Annotation of plasmid genes.

Good annotation of plasmid genomes is essential to maximise the value of the rapidly increasing volume of plasmid sequences. This short review highlights some of the current issues and suggests some ways forward. Where a well-studied related plasmid system exists we recommend that new annotation adheres to the convention already established for that system, so long as it is based on sound principles and solid experimental evidence, even if some of the new genes are more similar to homologues in different systems. Where a well-established model does not exist we provide generic gene names that reflect likely biochemical activity rather than overall purpose particularly, for example, where genes clearly belong to a type IV secretion system but it is not known whether they function in conjugative transfer or virulence. We also recommend that annotators use a whole system naming approach to avoid ending up with an illogical mixture of names from other systems based on the highest scoring match from a BLAST search. In addition, where function has not been experimentally established we recommend using just the locus tag, rather than a function-related gene name, while recording possible functions as notes rather than in a provisional name.

Value, but high costs in post-deposition data curation.

Discoverability of sequence data in primary data archives is proportional to the richness of contextual information associated with the data. Here, we describe an exercise in the improvement of contextual information surrounding sample records associated with metagenomics sequence reads available in the European Nucleotide Archive. We outline the annotation process and summarize findings of this effort aimed at increasing usability of publicly available environmental data. Furthermore, we emphasize the benefits of such an exercise and detail its costs. We conclude that such a third party annotation approach is expensive and has value as an element of curation, but should form only part of a more sustainable submitter-driven approach. Database URL: http://www.ebi.ac.uk/ena.

Complete Genome Sequences of Two Citrobacter rodentium Bacteriophages, CR8 and CR44b.

The complete genomes of two virulent phages infecting Citrobacter rodentium are reported here for the first time. Both bacteriophages were isolated from local sewage treatment plant effluents. Genome analyses revealed a close relationship between both phages and allowed their classification as members of the Autographivirinae subfamily in the T7-like genus.

A small predatory core genome in the divergent marine Bacteriovorax marinus SJ and the terrestrial Bdellovibrio bacteriovorus.

Bacteriovorax marinus SJ is a predatory delta-proteobacterium isolated from a marine environment. The genome sequence of this strain provides an interesting contrast to that of the terrestrial predatory bacterium Bdellovibrio bacteriovorus HD100. Based on their predatory lifestyle, Bacteriovorax were originally designated as members of the genus Bdellovibrio but subsequently were re-assigned to a new genus and family based on genetic and phenotypic differences. B. marinus attaches to gram-negative bacteria, penetrates through the cell wall to form a bdelloplast, in which it replicates, as shown using microscopy. Bacteriovorax is distinct, as it shares only 30% of its gene products with its closest sequenced relatives. Remarkably, 34% of predicted genes over 500 nt in length were completely unique with no significant matches in the databases. As expected, Bacteriovorax shares several characteristic loci with the other delta-proteobacteria. A geneset shared between Bacteriovorax and Bdellovibrio that is not conserved among other delta-proteobacteria such as Myxobacteria (which destroy prey bacteria externally via lysis), or the non-predatory Desulfo-bacteria and Geobacter species was identified. These 291 gene orthologues common to both Bacteriovorax and Bdellovibrio may be the key indicators of host-interaction predatory-specific processes required for prey entry. The locus from Bdellovibrio bacteriovorus is implicated in the switch from predatory to prey/host-independent growth. Although the locus is conserved in B. marinus, the sequence has only limited similarity. The results of this study advance understanding of both the similarities and differences between Bdellovibrio and Bacteriovorax and confirm the distant relationship between the two and their separation into different families.

Twenty-eight divergent polysaccharide loci specifying within- and amongst-strain capsule diversity in three strains of Bacteroides fragilis.

Comparison of the complete genome sequence of Bacteroides fragilis 638R, originally isolated in the USA, was made with two previously sequenced strains isolated in the UK (NCTC 9343) and Japan (YCH46). The presence of 10 loci containing genes associated with polysaccharide (PS) biosynthesis, each including a putative Wzx flippase and Wzy polymerase, was confirmed in all three strains, despite a lack of cross-reactivity between NCTC 9343 and 638R surface PS-specific antibodies by immunolabelling and microscopy. Genomic comparisons revealed an exceptional level of PS biosynthesis locus diversity. Of the 10 divergent PS-associated loci apparent in each strain, none is similar between NCTC 9343 and 638R. YCH46 shares one locus with NCTC 9343, confirmed by mAb labelling, and a second different locus with 638R, making a total of 28 divergent PS biosynthesis loci amongst the three strains. The lack of expression of the phase-variable large capsule (LC) in strain 638R, observed in NCTC 9343, is likely to be due to a point mutation that generates a stop codon within a putative initiating glycosyltransferase, necessary for the expression of the LC in NCTC 9343. Other major sequence differences were observed to arise from different numbers and variety of inserted extra-chromosomal elements, in particular prophages. Extensive horizontal gene transfer has occurred within these strains, despite the presence of a significant number of divergent DNA restriction and modification systems that act to prevent acquisition of foreign DNA. The level of amongst-strain diversity in PS biosynthesis loci is unprecedented.

The Citrobacter rodentium genome sequence reveals convergent evolution with human pathogenic Escherichia coli.

Citrobacter rodentium (formally Citrobacter freundii biotype 4280) is a highly infectious pathogen that causes colitis and transmissible colonic hyperplasia in mice. In common with enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC, respectively), C. rodentium exploits a type III secretion system (T3SS) to induce attaching and effacing (A/E) lesions that are essential for virulence. Here, we report the fully annotated genome sequence of the 5.3-Mb chromosome and four plasmids harbored by C. rodentium strain ICC168. The genome sequence revealed key information about the phylogeny of C. rodentium and identified 1,585 C. rodentium-specific (without orthologues in EPEC or EHEC) coding sequences, 10 prophage-like regions, and 17 genomic islands, including the locus for enterocyte effacement (LEE) region, which encodes a T3SS and effector proteins. Among the 29 T3SS effectors found in C. rodentium are all 22 of the core effectors of EPEC strain E2348/69. In addition, we identified a novel C. rodentium effector, named EspS. C. rodentium harbors two type VI secretion systems (T6SS) (CTS1 and CTS2), while EHEC contains only one T6SS (EHS). Our analysis suggests that C. rodentium and EPEC/EHEC have converged on a common host infection strategy through access to a common pool of mobile DNA and that C. rodentium has lost gene functions associated with a previous pathogenic niche.

Genomic and genetic analyses of diversity and plant interactions of Pseudomonas fluorescens.

BACKGROUND: Pseudomonas fluorescens are common soil bacteria that can improve plant health through nutrient cycling, pathogen antagonism and induction of plant defenses. The genome sequences of strains SBW25 and Pf0-1 were determined and compared to each other and with P. fluorescens Pf-5. A functional genomic in vivo expression technology (IVET) screen provided insight into genes used by P. fluorescens in its natural environment and an improved understanding of the ecological significance of diversity within this species. RESULTS: Comparisons of three P. fluorescens genomes (SBW25, Pf0-1, Pf-5) revealed considerable divergence: 61% of genes are shared, the majority located near the replication origin. Phylogenetic and average amino acid identity analyses showed a low overall relationship. A functional screen of SBW25 defined 125 plant-induced genes including a range of functions specific to the plant environment. Orthologues of 83 of these exist in Pf0-1 and Pf-5, with 73 shared by both strains. The P. fluorescens genomes carry numerous complex repetitive DNA sequences, some resembling Miniature Inverted-repeat Transposable Elements (MITEs). In SBW25, repeat density and distribution revealed 'repeat deserts' lacking repeats, covering approximately 40% of the genome. CONCLUSIONS: P. fluorescens genomes are highly diverse. Strain-specific regions around the replication terminus suggest genome compartmentalization. The genomic heterogeneity among the three strains is reminiscent of a species complex rather than a single species. That 42% of plant-inducible genes were not shared by all strains reinforces this conclusion and shows that ecological success requires specialized and core functions. The diversity also indicates the significant size of genetic information within the Pseudomonas pan genome.

Genome watch: What a scorcher!

This month's Genome Watch looks at the publication of four hyperthermophilic archaeal genomes, three of which belong to the Crenarchaeota phylum and one of which belongs to the newly defined Nanoarchaeota phylum.

The genome of Burkholderia cenocepacia J2315, an epidemic pathogen of cystic fibrosis patients.

Bacterial infections of the lungs of cystic fibrosis (CF) patients cause major complications in the treatment of this common genetic disease. Burkholderia cenocepacia infection is particularly problematic since this organism has high levels of antibiotic resistance, making it difficult to eradicate; the resulting chronic infections are associated with severe declines in lung function and increased mortality rates. B. cenocepacia strain J2315 was isolated from a CF patient and is a member of the epidemic ET12 lineage that originated in Canada or the United Kingdom and spread to Europe. The 8.06-Mb genome of this highly transmissible pathogen comprises three circular chromosomes and a plasmid and encodes a broad array of functions typical of this metabolically versatile genus, as well as numerous virulence and drug resistance functions. Although B. cenocepacia strains can be isolated from soil and can be pathogenic to both plants and man, J2315 is representative of a lineage of B. cenocepacia rarely isolated from the environment and which spreads between CF patients. Comparative analysis revealed that ca. 21% of the genome is unique in comparison to other strains of B. cenocepacia, highlighting the genomic plasticity of this species. Pseudogenes in virulence determinants suggest that the pathogenic response of J2315 may have been recently selected to promote persistence in the CF lung. The J2315 genome contains evidence that its unique and highly adapted genetic content has played a significant role in its success as an epidemic CF pathogen.

In silico identification of genes in bacteriophage DNA.

One of the most satisfying aspects of a genome sequencing project is the identification of the genes contained within it.These are of two types: those which encode tRNAs and those which produce proteins. After a general introduction on the properties of protein-encoding genes and the utility of the Basic Local Alignment Search Tool (BLASTX) to identify genes through homologs, a variety of tools are discussed by their creators. These include for genome annotation: GeneMark, Artemis, and BASys; and, for genome comparisons: Artemis Comparison Tool (ACT), Mauve, CoreGenes, and GeneOrder.

Genome sequence of a proteolytic (Group I) Clostridium botulinum strain Hall A and comparative analysis of the clostridial genomes.

Clostridium botulinum is a heterogeneous Gram-positive species that comprises four genetically and physiologically distinct groups of bacteria that share the ability to produce botulinum neurotoxin, the most poisonous toxin known to man, and the causative agent of botulism, a severe disease of humans and animals. We report here the complete genome sequence of a representative of Group I (proteolytic) C. botulinum (strain Hall A, ATCC 3502). The genome consists of a chromosome (3,886,916 bp) and a plasmid (16,344 bp), which carry 3650 and 19 predicted genes, respectively. Consistent with the proteolytic phenotype of this strain, the genome harbors a large number of genes encoding secreted proteases and enzymes involved in uptake and metabolism of amino acids. The genome also reveals a hitherto unknown ability of C. botulinum to degrade chitin. There is a significant lack of recently acquired DNA, indicating a stable genomic content, in strong contrast to the fluid genome of Clostridium difficile, which can form longer-term relationships with its host. Overall, the genome indicates that C. botulinum is adapted to a saprophytic lifestyle both in soil and aquatic environments. This pathogen relies on its toxin to rapidly kill a wide range of prey species, and to gain access to nutrient sources, it releases a large number of extracellular enzymes to soften and destroy rotting or decayed tissues.

The multidrug-resistant human pathogen Clostridium difficile has a highly mobile, mosaic genome.

We determined the complete genome sequence of Clostridium difficile strain 630, a virulent and multidrug-resistant strain. Our analysis indicates that a large proportion (11%) of the genome consists of mobile genetic elements, mainly in the form of conjugative transposons. These mobile elements are putatively responsible for the acquisition by C. difficile of an extensive array of genes involved in antimicrobial resistance, virulence, host interaction and the production of surface structures. The metabolic capabilities encoded in the genome show multiple adaptations for survival and growth within the gut environment. The extreme genome variability was confirmed by whole-genome microarray analysis; it may reflect the organism's niche in the gut and should provide information on the evolution of virulence in this organism.

Multireplicon genome architecture of Lactobacillus salivarius.

Lactobacillus salivarius subsp. salivarius strain UCC118 is a bacteriocin-producing strain with probiotic characteristics. The 2.13-Mb genome was shown by sequencing to comprise a 1.83 Mb chromosome, a 242-kb megaplasmid (pMP118), and two smaller plasmids. Megaplasmids previously have not been characterized in lactic acid bacteria or intestinal lactobacilli. Annotation of the genome sequence indicated an intermediate level of auxotrophy compared with other sequenced lactobacilli. No single-copy essential genes were located on the megaplasmid. However, contingency amino acid metabolism genes and carbohydrate utilization genes, including two genes for completion of the pentose phosphate pathway, were megaplasmid encoded. The megaplasmid also harbored genes for the Abp118 bacteriocin, a bile salt hydrolase, a presumptive conjugation locus, and other genes potentially relevant for probiotic properties. Two subspecies of L. salivarius are recognized, salivarius and salicinius, and we detected megaplasmids in both subspecies by pulsed-field gel electrophoresis of sizes ranging from 100 kb to 380 kb. The discovery of megaplasmids of widely varying size in L. salivarius suggests a possible mechanism for genome expansion or contraction to adapt to different environments.

The Chlamydophila abortus genome sequence reveals an array of variable proteins that contribute to interspecies variation.

The obligate intracellular bacterial pathogen Chlamydophila abortus strain S26/3 (formerly the abortion subtype of Chlamydia psittaci) is an important cause of late gestation abortions in ruminants and pigs. Furthermore, although relatively rare, zoonotic infection can result in acute illness and miscarriage in pregnant women. The complete genome sequence was determined and shows a high level of conservation in both sequence and overall gene content in comparison to other Chlamydiaceae. The 1,144,377-bp genome contains 961 predicted coding sequences, 842 of which are conserved with those of Chlamydophila caviae and Chlamydophila pneumoniae. Within this conserved Cp. abortus core genome we have identified the major regions of variation and have focused our analysis on these loci, several of which were found to encode highly variable protein families, such as TMH/Inc and Pmp families, which are strong candidates for the source of diversity in host tropism and disease causation in this group of organisms. Significantly, Cp. abortus lacks any toxin genes, and also lacks genes involved in tryptophan metabolism and nucleotide salvaging (guaB is present as a pseudogene), suggesting that the genetic basis of niche adaptation of this species is distinct from those previously proposed for other chlamydial species.

Extensive DNA inversions in the B. fragilis genome control variable gene expression.

The obligately anaerobic bacterium Bacteroides fragilis, an opportunistic pathogen and inhabitant of the normal human colonic microbiota, exhibits considerable within-strain phase and antigenic variation of surface components. The complete genome sequence has revealed an unusual breadth (in number and in effect) of DNA inversion events that potentially control expression of many different components, including surface and secreted components, regulatory molecules, and restriction-modification proteins. Invertible promoters of two different types (12 group 1 and 11 group 2) were identified. One group has inversion crossover (fix) sites similar to the hix sites of Salmonella typhimurium. There are also four independent intergenic shufflons that potentially alter the expression and function of varied genes. The composition of the 10 different polysaccharide biosynthesis gene clusters identified (7 with associated invertible promoters) suggests a mechanism of synthesis similar to the O-antigen capsules of Escherichia coli.

Transferable antibiotic resistance elements in Haemophilus influenzae share a common evolutionary origin with a diverse family of syntenic genomic islands.

Transferable antibiotic resistance in Haemophilus influenzae was first detected in the early 1970s. After this, resistance spread rapidly worldwide and was shown to be transferred by a large 40- to 60-kb conjugative element. Bioinformatics analysis of the complete sequence of a typical H. influenzae conjugative resistance element, ICEHin1056, revealed the shared evolutionary origin of this element. ICEHin1056 has homology to 20 contiguous sequences in the National Center for Biotechnology Information database. Systematic comparison of these homologous sequences resulted in identification of a conserved syntenic genomic island consisting of up to 33 core genes in 16 beta- and gamma-Proteobacteria. These diverse genomic islands shared a common evolutionary origin, insert into tRNA genes, and have diverged widely, with G+C contents ranging from 40 to 70% and amino acid homologies as low as 20 to 25% for shared core genes. These core genes are likely to account for the conjugative transfer of the genomic islands and may even encode autonomous replication. Accessory gene clusters were nestled among the core genes and encode the following diverse major attributes: antibiotic, metal, and antiseptic resistance; degradation of chemicals; type IV secretion systems; two-component signaling systems; Vi antigen capsule synthesis; toxin production; and a wide range of metabolic functions. These related genomic islands include the following well-characterized structures: SPI-7, found in Salmonella enterica serovar Typhi; PAP1 or pKLC102, found in Pseudomonas aeruginosa; and the clc element, found in Pseudomonas sp. strain B13. This is the first report of a diverse family of related syntenic genomic islands with a deep evolutionary origin, and our findings challenge the view that genomic islands consist only of independently evolving modules.

Genomic plasticity of the causative agent of melioidosis, Burkholderia pseudomallei.

Burkholderia pseudomallei is a recognized biothreat agent and the causative agent of melioidosis. This Gram-negative bacterium exists as a soil saprophyte in melioidosis-endemic areas of the world and accounts for 20% of community-acquired septicaemias in northeastern Thailand where half of those affected die. Here we report the complete genome of B. pseudomallei, which is composed of two chromosomes of 4.07 megabase pairs and 3.17 megabase pairs, showing significant functional partitioning of genes between them. The large chromosome encodes many of the core functions associated with central metabolism and cell growth, whereas the small chromosome carries more accessory functions associated with adaptation and survival in different niches. Genomic comparisons with closely and more distantly related bacteria revealed a greater level of gene order conservation and a greater number of orthologous genes on the large chromosome, suggesting that the two replicons have distinct evolutionary origins. A striking feature of the genome was the presence of 16 genomic islands (GIs) that together made up 6.1% of the genome. Further analysis revealed these islands to be variably present in a collection of invasive and soil isolates but entirely absent from the clonally related organism B. mallei. We propose that variable horizontal gene acquisition by B. pseudomallei is an important feature of recent genetic evolution and that this has resulted in a genetically diverse pathogenic species.

Comparative cell wall core biosynthesis in the mycolated pathogens, Mycobacterium tuberculosis and Corynebacterium diphtheriae.

The recent determination of the complete genome sequence of Corynebacterium diphtheriae, the aetiological agent of diphtheria, has allowed a detailed comparison of its physiology with that of its closest sequenced pathogenic relative Mycobacterium tuberculosis. Of major importance to the pathogenicity and resilience of the latter is its particularly complex cell envelope. The corynebacteria share many of the features of this extraordinary structure although to a lesser level of complexity. The cell envelope of M. tuberculosis has provided the molecular targets for several of the major anti-tubercular drugs. Given a backdrop of emerging multi-drug resistant strains of the organism (MDR-TB) and its continuing global threat to human health, the search for novel anti-tubercular agents is of paramount importance. The unique structure of this cell wall and the importance of its integrity to the viability of the organism suggest that the search for novel drug targets within the array of enzymes responsible for its construction may prove fruitful. Although the application of modern bioinformatics techniques to the 'mining' of the M. tuberculosis genome has already increased our knowledge of the biosynthesis and assembly of the mycobacterial cell wall, several issues remain uncertain. Further analysis by comparison with its relatives may bring clarity and aid the early identification of novel cellular targets for new anti-tuberculosis drugs. In order to facilitate this aim, this review intends to illustrate the broad similarities and highlight the structural differences between the two bacterial envelopes and discuss the genetics of their biosynthesis.

Shrinking genomics.

Two bacteria are featured this month, and both are at the lower end of the genome size scale. The first, Mycoplasma gallisepticum, belongs to a group of bacteria that have been studied both as important human and animal pathogens and in the pursuit of understanding the essential functions of a self-replicating minimal cell. The second, Nanoarchaeum equitans, is an obligate symbiont that only grows in co-culture with another archaeon. N. equitans seems to be the coelacanth of the microbial world--it has been assigned to a new phylum and represents a primitive form of prokaroytic life.