Phase-variable capsular polysaccharides and lipoproteins modify bacteriophage susceptibility in Bacteroides thetaiotaomicron

Phase-variable bacteria simultaneously express multiple capsules

Capsular polysaccharides (CPSs) protect bacteria from host and environmental factors. Many bacteria can express different CPSs and these CPSs are phase variable. For example, Bacteroides thetaiotaomicron (B. theta) is a prominent member of the human gut microbiome and expresses eight different capsular polysaccharides. Bacteria, including B. theta, have been shown to change their CPSs to adapt to various niches such as immune, bacteriophage, and antibiotic perturbations. However, there are limited tools to study CPSs and fundamental questions regarding phase variance, including if gut bacteria can express more than one capsule at the same time, remain unanswered. To better understand the roles of different CPSs, we generated a B. theta CPS1-specific antibody and a flow cytometry assay to detect CPS expression in individual bacteria in the gut microbiota. Using these novel tools, we report for the first time that bacteria can simultaneously express multiple CPSs. We also observed that nutrients such as glucose and salts had no effect on CPS expression. The ability to express multiple CPSs at the same time may provide bacteria with an adaptive advantage to thrive amid changing host and environmental conditions, especially in the intestine.

New technologies for developing phage-based tools to manipulate the human microbiome

Gut bacteria play an essential role in the human body by regulating multiple functions, producing essential metabolites, protecting against pathogen invasion, and much more. Conversely, changes in their community structure are linked to several gastrointestinal (GI) and non-GI conditions. Fortunately, these bacteria are amenable to external perturbations, but we need specific tools for their safe manipulation as nonspecific changes can cause unpredicted long-term consequences. Here, we mainly discuss recent advances in cultivation-independent technologies and argue their relevance to different key steps, that is, identifying the modulation targets and developing phage-based tools to precisely modulate gut bacteria and restore a sustainable microbiome in humans. We finally suggest multiple modulating strategies for different dysbiosis-associated diseases.

Proteomic and Transcriptomic Analysis of Microviridae φX174 Infection Reveals Broad Upregulation of Host Escherichia coli Membrane Damage and Heat Shock Responses

A major part of the healthy human gut microbiome is the Microviridae bacteriophage, exemplified by the model φX174 phage, and their E. coli hosts. Although much has been learned from studying φX174 over the last half-century, until this work, the E. coli host response to infection has never been investigated in detail.

Measuring host-bacteriophage dynamics is an important approach to understanding bacterial survival functions and responses to infection. The model Microviridae bacteriophage φX174 is endemic to the human gut and has been studied for over 70 years, but the host response to infection has never been investigated in detail. To address this gap in our understanding of this important interaction within our microbiome, we have measured host Escherichia coli C proteomic and transcriptomic response to φX174 infection. We used mass spectrometry and RNA sequencing (RNA-seq) to identify and quantify all 11 φX174 proteins and over 1,700 E. coli proteins, enabling us to comprehensively map host pathways involved in φX174 infection. Most notably, we see significant host responses centered on membrane damage and remodeling, cellular chaperone and translocon activity, and lipoprotein processing, which we speculate is due to the peptidoglycan-disruptive effects of the φX174 lysis protein E on MraY activity. We also observe the massive upregulation of small heat shock proteins IbpA/B, along with other heat shock pathway chaperones, and speculate on how the specific characteristics of holdase protein activity may be beneficial for viral infections. Together, this study enables us to begin to understand the proteomic and transcriptomic host responses of E. coli to Microviridae infections and contributes insights to the activities of this important model host-phage interaction.

IMPORTANCE A major part of the healthy human gut microbiome is the Microviridae bacteriophage, exemplified by the model φX174 phage, and their E. coli hosts. Although much has been learned from studying φX174 over the last half-century, until this work, the E. coli host response to infection has never been investigated in detail. We reveal the proteomic and transcriptomic pathways differentially regulated during the φX174 infection cycle and uncover the details of a coordinated cellular response to membrane damage that results in increased lipoprotein processing and membrane trafficking, likely due to the phage antibiotic-like lysis protein. We also reveal that small heat shock proteins IbpA/B are massively upregulated during infection and that these holdase chaperones are highly conserved across the domains of life, indicating that reliance on them is likely widespread across viruses.

Microbial Arsenal of Antiviral Defenses - Part I

Bacteriophages or phages are viruses that infect bacterial cells (for the scope of this review we will also consider viruses that infect Archaea). Constant threat of phage infection is a major force that shapes evolution of the microbial genomes. To withstand infection, bacteria had evolved numerous strategies to avoid recognition by phages or to directly interfere with phage propagation inside the cell. Classical molecular biology and genetic engineering have been deeply intertwined with the study of phages and host defenses. Nowadays, owing to the rise of phage therapy, broad application of CRISPR-Cas technologies, and development of bioinformatics approaches that facilitate discovery of new systems, phage biology experiences a revival. This review describes variety of strategies employed by microbes to counter phage infection, with a focus on novel systems discovered in recent years. First chapter covers defense associated with cell surface, role of small molecules, and innate immunity systems relying on DNA modification.

Infection with Bacteroides Phage BV01 Alters the Host Transcriptome and Bile Acid Metabolism in a Common Human Gut Microbe

Gut-associated phages are hypothesized to alter the abundance and activity of their bacterial hosts, contributing to human health and disease. Although temperate phages constitute a significant fraction of the gut virome, the effects of lysogenic infection are underexplored. We report that the temperate phage, Bacteroides phage BV01, broadly alters its host's transcriptome, the prominent human gut symbiont Bacteroides vulgatus. This alteration occurs through phage-induced repression of a tryptophan-rich sensory protein (TspO) and represses bile acid deconjugation. Because microbially modified bile acids are important signals for the mammalian host, this is a mechanism by which a phage may influence mammalian phenotypes. Furthermore, BV01 and its relatives in the proposed phage family Salyersviridae are ubiquitous in human gut metagenomes, infecting a broad range of Bacteroides hosts. These results demonstrate the complexity of phage-bacteria-mammal relationships and emphasize a need to better understand the role of temperate phages in the gut microbiome.

Isolation and characterisation of ΦcrAss002, a crAss-like phage from the human gut that infects Bacteroides xylanisolvens

BACKGROUND

The gut phageome comprises a complex phage community of thousands of individual strains, with a few highly abundant bacteriophages. CrAss-like phages, which infect bacteria of the order Bacteroidales, are the most abundant bacteriophage family in the human gut and make an important contribution to an individual's core virome. Based on metagenomic data, crAss-like phages form a family, with four sub-families and ten candidate genera. To date, only three representatives isolated in pure culture have been reported: ΦcrAss001 and two closely related phages DAC15 and DAC17; all are members of the less abundant candidate genus VI. The persistence at high levels of both crAss-like phage and their Bacteroidales hosts in the human gut has not been explained mechanistically, and this phage-host relationship can only be properly studied with isolated phage-host pairs from as many genera as possible.

RESULTS

Faeces from a healthy donor with high levels of crAss-like phage was used to initiate a faecal fermentation in a chemostat, with selected antibiotics chosen to inhibit rapidly growing bacteria and selectively enrich for Gram-negative Bacteroidales. This had the objective of promoting the simultaneous expansion of crAss-like phages on their native hosts. The levels of seven different crAss-like phages expanded during the fermentation, indicating that their hosts were also present in the fermenter. The enriched supernatant was then tested against individual Bacteroidales strains isolated from the same faecal sample. This resulted in the isolation of a previously uncharacterised crAss-like phage of candidate genus IV of the proposed Alphacrassvirinae sub-family, ΦcrAss002, that infects the gut commensal Bacteroides xylanisolvens. ΦcrAss002 does not form plaques or spots on lawns of sensitive cells, nor does it lyse liquid cultures, even at high titres. In keeping with the co-abundance of phage and host in the human gut, ΦcrAss002 and Bacteroides xylanisolvens can also co-exist at high levels when co-cultured in laboratory media.

CONCLUSIONS

We report the isolation and characterisation of ΦcrAss002, the first representative of the proposed Alphacrassvirinae sub-family of crAss-like phages. ΦcrAss002 cannot form plaques or spots on bacterial lawns but can co-exist with its host, Bacteroides xylanisolvens, at very high levels in liquid culture without impacting on bacterial numbers. Video abstract.

Thousands of previously unknown phages discovered in whole-community human gut metagenomes

BACKGROUND

Double-stranded DNA bacteriophages (dsDNA phages) play pivotal roles in structuring human gut microbiomes; yet, the gut virome is far from being fully characterized, and additional groups of phages, including highly abundant ones, continue to be discovered by metagenome mining. A multilevel framework for taxonomic classification of viruses was recently adopted, facilitating the classification of phages into evolutionary informative taxonomic units based on hallmark genes. Together with advanced approaches for sequence assembly and powerful methods of sequence analysis, this revised framework offers the opportunity to discover and classify unknown phage taxa in the human gut.

RESULTS

A search of human gut metagenomes for circular contigs encoding phage hallmark genes resulted in the identification of 3738 apparently complete phage genomes that represent 451 putative genera. Several of these phage genera are only distantly related to previously identified phages and are likely to found new families. Two of the candidate families, "Flandersviridae" and "Quimbyviridae", include some of the most common and abundant members of the human gut virome that infect Bacteroides, Parabacteroides, and Prevotella. The third proposed family, "Gratiaviridae," consists of less abundant phages that are distantly related to the families Autographiviridae, Drexlerviridae, and Chaseviridae. Analysis of CRISPR spacers indicates that phages of all three putative families infect bacteria of the phylum Bacteroidetes. Comparative genomic analysis of the three candidate phage families revealed features without precedent in phage genomes. Some "Quimbyviridae" phages possess Diversity-Generating Retroelements (DGRs) that generate hypervariable target genes nested within defense-related genes, whereas the previously known targets of phage-encoded DGRs are structural genes. Several "Flandersviridae" phages encode enzymes of the isoprenoid pathway, a lipid biosynthesis pathway that so far has not been known to be manipulated by phages. The "Gratiaviridae" phages encode a HipA-family protein kinase and glycosyltransferase, suggesting these phages modify the host cell wall, preventing superinfection by other phages. Hundreds of phages in these three and other families are shown to encode catalases and iron-sequestering enzymes that can be predicted to enhance cellular tolerance to reactive oxygen species.

CONCLUSIONS

Analysis of phage genomes identified in whole-community human gut metagenomes resulted in the delineation of at least three new candidate families of Caudovirales and revealed diverse putative mechanisms underlying phage-host interactions in the human gut. Addition of these phylogenetically classified, diverse, and distinct phages to public databases will facilitate taxonomic decomposition and functional characterization of human gut viromes. Video abstract.

Probing the "Dark Matter" of the Human Gut Phageome: Culture Assisted Metagenomics Enables Rapid Discovery and Host-Linking for Novel Bacteriophages

Recent years have been marked by the growing interest towards virulent and temperate bacteriophage populations inhabiting the human lower gastrointestinal tract – the gut phageome. A number of studies demonstrated high levels of specificity and temporal stability of individual gut phageomes, as well as their specific alterations in disease cohorts, in parallel with changes in the bacteriome. It has been speculated that phages might have an active role in shaping the taxonomic composition and functional properties of the human gut bacteriome. An overwhelming majority of gut bacteriophages, however, remain uncultured, unclassified, and their specific hosts and infection strategies are still unknown. They are often referred to as “the viral dark matter”. A possible breakthrough in understanding of the phageome can only become possible when a significant proportion of the “the viral dark matter” is identified and linked to bacterial hosts. Here, we describe a method that enables rapid discovery and host-linking of novel bacteriophages in the gut via a combination of serial enrichment cultures and shotgun metagenomics of viral DNA. Using this approach dozens of novel and previously known bacteriophages were detected, including the ones infecting difficult-to-culture anaerobic bacteria. The majority of phages failed to produce lysis and propagate on host cultures in traditional assays. The newly identified phages include representatives of Siphoviridae, Myoviridae, Podoviridae, and crAss-like viruses, infecting diverse bacterial taxa of Bacteroidetes, Firmicutes, Actinobacteria, Verrucomicrobia and Proteobacteria phyla. The proposed new method has a potential for high-throughput screening applications for mass discovery of new phages in different environments.

Post-translational suppression of the high affinity IgE receptor expression on mast cells by an intestinal bacterium

Mast cells, which express the high-affinity IgE receptor (FcεRI) on their surface, play a crucial role in inducing allergic inflammation. Since mast cells are activated by crosslinking of FcεRI with IgE and allergens, the cell surface expression level of FcεRI is an important factor in determining the sensitivity to allergens. Recently, the involvement of gut microbiota in the prevalence and regulation of allergy has attracted attention but the precise underlying mechanisms are not fully understood. In this study, the effect of intestinal bacteria on cell surface expression of FcεRI was examined. Bacteroides acidifaciens type A 43 specifically suppressed cell surface expression of FcεRI on mouse bone marrow-derived mast cells (BMMCs) without reduction in FcεRI α and β-chain mRNA and total protein expression. The suppressive effect required sustained exposure to this bacterium, with a corresponding reduction in Erk activation. Inhibition of Erk decreased cell surface distribution of FcεRI in BMMCs, at least in part, through facilitated endocytosis of FcεRI. These results indicate that B. acidifaciens type A 43 suppresses cell surface expression of FcεRI on mast cells in a post-translational manner via inhibition of Erk. The suppression of FcεRI expression on mast cells by specific bacteria might be the underlying mechanism involved in the regulation of allergy by gut microbiota.

cognac: rapid generation of concatenated gene alignments for phylogenetic inference from large, bacterial whole genome sequencing datasets

Background

The quantity of genomic data is expanding at an increasing rate. Tools for phylogenetic analysis which scale to the quantity of available data are required. To address this need, we present cognac, a user-friendly software package to rapidly generate concatenated gene alignments for phylogenetic analysis.

Results

We illustrate that cognac is able to rapidly identify phylogenetic marker genes using a data driven approach and efficiently generate concatenated gene alignments for very large genomic datasets. To benchmark our tool, we generated core gene alignments for eight unique genera of bacteria, including a dataset of over 11,000 genomes from the genus Escherichia producing an alignment with 1353 genes, which was constructed in less than 17 h.

Conclusions

We demonstrate that cognac presents an efficient method for generating concatenated gene alignments for phylogenetic analysis. We have released cognac as an R package (https://github.com/rdcrawford/cognac) with customizable parameters for adaptation to diverse applications.

TonB-dependent transporters in the Bacteroidetes: Unique domain structures and potential functions

The human gut microbiota endows the host with a wealth of metabolic functions central to health, one of which is the degradation and fermentation of complex carbohydrates. The Bacteroidetes are one of the dominant bacterial phyla of this community and possess an expanded capacity for glycan utilization. This is mediated via the coordinated expression of discrete polysaccharide utilization loci (PUL) that invariantly encode a TonB-dependent transporter (SusC) that works with a glycan-capturing lipoprotein (SusD). More broadly within Gram-negative bacteria, TonB-dependent transporters (TBDTs) are deployed for the uptake of not only sugars, but also more often for essential nutrients such as iron and vitamins. Here, we provide a comprehensive look at the repertoire of TBDTs found in the model gut symbiont Bacteroides thetaiotaomicron and the range of predicted functional domains associated with these transporters and SusD proteins for the uptake of both glycans and other nutrients. This atlas of the B. thetaiotaomicron TBDTs reveals that there are at least three distinct subtypes of these transporters encoded within its genome that are presumably regulated in different ways to tune nutrient uptake.

Does over a century of aerobic phage work provide a solid framework for the study of phages in the gut?

Bacterial viruses (bacteriophages, phages) of the gut have increasingly become a focus in microbiome studies, with an understanding that they are likely key players in health and disease. However, characterization of the virome remains largely based on bioinformatic approaches, with the impact of these viromes inferred based on a century of knowledge from aerobic phage work. Studying the phages infecting anaerobes is difficult, as they are often technically demanding to isolate and propagate. In this review, we primarily discuss the phages infecting three well-studied anaerobes in the gut: Bifidobacterium, Clostridia and Bacteroides, with a particular focus on the challenges in isolating and characterizing these phages. We contrast the lessons learned from these to other anaerobic work on phages infecting facultative anaerobes of the gut: Enterococcus and Lactobacillus. Phages from the gut do appear to adhere to the lessons learned from aerobic work, but the additional challenges of working on them has required ingenious new approaches to enable their study. This, in turn, has uncovered remarkable biology likely underpinning phage-host relationships in many stable environments.

Beyond antibacterials - exploring bacteriophages as antivirulence agents

Life-threatening infections caused by multidrug-resistant bacteria are becoming increasingly difficult to treat. There is growing interest in exploiting bacteriophages (or phages) to combat bacterial infections. Phages often target bacterial surface structures that may also be important for virulence. Upon phage challenge, these molecules may be lost or modified, resulting in phage resistance and possibly phenotypical conversion. Importantly, possible trade-offs may include lower fitness, increased sensitivity to antibiotics and immune defense mechanisms, and virulence attenuation. Although evolution of phage-resistance may be difficult to prevent, the trade-off phenomenon carries potential for antibacterial therapy. Here we present some insights into the molecular principles and significance of this coincidental interplay between phages, bacteria, and immune cells, and discuss the prospect of developing phage-derived products as antivirulence agents.

Shining Light on Human Gut Bacteriophages

The human gut is a complex environment that contains a multitude of microorganisms that are collectively termed the microbiome. Multiple factors have a role to play in driving the composition of human gut bacterial communities either toward homeostasis or the instability that is associated with many disease states. One of the most important forces are likely to be bacteriophages, bacteria-infecting viruses that constitute by far the largest portion of the human gut virome. Despite this, bacteriophages (phages) are the one of the least studied residents of the gut. This is largely due to the challenges associated with studying these difficult to culture entities. Modern high throughput sequencing technologies have played an important role in improving our understanding of the human gut phageome but much of the generated sequencing data remains uncharacterised. Overcoming this requires database-independent bioinformatic pipelines and even those phages that are successfully characterized only provide limited insight into their associated biological properties, and thus most viral sequences have been characterized as “viral dark matter.” Fundamental to understanding the role of phages in shaping the human gut microbiome, and in turn perhaps influencing human health, is how they interact with their bacterial hosts. An essential aspect is the isolation of novel phage-bacteria host pairs by direct isolation through various screening methods, which can transform in silico phages into a biological reality. However, this is also beset with multiple challenges including culturing difficulties and the use of traditional methods, such as plaquing, which may bias which phage-host pairs that can be successfully isolated. Phage-bacteria interactions may be influenced by many aspects of complex human gut biology which can be difficult to reproduce under laboratory conditions. Here we discuss some of the main findings associated with the human gut phageome to date including composition, our understanding of phage-host interactions, particularly the observed persistence of virulent phages and their hosts, as well as factors that may influence these highly intricate relationships. We also discuss current methodologies and bottlenecks hindering progression in this field and identify potential steps that may be useful in overcoming these hurdles.

Bacteroides thetaiotaomicron-Infecting Bacteriophage Isolates Inform Sequence-Based Host Range Predictions

Our emerging view of the gut microbiome largely focuses on bacteria, while less is known about other microbial components, such as bacteriophages (phages). Though phages are abundant in the gut, very few phages have been isolated from this ecosystem. Here, we report the genomes of 27 phages from the United States and Bangladesh that infect the prevalent human gut bacterium Bacteroides thetaiotaomicron. These phages are mostly distinct from previously sequenced phages with the exception of two, which are crAss-like phages. We compare these isolates to existing human gut metagenomes, revealing similarities to previously inferred phages and additional unexplored phage diversity. Finally, we use host tropisms of these phages to identify alleles of phage structural genes associated with infectivity. This work provides a detailed view of the gut's "viral dark matter" and a framework for future efforts to further integrate isolation- and sequencing-focused efforts to understand gut-resident phages.

Capsular Polysaccharide Cross-Regulation Modulates Bacteroides thetaiotaomicron Biofilm Formation

Bacteroides thetaiotaomicron is one of the most abundant gut symbiont species, whose contribution to host health through its ability to degrade dietary polysaccharides and mature the immune system is under intense scrutiny. In contrast, adhesion and biofilm formation, which are potentially involved in gut colonization and microbiota structure and stability, have hardly been investigated in this intestinal bacterium. To uncover B. thetaiotaomicron biofilm-related functions, we performed a transposon mutagenesis in the poorly biofilm-forming reference strain VPI-5482 and showed that capsule 4, one of the eight B. thetaiotaomicron capsules, hinders biofilm formation. We then showed that the production of capsules 1, 2, 3, 5, and 6 also inhibits biofilm formation and that decreased capsulation of the population correlated with increased biofilm formation, suggesting that capsules could be masking adhesive surface structures. In contrast, we showed that capsule 8 displayed intrinsic adhesive properties. Finally, we demonstrated that BT2934, the wzx homolog of the B. thetaiotaomicron glycosylation locus, competes with capsule production and impacts its adhesion capacity. This study therefore establishes B. thetaiotaomicron capsule regulation as a major determinant of B. thetaiotaomicron biofilm formation, providing new insights into how modulation of different B. thetaiotaomicron surface structures affects in vitro biofilm formation.IMPORTANCE The human gut harbors a complex bacterial community that plays important roles in host health and disease, including nutrient acquisition, maturation of the immune system, and resistance to infections. The capacity to adhere to surfaces and form communities called biofilms is believed to be important for niche colonization and maintenance of gut bacteria. However, little is known about the adhesion capacity of most gut bacteria. In this study, we investigated biofilm formation in Bacteroides thetaiotaomicron, one of the most abundant bacteria of the normal mammalian intestine. We identified that B. thetaiotaomicron capsules, a group of eight surface-exposed polysaccharidic layers mediating important interactions with the gut environment, are also major determinants of biofilm formation that mask or unmask adhesion factors. Studying how B. thetaiotaomicron regulates its adhesion properties will allow us to better understand the physiology and specific properties of this important gut symbiont within anaerobic biofilms.