phenotypic assays. The analysis confirmed that the PHE S. flexneri Type strains are phenotypically and genotypically distinct. Novel variants will continue to be added to this archive.

Recent advances in understanding enteric pathogenic Escherichia coli

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Deletion of pic results in decreased virulence for a clinical isolate of Shigella flexneri 2a from China

Background

Shigella is a major pathogen responsible for bacillary dysentery, a severe form of shigellosis. Severity of the disease depends on the virulence of the infecting strain. Shigella pathogenicity is a multi-gene phenomenon, involving the participation of genes on an unstable large virulence plasmid and chromosomal pathogenicity islands.

Results

A multiplex PCR (mPCR) assay was developed to detect S. flexneri 2a from rural regions of Zhengding (Hebei Province, China). We isolated and tested 86 strains using our mPCR assay, which targeted the ipaH, ial and set1B genes. A clinical strain of S. flexneri 2a 51 (SF51) containing ipaH and ial, but lacking set1B was found. The virulence of this strain was found to be markedly decreased. Further testing showed that the SF51 strain lacked pic. To investigate the role of pic in S. flexneri 2a infections, a pic knockout mutant (SF301-∆ pic) and two complementation strains, SF301-∆ pic/pPic and SF51/pPic, were created. Differences in virulence for SF51, SF301-∆ pic, SF301-∆ pic/pPic, SF51/pPic and S. flexneri 2a 301 (SF301) were compared. Compared with SF301, both SF51 and SF301-∆ pic exhibited lower levels of Hela cell invasion and resulted in reduced keratoconjunctivitis, with low levels of tissue damage seen in murine eye sections. The virulence of SF301-∆ pic and SF51 was partially recovered in vitro and in vivo through the addition of a complementary pic gene.

Conclusions

The pic gene appears to be involved in an increase in pathogenicity of S. flexneri 2a. This gene assists with bacterial invasion into host cells and alters inflammatory reactions.

Hemagglutinating activity is directly correlated with colonization ability of shigellae in suckling mouse model

The aim of the present study was to explore a new approach based on the hemagglutination (HA) assay to understand the colonization ability of Shigella spp. To study colonization ability, an animal model of 4-day-old suckling mouse, was exploited. We characterized the HA activity of 48 Shigella strains, with erythrocytes collected from rabbit, guinea pig, chicken, and sheep. Only rabbit and guinea pig erythrocytes showed positive HA reactions in most of the cases. On the basis of HA pattern, 4 strains from each serogroup were selected for in vivo colonization studies. Our results showed a positive correlation between HA activity and colonization ability of the strains belonging to different serogroups (groups A, B, C, and D) of Shigella. In all 4 serogroups, high HA titer was associated with greater intestinal colonization.

A Genomic Island in Salmonella enterica ssp. salamae provides new insights on the genealogy of the locus of enterocyte effacement

The genomic island encoding the locus of enterocyte effacement (LEE) is an important virulence factor of the human pathogenic Escherichia coli. LEE typically encodes a type III secretion system (T3SS) and secreted effectors capable of forming attaching and effacing lesions. Although prominent in the pathogenic E. coli such as serotype O157:H7, LEE has also been detected in Citrobacter rodentium, E. albertii, and although not confirmed, it is likely to also be in Shigella boydii. Previous phylogenetic analysis of LEE indicated the genomic island was evolving through stepwise acquisition of various components. This study describes a new LEE region from two strains of Salmonella enterica subspecies salamae serovar Sofia along with a phylogenetic analysis of LEE that provides new insights into the likely evolution of this genomic island. The Salmonella LEE contains 36 of the 41 genes typically observed in LEE within a genomic island of 49, 371 bp that encodes a total of 54 genes. A phylogenetic analysis was performed on the entire T3SS and four T3SS genes (escF, escJ, escN, and escV) to elucidate the genealogy of LEE. Phylogenetic analysis inferred that the previously known LEE islands are members of a single lineage distinct from the new Salmonella LEE lineage. The previously known lineage of LEE diverged between islands found in Citrobacter and those in Escherichia and Shigella. Although recombination and horizontal gene transfer are important factors in the genealogy of most genomic islands, the phylogeny of the T3SS of LEE can be interpreted with a bifurcating tree. It seems likely that the LEE island entered the Enterobacteriaceae through horizontal gene transfer as a single unit, rather than as separate subsections, which was then subjected to the forces of both mutational change and recombination.

Bone strength in children with growing pains: long-term follow-up

OBJECTIVES

To examine the changes in bone strength in a cohort of children with 'growing pains' (GP) after 5 years follow-up and the correlation with pain outcome.

METHODS

Bone strength was measured by quantitative ultrasound. Subjects were 39 children with GP previously studied. Controls were normograms based on the measurement of bone speed of sound in 1085 healthy children. Current GP status was assessed by parental questionnaires. Bone strength was compared with pain outcome.

RESULTS

We examined 30/39 (77%) patients after 5 years. Bone strength was significantly increased when compared to the first study (Z score 0.65±1.77 vs. -0.62±0.90, p<0.001). While overall there was no significant difference in the bone strength between the 16 (53%) patients whose GP resolved and the 14 (47%) who continued to have GP episodes (p=0.71), all 6 (20%) patients with a speed of sound Z-score <-1 continued to have GP (p=0.003).

CONCLUSIONS

Our findings that pain improves in most patients parallel to the increase in bone strength may support the hypothesis of GP representing in some patients a local overuse syndrome.

Infection strategies of enteric pathogenic Escherichia coli

Enteric Escherichia coli (E. coli) are both natural flora of humans and important pathogens causing significant morbidity and mortality worldwide. Traditionally enteric E. coli have been divided into 6 pathotypes, with further pathotypes often proposed. In this review we suggest expansion of the enteric E. coli into 8 pathotypes to include the emerging pathotypes of adherent invasive E. coli (AIEC) and Shiga-toxin producing enteroaggregative E. coli (STEAEC). The molecular mechanisms that allow enteric E. coli to colonize and cause disease in the human host are examined and for two of the pathotypes that express a type 3 secretion system (T3SS) we discuss the complex interplay between translocated effectors and manipulation of host cell signaling pathways that occurs during infection.

Analysis of synonymous codon usage inShigella flexneri2a strain 301 and otherShigellaandEscherichia colistrains

In this study, we analysed synonymous codon usage in Shigella flexneri 2a strain 301 (Sf301) and performed a comparative analysis of synonymous codon usage patterns in Sf301 and other strains of Shigella and Escherichia coli . Although there was a significant variety in codon usage bias among different Sf301 genes, there was a slight but observable codon usage bias that could primarily be attributable to mutational pressure and translational selection. In addition, the relative abundance of dinucleotides in Sf301 was observed to be independent of the overall base composition but was still caused by differential mutational pressure; this also shaped codon usage. By comparing the relative synonymous codon usage values across different Shigella and E. coli strains, we suggested that the synonymous codon usage pattern in the Shigella genomes was strain specific. This study represents a comprehensive analysis of Shigella codon usage patterns and provides a basic understanding of the mechanisms underlying codon usage bias.

Gene decay in Shigella as an incipient stage of host-adaptation

Background

Many facultative bacterial pathogens have undergone extensive gene decay processes, possibly due to lack of selection pressure during evolutionary conversion from free-living to intracellular lifestyle. Shigella, the causative agents of human shigellosis, have arisen from different E. coli-like ancestors independently by convergent paths. As these bacteria all have lost large numbers of genes by mutation or deletion, they can be used as ideal models for systematically studying the process of gene function loss in different bacteria living under similar selection pressures.

Methodologies/Principal Findings

We compared the sequenced Shigella genomes and re-defined decayed genes (pseudogenes plus deleted genes) in these bacteria. Altogether, 85 genes are commonly decayed in the five analyzed Shigella strains and 1456 genes are decayed in at least one Shigella strain. Genes coding for carbon utilization, cell motility, transporter or membrane proteins are prone to be inactivated. Decayed genes tend to concentrate in certain operons rather than distribute averagely across the whole genome. Genes in the decayed operon accumulated more non-synonymous mutations than the rest genes and meanwhile have lower expression levels.

Conclusions

Different Shigella lineages underwent convergent gene decay processes, and inactivation of one gene would lead to a lesser selection pressure for the other genes in the same operon. The pool of superfluous genes for Shigella may contain at least two thousand genes and the gene decay processes may still continue in Shigella until a minimum genome harboring only essential genes is reached.

Whole-genome phylogeny of Escherichia coli/Shigella group by feature frequency profiles (FFPs)

A whole-genome phylogeny of the Escherichia coli/Shigella group was constructed by using the feature frequency profile (FFP) method. This alignment-free approach uses the frequencies of l-mer features of whole genomes to infer phylogenic distances. We present two phylogenies that accentuate different aspects of E. coli/Shigella genomic evolution: (i) one based on the compositions of all possible features of length l = 24 (∼8.4 million features), which are likely to reveal the phenetic grouping and relationship among the organisms and (ii) the other based on the compositions of core features with low frequency and low variability (∼0.56 million features), which account for ∼69% of all commonly shared features among 38 taxa examined and are likely to have genome-wide lineal evolutionary signal. Shigella appears as a single clade when all possible features are used without filtering of noncore features. However, results using core features show that Shigella consists of at least two distantly related subclades, implying that the subclades evolved into a single clade because of a high degree of convergence influenced by mobile genetic elements and niche adaptation. In both FFP trees, the basal group of the E. coli/Shigella phylogeny is the B2 phylogroup, which contains primarily uropathogenic strains, suggesting that the E. coli/Shigella ancestor was likely a facultative or opportunistic pathogen. The extant commensal strains diverged relatively late and appear to be the result of reductive evolution of genomes. We also identify clade distinguishing features and their associated genomic regions within each phylogroup. Such features may provide useful information for understanding evolution of the groups and for quick diagnostic identification of each phylogroup.

An integrated approach for finding overlooked genes in Shigella

BACKGROUND

The completion of numerous genome sequences introduced an era of whole-genome study. However, many genes are missed during genome annotation, including small RNAs (sRNAs) and small open reading frames (sORFs). In order to improve genome annotation, we aimed to identify novel sRNAs and sORFs in Shigella, the principal etiologic agents of bacillary dysentery.

METHODOLOGY/PRINCIPAL FINDINGS

We identified 64 sRNAs in Shigella, which were experimentally validated in other bacteria based on sequence conservation. We employed computer-based and tiling array-based methods to search for sRNAs, followed by RT-PCR and northern blots, to identify nine sRNAs in Shigella flexneri strain 301 (Sf301) and 256 regions containing possible sRNA genes. We found 29 candidate sORFs using bioinformatic prediction, array hybridization and RT-PCR verification. We experimentally validated 557 (57.9%) DOOR operon predictions in the chromosomes of Sf301 and 46 (76.7%) in virulence plasmid.We found 40 additional co-expressed gene pairs that were not predicted by DOOR.

CONCLUSIONS/SIGNIFICANCE

We provide an updated and comprehensive annotation of the Shigella genome. Our study increased the expected numbers of sORFs and sRNAs, which will impact on future functional genomics and proteomics studies. Our method can be used for large scale reannotation of sRNAs and sORFs in any microbe with a known genome sequence.

Reductive divergence of enterobacterial repetitive intergenic consensus sequences among Gammaproteobacteria genomes

Enterobacterial repetitive intergenic consensus (ERIC) sequence is a transcription-modulating, nonautonomous, miniature inverted-repeat transposable element. Its origin and the mechanism of highly varying incidences, limited to Enterobacteriaceae and Vibrionaceae, have not been identified. In this study, distribution and divergence of ERICs along bacterial taxonomie units were analyzed. ERICs were found among five families of gammaproteobacteria, with the copy numbers varying with exponential increments. The variability was explained by genus (45%) and species (36%) affiliations, indicating that copy numbers are specific to sub-family taxa. ERICs were interspersed in genomes with considerable divergences. Locations of ERICs in a genome appeared to be strongly conserved in a strain, moderately in a species or a genus, and weakly in a family. ERICs in different species of a genus were from the identical population of sequences while ERICs in different genera of a family were nearly identical. However, ERICs in different families formed distinct monophylectic groups, implying vertical transmission of diverging population of sequences. In spite of large difference in copy numbers, overall intra-genome evolutionary distances among ERICs were similar among different species, except for a few genomes. The exceptions substantiated hypotheses of genetic drifts and horizontal gene transfers of mobility capacity. Therefore, the confined, variable distribution of ERIC could be explained as a two-step evolution: introduction and proliferation of ERIC in one of the progenitors of gammaproteobacteria, followed by vertical transmission under negative selection. Deterioration of sequences and reduction in copy number were concluded to be the predominant patterns in the evolution of ERIC loci.

progressiveMauve: multiple genome alignment with gene gain, loss and rearrangement

Background

Multiple genome alignment remains a challenging problem. Effects of recombination including rearrangement, segmental duplication, gain, and loss can create a mosaic pattern of homology even among closely related organisms.

Methodology/Principal Findings

We describe a new method to align two or more genomes that have undergone rearrangements due to recombination and substantial amounts of segmental gain and loss (flux). We demonstrate that the new method can accurately align regions conserved in some, but not all, of the genomes, an important case not handled by our previous work. The method uses a novel alignment objective score called a sum-of-pairs breakpoint score, which facilitates accurate detection of rearrangement breakpoints when genomes have unequal gene content. We also apply a probabilistic alignment filtering method to remove erroneous alignments of unrelated sequences, which are commonly observed in other genome alignment methods. We describe new metrics for quantifying genome alignment accuracy which measure the quality of rearrangement breakpoint predictions and indel predictions. The new genome alignment algorithm demonstrates high accuracy in situations where genomes have undergone biologically feasible amounts of genome rearrangement, segmental gain and loss. We apply the new algorithm to a set of 23 genomes from the genera Escherichia, Shigella, and Salmonella. Analysis of whole-genome multiple alignments allows us to extend the previously defined concepts of core- and pan-genomes to include not only annotated genes, but also non-coding regions with potential regulatory roles. The 23 enterobacteria have an estimated core-genome of 2.46Mbp conserved among all taxa and a pan-genome of 15.2Mbp. We document substantial population-level variability among these organisms driven by segmental gain and loss. Interestingly, much variability lies in intergenic regions, suggesting that the Enterobacteriacae may exhibit regulatory divergence.

Conclusions

The multiple genome alignments generated by our software provide a platform for comparative genomic and population genomic studies. Free, open-source software implementing the described genome alignment approach is available from http://gel.ahabs.wisc.edu/mauve.

Gene and genome trees conflict at many levels

Horizontal gene transfer (HGT) plays a significant role in microbial evolution. It can accelerate the adaptation of an organism, it can generate new metabolic pathways and it can completely remodel an organism's genome. We examine 27 closely related genomes from the YESS group of gamma proteobacteria and a variety of four-taxon datasets from a diverse range of prokaryotes in order to explore the kinds of effects HGT has had on these organisms.

Genome sequences of Escherichia coli B strains REL606 and BL21(DE3)

Escherichia coli K-12 and B have been the subjects of classical experiments from which much of our understanding of molecular genetics has emerged. We present here complete genome sequences of two E. coli B strains, REL606, used in a long-term evolution experiment, and BL21(DE3), widely used to express recombinant proteins. The two genomes differ in length by 72,304 bp and have 426 single base pair differences, a seemingly large difference for laboratory strains having a common ancestor within the last 67 years. Transpositions by IS1 and IS150 have occurred in both lineages. Integration of the DE3 prophage in BL21(DE3) apparently displaced a defective prophage in the lambda attachment site of B. As might have been anticipated from the many genetic and biochemical experiments comparing B and K-12 over the years, the B genomes are similar in size and organization to the genome of E. coli K-12 MG1655 and have >99% sequence identity over approximately 92% of their genomes. E. coli B and K-12 differ considerably in distribution of IS elements and in location and composition of larger mobile elements. An unexpected difference is the absence of a large cluster of flagella genes in B, due to a 41 kbp IS1-mediated deletion. Gene clusters that specify the LPS core, O antigen, and restriction enzymes differ substantially, presumably because of horizontal transfer. Comparative analysis of 32 independently isolated E. coli and Shigella genomes, both commensals and pathogenic strains, identifies a minimal set of genes in common plus many strain-specific genes that constitute a large E. coli pan-genome.

Cryptic lineages of the genus Escherichia

Extended multilocus sequence typing (MLST) analysis of atypical Escherichia isolates was used to identify five novel phylogenetic clades (CI to CV) among isolates from environmental, human, and animal sources. Analysis of individual housekeeping loci showed that E. coli and its sister clade, CI, remain largely indistinguishable and represent nascent evolutionary lineages. Conversely, clades of similar age (CIII and CIV) were found to be phylogenetically distinct. When all Escherichia lineages (named and unnamed) were evaluated, we found evidence that Escherichia fergusonii has evolved at an accelerated rate compared to E. coli, CI, CIII, CIV, and CV, suggesting that this species is younger than estimated by the molecular clock method. Although the five novel clades were phylogenetically distinct, we were unable to identify a discriminating biochemical marker for all but one of them (CIII) with traditional phenotypic profiling. CIII had a statistically different phenotype from E. coli that resulted from the loss of sucrose and sorbitol fermentation and lysine utilization. The lack of phenotypic distinction has likely hindered the ability to differentiate these clades from typical E. coli, and so their ecological significance and importance for applied and clinical microbiology are yet to be determined. However, our sampling suggests that CIII, CIV, and CV represent environmentally adapted Escherichia lineages that may be more abundant outside the host gastrointestinal tract.

GO4genome: a prokaryotic phylogeny based on genome organization

Determining the phylogeny of closely related prokaryotes may fail in an analysis of rRNA or a small set of sequences. Whole-genome phylogeny utilizes the maximally available sample space. For a precise determination of genome similarity, two aspects have to be considered when developing an algorithm of whole-genome phylogeny: (1) gene order conservation is a more precise signal than gene content; and (2) when using sequence similarity, failures in identifying orthologues or the in situ replacement of genes via horizontal gene transfer may give misleading results. GO4genome is a new paradigm, which is based on a detailed analysis of gene function and the location of the respective genes. For characterization of genes, the algorithm uses gene ontology enabling a comparison of function independent of evolutionary relationship. After the identification of locally optimal series of gene functions, their length distribution is utilized to compute a phylogenetic distance. The outcome is a classification of genomes based on metabolic capabilities and their organization. Thus, the impact of effects on genome organization that are not covered by methods of molecular phylogeny can be studied. Genomes of strains belonging to Escherichia coli, Shigella, Streptococcus, Methanosarcina, and Yersinia were analyzed. Differences from the findings of classical methods are discussed.

The temporal dynamics of slightly deleterious mutations in Escherichia coli and Shigella spp

The Shigella are recently emerged clones of Escherichia coli, which have independently adopted an intracellular pathogenic lifestyle. We examined the molecular evolutionary consequences of this niche specialization by comparing the normalized, directional frequency profiles of unique polymorphisms within 2,098 orthologues representing the intersection of five E. coli and four Shigella genomes. We note a surfeit of AT-enriching changes (GC-->AT), transversions, and nonsynonymous changes in the Shigella genomes. By examining these differences within a temporal framework, we conclude that our results are consistent with relaxed or inefficient selection in Shigella owing to a reduced effective population size. Alternative interpretations, and the interesting exception of Shigella sonnei, are discussed. Finally, this analysis lends support to the view that nucleotide composition typically does not lie at mutational equilibrium but that selection plays a role in maintaining a higher GC content than would result solely from mutation bias. This argument sheds light on the enrichment of adenine and thymine in the genomes of bacterial endosymbionts where purifying selection is very weak.