Bacteriophage P1 pac sites inserted into the chromosome greatly increase packaging and transduction of Escherichia coli genomic DNA

Phage-host co-evolution has led to distinct generalized transduction strategies

Generalized transduction is pivotal in bacterial evolution but lacks comprehensive understanding regarding the facilitating features and variations among phages. We addressed this gap by sequencing and comparing the transducing particle content of three different Salmonella Typhimurium phages (i.e. Det7, ES18 and P22) that share a headful packaging mechanism that is typically initiated from a cognate pac site within the phage chromosome. This revealed substantial disparities in both the extent and content of transducing particles among these phages. While Det7 outperformed ES18 in terms of relative number of transducing particles, both phages contrasted with P22 in terms of content. In fact, we found evidence for the presence of conserved P22 pac-like sequences in the host chromosome that direct tremendously increased packaging and transduction frequencies of downstream regions by P22. More specifically, a ca. 561 kb host region between oppositely oriented pac-like sequences in the purF and minE loci was identified as highly packaged and transduced during both P22 prophage induction and lytic infection. Our findings underscore the evolution of phage transducing capacity towards attenuation, promiscuity or directionality, and suggest that pac-like sequences in the host chromosome could become selected as sites directing high frequency of transduction.

Interaction of bacteriophage P1 with an epiphytic Pantoea agglomerans strain-the role of the interplay between various mobilome elements

P1 is a model, temperate bacteriophage of the 94 kb genome. It can lysogenize representatives of the Enterobacterales order. In lysogens, it is maintained as a plasmid. We tested P1 interactions with the biocontrol P. agglomerans L15 strain to explore the utility of P1 in P. agglomerans genome engineering. A P1 derivative carrying the Tn9 (cmR) transposon could transfer a plasmid from Escherichia coli to the L15 cells. The L15 cells infected with this derivative formed chloramphenicol-resistant colonies. They could grow in a liquid medium with chloramphenicol after adaptation and did not contain prophage P1 but the chromosomally inserted cmR marker of P1 Tn9 (cat). The insertions were accompanied by various rearrangements upstream of the Tn9 cat gene promoter and the loss of IS1 (IS1L) from the corresponding region. Sequence analysis of the L15 strain genome revealed a chromosome and three plasmids of 0.58, 0.18, and 0.07 Mb. The largest and the smallest plasmid appeared to encode partition and replication incompatibility determinants similar to those of prophage P1, respectively. In the L15 derivatives cured of the largest plasmid, P1 with Tn9 could not replace the smallest plasmid even if selected. However, it could replace the smallest and the largest plasmid of L15 if its Tn9 IS1L sequence driving the Tn9 mobility was inactivated or if it was enriched with an immobile kanamycin resistance marker. Moreover, it could develop lytically in the L15 derivatives cured of both these plasmids. Clearly, under conditions of selection for P1, the mobility of the P1 selective marker determines whether or not the incoming P1 can outcompete the incompatible L15 resident plasmids. Our results demonstrate that P. agglomerans can serve as a host for bacteriophage P1 and can be engineered with the help of this phage. They also provide an example of how antibiotics can modify the outcome of horizontal gene transfer in natural environments. Numerous plasmids of Pantoea strains appear to contain determinants of replication or partition incompatibility with P1. Therefore, P1 with an immobile selective marker may be a tool of choice in curing these strains from the respective plasmids to facilitate their functional analysis.

Compounding Achromobacter Phages for Therapeutic Applications

Achromobacter species colonization of Cystic Fibrosis respiratory airways is an increasing concern. Two adult patients with Cystic Fibrosis colonized by Achromobacter xylosoxidans CF418 or Achromobacter ruhlandii CF116 experienced fatal exacerbations. Achromobacter spp. are naturally resistant to several antibiotics. Therefore, phages could be valuable as therapeutics for the control of Achromobacter. In this study, thirteen lytic phages were isolated and characterized at the morphological and genomic levels for potential future use in phage therapy. They are presented here as the Achromobacter Kumeyaay phage collection. Six distinct Achromobacter phage genome clusters were identified based on a comprehensive phylogenetic analysis of the Kumeyaay collection as well as the publicly available Achromobacter phages. The infectivity of all phages in the Kumeyaay collection was tested in 23 Achromobacter clinical isolates; 78% of these isolates were lysed by at least one phage. A cryptic prophage was induced in Achromobacter xylosoxidans CF418 when infected with some of the lytic phages. This prophage genome was characterized and is presented as Achromobacter phage CF418-P1. Prophage induction during lytic phage preparation for therapy interventions require further exploration. Large-scale production of phages and removal of endotoxins using an octanol-based procedure resulted in a phage concentrate of 1 × 109 plaque-forming units per milliliter with an endotoxin concentration of 65 endotoxin units per milliliter, which is below the Food and Drugs Administration recommended maximum threshold for human administration. This study provides a comprehensive framework for the isolation, bioinformatic characterization, and safe production of phages to kill Achromobacter spp. in order to potentially manage Cystic Fibrosis (CF) pulmonary infections.

P1 Bacteriophage-Enabled Delivery of CRISPR-Cas9 Antimicrobial Activity Against Shigella flexneri

The discovery of clustered, regularly interspaced, short palindromic repeats (CRISPR) and the Cas9 RNA-guided nuclease provides unprecedented opportunities to selectively kill specific populations or species of bacteria. However, the use of CRISPR-Cas9 to clear bacterial infections in vivo is hampered by the inefficient delivery of cas9 genetic constructs into bacterial cells. Here, we use a broad-host-range P1-derived phagemid to deliver the CRISPR-Cas9 chromosomal-targeting system into Escherichia coli and the dysentery-causing Shigella flexneri to achieve DNA sequence-specific killing of targeted bacterial cells. We show that genetic modification of the helper P1 phage DNA packaging site (pac) significantly enhances the purity of packaged phagemid and improves the Cas9-mediated killing of S. flexneri cells. We further demonstrate that P1 phage particles can deliver chromosomal-targeting cas9 phagemids into S. flexneri in vivo using a zebrafish larvae infection model, where they significantly reduce the bacterial load and promote host survival. Our study highlights the potential of combining P1 bacteriophage-based delivery with the CRISPR chromosomal-targeting system to achieve DNA sequence-specific cell lethality and efficient clearance of bacterial infection.

Efficient traceless modification of the P1 bacteriophage genome through homologous recombination with enrichment in double recombinants: A new perspective on the functional annotation of uncharacterized phage genes

The advent of high-throughput omic technologies has caused unprecedented progress in research on bacteriophages, the most abundant and still the least explored entities on earth. Despite the growing number of phage genomes sequenced and the rejuvenation of interest in phage therapy, the progress in the functional analysis of phage genes is slow. Simple and efficient techniques of phage genome targeted mutagenesis that would allow one to knock out particular genes precisely without polar effects in order to study the effect of these knock-outs on phage functions are lacking. Even in the case of model phages, the functions of approximately half of their genes are unknown. P1 is an enterobacterial temperate myophage of clinical significance, which lysogenizes cells as a plasmid. It has a long history of studies, serves as a model in basic research, is a gene transfer vector, and is a source of genetic tools. Its gene products have structural homologs in several other phages. In this perspective article, we describe a simple and efficient procedure of traceless P1 genome modification that could also serve to acquire targeted mutations in the genomes of certain other temperate phages and speed up functional annotations of phage genes.

Novel Salmonella Phage, vB_Sen_STGO-35-1, Characterization and Evaluation in Chicken Meat

Salmonellosis is one of the most frequently reported zoonotic foodborne diseases worldwide, and poultry is the most important reservoir of Salmonella enterica serovar Enteritidis. The use of lytic bacteriophages (phages) to reduce foodborne pathogens has emerged as a promising biocontrol intervention for Salmonella spp. Here, we describe and evaluate the newly isolated Salmonella phage STGO-35-1, including: (i) genomic and phenotypic characterization, (ii) an analysis of the reduction of Salmonella in chicken meat, and (iii) genome plasticity testing. Phage STGO-35-1 represents an unclassified siphovirus, with a length of 47,483 bp, a G + C content of 46.5%, a headful strategy of packaging, and a virulent lifestyle. Phage STGO-35-1 reduced S. Enteritidis counts in chicken meat by 2.5 orders of magnitude at 4 °C. We identified two receptor-binding proteins with affinity to LPS, and their encoding genes showed plasticity during an exposure assay. Phenotypic, proteomic, and genomic characteristics of STGO-35-1, as well as the Salmonella reduction in chicken meat, support the potential use of STGO-35-1 as a targeted biocontrol agent against S. Enteritidis in chicken meat. Additionally, computational analysis and a short exposure time assay allowed us to predict the plasticity of genes encoding putative receptor-binding proteins.

Genetic Modification of Sodalis Species by DNA Transduction

Bacteriophages (phages) are ubiquitous in nature. These viruses play a number of central roles in microbial ecology and evolution by, for instance, promoting horizontal gene transfer (HGT) among bacterial species. The ability of phages to mediate HGT through transduction has been widely exploited as an experimental tool for the genetic study of bacteria. As such, bacteriophage P1 represents a prototypical generalized transducing phage with a broad host range that has been extensively employed in the genetic manipulation of Escherichia coli and a number of other model bacterial species. Here we demonstrate that P1 is capable of infecting, lysogenizing, and promoting transduction in members of the bacterial genus Sodalis, including the maternally inherited insect endosymbiont Sodalis glossinidius. While establishing new tools for the genetic study of these bacterial species, our results suggest that P1 may be used to deliver DNA to many Gram-negative endosymbionts in their insect host, thereby circumventing a culturing requirement to genetically manipulate these organisms.

IMPORTANCE A large number of economically important insects maintain intimate associations with maternally inherited endosymbiotic bacteria. Due to the inherent nature of these associations, insect endosymbionts cannot be usually isolated in pure culture or genetically manipulated. Here we use a broad-host-range bacteriophage to deliver exogenous DNA to an insect endosymbiont and a closely related free-living species. Our results suggest that broad-host-range bacteriophages can be used to genetically alter insect endosymbionts in their insect host and, as a result, bypass a culturing requirement to genetically alter these bacteria.

Horizontal genetic exchange of chromosomally encoded markers between Campylobacter jejuni cells

Many epidemiological studies provide us with the evidence of horizontal gene transfer (HGT) contributing to the bacterial genomic diversity that benefits the bacterial populations with increased ability to adapt to the dynamic environments. Campylobacter jejuni, a major cause of acute enteritis in the U.S., often linked with severe post-infection neuropathies, has been reported to exhibit a non-clonal population structure and comparatively higher strain-level genetic variation. In this study, we provide evidence of the HGT of chromosomally encoded genetic markers between C. jejuni cells in the biphasic MH medium. We used two C. jejuni NCTC-11168 mutants harbouring distinct antibiotic-resistance genes [chloramphenicol (Cm) and kanamycin (Km)] present at two different neutral genomic loci. Cultures of both marker strains were mixed together and incubated for 5 hrs, then plated on MH agar plates supplemented with both antibiotics. The recombinant cells with double antibiotic markers were generated at the frequency of 0.02811 ± 0.0035% of the parental strains. PCR assays using locus-specific primers confirmed that transfer of the antibiotic-resistance genes was through homologous recombination. Also, the addition of chicken cecal content increased the recombination efficiency approximately up to 10-fold as compared to the biphasic MH medium (control) at P < 0.05. Furthermore, treating the co-culture with DNase I decreased the available DNA, which in turn significantly reduced recombination efficiency by 99.92% (P < 0.05). We used the cell-free supernatant of 16 hrs-culture of Wild-type C. jejuni as a template for PCR and found DNA sequences from six different genomic regions were easily amplified, indicating the presence of released chromosomal DNA in the culture supernatant. Our findings suggest that HGT in C. jejuni is facilitated in the chicken gut environment contributing to in vivo genomic diversity. Additionally, C. jejuni might have an active mechanism to release its chromosomal DNA into the extracellular environment, further expediting HGT in C. jejuni populations.

Large Preferred Region for Packaging of Bacterial DNA by phiC725A, a Novel Pseudomonas aeruginosa F116-Like Bacteriophage

Bacteriophage vB_PaeP_PAO1_phiC725A (short name phiC725A) was isolated following mitomycin C induction of C7-25, a clinical Pseudomonas aeruginosa strain carrying phiC725A as a prophage. The phiC725A genome sequence shows similarity to F116, a P. aeruginosa podovirus capable of generalized transduction. Likewise, phiC725A is a podovirus with long tail fibers. PhiC725A was able to lysogenize two additional P. aeruginosa strains in which it was maintained both as a prophage and in an episomal state. Investigation by deep sequencing showed that bacterial DNA carried inside phage particles originated predominantly from a 700-800kb region, immediately flanking the attL prophage insertion site, whether the phages were induced from a lysogen or recovered after infection. This indicates that during productive replication, recombination of phage genomes with the bacterial chromosome at the att site occurs occasionally, allowing packaging of adjacent bacterial DNA.

Phage-mediated horizontal gene transfer of both prophage and heterologous DNA by ϕBB-1, a bacteriophage of Borrelia burgdorferi

Horizontal gene transfer (HGT) in Borrelia burgdorferi, the Lyme disease agent, is likely mediated by bacteriophage. Studies of the B. burgdorferi phage, ϕBB-1 and its role in HGT have been hindered by the lack of an assay for readily characterizing phage-mediated DNA movement (transduction). Here we describe an in vitro assay in which a clone of B. burgdorferi strain CA-11.2A encoding kanamycin resistance on a ϕBB-1 prophage is co-cultured with different clones encoding gentamicin resistance on a shuttle vector; transduction is monitored by enumerating colonies selected in the presence of both kanamycin and gentamicin. When both clones used in the assay were derived from CA-11.2A, the frequency of transduction was 1.23 × 10^-6 transductants per cell, and could be increased 5-fold by exposing the phage-producing strain to 5% ethanol. Transduction was also demonstrated between the CA-11.2A clone and clones of both high-passage B. burgdorferi strain B31 and low-passage, virulent B. burgdorferi strain 297, although with lower transduction frequencies. The transductant in the 297 background produced phage capable of transducing another B. burgdorferi clone: this is the first experimental demonstration of transduction from a clone of a virulent strain. In addition to prophage DNA, small Escherichia coli-derived shuttle vectors were also transduced between co-cultured B. burgdorferi strains, suggesting both a broad role for the phage in the HGT of heterologous DNA and a potential use of the phage as a molecular tool. These results enhance our understanding of phage-mediated transduction as a mechanism of HGT in the Lyme disease spirochetes. Furthermore, the reagents and techniques developed herein will facilitate future studies of phage-mediated HGT, especially within the tick vector and vertebrate host.

Mechanisms of DNA Packaging by Large Double-Stranded DNA Viruses

Translocation of viral double-stranded DNA (dsDNA) into the icosahedral prohead shell is catalyzed by TerL, a motor protein that has ATPase, endonuclease, and translocase activities. TerL, following endonucleolytic cleavage of immature viral DNA concatemer recognized by TerS, assembles into a pentameric ring motor on the prohead's portal vertex and uses ATP hydrolysis energy for DNA translocation. TerL's N-terminal ATPase is connected by a hinge to the C-terminal endonuclease. Inchworm models propose that modest domain motions accompanying ATP hydrolysis are amplified, through changes in electrostatic interactions, into larger movements of the C-terminal domain bound to DNA. In phage ϕ29, four of the five TerL subunits sequentially hydrolyze ATP, each powering translocation of 2.5 bp. After one viral genome is encapsidated, the internal pressure signals termination of packaging and ejection of the motor. Current focus is on the structures of packaging complexes and the dynamics of TerL during DNA packaging, endonuclease regulation, and motor mechanics.