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Background The increasing number of completely sequenced bacterial genomes allows comparing their architecture and genetic makeup. Such new information highlights the crucial role of lateral genetic exchanges in bacterial evolution and speciation. Results Here we analyzed the twelve sequenced genomes of Streptococcus pyogenes by a naïve approach that examines the preferential nucleotide usage along the chromosome, namely the usage of G versus C (GC-skew) and T versus A (TA-skew). The cumulative GC-skew plot presented an inverted V-shape composed of two symmetrical linear segments, where the minimum and maximum corresponded to the origin and terminus of DNA replication. In contrast, the cumulative TA-skew presented a V-shape, which segments were interrupted by several steep slopes regions (SSRs), indicative of a different nucleotide composition bias. Each S. pyogenes genome contained up to nine individual SSRs, encompassing all described strain-specific prophages. In addition, each genome contained a similar unique non-phage SSR, the core of which consisted of 31 highly homologous genes. This core includes the M-protein, other mga -related factors and other virulence genes, totaling ten intrinsic virulence genes. In addition to a high content in virulence-related genes and to a peculiar nucleotide bias, this SSR, which is 47 kb-long in a M1GAS strain, harbors direct repeats and a tRNA gene, suggesting a mobile element. Moreover, its complete absence in a M-protein negative group A Streptococcus natural isolate demonstrates that it could be spontaneously lost, but in vitro deletion experiments indicates that its excision occurred at very low rate. The stability of this SSR, combined to its presence in all sequenced S. pyogenes sequenced genome, suggests that it results from an ancient acquisition. Conclusion Thus, this non-phagic SSR is compatible with a pathogenicity island, acquired before S. pyogenes speciation. Its potential excision might bear relevance for vaccine development, because vaccines targeting M-protein might select for M-protein-negative variants that still carry other virulence determinants.

Recombineering (recombination-mediated genetic engineering) is a powerful strategy for bacterial genomic DNA and plasmid DNA modifications. CoS-MAGE improved over MAGE (multiplex automated genome engineering) by co-electroporation of an antibiotic resistance repair oligo along with the oligos for modification of the Escherichia coli chromosome. After several cycles of recombineering, the sub-population of mutants were selected among the antibiotic resistant colonies. However, a pre-generated strain with mutS deletion and multiple inactivated antibiotic resistance genes integration is required. Herein, CoS-MAGE was modified by employing a single copy BAC vector harboring a bla-mkan cassette and a Red helper vector cloned with dominant mutL E32K , thus bypassing the utilization of the pre-generated strain. The proof-of-concept of the new strategy, CoS-BAC-MAGE, was demonstrated via the mutation of non-essential genes, essential genes, and AT rich regions of the wild type strain E. coli MG1655. With this system, an editing efficiency of 60% was realized. Furthermore, by toggling between two antibiotic resistance genes (one active, the other defective) on the BAC, sequential mutations were achieved without the requirement of BAC vector elimination and re-transformation. Via CoS-BAC-MAGE, simultaneously mutations of three sites were obtained in a day. We envision that CoS-BAC-MAGE will be a practical improvement for the generation of chromosomal mutations using the Cos-MAGE approach.

Escherichia coli strains W3110 and BL21 were engineered for the production of plasmid DNA (pDNA) under aerobic and transitions to microaerobic conditions. The gene coding for recombinase A (recA) was deleted in both strains. In addition, the Vitreoscilla hemoglobin (VHb) gene (vgb) was chromosomally inserted and constitutively expressed in each E. coli recA mutant and wild type. The recA inactivation increased the supercoiled pDNA fraction (SCF) in both strains, while VHb expression improved the pDNA production in W3110, but not in BL21. Therefore, a codon-optimized version of vgb was inserted in strain BL21recA−, which, together with W3110recA−vgb+, was tested in cultures with shifts from aerobic to oxygen-limited regimes. VHb expression lowered the accumulation of fermentative by-products in both strains. VHb-expressing cells displayed higher oxidative activity as indicated by the Redox Sensor Green fluorescence, which was more intense in BL21 than in W3110. Furthermore, VHb expression did not change pDNA production in W3110, but decreased it in BL21. These results are useful for understanding the physiological effects of VHb expression in two industrially relevant E. coli strains, and for the selection of a host for pDNA production.

Sinorhizobium meliloti is an α-proteobacterium that forms agronomically important N2-fixing root nodules in legumes. We report here the complete sequence of the largest constituent of its genome, a 62.7% GC-rich 3,654,135-bp circular chromosome. Annotation allowed assignment of a function to 59% of the 3,341 predicted protein-coding ORFs, the rest exhibiting partial, weak, or no similarity with any known sequence. Unexpectedly, the level of reiteration within this replicon is low, with only two genes duplicated with more than 90% nucleotide sequence identity, transposon elements accounting for 2.2% of the sequence, and a few hundred short repeated palindromic motifs (RIME1, RIME2, and C) widespread over the chromosome. Three regions with a significantly lower GC content are most likely of external origin. Detailed annotation revealed that this replicon contains all housekeeping genes except two essential genes that are located on pSymB. Amino acid/peptide transport and degradation and sugar metabolism appear as two major features of the S. meliloti chromosome. The presence in this replicon of a large number of nucleotide cyclases with a peculiar structure, as well as of genes homologous to virulence determinants of animal and plant pathogens, opens perspectives in the study of this bacterium both as a free-living soil microorganism and as a plant symbiont.

Chromosomes must be highly compacted and organized within cells, but how this is achieved in vivo remains poorly understood. We report the use of chromosome conformation capture coupled with deep sequencing (Hi-C) to map the structure of bacterial chromosomes. Analysis of Hi-C data and polymer modeling indicates that the Caulobacter crescentus chromosome consists of multiple, largely independent spatial domains that are probably composed of supercoiled plectonemes arrayed into a bottle brush-like fiber. These domains are stable throughout the cell cycle and are reestablished concomitantly with DNA replication. We provide evidence that domain boundaries are established by highly expressed genes and the formation of plectoneme-free regions, whereas the histone-like protein HU and SMC (structural maintenance of chromosomes) promote short-range compaction and the colinearity of chromosomal arms, respectively. Collectively, our results reveal general principles for the organization and structure of chromosomes in vivo.

Molecular and genomic surveillance systems for bacterial pathogens currently rely on tracking clonally evolving lineages. By contrast, plasmids are usually excluded or analyzed with low-resolution techniques, despite being the primary vectors of antibiotic resistance genes across many key pathogens. Here, we used a combination of long- and short-read sequence data of Klebsiella pneumoniae isolates (n = 1,717) from a European survey to perform an integrated, continent-wide study of chromosomal and plasmid diversity. This revealed three contrasting modes of dissemination used by carbapenemase genes, which confer resistance to last-line carbapenems. First, bla OXA-48-like genes have spread primarily via the single epidemic pOXA-48–like plasmid, which emerged recently in clinical settings and spread rapidly to numerous lineages. Second, bla VIM and bla NDM genes have spread via transient associations of many diverse plasmids with numerous lineages. Third, bla KPC genes have transmitted predominantly by stable association with one successful clonal lineage (ST258/512) yet have been mobilized among diverse plasmids within this lineage. We show that these plasmids, which include pKpQIL-like and IncX3 plasmids, have a long association (and are coevolving) with the lineage, although frequent recombination and rearrangement events between them have led to a complex array of mosaic plasmids carrying bla KPC. Taken altogether, these results reveal the diverse trajectories of antibiotic resistance genes in clinical settings, summarized as using one plasmid/multiple lineages, multiple plasmids/multiple lineages, and multiple plasmids/one lineage. Our study provides a framework for the much needed incorporation of plasmid data into genomic surveillance systems, an essential step toward a more comprehensive understanding of resistance spread.

Bacteriophages are the main vehicle for gene swapping in bacteria, notoriously of pathogenicity islands and antibiotic resistance genes. Chen et al. noticed that the Staphylococcus aureus prophages do not excise from their host's genome until very late in their life cycles (see the Perspective by Davidson). Thus, the phage DNA is amplified while embedded in the bacterial chromosome. The resulting concatemers are processively packed into virus capsules while still integrated in the host chromosome. Each virion is only set loose when the capsule has reached physical capacity—a process called “headful” packaging. In situ amplification maximizes viral replication, and the headful mechanism means adjacent bacterial-host DNA also gets grabbed to fill the capsule. This process ensures that host genes are transmitted along with the phage. Science , this issue p. 207 ; see also p. 152 Staphylococcus aureus phages amplify and package while chromosomally integrated such that host DNA becomes incorporated in the virus particle. Genetic transduction is a major evolutionary force that underlies bacterial adaptation. Here we report that the temperate bacteriophages of Staphylococcus aureus engage in a distinct form of transduction we term lateral transduction. Staphylococcal prophages do not follow the previously described excision-replication-packaging pathway but instead excise late in their lytic program. Here, DNA packaging initiates in situ from integrated prophages, and large metameric spans including several hundred kilobases of the S. aureus genome are packaged in phage heads at very high frequency. In situ replication before DNA packaging creates multiple prophage genomes so that lateral-transducing particles form during normal phage maturation, transforming parts of the S. aureus chromosome into hypermobile regions of gene transfer.

Plasmids play an important role in bacterial genome evolution by transferring genes between lineages. Fitness costs associated with plasmid carriage are expected to be a barrier to gene exchange, but the causes of plasmid fitness costs are poorly understood. Single compensatory mutations are often sufficient to completely ameliorate plasmid fitness costs, suggesting that such costs are caused by specific genetic conflicts rather than generic properties of plasmids, such as their size, metabolic burden, or gene expression level. By combining the results of experimental evolution with genetics and transcriptomics, we show here that fitness costs of 2 divergent large plasmids in Pseudomonas fluorescens are caused by inducing maladaptive expression of a chromosomal tailocin toxin operon. Mutations in single genes unrelated to the toxin operon, and located on either the chromosome or the plasmid, ameliorated the disruption associated with plasmid carriage. We identify one of these compensatory loci, the chromosomal gene PFLU4242 , as the key mediator of the fitness costs of both plasmids, with the other compensatory loci either reducing expression of this gene or mitigating its deleterious effects by up-regulating a putative plasmid-borne ParAB operon. The chromosomal mobile genetic element Tn6291, which uses plasmids for transmission, remained up-regulated even in compensated strains, suggesting that mobile genetic elements communicate through pathways independent of general physiological disruption. Plasmid fitness costs caused by specific genetic conflicts are unlikely to act as a long-term barrier to horizontal gene transfer (HGT) due to their propensity for amelioration by single compensatory mutations, helping to explain why plasmids are so common in bacterial genomes.

Key Points This Review discusses natural bacterial transformation, highlighting the common and divergent features that exist in a phylogenetically diverse range of naturally transformable species. Transformation is defined as the uptake of foreign DNA as single strands and its subsequent integration into the bacterial chromosome by homologous recombination. The mechanisms of uptake and integration, which are largely conserved among species, are highlighted and their conservation is explored. In contrast to DNA-uptake mechanisms, the regulation of the ability to transform (which is known as competence) and the signals that induce competence vary widely between species; the range of mechanisms that are involved are discussed. The roles of competence and imported DNA are also considered, and we argue that evidence so far generally points towards a role for transformation in the generation of genetic diversity or in chromosomal repair, rather than a nutritional role. Finally, we explore the future prospects in this field of research, detailing several case studies of species that have recently been shown to be transformable and the potential difficulties in demonstrating transformability in a new species. In this Review, Claverys and colleagues describe the divergent and common principles that govern the transformation process in phylogenetically distinct bacteria and discuss the potential role of imported DNA in generating genetic diversity. They also discuss how this information can be used for the prediction of new transformable species. Natural bacterial transformation involves the internalization and chromosomal integration of DNA and has now been documented in ∼80 species. Recent advances have established that phylogenetically distant species share conserved uptake and processing proteins but differ in the inducing cues and regulatory mechanisms that are involved. In this Review, we highlight divergent and common principles that govern the transformation process in different bacteria. We discuss how this cumulative knowledge enables the prediction of new transformable species and supports the idea that the main role of internalized DNA is in the generation of genetic diversity or in chromosome repair rather than in nutrition.

Once episomal, the hybrid molecule replicates like a normal viral genome would, and if it has retained a cos site, the concatameric hybrid DNA can be packaged into phage heads by the cos-type packaging machinery and transferred to new host cells (Fig 2). Because aberrant excision is rare, and the bacterial DNA that can be transferred is restricted, specialized transduction is believed to make only a small contribution to all phage-mediated gene transfer. [...]to offset the predictably catastrophic effects (splitting the viral genome in two) of in situ DNA packaging, in situ theta replication creates multiple integrated genomes so that both in situ DNA packaging and phage maturation can proceed in parallel. SaPI-encoded small terminases then form hetero-oligomers with phage large terminases to form new terminase enzymes that recognize SaPI pac sites (instead of phage pac sites), enabling SaPIs to hijack the phage packaging machinery to encapsidate their own genomes into infective phage-derived particles that are transferred at extremely high frequencies, both intra- and intergenerically (Fig 3) [21, 22]. [...]SaPI-like elements appear to be widespread, as phage-inducible chromosomal islands (PICIs) have now been discovered in both gram-positive and gram-negative bacteria [23, 24]. [...]an important distinction is that although phage pseudo-pac sites appear to be located in random positions, SaPI pseudo-pac sites are often linked to and direct the unidirectional packaging of S. aureus genes that are associated with virulence and disease. [...]although they are relatively small pathogenicity islands (generally 14–16 kb), island-mediated generalized transduction links the SaPIs to a much broader repertoire of virulence determinants than they can carry themselves.