Signals triggering prophage induction in the gut microbiota

RNA-based regulation in bacteria-phage interactions

Interactions of bacteria with their viruses named bacteriophages or phages shape the bacterial genome evolution and contribute to the diversity of phages. RNAs have emerged as key components of several anti-phage defense systems in bacteria including CRISPR-Cas, toxin-antitoxin and abortive infection. Frequent association with mobile genetic elements and interplay between different anti-phage defense systems are largely discussed. Newly discovered defense systems such as retrons and CBASS include RNA components. RNAs also perform their well-recognized regulatory roles in crossroad of phage-bacteria regulatory networks. Both regulatory and defensive function can be sometimes attributed to the same RNA molecules including CRISPR RNAs. This review presents the recent advances on the role of RNAs in the bacteria-phage interactions with a particular focus on clostridial species including an important human pathogen, Clostridioides difficile.

Appelmans protocol - A directed in vitro evolution enables induction and recombination of prophages with expanded host range

Infections caused by carbapenem-resistant Acinetobacter baumannii (CRAB) present significant healthcare challenges due to limited treatment options. Bacteriophage (phage) therapy offers potential as an alternative treatment. However, the high host specificity of phages poses challenges for their therapeutic application. To broaden the phage spectrum, laboratory-based phage training using the Appelmans protocol was employed in this study. As a result, the protocol successfully expanded the host range of a phage cocktail targeting CRAB. Further analysis revealed that the expanded host range phages isolated from the output cocktail were identified as recombinant derivatives originating from prophages induced from encountered bacterial strains. These findings provide valuable genetic insights into the protocol's mechanism when applied to phages infecting A. baumannii strains that have never been investigated before. However, it is noteworthy that the expanded host range phages obtained from this protocol exhibited limited stability, raising concerns about their suitability for therapeutic purposes.

The Diversity of Bacteriophages in the Human Gut

Bacteriophages, commonly referred to as phages, are viruses that infect bacteria and are among the most numerous microorganisms on the planet. They occur throughout nature occupying every habitat where their bacterial hosts can be found. Within these communities, phages are responsible for shaping the bacterial community structure and function through their interactions. Phages shape the community structure and function within the human gut but are also able to influence the human host. As such, there is increased interest in understanding the composition and activity of the gastrointestinal phages, although these studies have been hindered by the difficulties accompanying the study of the human gut. Here, we summarize the methods and findings pertaining to the diversity of the human gastrointestinal phages.

Precision run-on sequencing (PRO-seq) for microbiome transcriptomics

Bacteria respond to environmental stimuli through precise regulation of transcription initiation and elongation. Bulk RNA sequencing primarily characterizes mature transcripts, so to identify actively transcribed loci we need to capture RNA polymerase (RNAP) complexed with nascent RNA. However, such capture methods have only previously been applied to culturable, genetically tractable organisms such as E. coli and B. subtilis. Here we apply precision run-on sequencing (PRO-seq) to profile nascent transcription in cultured E. coli and diverse uncultured bacteria. We demonstrate that PRO-seq can characterize the transcription of small, structured, or post-transcriptionally modified RNAs, which are often absent from bulk RNA-seq libraries. Applying PRO-seq to the human microbiome highlights taxon-specific RNAP pause motifs and pause-site distributions across non-coding RNA loci that reflect structure-coincident pausing. We also uncover concurrent transcription and cleavage of CRISPR guide RNAs and transfer RNAs. We demonstrate the utility of PRO-seq for exploring transcriptional dynamics in diverse microbial communities.

The mobilome of Lactobacillus crispatus M247 includes two novel genetic elements: Tn7088 coding for a putative bacteriocin and the siphovirus prophage ΦM247

Lactobacillus crispatus is a member of the vaginal and gastrointestinal human microbiota. Here we determined the complete genome sequence of the probiotic strain M247 combining Nanopore and Illumina technologies. The M247 genome is organized in one circular chromosome of 2 336 109 bp, with a GC content of 37.04 % and 2303 ORFs, of which 1962 could be annotated. Analysis of the M247 mobilome, which accounts for 14 % of the whole genome, revealed the presence of: (i) Tn7088, a novel 14 105 bp long integrative and mobilizable element (IME) containing 16 ORFs; (ii) ΦM247, a novel 42 510 bp long siphovirus prophage containing 52 ORFs; (iii) three clustered regularly interspaced short palindromic repeats (CRISPRs); and (iv) 226 insertion sequences (ISs) belonging to 14 different families. Tn7088 has a modular organization including a mobilization module encoding FtsK homologous proteins and a relaxase, an integration/excision module coding for an integrase and an excisionase, and an adaptation module coding for a class I bacteriocin and homologous to the listeriolysin S (lls) locus of Listeria monocytogenes. Genome-wide homology search analysis showed the presence of Tn7088-like elements in 12 out of 23 L. crispatus complete public genomes. Mobilization and integration/excision modules are essentially conserved, while the adaptation module is variable since it is the target site for the integration of different ISs. Prophage ΦM247 contains genes for phage structural proteins, DNA replication and packaging, lysogenic and lytic cycles. ΦM247-like prophages are present in seven L. crispatus complete genomes, with sequence variability mainly due to the integration of ISs. PCR and sequencing showed that the Tn7088 IME excises from the M247 chromosome producing a circular form at a concentration of 4.32×10-5 copies per chromosome, and reconstitution of the Tn7088 chromosomal target site occurred at 6.65×10-4 copies per chromosome. The ΦM247 prophage produces an excised form and a reconstituted target site at a level of 3.90×10-5 and 2.48×10-5 copies per chromosome, respectively. This study identified two novel genetic elements in L. crispatus. Tn7088 represents the first example of an IME carrying a biosynthetic gene cluster for a class I bacteriocin in L. crispatus.

Widespread and largely unknown prophage activity, diversity, and function in two genera of wheat phyllosphere bacteria

Environmental bacteria host an enormous number of prophages, but their diversity and natural functions remain largely elusive. Here, we investigate prophage activity and diversity in 63 Erwinia and Pseudomonas strains isolated from flag leaves of wheat grown in a single field. Introducing and validating Virion Induction Profiling Sequencing (VIP-Seq), we identify and quantify the activity of 120 spontaneously induced prophages, discovering that some phyllosphere bacteria produce more than 108 virions/mL in overnight cultures, with significant induction also observed in planta. Sequence analyses and plaque assays reveal E. aphidicola prophages contribute a majority of intraspecies genetic diversity and divide their bacterial hosts into antagonistic factions engaged in widespread microbial warfare, revealing the importance of prophage-mediated microdiversity. When comparing spontaneously active prophages with predicted prophages we also find insertion sequences are strongly correlated with non-active prophages. In conclusion, we discover widespread and largely unknown prophage diversity and function in phyllosphere bacteria.

Gut virome profiling identifies an association between temperate phages and colorectal cancer promoted by Helicobacter pylori infection

Colorectal cancer (CRC) is one of the most commonly diagnosed cancers worldwide. While a close correlation between chronic Helicobacter pylori infection and CRC has been reported, the role of the virome has been overlooked. Here, we infected Apc-mutant mouse models and C57BL/6 mice with H. pylori and conducted a comprehensive metagenomics analysis of H. pylori-induced changes in lower gastrointestinal tract bacterial and viral communities. We observed an expansion of temperate phages in H. pylori infected Apc+/1638N mice at the early stage of carcinogenesis. Some of the temperate phages were predicted to infect bacteria associated with CRC, including Enterococcus faecalis. We also observed a high prevalence of virulent genes, such as flgJ, cwlJ, and sleB, encoded by temperate phages. In addition, we identified phages associated with pre-onset and onset of H. pylori-promoted carcinogenesis. Through co-occurrence network analysis, we found strong associations between the viral and bacterial communities in infected mice before the onset of carcinogenesis. These findings suggest that the expansion of temperate phages, possibly caused by prophage induction triggered by H. pylori infection, may have contributed to the development of CRC in mice by interacting with the bacterial community.

An Eco-evolutionary Model on Surviving Lysogeny Through Grounding and Accumulation of Prophages

Temperate phages integrate into the bacterial genomes propagating along with the bacterial genomes. Multiple phage elements, representing diverse prophages, are present in most bacterial genomes. The evolutionary events and the ecological dynamics underlying the accumulation of prophage elements in bacterial genomes have yet to be understood. Here, we show that the local wastewater had 7% of lysogens (hosting mitomycin C-inducible prophages), and they showed resistance to superinfection by their corresponding lysates. Genomic analysis of four lysogens and four non-lysogens revealed the presence of multiple prophages (belonging to Myoviridae and Siphoviridae) in both lysogens and non-lysogens. For large-scale comparison, 2180 Escherichia coli genomes isolated from various sources across the globe and 523 genomes specifically isolated from diverse wastewaters were analyzed. A total of 15,279 prophages were predicted among 2180 E. coli genomes and 2802 prophages among 523 global wastewater isolates, with a mean of ~ 5 prophages per genome. These observations indicate that most putative prophages are relics of past bacteria-phage conflicts; they are "grounded" prophages that cannot excise from the bacterial genome. Prophage distribution analysis based on the sequence homology suggested the random distribution of E. coli prophages within and between E. coli clades. The independent occurrence pattern of these prophages indicates extensive horizontal transfers across the genomes. We modeled the eco-evolutionary dynamics to reconstruct the events that could have resulted in the prophage accumulation accounting for infection, superinfection immunity, and grounding. In bacteria-phage conflicts, the bacteria win by grounding the prophage, which could confer superinfection immunity.

Dietary D-xylose promotes intestinal health by inducing phage production in Escherichia coli

Elimination of specific enteropathogenic microorganisms is critical to gut health. However, the complexity of the gut community makes it challenging to target specific bacterial organisms. Accumulating evidence suggests that various foods can change the abundance of intestinal bacteria by modulating prophage induction. By using pathogenic Escherichia coli (E. coli) ATCC 25922 as a model in this research, we explored the potential of dietary modulation of prophage induction and subsequent bacterial survival. Among a panel of sugars tested in vitro, D-xylose was shown to efficiently induce prophages in E. coli ATCC 25922, which depends, in part, upon the production of D-lactic acid. In an enteric mouse model, prophage induction was found to be further enhanced in response to propionic acid. Dietary D-xylose increased the proportion of Clostridia which converted D-lactic acid to propionic acid. Intestinal propionic acid levels were diminished, following either oral gavage with the dehydrogenase gene (ldhA)-deficient E. coli ATCC 25922 or depletion of intestinal Clostridia by administration of streptomycin. D-Xylose metabolism and exposure to propionic acid triggered E. coli ATCC 25922 SOS response that promoted prophage induction. E. coli ATCC 25922 mutant of RecA, a key component of SOS system, exhibited decreased phage production. These findings suggest the potential of using dietary components that can induce prophages as antimicrobial alternatives for disease control and prevention by targeted elimination of harmful gut bacteria.

The role of bacteriophages in shaping bacterial composition and diversity in the human gut

The microbiota of the gut has continued to co-evolve alongside their human hosts conferring considerable health benefits including the production of nutrients, drug metabolism, modulation of the immune system, and playing an antagonistic role against pathogen invasion of the gastrointestinal tract (GIT). The gut is said to provide a habitat for diverse groups of microorganisms where they all co-habit and interact with one another and with the immune system of humans. Phages are bacterial parasites that require the host metabolic system to replicate via the lytic or lysogenic cycle. The phage and bacterial populations are regarded as the most dominant in the gut ecosystem. As such, among the various microbial interactions, the phage-bacteria interactions, although complex, have been demonstrated to co-evolve over time using different mechanisms such as predation, lysogenic conversion, and phage induction, alongside counterdefense by the bacterial population. With the help of models and dynamics of phage-bacteria interactions, the complexity behind their survival in the gut ecosystem was demystified, and their roles in maintaining gut homeostasis and promoting the overall health of humans were elucidated. Although the treatment of various gastrointestinal infections has been demonstrated to be successful against multidrug-resistant causative agents, concerns about this technique are still very much alive among researchers owing to the potential for phages to evolve. Since a dearth of knowledge exists regarding the use of phages for therapeutic purposes, more studies involving experimental models and clinical trials are needed to widen the understanding of bacteria-phage interactions and their association with immunological responses in the gut of humans.

Structural and functional features of a broad-spectrum prophage-encoded enzybiotic from Enterococcus faecium

Multidrug-resistant (MDR) bacteria have become a growing threat to public health. The gram-positive Enterococcus faecium is classified by WHO as a high-priority pathogen among the global priority list of antibiotic-resistant bacteria. Peptidoglycan-degrading enzymes (PDEs), also known as enzybiotics, are useful bactericidal agents in the fight against resistant bacteria. In this work, a genome-based screening approach of the genome of E. faecium allowed the identification of a putative PDE gene with predictive amidase activity (EfAmi1; EC 3.5.1.28) in a prophage-integrated sequence. EfAmi1 is composed by two domains: a N-terminal Zn²⁺-dependent N-acetylmuramoyl-L-alanine amidase-2 (NALAA-2) domain and a C-terminal domain with unknown structure and function. The full-length gene of EfAmi1 was cloned and expressed as a 6xHis-tagged protein in E. coli. EfAmi1 was produced as a soluble protein, purified, and its lytic and antimicrobial activities were investigated using turbidity reduction and Kirby-Bauer disk-diffusion assays against clinically isolated bacterial pathogens. The crystal structure of the N-terminal amidase-2 domain was determined using X-ray crystallography at 1.97 Å resolution. It adopts a globular fold with several α-helices surrounding a central five-stranded β-sheet. Sequence comparison revealed a cluster of conserved amino acids that defines a putative binding site for a buried zinc ion. The results of the present study suggest that EfAmi1 displays high lytic and antimicrobial activity and may represent a promising new antimicrobial in the post-antibiotic era.

Horizontal gene transfer among host-associated microbes

Horizontal gene transfer is an important evolutionary force, facilitating bacterial diversity. It is thought to be pervasive in host-associated microbiomes, where bacterial densities are high and mobile elements are frequent. These genetic exchanges are also key for the rapid dissemination of antibiotic resistance. Here, we review recent studies that have greatly extended our knowledge of the mechanisms underlying horizontal gene transfer, the ecological complexities of a network of interactions involving bacteria and their mobile elements, and the effect of host physiology on the rates of genetic exchanges. Furthermore, we discuss other, fundamental challenges in detecting and quantifying genetic exchanges in vivo, and how studies have contributed to start overcoming these challenges. We highlight the importance of integrating novel computational approaches and theoretical models with experimental methods where multiple strains and transfer elements are studied, both in vivo and in controlled conditions that mimic the intricacies of host-associated environments.

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.

Signals triggering prophage induction in the gut microbiota

Compared to bacteria of the gut microbiota, bacteriophages are still poorly characterised, and their physiological importance is far less known. Temperate phages are probably a major actor in the gut, as it is estimated that 80% of intestinal bacteria are lysogens, meaning that they are carrying prophages. In addition, prophage induction rates are higher in the gut than in vitro. However, studies on the signals leading to prophage induction have essentially focused on genotoxic agents with poor relevance for this environment. In this review, we sum up recent findings about signals able to trigger prophage induction in the gut. Three categories of signals are at play: those originating from interactions between intestinal microbes, those from the human or animal host physiology and those from external intakes. These recent results highlight the diversity of factors influencing prophage induction in the gut, and start to unveil ways by which microbiota composition may be modulated.