A set of recombineering plasmids for gram-negative bacteria

We have constructed a set of plasmids that can be used to express recombineering functions in some gram-negative bacteria, thereby facilitating in vivo genetic manipulations. These plasmids include an origin of replication and a segment of the bacteriophage lambda genome comprising the red genes (exo, bet and gam) under their native control. These constructs do not require the anti-termination event normally required for Red expression, making their application more likely in divergent species. Some of the plasmids have temperature-sensitive replicons to simplify curing. In creating these vectors we developed two useful recombineering applications. Any gene linked to a drug marker can be retrieved by gap-repair using only a plasmid origin and target homologies. A plasmid origin of replication can be changed to a different origin by targeted replacement, to potentially alter its copy number and host range. Both these techniques will prove useful for manipulation of plasmids in vivo. Most of the Red plasmid constructs catalyzed efficient recombination in E. coli with a low level of uninduced background recombination. These Red plasmids have been successfully tested in Salmonella, and we anticipate that that they will provide efficient recombination in other related gram-negative bacteria.

Enterococcus faecalis prophage dynamics and contributions to pathogenic traits

Polylysogeny is frequently considered to be the result of an adaptive evolutionary process in which prophages confer fitness and/or virulence factors, thus making them important for evolution of both bacterial populations and infectious diseases. The Enterococcus faecalis V583 isolate belongs to the high-risk clonal complex 2 that is particularly well adapted to the hospital environment. Its genome carries 7 prophage-like elements (V583-pp1 to -pp7), one of which is ubiquitous in the species. In this study, we investigated the activity of the V583 prophages and their contribution to E. faecalis biological traits. We systematically analyzed the ability of each prophage to excise from the bacterial chromosome, to replicate and to package its DNA. We also created a set of E. faecalis isogenic strains that lack from one to all six non-ubiquitous prophages by mimicking natural excision. Our work reveals that prophages of E. faecalis V583 excise from the bacterial chromosome in the presence of a fluoroquinolone, and are able to produce active phage progeny. Intricate interactions between V583 prophages were also unveiled: i) pp7, coined EfCIV583 for E. faecalis chromosomal island of V583, hijacks capsids from helper phage 1, leading to the formation of distinct virions, and ii) pp1, pp3 and pp5 inhibit excision of pp4 and pp6. The hijacking exerted by EfCIV583 on helper phage 1 capsids is the first example of molecular piracy in Gram positive bacteria other than staphylococci. Furthermore, prophages encoding platelet-binding-like proteins were found to be involved in adhesion to human platelets, considered as a first step towards the development of infective endocarditis. Our findings reveal not only a role of E. faecalis V583 prophages in pathogenicity, but also provide an explanation for the correlation between antibiotic usage and E. faecalis success as a nosocomial pathogen, as fluoriquinolone may provoke release of prophages and promote gene dissemination among isolates.

Enterococcus faecalis is a member of the core-microbiome of the human gastrointestinal tract. In the last decades however, this bacterial species has emerged as a major cause of hospital-acquired infections worldwide. Some isolates are particularly adapted to the hospital environment, and this adaptation was recently linked with enrichment in mobile genetic elements including prophages, which are chromosomal integrated genomes of bacterial viruses. We characterized the biological prophage activity in an E. faecalis strain of clinical origin that harbors 7 prophages. Six active prophages exhibit intricate interactions, one of which is involved in a molecular piracy phenomenon. We also established, for the first time, a direct correlation between prophage and adhesion to human platelets, an initial step towards infective endocarditis. Finally, we showed that fluoroquinolone increases prophage activity and can thus contribute to horizontal gene spreading. Overall, we provide evidence that prophages are key players in E. faecalis evolution towards pathogenicity.

Structure and lytic activity of a Bacillus anthracis prophage endolysin

We report a structural and functional analysis of the lambda prophage Ba02 endolysin (PlyL) encoded by the Bacillus anthracis genome. We show that PlyL comprises two autonomously folded domains, an N-terminal catalytic domain and a C-terminal cell wall-binding domain. We determined the crystal structure of the catalytic domain; its three-dimensional fold is related to that of the cell wall amidase, T7 lysozyme, and contains a conserved zinc coordination site and other components of the catalytic machinery. We demonstrate that PlyL is an N-acetylmuramoyl-L-alanine amidase that cleaves the cell wall of several Bacillus species when applied exogenously. We show, unexpectedly, that the catalytic domain of PlyL cleaves more efficiently than the full-length protein, except in the case of Bacillus cereus, and using GFP-tagged cell wall-binding domain, we detected strong binding of the cell wall-binding domain to B. cereus but not to other species tested. We further show that a related endolysin (Ply21) from the B. cereus phage, TP21, shows a similar pattern of behavior. To explain these data, and the species specificity of PlyL, we propose that the C-terminal domain inhibits the activity of the catalytic domain through intramolecular interactions that are relieved upon binding of the C-terminal domain to the cell wall. Furthermore, our data show that (when applied exogenously) targeting of the enzyme to the cell wall is not a prerequisite of its lytic activity, which is inherently high. These results may have broad implications for the design of endolysins as therapeutic agents.

Moonlighting bacteriophage proteins derepress staphylococcal pathogenicity islands

Staphylococcal superantigen-carrying pathogenicity islands (SaPIs) are discrete, chromosomally integrated units of approximately 15 kilobases that are induced by helper phages to excise and replicate. SaPI DNA is then efficiently encapsidated in phage-like infectious particles, leading to extremely high frequencies of intra- as well as intergeneric transfer. In the absence of helper phage lytic growth, the island is maintained in a quiescent prophage-like state by a global repressor, Stl, which controls expression of most of the SaPI genes. Here we show that SaPI derepression is effected by a specific, non-essential phage protein that binds to Stl, disrupting the Stl-DNA complex and thereby initiating the excision-replication-packaging cycle of the island. Because SaPIs require phage proteins to be packaged, this strategy assures that SaPIs will be transferred once induced. Several different SaPIs are induced by helper phage 80alpha and, in each case, the SaPI commandeers a different non-essential phage protein for its derepression. The highly specific interactions between different SaPI repressors and helper-phage-encoded antirepressors represent a remarkable evolutionary adaptation involved in pathogenicity island mobilization.

A chromosomally integrated bacteriophage in invasive meningococci

Cerebrospinal meningitis is a feared disease that can cause the death of a previously healthy individual within hours. Paradoxically, the causative agent, Neisseria meningitidis, is a common inhabitant of the human nasopharynx, and as such, may be considered a normal, commensal organism. Only in a small proportion of colonized people do the bacteria invade the bloodstream, from where they can cross the blood–brain barrier to cause meningitis. Furthermore, most meningococcal disease is caused by bacteria belonging to only a few of the phylogenetic groups among the large number that constitute the population structure of this genetically variable organism. However, the genetic basis for the differences in pathogenic potential remains elusive. By performing whole genome comparisons of a large collection of meningococcal isolates of defined pathogenic potential we brought to light a meningococcal prophage present in disease-causing bacteria. The phage, of the filamentous family, excises from the chromosome and is secreted from the bacteria via the type IV pilin secretin. Therefore, this element, by spreading among the population, may promote the development of new epidemic clones of N. meningitidis that are capable of breaking the normal commensal relationship with humans and causing invasive disease.

Genetics of Borrelia burgdorferi

The spirochetes in the Borrelia burgdorferi sensu lato genospecies group cycle in nature between tick vectors and vertebrate hosts. The current assemblage of B. burgdorferi sensu lato, of which three species cause Lyme disease in humans, originated from a rapid species radiation that occurred near the origin of the clade. All of these species share a unique genome structure that is highly segmented and predominantly composed of linear replicons. One of the circular plasmids is a prophage that exists as several isoforms in each cell and can be transduced to other cells, likely contributing to an otherwise relatively anemic level of horizontal gene transfer, which nevertheless appears to be adequate to permit strong natural selection and adaptation in populations of B. burgdorferi. Although the molecular genetic toolbox is meager, several antibiotic-resistant mutants have been isolated, and the resistance alleles, as well as some exogenous genes, have been fashioned into markers to dissect gene function. Genetic studies have probed the role of the outer membrane lipoprotein OspC, which is maintained in nature by multiple niche polymorphisms and negative frequency-dependent selection. One of the most intriguing genetic systems in B. burgdorferi is vls recombination, which generates antigenic variation during infection of mammalian hosts.

Genetic basis of growth adaptation of Escherichia coli after deletion of pgi, a major metabolic gene

Bacterial survival requires adaptation to different environmental perturbations such as exposure to antibiotics, changes in temperature or oxygen levels, DNA damage, and alternative nutrient sources. During adaptation, bacteria often develop beneficial mutations that confer increased fitness in the new environment. Adaptation to the loss of a major non-essential gene product that cripples growth, however, has not been studied at the whole-genome level. We investigated the ability of Escherichia coli K-12 MG1655 to overcome the loss of phosphoglucose isomerase (pgi) by adaptively evolving ten replicates of E. coli lacking pgi for 50 days in glucose M9 minimal medium and by characterizing endpoint clones through whole-genome re-sequencing and phenotype profiling. We found that 1) the growth rates for all ten endpoint clones increased approximately 3-fold over the 50-day period; 2) two to five mutations arose during adaptation, most frequently in the NADH/NADPH transhydrogenases udhA and pntAB and in the stress-associated sigma factor rpoS; and 3) despite similar growth rates, at least three distinct endpoint phenotypes developed as defined by different rates of acetate and formate secretion. These results demonstrate that E. coli can adapt to the loss of a major metabolic gene product with only a handful of mutations and that adaptation can result in multiple, alternative phenotypes.

The pKO2 linear plasmid prophage of Klebsiella oxytoca

Temperate bacteriophages with plasmid prophages are uncommon in nature, and of these only phages N15 and PY54 are known to have a linear plasmid prophage with closed hairpin telomeres. We report here the complete nucleotide sequence of the 51,601-bp Klebsiella oxytoca linear plasmid pKO2, and we demonstrate experimentally that it is also a prophage. We call this bacteriophage phiKO2. An analysis of the 64 predicted phiKO2 genes indicate that it is a fairly close relative of phage N15; they share a mosaic relationship that is typical of different members of double-stranded DNA tailed-phage groups. Although the head, tail shaft, and lysis genes are not recognizably homologous between these phages, other genes such as the plasmid partitioning, replicase, prophage repressor, and protelomerase genes (and their putative targets) are so similar that we predict that they must have nearly identical DNA binding specificities. The phiKO2 virion is unusual in that its phage lambda-like tails have an exceptionally long (3,433 amino acids) central tip tail fiber protein. The phiKO2 genome also carries putative homologues of bacterial dinI and umuD genes, both of which are involved in the host SOS response. We show that these divergently transcribed genes are regulated by LexA protein binding to a single target site that overlaps both promoters.

Abortive infection mechanisms and prophage sequences significantly influence the genetic makeup of emerging lytic lactococcal phages

In this study, we demonstrated the remarkable genome plasticity of lytic lactococcal phages that allows them to rapidly adapt to the dynamic dairy environment. The lytic double-stranded DNA phage ul36 was used to sequentially infect a wild-type strain of Lactococcus lactis and two isogenic derivatives with genes encoding two phage resistance mechanisms, AbiK and AbiT. Four phage mutants resistant to one or both Abi mechanisms were isolated. Comparative analysis of their complete genomes, as well as morphological observations, revealed that phage ul36 extensively evolved by large-scale homologous and nonhomologous recombination events with the inducible prophage present in the host strain. One phage mutant exchanged as much as 79% of its genome compared to the core genome of ul36. Thus, natural phage defense mechanisms and prophage elements found in bacterial chromosomes contribute significantly to the evolution of the lytic phage population.

Bacterial retrons encode phage-defending tripartite toxin-antitoxin systems

Retrons are prokaryotic genetic retroelements encoding a reverse transcriptase that produces multi-copy single-stranded DNA1 (msDNA). Despite decades of research on the biosynthesis of msDNA2, the function and physiological roles of retrons have remained unknown. Here we show that Retron-Sen2 of Salmonella enterica serovar Typhimurium encodes an accessory toxin protein, STM14_4640, which we renamed as RcaT. RcaT is neutralized by the reverse transcriptase-msDNA antitoxin complex, and becomes active upon perturbation of msDNA biosynthesis. The reverse transcriptase is required for binding to RcaT, and the msDNA is required for the antitoxin activity. The highly prevalent RcaT-containing retron family constitutes a new type of tripartite DNA-containing toxin-antitoxin system. To understand the physiological roles of such toxin-antitoxin systems, we developed toxin activation-inhibition conjugation (TAC-TIC), a high-throughput reverse genetics approach that identifies the molecular triggers and blockers of toxin-antitoxin systems. By applying TAC-TIC to Retron-Sen2, we identified multiple trigger and blocker proteins of phage origin. We demonstrate that phage-related triggers directly modify the msDNA, thereby activating RcaT and inhibiting bacterial growth. By contrast, prophage proteins circumvent retrons by directly blocking RcaT. Consistently, retron toxin-antitoxin systems act as abortive infection anti-phage defence systems, in line with recent reports3,4. Thus, RcaT retrons are tripartite DNA-regulated toxin-antitoxin systems, which use the reverse transcriptase-msDNA complex both as an antitoxin and as a sensor of phage protein activities.

LexA Cleavage Is Required for CTX Prophage Induction

The physiologic conditions and molecular interactions that control phage production have been studied in few temperate phages. We investigated the mechanisms that regulate production of CTXphi, a temperate filamentous phage that infects Vibrio cholerae and encodes cholera toxin. In CTXphi lysogens, the activity of P(rstA), the only CTXphi promoter required for CTX prophage development, is repressed by RstR, the CTXvphi repressor. We found that the V. cholerae SOS response regulates CTXvphi production. The molecular mechanism by which this cellular response to DNA damage controls CTXphi production differs from that by which the E. coli SOS response controls induction of many prophages. UV-stimulated CTXphi production required RecA-dependent autocleavage of LexA, a repressor that controls expression of numerous host DNA repair genes. LexA and RstR both bind to and repress P(rstA). Thus, CTXphi production is controlled by a cellular repressor whose activity is regulated by the cell's response to DNA damage.

Use of the lambda Red recombinase system to produce recombinant prophages carrying antibiotic resistance genes

Background

The Red recombinase system of bacteriophage lambda has been used to inactivate chromosomal genes in E. coli K-12 through homologous recombination using linear PCR products. The aim of this study was to induce mutations in the genome of some temperate Shiga toxin encoding bacteriophages. When phage genes are in the prophage state, they behave like chromosomal genes. This enables marker genes, such as antibiotic resistance genes, to be incorporated into the stx gene. Once the phages' lytic cycle is activated, recombinant Shiga toxin converting phages are produced. These phages can transfer the marker genes to the bacteria that they infect and convert. As the Red system's effectiveness decreased when used for our purposes, we had to introduce significant variations to the original method. These modifications included: confirming the stability of the target stx gene increasing the number of cells to be transformed and using a three-step PCR method to produce the amplimer containing the antibiotic resistance gene.

Results

Seven phages carrying two different antibiotic resistance genes were derived from phages that are directly involved in the pathogenesis of Shiga toxin-producing strains, using this modified protocol.

Conclusion

This approach facilitates exploration of the transduction processes and is a valuable tool for studying phage-mediated horizontal gene transfer.

LambdaSa1 and LambdaSa2 prophage lysins of Streptococcus agalactiae

Putative N-acetylmuramyl-l-alanine amidase genes from LambdaSa1 and LambdaSa2 prophages of Streptococcus agalactiae were cloned and expressed in Escherichia coli. The purified enzymes lysed the cell walls of Streptococcus agalactiae, Streptococcus pneumoniae, and Staphylococcus aureus. The peptidoglycan digestion products in the cell wall lysates were not consistent with amidase activity. Instead, the structure of the muropeptide digestion fragments indicated that both the LambdaSa1 and LambdaSa2 lysins exhibited gamma-d-glutaminyl-l-lysine endopeptidase activity. The endopeptidase cleavage specificity of the lysins was confirmed using a synthetic peptide substrate corresponding to a portion of the stem peptide and cross bridge of Streptococcus agalactiae peptidoglycan. The LambdaSa2 lysin also displayed beta-d-N-acetylglucosaminidase activity.

Genetic and Molecular Analysis of GogB, a Phage-encoded Type III-secreted Substrate in Salmonella enterica Serovar Typhimurium with Autonomous Expression from its Associated Phage

Salmonella enterica serovar Typhimurium is lysogenized by several temperate bacteriophages that encode lysogenic conversion genes, which can act as virulence factors during infection and contribute to the genetic diversity and pathogenic potential of the lysogen. We have investigated the temperate bacteriophage called Gifsy-1 in S.enterica serovar Typhimurium and show here that the product of the gogB gene encoded within this phage shares similarity with proteins from other Gram-negative pathogens. The amino-terminal portion of GogB shares similarity with leucine-rich repeat-containing virulence-associated proteins from other Gram-negative pathogens, whereas the carboxyl-terminal portion of GogB shares similarity with uncharacterized proteins in other pathogens. We show that GogB is secreted by both type III secretion systems encoded in Salmonella Pathogenicity Island-1 (SPI-1) and SPI-2 but translocation into host cells is a SPI-2-mediated process. Once translocated, GogB localizes to the cytoplasm of infected host cells. The genetic regulation of gogB in Salmonella is influenced by the transcriptional activator, SsrB, under SPI-2-inducing conditions, but the modular nature of the gogB gene allows for autonomous expression and type III secretion following horizontal gene transfer into a heterologous pathogen. These data define the first autonomously expressed lysogenic conversion gene within Gifsy-1 that acts as a modular and promiscuous type III-secreted substrate of the infection process.

Comparative genomics and transcriptional analysis of prophages identified in the genomes of Lactobacillus gasseri, Lactobacillus salivarius, and Lactobacillus casei

Lactobacillus gasseri ATCC 33323, Lactobacillus salivarius subsp. salivarius UCC 118, and Lactobacillus casei ATCC 334 contain one (LgaI), four (Sal1, Sal2, Sal3, Sal4), and one (Lca1) distinguishable prophage sequences, respectively. Sequence analysis revealed that LgaI, Lca1, Sal1, and Sal2 prophages belong to the group of Sfi11-like pac site and cos site Siphoviridae, respectively. Phylogenetic investigation of these newly described prophage sequences revealed that they have not followed an evolutionary development similar to that of their bacterial hosts and that they show a high degree of diversity, even within a species. The attachment sites were determined for all these prophage elements; LgaI as well as Sal1 integrates in tRNA genes, while prophage Sal2 integrates in a predicted arginino-succinate lyase-encoding gene. In contrast, Lca1 and the Sal3 and Sal4 prophage remnants are integrated in noncoding regions in the L. casei ATCC 334 and L. salivarius UCC 118 genomes. Northern analysis showed that large parts of the prophage genomes are transcriptionally silent and that transcription is limited to genome segments located near the attachment site. Finally, pulsed-field gel electrophoresis followed by Southern blot hybridization with specific prophage probes indicates that these prophage sequences are narrowly distributed within lactobacilli.

The prophages of Lactobacillus johnsonii NCC 533: comparative genomics and transcription analysis

Two non-inducible, but apparently complete prophages were identified in the genome of the sequenced Lactobacillus johnsonii strain NCC 533. The 38- and 40-kb-long prophages Lj928 and Lj965 represent distinct lineages of Sfi11-like pac-site Siphoviridae unrelated at the DNA sequence level. The deduced structural proteins from Lj928 demonstrated aa sequence identity with Lactococcus lactis phage TP901-1, while Lj965 shared sequence links with Streptococcus thermophilus phage O1205. With the exception of tRNA genes, inserted between DNA replication and DNA packaging genes, the transcription of the prophage was restricted to the genome segments near both attachment sites. Transcribed genes unrelated to phage functions were inserted between the phage repressor and integrase genes; one group of genes shared sequence relatedness with a mobile DNA element in Staphylococcus aureus. A short, but highly transcribed region was located between the phage lysin and right attachment site; it lacked a protein-encoding function in one prophage.

Prophage induction, but not production of phage particles, is required for lethal disease in a microbiome-replete murine model of enterohemorrhagic E. coli infection

Enterohemorrhagic Escherichia coli (EHEC) colonize intestinal epithelium by generating characteristic attaching and effacing (AE) lesions. They are lysogenized by prophage that encode Shiga toxin 2 (Stx2), which is responsible for severe clinical manifestations. As a lysogen, prophage genes leading to lytic growth and stx2 expression are repressed, whereas induction of the bacterial SOS response in response to DNA damage leads to lytic phage growth and Stx2 production both in vitro and in germ-free or streptomycin-treated mice. Some commensal bacteria diminish prophage induction and concomitant Stx2 production in vitro, whereas it has been proposed that phage-susceptible commensals may amplify Stx2 production by facilitating successive cycles of infection in vivo. We tested the role of phage induction in both Stx production and lethal disease in microbiome-replete mice, using our mouse model encompassing the murine pathogen Citrobacter rodentium lysogenized with the Stx2-encoding phage Φstx2dact. This strain generates EHEC-like AE lesions on the murine intestine and causes lethal Stx-mediated disease. We found that lethal mouse infection did not require that Φstx2dact infect or lysogenize commensal bacteria. In addition, we detected circularized phage genomes, potentially in the early stage of replication, in feces of infected mice, confirming that prophage induction occurs during infection of microbiota-replete mice. Further, C. rodentium (Φstx2dact) mutants that do not respond to DNA damage or express stx produced neither high levels of Stx2 in vitro or lethal infection in vivo, confirming that SOS induction and concomitant expression of phage-encoded stx genes are required for disease. In contrast, C. rodentium (Φstx2dact) mutants incapable of prophage genome excision or of packaging phage genomes retained the ability to produce Stx in vitro, as well as to cause lethal disease in mice. Thus, in a microbiome-replete EHEC infection model, lytic induction of Stx-encoding prophage is essential for lethal disease, but actual phage production is not.

Purification and characterization of the repressor of the shiga toxin-encoding bacteriophage 933W: DNA binding, gene regulation, and autocleavage

The genes encoding Shiga toxin (stx), the major virulence factor of Shiga toxin-encoding Escherichia coli (STEC) strains, are carried on lambdoid prophages resident in all known STEC strains. The stx genes are expressed only during lytic growth of these temperate bacteriophages. We cloned the gene encoding the repressor of the Shiga toxin-encoding bacteriophage 933W and examined the DNA binding and transcriptional regulatory activities of the overexpressed, purified protein. Typical of nearly all lambdoid phage repressors, 933W repressor binds to three sites in 933W right operator (OR). Also typical, when bound at OR, 933W repressor functions as an activator at the PRM promoter and a repressor at the PR promoter. In contrast to other lambdoid bacteriophages, 933W left operator (OL) contains only two repressor binding sites, but the OL-bound repressor still efficiently represses PL transcription. Lambdoid prophage induction requires inactivation of the repressor's DNA binding activity. In all phages examined thus far, this inactivation requires a RecA-stimulated repressor autoproteolysis event, with cleavage occurring precisely in an Ala-Gly dipeptide sequence that is found within a "linker " region that joins the two domains of these proteins. However, 933W repressor protein contains neither an Ala-Gly nor an alternative Cys-Gly dipeptide cleavage site anywhere in its linker sequence. We show here that the autocleavage occurs at a Leu-Gly dipeptide. Thus, the specificity of the repressor autocleavage site is more variable than thought previously.

Borrelia burgdorferi EbfC, a novel, chromosomally encoded protein, binds specific DNA sequences adjacent to erp loci on the spirochete's resident cp32 prophages

All examined isolates of the Lyme disease spirochete, Borrelia burgdorferi, naturally maintain numerous variants of a prophage family as circular cp32 episomes. Each cp32 carries a locus encoding one or two different Erp outer membrane, surface-exposed lipoproteins. Many of the Erp proteins bind a host complement regulator, factor H, which is hypothesized to protect the spirochete from complement-mediated killing. We now describe the isolation and characterization of a novel, chromosomally encoded protein, EbfC, that binds specific DNA sequences located immediately 5' of all erp loci. This is one of the first site-specific DNA-binding proteins to be identified in any spirochete. The location of the ebfC gene on the B. burgdorferi chromosome suggests that the cp32 prophages have evolved to use this bacterial host protein for their own benefit and that EbfC probably plays additional roles in the bacterium. A wide range of other bacteria encode homologs of EbfC, none of which have been well characterized, so demonstration that B. burgdorferi EbfC is a site-specific DNA-binding protein has broad implications across the eubacterial kingdom.

A super-family of transcriptional activators regulates bacteriophage packaging and lysis in Gram-positive bacteria

The propagation of bacteriophages and other mobile genetic elements requires exploitation of the phage mechanisms involved in virion assembly and DNA packaging. Here, we identified and characterized four different families of phage-encoded proteins that function as activators required for transcription of the late operons (morphogenetic and lysis genes) in a large group of phages infecting Gram-positive bacteria. These regulators constitute a super-family of proteins, here named late transcriptional regulators (Ltr), which share common structural, biochemical and functional characteristics and are unique to this group of phages. They are all small basic proteins, encoded by genes present at the end of the early gene cluster in their respective phage genomes and expressed under cI repressor control. To control expression of the late operon, the Ltr proteins bind to a DNA repeat region situated upstream of the terS gene, activating its transcription. This involves the C-terminal part of the Ltr proteins, which control specificity for the DNA repeat region. Finally, we show that the Ltr proteins are the only phage-encoded proteins required for the activation of the packaging and lysis modules. In summary, we provide evidence that phage packaging and lysis is a conserved mechanism in Siphoviridae infecting a wide variety of Gram-positive bacteria.

Evolutionary relationships among actinophages and a putative adaptation for growth in Streptomyces spp

The genome sequences of eight Streptomyces phages are presented, four of which were isolated for this study. Phages R4, TG1, Hau3, and SV1 were isolated previously and have been exploited as tools for understanding and genetically manipulating Streptomyces spp. We also extracted five apparently intact prophages from recent Streptomyces spp. genome projects and, together with six phage genomes in the database, we analyzed all 19 Streptomyces phage genomes with a view to understanding their relationships to each other and to other actinophages, particularly the mycobacteriophages. Fifteen of the Streptomyces phages group into four clusters of related genomes. Although the R4-like phages do not share nucleotide sequence similarity with other phages, they clearly have common ancestry with cluster A mycobacteriophages, sharing many protein homologues, common gene syntenies, and similar repressor-stoperator regulatory systems. The R4-like phage Hau3 and the prophage StrepC.1 (from Streptomyces sp. strain C) appear to have hijacked a unique adaptation of the streptomycetes, i.e., use of the rare UUA codon, to control translation of the essential phage protein, the terminase. The Streptomyces venezuelae generalized transducing phage SV1 was used to predict the presence of other generalized transducing phages for different Streptomyces species.

Type II heat-labile enterotoxins from 50 diverse Escherichia coli isolates belong almost exclusively to the LT-IIc family and may be prophage encoded

Some enterotoxigenic Escherichia coli (ETEC) produce a type II heat-labile enterotoxin (LT-II) that activates adenylate cyclase in susceptible cells but is not neutralized by antisera against cholera toxin or type I heat-labile enterotoxin (LT-I). LT-I variants encoded by plasmids in ETEC from humans and pigs have amino acid sequences that are ≥ 95% identical. In contrast, LT-II toxins are chromosomally encoded and are much more diverse. Early studies characterized LT-IIa and LT-IIb variants, but a novel LT-IIc was reported recently. Here we characterized the LT-II encoding loci from 48 additional ETEC isolates. Two encoded LT-IIa, none encoded LT-IIb, and 46 encoded highly related variants of LT-IIc. Phylogenetic analysis indicated that the predicted LT-IIc toxins encoded by these loci could be assigned to 6 subgroups. The loci corresponding to individual toxins within each subgroup had DNA sequences that were more than 99% identical. The LT-IIc subgroups appear to have arisen by multiple recombinational events between progenitor loci encoding LT-IIc1- and LT-IIc3-like variants. All loci from representative isolates encoding the LT-IIa, LT-IIb, and each subgroup of LT-IIc enterotoxins are preceded by highly-related genes that are between 80 and 93% identical to predicted phage lysozyme genes. DNA sequences immediately following the B genes differ considerably between toxin subgroups, but all are most closely related to genomic sequences found in predicted prophages. Together these data suggest that the LT-II loci are inserted into lambdoid type prophages that may or may not be infectious. These findings raise the possibility that production of LT-II enterotoxins by ETEC may be determined by phage conversion and may be activated by induction of prophage, in a manner similar to control of production of Shiga-like toxins by converting phages in isolates of enterohemmorhagic E. coli.

Genome sequencing unveils a novel sea enterotoxin-carrying PVL phage in Staphylococcus aureus ST772 from India

Staphylococcus aureus is a major human pathogen, first recognized as a leading cause of hospital-acquired infections. Community-associated S. aureus (CA-SA) pose a greater threat due to increase in severity of infection and disease among children and healthy adults. CA-SA strains in India are genetically diverse, among which is the sequence type (ST) 772, which has now spread to Australia, Europe and Japan. Towards understanding the genetic characteristics of ST772, we obtained draft genome sequences of five relevant clinical isolates and studied the properties of their PVL-carrying prophages, whose presence is a defining hallmark of CA-SA. We show that this is a novel prophage, which carries the structural genes of the hlb-carrying prophage and includes the sea enterotoxin. This architecture probably emerged early within the ST772 lineage, at least in India. The sea gene, unique to ST772 PVL, despite having promoter sequence characteristics typical of low expression, appears to be highly expressed during early phase of growth in laboratory conditions. We speculate that this might be a consequence of its novel sequence context. The crippled nature of the hlb-converting prophage in ST772 suggests that widespread mobility of the sea enterotoxin might be a selective force behind its 'transfer' to the PVL prophage. Wild type ST772 strains induced strong proliferative responses as well as high cytotoxic activity against neutrophils, likely mediated by superantigen SEA and the PVL toxin respectively. Both proliferation and cytotoxicity were markedly reduced in a cured ST772 strain indicating the impact of the phage on virulence. The presence of SEA alongside the genes for the immune system-modulating PVL toxin may contribute to the success and virulence of ST772.

Lysogenic host-virus interactions in SAR11 marine bacteria

Host-virus interactions structure microbial communities, drive biogeochemical cycles and enhance genetic diversity in nature1,2. Hypotheses proposed to explain the range of interactions that mediate these processes often invoke lysogeny3-6, a latent infection strategy used by temperate bacterial viruses to replicate in host cells until an induction event triggers the production and lytic release of free viruses. Most cultured bacteria harbour temperate viruses in their genomes (prophage)7. The absence of prophages in cultures of the dominant lineages of marine bacteria has contributed to an ongoing debate over the ecological significance of lysogeny and other viral life strategies in nature6,8-15. Here, we report the discovery of prophages in cultured SAR11, the ocean's most abundant clade of heterotrophic bacteria16,17. We show the concurrent production of cells and viruses, with enhanced virus production under carbon-limiting growth conditions. Evidence that related prophages are broadly distributed in the oceans suggests that similar interactions have contributed to the evolutionary success of SAR11 in nutrient-limited systems.

Digital phagograms: predicting phage infectivity through a multilayer machine learning approach

Machine learning has been broadly implemented to investigate biological systems. In this regard, the field of phage biology has embraced machine learning to elucidate and predict phage-host interactions, based on receptor-binding proteins, (anti-)defense systems, prophage detection, and life cycle recognition. Here, we highlight the enormous potential of integrating information from omics data with insights from systems biology to better understand phage-host interactions. We conceptualize and discuss the potential of a multilayer model that mirrors the phage infection process, integrating adsorption, bacterial pan-immune components and hijacking of the bacterial metabolism to predict phage infectivity. In the future, this model can offer insights into the underlying mechanisms of the infection process, and digital phagograms can support phage cocktail design and phage engineering.