The genome sequence of Escherichia coli tailed phage D6 and the diversity of Enterobacteriales circular plasmid prophages

Bacteriophages and bacterial extracellular vesicles, threat or opportunity?

Emergence of antimicrobial-resistant bacteria (AMR) is one of the health major problems worldwide. The scientists are looking for a novel method to treat infectious diseases. Phage therapy is considered a suitable approach for treating infectious diseases. However, there are different challenges in this way. Some biological aspects can probably influence on therapeutic results and further investigations are necessary to reach a successful phage therapy. Bacteriophage activity can influence by bacterial defense system. Bacterial extracellular vesicles (BEVs) are one of the bacterial defense mechanisms which can modify the results of bacteriophage activity. BEVs have the significant roles in the gene transferring, invasion, escape, and spreading of bacteriophages. In this review, the defense mechanisms of bacteria against bacteriophages, especially BEVs secretion, the hidden linkage of BEVs and bacteriophages, and its possible consequences on the bacteriophage activity as well phage therapy will be discussed.

Presence of phage-plasmids in multiple serovars of Salmonella enterica

Evidence is accumulating in the literature that the horizontal spread of antimicrobial resistance (AMR) genes mediated by bacteriophages and bacteriophage-like plasmid (phage-plasmid) elements is much more common than previously envisioned. For instance, we recently identified and characterized a circular P1-like phage-plasmid harbouring a bla CTX-M-15 gene conferring extended-spectrum beta-lactamase (ESBL) resistance in Salmonella enterica serovar Typhi. As the prevalence and epidemiological relevance of such mechanisms has never been systematically assessed in Enterobacterales, in this study we carried out a follow-up retrospective analysis of UK Salmonella isolates previously sequenced as part of routine surveillance protocols between 2016 and 2021. Using a high-throughput bioinformatics pipeline we screened 47 784 isolates for the presence of the P1 lytic replication gene repL, identifying 226 positive isolates from 25 serovars and demonstrating that phage-plasmid elements are more frequent than previously thought. The affinity for phage-plasmids appears highly serovar-dependent, with several serovars being more likely hosts than others; most of the positive isolates (170/226) belonged to S. Typhimurium ST34 and ST19. The phage-plasmids ranged between 85.8 and 98.2 kb in size, with an average length of 92.1 kb; detailed analysis indicated a high amount of diversity in gene content and genomic architecture. In total, 132 phage-plasmids had the p0111 plasmid replication type, and 94 the IncY type; phylogenetic analysis indicated that both horizontal and vertical gene transmission mechanisms are likely to be involved in phage-plasmid propagation. Finally, phage-plasmids were present in isolates that were resistant and non-resistant to antimicrobials. In addition to providing a first comprehensive view of the presence of phage-plasmids in Salmonella, our work highlights the need for a better surveillance and understanding of phage-plasmids as AMR carriers, especially through their characterization with long-read sequencing.

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.

Analysis and culturing of the prototypic crAssphage reveals a phage-plasmid lifestyle

The prototypic crAssphage (Carjivirus communis) is one of the most abundant, prevalent, and persistent gut bacteriophages, yet it remains uncultured and its lifestyle uncharacterized. For the last decade, crAssphage has escaped plaque-dependent culturing efforts, leading us to investigate alternative lifestyles that might explain its widespread success. Through genomic analyses and culturing, we find that crAssphage uses a phage-plasmid lifestyle to persist extrachromosomally. Plasmid-related genes are more highly expressed than those implicated in phage maintenance. Leveraging this finding, we use a plaque-free culturing approach to measure crAssphage replication in culture with Phocaeicola vulgatus, Phocaeicola dorei, and Bacteroides stercoris, revealing a broad host range. We demonstrate that crAssphage persists with its hosts in culture without causing major cell lysis events or integrating into host chromosomes. The ability to switch between phage and plasmid lifestyles within a wide range of hosts contributes to the prolific nature of crAssphage in the human gut microbiome.

Phage-plasmids promote recombination and emergence of phages and plasmids

Phages and plasmids are regarded as distinct types of mobile genetic elements that drive bacterial evolution by horizontal gene transfer. However, the distinction between both types is blurred by the existence of elements known as prophage-plasmids or phage-plasmids, which transfer horizontally between cells as viruses and vertically within cellular lineages as plasmids. Here, we study gene flow between the three types of elements. We show that the gene repertoire of phage-plasmids overlaps with those of phages and plasmids. By tracking recent recombination events, we find that phage-plasmids exchange genes more frequently with plasmids than with phages, and that direct gene exchange between plasmids and phages is less frequent in comparison. The results suggest that phage-plasmids can mediate gene flow between plasmids and phages, including exchange of mobile element core functions, defense systems, and antibiotic resistance. Moreover, a combination of gene transfer and gene inactivation may result in the conversion of elements. For example, gene loss turns P1-like phage-plasmids into integrative prophages or into plasmids (that are no longer phages). Remarkably, some of the latter have acquired conjugation-related functions to became mobilisable by conjugation. Thus, our work indicates that phage-plasmids can play a key role in the transfer of genes across mobile elements within their hosts, and can act as intermediates in the conversion of one type of element into another.

Mutation-induced infections of phage-plasmids

Phage-plasmids are extra-chromosomal elements that act both as plasmids and as phages, whose eco-evolutionary dynamics remain poorly constrained. Here, we show that segregational drift and loss-of-function mutations play key roles in the infection dynamics of a cosmopolitan phage-plasmid, allowing it to create continuous productive infections in a population of marine Roseobacter. Recurrent loss-of-function mutations in the phage repressor that controls prophage induction leads to constitutively lytic phage-plasmids that spread rapidly throughout the population. The entire phage-plasmid genome is packaged into virions, which were horizontally transferred by re-infecting lysogenized cells, leading to an increase in phage-plasmid copy number and to heterozygosity in a phage repressor locus in re-infected cells. However, the uneven distribution of phage-plasmids after cell division (i.e., segregational drift) leads to the production of offspring carrying only the constitutively lytic phage-plasmid, thus restarting the lysis-reinfection-segregation life cycle. Mathematical models and experiments show that these dynamics lead to a continuous productive infection of the bacterial population, in which lytic and lysogenic phage-plasmids coexist. Furthermore, analyses of marine bacterial genome sequences indicate that the plasmid backbone here can carry different phages and disseminates trans-continentally. Our study highlights how the interplay between phage infection and plasmid genetics provides a unique eco-evolutionary strategy for phage-plasmids.

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.

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.

Characterization of a P1-bacteriophage-like plasmid (phage-plasmid) harbouring bla CTX-M-15 in Salmonella enterica serovar Typhi

Antimicrobial-resistance (AMR) genes can be transferred between microbial cells via horizontal gene transfer (HGT), which involves mobile and integrative elements such as plasmids, bacteriophages, transposons, integrons and pathogenicity islands. Bacteriophages are found in abundance in the microbial world, but their role in virulence and AMR has not fully been elucidated in the Enterobacterales. With short-read sequencing paving the way to systematic high-throughput AMR gene detection, long-read sequencing technologies now enable us to establish how such genes are structurally connected into meaningful genomic units, raising questions about how they might cooperate to achieve their biological function. Here, we describe a novel ~98 kbp circular P1-bacteriophage-like plasmid termed ph681355 isolated from a clinical Salmonella enterica serovar Typhi isolate. It carries bla _CTX-M-15, an IncY plasmid replicon (repY gene) and the ISEcP1 mobile element and is, to our knowledge, the first reported P1-bacteriophage-like plasmid (phage-plasmid) in S. enterica Typhi. We compared ph681355 to two previously described phage-plasmids, pSJ46 from S. enterica serovar Indiana and pMCR-1-P3 from Escherichia coli, and found high nucleotide similarity across the backbone. However, we saw low ph681355 backbone similarity to plasmid p60006 associated with the extensively drug-resistant S. enterica Typhi outbreak isolate in Pakistan, providing evidence of an alternative route for bla _CTX-M-15 transmission. Our discovery highlights the importance of utilizing long-read sequencing in interrogating bacterial genomic architecture to fully understand AMR mechanisms and their clinical relevance. It also raises questions regarding how widespread bacteriophage-mediated HGT might be, suggesting that the resulting genomic plasticity might be higher than previously thought.

RecBCD enzyme and Chi recombination hotspots as determinants of self vs. non-self: Myths and mechanisms

Bacteria face a challenge when DNA enters their cells by transformation, mating, or phage infection. Should they treat this DNA as an invasive foreigner and destroy it, or consider it one of their own and potentially benefit from incorporating new genes or alleles to gain useful functions? It is frequently stated that the short nucleotide sequence Chi (5' GCTGGTGG 3'), a hotspot of homologous genetic recombination recognized by Escherichia coli's RecBCD helicase-nuclease, allows E. coli to distinguish its DNA (self) from any other DNA (non-self) and to destroy non-self DNA, and that Chi is "over-represented" in the E. coli genome. We show here that these latter statements (dogmas) are not supported by available evidence. We note Chi's wide-spread occurrence and activity in distantly related bacterial species and phages. We illustrate multiple, highly non-random features of the genomes of E. coli and coliphage P1 that account for Chi's high frequency and genomic position, leading us to propose that P1 selects for Chi's enhancement of recombination, whereas E. coli selects for the preferred codons in Chi. We discuss other, previously described mechanisms for self vs. non-self determination involving RecBCD and for RecBCD's destruction of DNA that cannot recombine, whether foreign or domestic, with or without Chi.

Genome sequencing and comparative analysis of Wolbachia strain wAlbA reveals Wolbachia-associated plasmids are common

Wolbachia are widespread maternally-transmitted bacteria of arthropods that often spread by manipulating their host's reproduction through cytoplasmic incompatibility (CI). Their invasive potential is currently being harnessed in field trials aiming to control mosquito-borne diseases. Wolbachia genomes commonly harbour prophage regions encoding the cif genes which confer their ability to induce CI. Recently, a plasmid-like element was discovered in wPip, a Wolbachia strain infecting Culex mosquitoes; however, it is unclear how common such extra-chromosomal elements are in Wolbachia. Here we sequenced the complete genome of wAlbA, a strain of the symbiont found in Aedes albopictus, after eliminating the co-infecting and higher density wAlbB strain that previously made sequencing of wAlbA challenging. We show that wAlbA is associated with two new plasmids and identified additional Wolbachia plasmids and related chromosomal islands in over 20% of publicly available Wolbachia genome datasets. These plasmids encode a variety of accessory genes, including several phage-like DNA packaging genes as well as genes potentially contributing to host-symbiont interactions. In particular, we recovered divergent homologues of the cif genes in both Wolbachia- and Rickettsia-associated plasmids. Our results indicate that plasmids are common in Wolbachia and raise fundamental questions around their role in symbiosis. In addition, our comparative analysis provides useful information for the future development of genetic tools to manipulate and study Wolbachia symbionts.

Comparative genome analysis of Vagococcus fluvialis reveals abundance of mobile genetic elements in sponge-isolated strains

BACKGROUND

Vagococcus fluvialis is a species of lactic acid bacteria found both free-living in river and seawater and associated to hosts, such as marine sponges. This species has been greatly understudied, with no complete genome assembly available to date, which is essential for the characterisation of the mobilome.

RESULTS

We sequenced and assembled de novo the complete genome sequences of five V. fluvialis isolates recovered from marine sponges. Pangenome analysis of the V. fluvialis species (total of 17 genomes) showed a high intraspecific diversity, with 45.5% of orthologous genes found to be strain specific. Despite this diversity, analyses of gene functions clustered all V. fluvialis species together and separated them from other sequenced Vagococcus species. V. fluvialis strains from different habitats were highly similar in terms of functional diversity but the sponge-isolated strains were enriched in several functions related to the marine environment. Furthermore, sponge-isolated strains carried a significantly higher number of mobile genetic elements (MGEs) compared to previously sequenced V. fluvialis strains from other environments. Sponge-isolated strains carried up to 4 circular plasmids each, including a 48-kb conjugative plasmid. Three of the five strains carried an additional circular extrachromosomal sequence, assumed to be an excised prophage as it contained mainly viral genes and lacked plasmid replication genes. Insertion sequences (ISs) were up to five times more abundant in the genomes of sponge-isolated strains compared to the others, including several IS families found exclusively in these genomes.

CONCLUSIONS

Our findings highlight the dynamics and plasticity of the V. fluvialis genome. The abundance of mobile genetic elements in the genomes of sponge-isolated V. fluvialis strains suggests that the mobilome might be key to understanding the genomic signatures of symbiosis in bacteria.

Complete genome sequences and genomic characterization of five plasmids harbored by environmentally persistent Cronobacter sakazakii strains ST83 H322 and ST64 GK1025B obtained from powdered infant formula manufacturing facilities

Background

Cronobacter sakazakii is a foodborne pathogen that causes septicemia, meningitis, and necrotizing enterocolitis in neonates and infants. The current research details the full genome sequences of two extremely persistent C. sakazakii strains (H322 and GK1025B) isolated from powdered infant formula (PIF) manufacturing settings. In addition, the genetic attributes associated with five plasmids, pH322_1, pH322_2, pGK1025B_1, pGK1025B_2, and pGK1025B_3 are described.

Materials and Methods

Using PacBio single-molecule real-time (SMRT^®) sequencing technology, whole genome sequence (WGS) assemblies of C. sakazakii H322 [Sequence type (ST)83, clonal complex [CC] 83) and GK1025B (ST64, CC64) were generated. Plasmids, also sequenced, were aligned with phylogenetically related episomes to determine, and identify conserved and missing genomic regions.

Results

A truncated ~ 13 Kbp type 6 secretion system (T6SS) gene cluster harbored on virulence plasmids pH322_2 and pGK1025B_2, and a second large deletion (~ 6 Kbp) on pH322_2, which included genes for a tyrosine-type recombinase/integrase, a hypothetical protein, and a phospholipase D was identified. Within the T6SS of pH322_2 and pGK1025B_2, an arsenic resistance operon was identified which is in common with that of plasmids pSP291_1 and pESA3. In addition, PHASTER analysis identified an intact 96.9 Kbp Salmonella SSU5 prophage gene cluster in pH322_1 and pGK1025B_1 and showed that these two plasmids were phylogenetically related to C. sakazakii plasmids: pCS1, pCsa767a, pCsaC757b, pCsaC105731a. Plasmid pGK1025B_3 was identified as a novel conjugative Cronobacter plasmid. Furthermore, WGS analysis identified a ~ 16.4 Kbp type 4 secretion system gene cluster harbored on pGK1025B_3, which contained a phospholipase D gene, a key virulence factor in several host–pathogen diseases.

Conclusion

These data provide high resolution information on C. sakazakii genomes and emphasizes the need for furthering surveillance studies to link genotype to phenotype of strains from previous investigations. These results provide baseline data necessary for future in-depth investigations of C. sakazakii that colonize PIF manufacturing facility settings and genomic analyses of these two C. sakazakii strains and five associated plasmids will contribute to a better understanding of this pathogen's survival and persistence within various “built environments” like PIF manufacturing facilities.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13099-022-00500-5.

New Insights into the Structure and Assembly of Bacteriophage P1

Bacteriophage P1 is the premier transducing phage of E. coli. Despite its prominence in advancing E. coli genetics, modern molecular techniques have not been applied to thoroughly understand P1 structure. Here, we report the proteome of the P1 virion as determined by liquid chromatography tandem mass-spectrometry. Additionally, a library of single-gene knockouts identified the following five previously unknown essential genes: pmgA, pmgB, pmgC, pmgG, and pmgR. In addition, proteolytic processing of the major capsid protein is a known feature of P1 morphogenesis, and we identified the processing site by N-terminal sequencing to be between E120 and S121, producing a 448-residue, 49.3 kDa mature peptide. Furthermore, the P1 defense against restriction (Dar) system consists of six known proteins that are incorporated into the virion during morphogenesis. The largest of these, DarB, is a 250 kDa protein that is believed to translocate into the cell during infection. DarB deletions indicated the presence of an N-terminal packaging signal, and the N-terminal 30 residues of DarB are shown to be sufficient for directing a heterologous reporter protein to the capsid. Taken together, the data expand on essential structural P1 proteins as well as introduces P1 as a nanomachine for cellular delivery.

A Plasmid With Conserved Phage Genes Helps Klebsiella pneumoniae Defend Against the Invasion of Transferable DNA Elements at the Cost of Reduced Virulence

Klebsiella pneumoniae exhibits extensive phenotypic and genetic diversity. Higher plasmid loads in the cell were supposed to play an key role in its genome diversity. Although some plasmids are widely distributed in Kp populations, they are poorly recognized. A plasmid named p2 in strain Kp1604 was predicted to be an intact prophage like Salmonella phage SSU5. However, our study showed that p2 was specifically packaged into membrane vesicles (MVs) rather than phage particles triggered by mitomycin C and subinhibitory concentrations of antibiotics. p2-minus mutant Kp1604Δp2 did not affect MV production. Compared with Kp1604, the capacity of plasmid uptake and the amount of phage burst of Kp1604Δp2 were improved. Moreover, virulence of Kp1604Δp2 also increased. Our results indicated that p2 could contribute to the host defense against the invasion of transferable DNA elements at the cost of reduced virulence. Further study on the mechanism will help us understand how it provides adaptive phenotypes to host evolution.

Induction and Elimination of Prophages Using CRISPR Interference

Prophages are widely spread among bacterial genomes, and they can have positive or negative effects on their hosts. A key aspect in the study of prophages is the discovery of their induction signals. Prophage induction can occur by inactivating a phage transcriptional repressor, which is responsible for maintaining the lysogenic state. This repressor can be inactivated through the bacterial SOS response. However, the induction signals for numerous prophages do not involve the SOS system, and therefore significant efforts are needed to identify these conditions. Similarly, curing bacterial strains of inducible prophages is a tedious process, requiring the screening of several colonies. Here, we investigated whether transcriptional silencing of a prophage repressor using CRISPR interference (CRISPRi) would lead to prophage induction. Using Escherichia coli phages λ and P2 as models, we demonstrated the efficiency of CRISPRi for prophage induction and for curing lysogenic strains of their prophages.

Long-read-sequenced reference genomes of the seven major lineages of enterotoxigenic Escherichia coli (ETEC) circulating in modern time

Enterotoxigenic Escherichia coli (ETEC) is an enteric pathogen responsible for the majority of diarrheal cases worldwide. ETEC infections are estimated to cause 80,000 deaths annually, with the highest rates of burden, ca 75 million cases per year, amongst children under 5 years of age in resource-poor countries. It is also the leading cause of diarrhoea in travellers. Previous large-scale sequencing studies have found seven major ETEC lineages currently in circulation worldwide. We used PacBio long-read sequencing combined with Illumina sequencing to create high-quality complete reference genomes for each of the major lineages with manually curated chromosomes and plasmids. We confirm that the major ETEC lineages all harbour conserved plasmids that have been associated with their respective background genomes for decades, suggesting that the plasmids and chromosomes of ETEC are both crucial for ETEC virulence and success as pathogens. The in-depth analysis of gene content, synteny and correct annotations of plasmids will elucidate other plasmids with and without virulence factors in related bacterial species. These reference genomes allow for fast and accurate comparison between different ETEC strains, and these data will form the foundation of ETEC genomics research for years to come.

Molecular characterization of plasmids encoding blaCTX-M from faecal Escherichia coli in travellers returning to the UK from South Asia

BACKGROUND

The global prevalence of extended-spectrum beta-lactamase-producing Escherichia coli is rising and is dominated by bla_CTX-M spread by plasmids. Travellers to South Asia from Western Europe have high rates of acquisition of faecal CTX-M-producing E. coli (CTX-M-EC).

AIMS

To determine the conjugative ability of CTX-M-EC acquired by healthy volunteers after travel to South Asia, the proportion of travel-acquired CTX-M-EC where bla_CTX-M is encoded on a plasmid vs on the bacterial chromosome, and the relatedness of travel-acquired CTX-M-EC plasmids to previously sequenced plasmids.

METHODS

Faecal samples were collected pre- and post-travel from 23 volunteers who visited South Asia, and CTX-M-EC were cultured. After short- and long-read sequencing, 10 plasmid sequences were identified and compared with previously sequenced plasmids in GenBank. Conjugation to E. coli K-12 was undertaken using filter mating.

FINDINGS

Thirty-five percent of CTX-M-EC isolates tested transferred the bla_CTX-M plasmid by conjugation. Travel-acquired CTX-M-EC carried bla_CTX-M on a plasmid in 62% of isolates, whereas 38% of isolates had bla_CTX-M on the chromosome. CTX-M-EC plasmids acquired after travel to South Asia had close homology to previously described epidemic plasmids which are widely disseminated in humans, animals and the natural environment.

CONCLUSION

Globally successful epidemic plasmids are involved in the spread of CTX-M-EC. Targeted strategies may be used to displace such plasmids from the host strain as part of efforts in infection prevention and control in healthcare settings. Bacteria with bla_CTX-M plasmids were readily acquired by healthy volunteers, and were carried on return to the UK, providing opportunities for onward dissemination.

Putative plasmid prophages of Bacillus cereus sensu lato may hold the key to undiscovered phage diversity

Bacteriophages are bacterial viruses and the most abundant biological entities on Earth. Temperate bacteriophages can form prophages stably maintained in the host population: they either integrate into the host genome or replicate as plasmids in the host cytoplasm. As shown, tailed temperate bacteriophages may form circular plasmid prophages in many bacterial species of the taxa Firmicutes, Gammaproteobacteria and Spirochaetes. The actual number of such prophages is thought to be underestimated for two main reasons: first, in bacterial whole genome-sequencing assemblies, they are difficult to distinguish from actual plasmids; second, there is an absence of experimental studies which are vital to confirm their existence. In Firmicutes, such prophages appear to be especially numerous. In the present study, we identified 23 genomes from species of the Bacillus cereus group that were deposited in GenBank as plasmids and may belong to plasmid prophages with little or no homology to known viruses. We consider these putative prophages worth experimental assays since it will broaden our knowledge of phage diversity and suggest that more attention be paid to such molecules in all bacterial sequencing projects as this will help in identifying previously unknown phages.

Bacteria have numerous distinctive groups of phage-plasmids with conserved phage and variable plasmid gene repertoires

Plasmids and temperate phages are key contributors to bacterial evolution. They are usually regarded as very distinct. However, some elements, termed phage–plasmids, are known to be both plasmids and phages, e.g. P1, N15 or SSU5. The number, distribution, relatedness and characteristics of these phage–plasmids are poorly known. Here, we screened for these elements among ca. 2500 phages and 12000 plasmids and identified 780 phage–plasmids across very diverse bacterial phyla. We grouped 92% of them by similarity of gene repertoires to eight defined groups and 18 other broader communities of elements. The existence of these large groups suggests that phage–plasmids are ancient. Their gene repertoires are large, the average element is larger than an average phage or plasmid, and they include slightly more homologs to phages than to plasmids. We analyzed the pangenomes and the genetic organization of each group of phage–plasmids and found the key phage genes to be conserved and co-localized within distinct groups, whereas genes with homologs in plasmids are much more variable and include most accessory genes. Phage–plasmids are a sizeable fraction of the sequenced plasmids (∼7%) and phages (∼5%), and could have key roles in bridging the genetic divide between phages and other mobile genetic elements.

The Age of Phage: Friend or Foe in the New Dawn of Therapeutic and Biocontrol Applications?

Extended overuse and misuse of antibiotics and other antibacterial agents has resulted in an antimicrobial resistance crisis. Bacteriophages, viruses that infect bacteria, have emerged as a legitimate alternative antibacterial agent with a wide scope of applications which continue to be discovered and refined. However, the potential of some bacteriophages to aid in the acquisition, maintenance, and dissemination of negatively associated bacterial genes, including resistance and virulence genes, through transduction is of concern and requires deeper understanding in order to be properly addressed. In particular, their ability to interact with mobile genetic elements such as plasmids, genomic islands, and integrative conjugative elements (ICEs) enables bacteriophages to contribute greatly to bacterial evolution. Nonetheless, bacteriophages have the potential to be used as therapeutic and biocontrol agents within medical, agricultural, and food processing settings, against bacteria in both planktonic and biofilm environments. Additionally, bacteriophages have been deployed in developing rapid, sensitive, and specific biosensors for various bacterial targets. Intriguingly, their bioengineering capabilities show great promise in improving their adaptability and effectiveness as biocontrol and detection tools. This review aims to provide a balanced perspective on bacteriophages by outlining advantages, challenges, and future steps needed in order to boost their therapeutic and biocontrol potential, while also providing insight on their potential role in contributing to bacterial evolution and survival.

Genomic, Morphological and Functional Characterization of Virulent Bacteriophage IME-JL8 Targeting Citrobacter freundii

Citrobacter freundii refers to a fish pathogen extensively reported to be able to cause injury and high mortality. Phage therapy is considered a process to alternatively control bacterial infections and contaminations. In the present study, the isolation of a virulent bacteriophage IME-JL8 isolated from sewage was presented, and such bacteriophage was characterized to be able to infect Citrobacter freundii specifically. Phage IME-JL8 has been classified as the member of the Siphoviridae family, which exhibits the latent period of 30–40 min. The pH and thermal stability of phage IME-JL8 demonstrated that this bacteriophage achieved a pH range of 4–10 as well as a temperature range of 4, 25, and 37°C. As revealed from the results of whole genomic sequence analysis, IME-JL8 covers a double-stranded genome of 49,838 bp (exhibiting 47.96% G+C content), with 80 putative coding sequences contained. No bacterial virulence- or lysogenesis-related ORF was identified in the IME-JL8 genome, so it could be applicable to phage therapy. As indicated by the in vitro experiments, phage IME-JL8 is capable of effectively removing bacteria (the colony count decreased by 6.8 log units at 20 min), and biofilm can be formed in 24 h. According to the in vivo experiments, administrating IME-JL8 (1 × 10⁷ PFU) was demonstrated to effectively protect the fish exhibiting a double median lethal dose (2 × 10⁹ CFU/carp). Moreover, the phage treatment led to the decline of pro-inflammatory cytokines in carp with lethal infections. IME-JL8 was reported to induce efficient lysis of Citrobacter freundii both in vitro and in vivo, thereby demonstrating its potential as an alternative treatment strategy for infections attributed to Citrobacter freundii.

Whole-genome epidemiology links phage-mediated acquisition of a virulence gene to the clonal expansion of a pandemic Salmonella enterica serovar Typhimurium clone

Epidemic and pandemic clones of bacterial pathogens with distinct characteristics continually emerge, replacing those previously dominant through mechanisms that remain poorly characterized. Here, whole-genome-sequencing-powered epidemiology linked horizontal transfer of a virulence gene, sopE, to the emergence and clonal expansion of a new epidemic Salmonella enterica serovar Typhimurium (S. Typhimurium) clone. The sopE gene is sporadically distributed within the genus Salmonella and rare in S. enterica Typhimurium lineages, but was acquired multiple times during clonal expansion of the currently dominant pandemic monophasic S. Typhimurium sequence type (ST) 34 clone. Ancestral state reconstruction and time-scaled phylogenetic analysis indicated that sopE was not present in the common ancestor of the epidemic clade, but later acquisition resulted in increased clonal expansion of sopE-containing clones that was temporally associated with emergence of the epidemic, consistent with increased fitness. The sopE gene was mainly associated with a temperate bacteriophage mTmV, but recombination with other bacteriophage and apparent horizontal gene transfer of the sopE gene cassette resulted in distribution among at least four mobile genetic elements within the monophasic S. enterica Typhimurium ST34 epidemic clade. The mTmV prophage lysogenic transfer to other S. enterica serovars in vitro was limited, but included the common pig-associated S. enterica Derby (S. Derby). This may explain mTmV in S. Derby co-circulating on farms with monophasic S. Typhimurium ST34, highlighting the potential for further transfer of the sopE virulence gene in nature. We conclude that whole-genome epidemiology pinpoints potential drivers of evolutionary and epidemiological dynamics during pathogen emergence, and identifies targets for subsequent research in epidemiology and bacterial pathogenesis.

Phage Therapy: Consider the Past, Embrace the Future

Antibiotic-resistant bacteria infections pose a threat to public health. Considering the difficulty in developing new antibiotics, it is an urgent need to develop alternative therapies against bacterial pathogens. Bacteriophages (phages) are evaluated as potential substitutes or adjuncts of antibiotics because they are abundant in nature and could specifically lyse bacteria. In this review, we briefly introduce phage therapy and its advantages compared with traditional antibiotic therapy. We also summarize new emerging phage technologies, such as CRISPR-Cas, synthetic phages, etc., and discuss some possible obstacles and potential risks in the application process. We believe that, with the advancement in synthetic biology and delivery technology, phage therapy has broad prospects in the future.

DNA Packaging and Genomics of the Salmonella 9NA-Like Phages

We present the genome sequences of Salmonella enterica tailed phages Sasha, Sergei, and Solent. These phages, along with Salmonella phages 9NA, FSL_SP-062, and FSL_SP-069 and the more distantly related Proteus phage PmiS-Isfahan, have similarly sized genomes of between 52 and 57 kbp in length that are largely syntenic. Their genomes also show substantial genome mosaicism relative to one another, which is common within tailed phage clusters. Their gene content ranges from 80 to 99 predicted genes, of which 40 are common to all seven and form the core genome, which includes all identifiable virion assembly and DNA replication genes. The total number of gene types (pangenome) in the seven phages is 176, and 59 of these are unique to individual phages. Their core genomes are much more closely related to one another than to the genome of any other known phage, and they comprise a well-defined cluster within the family Siphoviridae To begin to characterize this group of phages in more experimental detail, we identified the genes that encode the major virion proteins and examined the DNA packaging of the prototypic member, phage 9NA. We show that it uses a pac site-directed headful packaging mechanism that results in virion chromosomes that are circularly permuted and about 13% terminally redundant. We also show that its packaging series initiates with double-stranded DNA cleavages that are scattered across a 170-bp region and that its headful measuring device has a precision of ±1.8%.IMPORTANCE The 9NA-like phages are clearly highly related to each other but are not closely related to any other known phage type. This work describes the genomes of three new 9NA-like phages and the results of experimental analysis of the proteome of the 9NA virion and DNA packaging into the 9NA phage head. There is increasing interest in the biology of phages because of their potential for use as antibacterial agents and for their ecological roles in bacterial communities. 9NA-like phages that infect two bacterial genera have been identified to date, and related phages infecting additional Gram-negative bacterial hosts are likely to be found in the future. This work provides a foundation for the study of these phages, which will facilitate their study and potential use.

Population analysis of D6-like plasmid prophage variants associated with specific IncC plasmid types in the emerging Salmonella Typhimurium ST213 genotype

The Salmonella enterica serovar Typhimurium sequence type 213 (ST213) emerged as a predominant genotype in Mexico. It is characterized by harboring multidrug resistance (MDR) IncC plasmids (previously IncA/C) and the lack of the Salmonella virulence plasmid (pSTV). Here we show that the D6-like plasmid prophage is present in most of the ST213 strains. We used the reported nucleotide sequence of YU39 plasmid (pYU39_89) to design a PCR typing scheme for the D6-like plasmid prophages, and determined the complete nucleotide sequences for the D6-like prophages of three additional ST213 strains (YU07-18, SL26 and SO21). Two prophage variants were described: i) a complete prophage, containing homologous sequences for most of the genetic modules described in P1 and D6 phages, which most likely allow for the lytic and lysogenic lifestyles; and ii) an incomplete prophage, lacking a 15 kb region containing morphogenesis genes, suggesting that it is defective. The tail fiber gene inversion region was the most divergent one between D6 and pYU39_89 genomes, suggesting the production of a distinct set of tail fibers, which could be involved in host range preferences. A glutaminyl-tRNA synthetase gene (glnS), which could be involved in providing host cell increased fitness or plasmid maintenance functions, was found in all D6-like genomes. Population level analysis revealed a biogeographic pattern of distribution of these plasmid-phages and specific associations with variants of MDR IncC plasmids. Statistically significant associations were found between the two prophage variants (p75 or p89), the type of IncC plasmids (I or II) and geographic isolation regions (Sonora, San Luis Potosí, Michoacán and Yucatán). This work integrates results from molecular typing, genomics and epidemiology to provide a broad overview for the evolution of an emergent Salmonella genotype.

Prophages in Salmonella enterica: a driving force in reshaping the genome and physiology of their bacterial host?

Thanks to the exponentially increasing number of publicly available bacterial genome sequences, one can now estimate the important contribution of integrated viral sequences to the diversity of bacterial genomes. Indeed, temperate bacteriophages are able to stably integrate the genome of their host through site‐specific recombination and transmit vertically to the host siblings. Lysogenic conversion has been long acknowledged to provide additional functions to the host, and particularly to bacterial pathogen genomes where prophages contribute important virulence factors. This review aims particularly at highlighting the current knowledge and questions about lysogeny in Salmonella genomes where functional prophages are abundant, and where genetic interactions between host and prophages are of particular importance for human health considerations.

Molecular and Evolutionary Determinants of Bacteriophage Host Range

The host range of a bacteriophage is the taxonomic diversity of hosts it can successfully infect. Host range, one of the central traits to understand in phages, is determined by a range of molecular interactions between phage and host throughout the infection cycle. While many well studied model phages seem to exhibit a narrow host range, recent ecological and metagenomics studies indicate that phages may have specificities that range from narrow to broad. There is a growing body of studies on the molecular mechanisms that enable phages to infect multiple hosts. These mechanisms, and their evolution, are of considerable importance to understanding phage ecology and the various clinical, industrial, and biotechnological applications of phage. Here we review knowledge of the molecular mechanisms that determine host range, provide a framework defining broad host range in an evolutionary context, and highlight areas for additional research.

Genome sequencing and functional characterization of the non-pathogenic Klebsiella pneumoniae KpGe bacteria

Klebsiella pneumoniae is an extensively studied human pathogen responsible for a wide variety of infections. Dictyostelium discoideum is a model host organism employed to study many facets of the complex interactions between phagocytic cells and bacteria. Historically, a non-pathogenic strain of K. pneumoniae has been used to feed Dictyostelium amoebae, and more recently to study cellular mechanisms involved in bacterial recognition, ingestion and killing. Here we provide the full genome sequence and functional characterization of this non-pathogenic KpGe strain.