Phage production is blocked in the adherent-invasive Escherichia coli LF82 upon macrophage infection

Adherent-invasive Escherichia coli (AIEC) strains are frequently recovered from stools of patients with dysbiotic microbiota. They have remarkable properties of adherence to the intestinal epithelium, and survive better than other E. coli in macrophages. The best studied of these AIEC is probably strain LF82, which was isolated from a Crohn's disease patient. This strain contains five complete prophages, which have not been studied until now. We undertook their analysis, both in vitro and inside macrophages, and show that all of them form virions. The Gally prophage is by far the most active, generating spontaneously over 108 viral particles per mL of culture supernatants in vitro, more than 100-fold higher than the other phages. Gally is also over-induced after a genotoxic stress generated by ciprofloxacin and trimethoprim. However, upon macrophage infection, a genotoxic environment, this over-induction is not observed. Analysis of the transcriptome and key steps of its lytic cycle in macrophages suggests that the excision of the Gally prophage continues to be repressed in macrophages. We conclude that strain LF82 has evolved an efficient way to block the lytic cycle of its most active prophage upon macrophage infection, which may participate to its good survival in macrophages.

The Crohn's disease-related bacterial strain LF82 assembles biofilm-like communities to protect itself from phagolysosomal attack

Patients with Crohn's disease exhibit abnormal colonization of the intestine by adherent invasive E. coli (AIEC). They adhere to epithelial cells, colonize them and survive inside macrophages. It appeared recently that AIEC LF82 adaptation to phagolysosomal stress involves a long lag phase in which many LF82 cells become antibiotic tolerant. Later during infection, they proliferate in vacuoles and form colonies harboring dozens of LF82 bacteria. In the present work, we investigated the mechanism sustaining this phase of growth. We found that intracellular LF82 produced an extrabacterial matrix that acts as a biofilm and controls the formation of LF82 intracellular bacterial communities (IBCs) for several days post infection. We revealed the crucial role played by the pathogenicity island encoding the yersiniabactin iron capture system to form IBCs and for optimal LF82 survival. These results illustrate that AIECs use original strategies to establish their replicative niche within macrophages.

Imaging the Cell Cycle of Pathogen E. coli During Growth in Macrophage

The study of the bacterial cell cycle at the single cell level can not only give insights on the fitness of the bacterial population but also reveal heterogeneous behavior. Typically, the DNA replication, the cell division, and the nucleoid conformation are appropriate representatives of the bacterial cell cycle. Because bacteria rapidly adapt their growth rate to environmental changes, the measure of cell cycle parameters gives valuable insights for the study of bacterial stress response or host-pathogen interactions. Here we describe methods to first introduce fluorescent fusion proteins and fluorescent tag within the chromosome of pathogenic bacteria to study these cell cycle steps; then to follow them within macrophages using a confocal spinning disk microscope.

Differential management of the replication terminus regions of the two Vibrio cholerae chromosomes during cell division

The replication terminus region (Ter) of the unique chromosome of most bacteria locates at mid-cell at the time of cell division. In several species, this localization participates in the necessary coordination between chromosome segregation and cell division, notably for the selection of the division site, the licensing of the division machinery assembly and the correct alignment of chromosome dimer resolution sites. The genome of Vibrio cholerae, the agent of the deadly human disease cholera, is divided into two chromosomes, chrI and chrII. Previous fluorescent microscopy observations suggested that although the Ter regions of chrI and chrII replicate at the same time, chrII sister termini separated before cell division whereas chrI sister termini were maintained together at mid-cell, which raised questions on the management of the two chromosomes during cell division. Here, we simultaneously visualized the location of the dimer resolution locus of each of the two chromosomes. Our results confirm the late and early separation of chrI and chrII Ter sisters, respectively. They further suggest that the MatP/matS macrodomain organization system specifically delays chrI Ter sister separation. However, TerI loci remain in the vicinity of the cell centre in the absence of MatP and a genetic assay specifically designed to monitor the relative frequency of sister chromatid contacts during constriction suggest that they keep colliding together until the very end of cell division. In contrast, we found that even though it is not able to impede the separation of chrII Ter sisters before septation, the MatP/matS macrodomain organization system restricts their movement within the cell and permits their frequent interaction during septum constriction.

The genome of Vibrio cholerae is divided into two circular chromosomes, chrI and chrII. ChrII is derived from a horizontally acquired mega-plasmid, which raised questions on the necessary coordination of the processes that ensure its segregation with the cell division cycle. Here, we show that the MatP/matS macrodomain organization system impedes the separation of sister copies of the terminus region of chrI before the initiation of septum constriction. In its absence, however, chrI sister termini remain sufficiently close to mid-cell to be processed by the FtsK cell division translocase. In contrast, we show that MatP cannot impede the separation of chrII sister termini before constriction. However, it restricts their movements within the cell, which allows for their processing by FtsK at the time of cell division. These results suggest that multiple redundant factors, including MatP in the enterobacteriaceae and the Vibrios, ensure that sister copies of the terminus region of bacterial chromosomes remain sufficiently close to mid-cell to be processed by FtsK.

The two Cis-acting sites, parS1 and oriC1, contribute to the longitudinal organisation of Vibrio cholerae chromosome I

The segregation of bacterial chromosomes follows a precise choreography of spatial organisation. It is initiated by the bipolar migration of the sister copies of the replication origin (ori). Most bacterial chromosomes contain a partition system (Par) with parS sites in close proximity to ori that contribute to the active mobilisation of the ori region towards the old pole. This is thought to result in a longitudinal chromosomal arrangement within the cell. In this study, we followed the duplication frequency and the cellular position of 19 Vibrio cholerae genome loci as a function of cell length. The genome of V. cholerae is divided between two chromosomes, chromosome I and II, which both contain a Par system. The ori region of chromosome I (ori_I) is tethered to the old pole, whereas the ori region of chromosome II is found at midcell. Nevertheless, we found that both chromosomes adopted a longitudinal organisation. Chromosome I extended over the entire cell while chromosome II extended over the younger cell half. We further demonstrate that displacing parS sites away from the ori_I region rotates the bulk of chromosome I. The only exception was the region where replication terminates, which still localised to the septum. However, the longitudinal arrangement of chromosome I persisted in Par mutants and, as was reported earlier, the ori region still localised towards the old pole. Finally, we show that the Par-independent longitudinal organisation and ori_I polarity were perturbed by the introduction of a second origin. Taken together, these results suggest that the Par system is the major contributor to the longitudinal organisation of chromosome I but that the replication program also influences the arrangement of bacterial chromosomes.

The FtsK Family of DNA Pumps

Interest for proteins of the FtsK family initially arose from their implication in many primordial processes in which DNA needs to be transported from one cell compartment to another in eubacteria. In the first section of this chapter, we address a list of the cellular functions of the different members of the FtsK family that have been so far studied. Soon after their discovery, interest for the FstK proteins spread because of their unique biochemical properties: most DNA transport systems rely on the assembly of complex multicomponent machines. In contrast, six FtsK proteins are sufficient to assemble into a fast and powerful DNA pump; the pump transports closed circular double stranded DNA molecules without any covalent-bond breakage nor topological alteration; transport is oriented despite the intrinsic symmetrical nature of the double stranded DNA helix and can occur across cell membranes. The different activities required for the oriented transport of DNA across cell compartments are achieved by three separate modules within the FtsK proteins: a DNA translocation module, an orientation module and an anchoring module. In the second part of this chapter, we review the structural and biochemical properties of these different modules.

Replication regulation of Vibrio cholerae chromosome II involves initiator binding to the origin both as monomer and as dimer

The origin region of Vibrio cholerae chromosome II (chrII) resembles plasmid origins that have repeated initiator-binding sites (iterons). Iterons are essential for initiation as well as preventing over-initiation of plasmid replication. In chrII, iterons are also essential for initiation but over-initiation is prevented by sites called 39-mers. Both iterons and 39-mers are binding sites of the chrII specific initiator, RctB. Here, we have isolated RctB mutants that permit over-initiation in the presence of 39-mers. Characterization of two of the mutants showed that both are defective in 39-mer binding, which helps to explain their over-initiation phenotype. In vitro, RctB bound to 39-mers as monomers, and to iterons as both monomers and dimers. Monomer binding to iterons increased in both the mutants, suggesting that monomers are likely to be the initiators. We suggest that dimers might be competitive inhibitors of monomer binding to iterons and thus help control replication negatively. ChrII replication was found to be dependent on chaperones DnaJ and DnaK in vivo. The chaperones preferentially improved dimer binding in vitro, further suggesting the importance of dimer binding in the control of chrII replication.

DNA adenine methylation is required to replicate both Vibrio cholerae chromosomes once per cell cycle

DNA adenine methylation is widely used to control many DNA transactions, including replication. In Escherichia coli, methylation serves to silence newly synthesized (hemimethylated) sister origins. SeqA, a protein that binds to hemimethylated DNA, mediates the silencing, and this is necessary to restrict replication to once per cell cycle. The methylation, however, is not essential for replication initiation per se but appeared so when the origins (oriI and oriII) of the two Vibrio cholerae chromosomes were used to drive plasmid replication in E. coli. Here we show that, as in the case of E. coli, methylation is not essential for oriI when it drives chromosomal replication and is needed for once-per-cell-cycle replication in a SeqA-dependent fashion. We found that oriII also needs SeqA for once-per-cell-cycle replication and, additionally, full methylation for efficient initiator binding. The requirement for initiator binding might suffice to make methylation an essential function in V. cholerae. The structure of oriII suggests that it originated from a plasmid, but unlike plasmids, oriII makes use of methylation for once-per-cell-cycle replication, the norm for chromosomal but not plasmid replication.

Bacteria usually have one chromosome but can have extrachromosomal replicons, called plasmids. Although normally dispensable, plasmids can confer adaptive advantage to cells in stressful environments. Bacteria can also have multiple chromosomes, each carrying essential genes, as in eukaryotes. In all organisms, chromosomes duplicate once before the cells divide so that the daughter cells can receive equal genetic dowry, but this is not usually the case with bacterial plasmids. Vibrio cholerae, the causative agent for the disease cholera, has a typical bacterial chromosome like the chromosome of the well-studied bacterium Escherichia coli and has a second chromosome with many signatures indicating its origin from a plasmid. Here we show that, in spite of the distinct nature of the two chromosomes, they both duplicate once per cell cycle, and they both require DNA adenine methylation for this purpose. Our study suggests that once-per-cell-cycle replication is a necessary feature of a chromosome in multichromosome bacteria, and provides a paradigm of how methylation could endow extrachromosomal replicons with the capacity to duplicate like chromosomes.

Identification of key structural determinants of the IntI1 integron integrase that influence attC x attI1 recombination efficiency

The integron platform codes for an integrase (IntI) from the tyrosine family of recombinases that mediates recombination between a proximal double-strand recombination site, attI and a single-strand target recombination site, attC. The attI site is only recognized by its cognate integrase, while the various tested attCs sites are recombined by several different IntI integrases. We have developed a genetic system to enrich and select mutants of IntI1 that provide a higher yield of recombination in order to identify key protein structural elements important for attC × attI1 recombination. We isolated mutants with higher activity on wild type and mutant attC sites. Interestingly, three out of four characterized IntI1 mutants selected on different substrates are mutants of the conserved aspartic acid in position 161. The IntI1 model we made based on the VchIntIA 3D structure suggests that substitution at this position, which plays a central role in multimer assembly, can increase or decrease the stability of the complex and accordingly influence the rate of attI × attC recombination versus attC × attC. These results suggest that there is a balance between the specificity of the protein and the protein/protein interactions in the recombination synapse.

Structural basis for broad DNA-specificity in integron recombination

Lateral DNA transfer--the movement of genetic traits between bacteria--has a profound impact on genomic evolution and speciation. The efficiency with which bacteria incorporate genetic information reflects their capacity to adapt to changing environmental conditions. Integron integrases are proteins that mediate site-specific DNA recombination between a proximal primary site (attI) and a secondary target site (attC) found within mobile gene cassettes encoding resistance or virulence factors. The lack of sequence conservation among attC sites has led to the hypothesis that a sequence-independent structural recognition determinant must exist within attC. Here we report the crystal structure of an integron integrase bound to an attC substrate. The structure shows that DNA target site recognition and high-order synaptic assembly are not dependent on canonical DNA but on the position of two flipped-out bases that interact in cis and in trans with the integrase. These extrahelical bases, one of which is required for recombination in vivo, originate from folding of the bottom strand of attC owing to its imperfect internal dyad symmetry. The mechanism reported here supports a new paradigm for how sequence-degenerate single-stranded genetic material is recognized and exchanged between bacteria.

Integron cassette insertion: a recombination process involving a folded single strand substrate

Integrons play a major role in the dissemination of antibiotic resistance genes among Gram-negative pathogens. Integron gene cassettes form circular intermediates carrying a recombination site, attC, and insert into an integron platform at a second site, attI, in a reaction catalyzed by an integron-specific integrase IntI. The IntI1 integron integrase preferentially binds to the 'bottom strand' of single-stranded attC. We have addressed the insertion mechanism in vivo using a recombination assay exploiting plasmid conjugation to exclusively deliver either the top or bottom strand of different integrase recombination substrates. Recombination of a single-stranded attC site with an attI site was 1000-fold higher for one strand than for the other. Conversely, following conjugative transfer of either attI strand, recombination with attC is highly unfavorable. These results and those obtained using mutations within a putative attC stem-and-loop strongly support a novel integron cassette insertion model in which the single bottom attC strand adopts a folded structure generating a double strand recombination site. Thus, recombination would insert a single strand cassette, which must be subsequently processed.

A new family of mobilizable suicide plasmids based on broad host range R388 plasmid (IncW) and RP4 plasmid (IncPalpha) conjugative machineries and their cognate Escherichia coli host strains

We describe the construction of the pSW family of conditionally replicating plasmids which are based on the IncX oriV origin (oriV(R6Kgamma)) of replication that is dependent on the pir-encoded protein. We constructed several Escherichia coli derivatives expressing pir from different chromosomal loci, and the pir gene could be transduced by phage P1 to any E. coli strain. These chromosomal constructions generate dapA and thyA knockouts, which lead to diaminopimelate or thymidine auxotrophies, respectively, and they serve to provide absolute counterselection even in rich media. These strains can be easily counterselected if used in plasmid transfer experiments into markerless recipients, and they have been demonstrated to work efficiently in E. coli xVibrio or E. coli xBartonella matings. We constructed different pSW plasmids carrying either the oriT(RP4) or the oriT(R388), and we demonstrated that these derivatives can be efficiently transferred using RP4 and R388 conjugation machineries, respectively. We also constructed two plasmids expressing the R388 conjugation machinery, but lacking the oriT(R388). We demonstrated that these plasmids enabled efficient and exclusive transfer of a pSW-oriT(R388) derivative from E. coli to V. cholerae, and we offer an alternative to the popular RP4-based delivery system.