Structure of the host-recognition device of Staphylococcus aureus phage ϕ11

Evaluation of bacteriophage ϕ11 host recognition protein and its host-binding peptides for diagnosing/targeting Staphylococcus aureus infections

BACKGROUND

Evaluating the potential of using both synthetic and biological products as targeting agents for the diagnosis, imaging, and treatment of infections due to particularly antibiotic-resistant pathogens is important for controlling infections. This study examined the interaction between Gp45, a receptor-binding protein of the ϕ11 lysogenic phage, and its host Staphylococcus aureus (S. aureus), a common cause of nosocomial infections.

METHODS

Using molecular dynamics and docking simulations, this study identified the peptides that bind to S. aureus wall teichoic acids via Gp45. It compared the binding affinity of Gp45 and the two highest-scoring peptide sequences (P1 and P3) and their scrambled forms using microscopy, spectroscopy, and ELISA.

RESULTS

It was found that rGp45 (recombinant Gp45) and chemically synthesised P1 had a higher binding affinity for S. aureus compared with all other peptides, except for Escherichia coli. Furthermore, rGp45 had a capture efficiency of > 86%; P1 had a capture efficiency of > 64%.

CONCLUSION

These findings suggest that receptor-binding proteins such as rGp45, which provide a critical initiation of the phage life cycle for host adsorption, might play an important role in the diagnosis, imaging, and targeting of bacterial infections. Studying such proteins could accordingly enable the development of effective strategies for controlling infections.

A Novel Subcluster of Closely Related Bacillus Phages with Distinct Tail Fiber/Lysin Gene Combinations

Bacteriophages (phages) are the most numerous entities on Earth, but we have only scratched the surface of describing phage diversity. We isolated seven Bacillus subtilis phages from desert soil in the southwest United States and then sequenced and characterized their genomes. Comparative analyses revealed high nucleotide and amino acid similarity between these seven phages, which constitute a novel subcluster. Interestingly, the tail fiber and lysin genes of these phages seem to come from different origins and carry out slightly different functions. These genes were likely acquired by this subcluster of phages via horizontal gene transfer. In conjunction with host range assays, our data suggest that these phages are adapting to hosts with different cell walls.

Asymmetric Structure of Podophage GP4 Reveals a Novel Architecture of Three Types of Tail Fibers

Bacteriophage tail fibers (or called tail spikes) play a critical role in the early stage of infection by binding to the bacterial surface. Podophages with known structures usually possess one or two types of fibers. Here, we resolved an asymmetric structure of the podophage GP4 to near-atomic resolution by cryo-EM. Our structure revealed a symmetry-mismatch relationship between the components of the GP4 tail with previously unseen topologies. In detail, two dodecameric adaptors (adaptors I and II), a hexameric nozzle, and a tail needle form a conserved tail body connected to a dodecameric portal occupying a unique vertex of the icosahedral head. However, five chain-like extended fibers (fiber I) and five tulip-like short fibers (fiber II) are anchored to a 15-fold symmetric fiber-tail adaptor, encircling the adaptor I, and six bamboo-like trimeric fibers (fiber III) are connected to the nozzle. Five fibers I, each composed of five dimers of the protein gp80 linked by an elongated rope protein, are attached to the five edges of the tail vertex of the icosahedral head. In this study, we identified a new structure of the podophage with three types of tail fibers, and such phages with different types of fibers may have a broad host range and/or infect host cells with considerably high efficiency, providing evolutionary advantages in harsh environments.

Phage Adsorption to Gram-Positive Bacteria

The phage life cycle is a multi-stage process initiated by the recognition and attachment of the virus to its bacterial host. This adsorption step depends on the specific interaction between bacterial structures acting as receptors and viral proteins called Receptor Binding Proteins (RBP). The adsorption process is essential as it is the first determinant of phage host range and a sine qua non condition for the subsequent conduct of the life cycle. In phages belonging to the Caudoviricetes class, the capsid is attached to a tail, which is the central player in the adsorption as it comprises the RBP and accessory proteins facilitating phage binding and cell wall penetration prior to genome injection. The nature of the viral proteins involved in host adhesion not only depends on the phage morphology (i.e., myovirus, siphovirus, or podovirus) but also the targeted host. Here, we give an overview of the adsorption process and compile the available information on the type of receptors that can be recognized and the viral proteins taking part in the process, with the primary focus on phages infecting Gram-positive bacteria.

Phage-encoded carbohydrate-interacting proteins in the human gut

In the human gastrointestinal tract, the gut mucosa and the bacterial component of the microbiota interact and modulate each other to accomplish a variety of critical functions. These include digestion aid, maintenance of the mucosal barrier, immune regulation, and production of vitamins, hormones, and other metabolites that are important for our health. The mucus lining of the gut is primarily composed of mucins, large glycosylated proteins with glycosylation patterns that vary depending on factors including location in the digestive tract and the local microbial population. Many gut bacteria have evolved to reside within the mucus layer and thus encode mucus-adhering and -degrading proteins. By doing so, they can influence the integrity of the mucus barrier and therefore promote either health maintenance or the onset and progression of some diseases. The viral members of the gut - mostly composed of bacteriophages - have also been shown to have mucus-interacting capabilities, but their mechanisms and effects remain largely unexplored. In this review, we discuss the role of bacteriophages in influencing mucosal integrity, indirectly via interactions with other members of the gut microbiota, or directly with the gut mucus via phage-encoded carbohydrate-interacting proteins. We additionally discuss how these phage-mucus interactions may influence health and disease states.

High-resolution cryo-EM structure of the Shigella virus Sf6 genome delivery tail machine

Sf6 is a bacterial virus that infects the human pathogen Shigella flexneri. Here, we describe the cryo-electron microscopy structure of the Sf6 tail machine before DNA ejection, which we determined at a 2.7-angstrom resolution. We built de novo structures of all tail components and resolved four symmetry-mismatched interfaces. Unexpectedly, we found that the tail exists in two conformations, rotated by ~6° with respect to the capsid. The two tail conformers are identical in structure but differ solely in how the portal and head-to-tail adaptor carboxyl termini bond with the capsid at the fivefold vertex, similar to a diamond held over a five-pronged ring in two nonidentical states. Thus, in the mature Sf6 tail, the portal structure does not morph locally to accommodate the symmetry mismatch but exists in two energetic minima rotated by a discrete angle. We propose that the design principles of the Sf6 tail are conserved across P22-like Podoviridae.

Structure and Topology Prediction of Phage Adhesion Devices Using AlphaFold2: The Case of Two Oenococcus oeni Phages

Lactic acid bacteria (LAB) are important microorganisms in food fermentation. In the food industry, bacteriophages (phages or bacterial viruses) may cause the disruption of LAB-dependent processes with product inconsistencies and economic losses. LAB phages use diverse adhesion devices to infect their host, yet the overall picture of host-binding mechanisms remains incomplete. Here, we aimed to determine the structure and topology of the adhesion devices of two lytic siphophages, OE33PA and Vinitor162, infecting the wine bacteria Oenococcus oeni. These phages possess adhesion devices with a distinct composition and morphology and likely use different infection mechanisms. We primarily used AlphaFold2, an algorithm that can predict protein structure with unprecedented accuracy, to obtain a 3D model of the adhesion devices' components. Using our prior knowledge of the architecture of the LAB phage host-binding machineries, we also reconstituted the topology of OE33PA and Vinitor162 adhesion devices. While OE33PA exhibits original structures in the assembly of its bulky adhesion device, Vinitor162 harbors several carbohydrate-binding modules throughout its long and extended adhesion device. Overall, these results highlight the ability of AlphaFold2 to predict protein structures and illustrate its great potential in the study of phage structures and host-binding mechanisms.

Overview of the risks of Staphylococcus aureus infections and their control by bacteriophages and bacteriophage-encoded products

Staphylococcus aureus is the leading cause of secondary infections in hospitals and a challenging pathogen in food industries. Decades after it was first reported, β-lactam-resistant S. aureus remains a subject of intense research owing to the ever-increasing issue of drug resistance. S. aureus bacteriophages (phages) or their encoded products are considered an alternative to antibiotics as they have been shown to be effective in treating some S. aureus-associated infections. In this review, we present a concise collection of the literature on the pathogenic potential of S. aureus and examine the prospects of using S. aureus phages and their encoded products as antimicrobials.

Staphylococcus epidermidis Phages Transduce Antimicrobial Resistance Plasmids and Mobilize Chromosomal Islands

Multidrug-resistant strains of S. epidermidis emerge in both nosocomial and livestock environments as the most important pathogens among coagulase-negative staphylococcal species. The study of transduction by phages is essential to understanding how virulence and antimicrobial resistance genes spread in originally commensal bacterial populations.

Staphylococcus epidermidis is a leading opportunistic pathogen causing nosocomial infections that is notable for its ability to form a biofilm and for its high rates of antibiotic resistance. It serves as a reservoir of multiple antimicrobial resistance genes that spread among the staphylococcal population by horizontal gene transfer such as transduction. While phage-mediated transduction is well studied in Staphylococcus aureus, S. epidermidis transducing phages have not been described in detail yet. Here, we report the characteristics of four phages, 27, 48, 456, and 459, previously used for S. epidermidis phage typing, and the newly isolated phage E72, from a clinical S. epidermidis strain. The phages, classified in the family Siphoviridae and genus Phietavirus, exhibited an S. epidermidis-specific host range, and together they infected 49% of the 35 strains tested. A whole-genome comparison revealed evolutionary relatedness to transducing S. aureus phietaviruses. In accordance with this, all the tested phages were capable of transduction with high frequencies up to 10⁻⁴ among S. epidermidis strains from different clonal complexes. Plasmids with sizes from 4 to 19 kb encoding resistance to streptomycin, tetracycline, and chloramphenicol were transferred. We provide here the first evidence of a phage-inducible chromosomal island transfer in S. epidermidis. Similarly to S. aureus pathogenicity islands, the transfer was accompanied by phage capsid remodeling; however, the interfering protein encoded by the island was distinct. Our findings underline the role of S. epidermidis temperate phages in the evolution of S. epidermidis strains by horizontal gene transfer, which can also be utilized for S. epidermidis genetic studies.

IMPORTANCE Multidrug-resistant strains of S. epidermidis emerge in both nosocomial and livestock environments as the most important pathogens among coagulase-negative staphylococcal species. The study of transduction by phages is essential to understanding how virulence and antimicrobial resistance genes spread in originally commensal bacterial populations. In this work, we provide a detailed description of transducing S. epidermidis phages. The high transduction frequencies of antimicrobial resistance plasmids and the first evidence of chromosomal island transfer emphasize the decisive role of S. epidermidis phages in attaining a higher pathogenic potential of host strains. To date, such importance has been attributed only to S. aureus phages, not to those of coagulase-negative staphylococci. This study also proved that the described transducing bacteriophages represent valuable genetic modification tools in S. epidermidis strains where other methods for gene transfer fail.

Reprogramming bacteriophage host range: design principles and strategies for engineering receptor binding proteins

Bacteriophages (phages) use specialized tail machinery to deliver proteins and genetic material into a bacterial cell during infection. Attached at the distal ends of their tails are receptor binding proteins (RBPs) that recognize specific molecules exposed on host bacteria surfaces. Since the therapeutic capacity of naturally occurring phages is often limited by narrow host ranges, there is significant interest in expanding their host range via directed evolution or structure-guided engineering of their RBPs. Here, we describe the design principles of different RBP engineering platforms and draw attention to the mechanisms linking RBP binding and the correct spatial and temporal attachment of the phage to the bacterial surface. A deeper understanding of these mechanisms will directly benefit future engineering of more effective phage-based therapeutics.

β-Lactams against the Fortress of the Gram-Positive Staphylococcus aureus Bacterium

The biological diversity of the unicellular bacteria-whether assessed by shape, food, metabolism, or ecological niche-surely rivals (if not exceeds) that of the multicellular eukaryotes. The relationship between bacteria whose ecological niche is the eukaryote, and the eukaryote, is often symbiosis or stasis. Some bacteria, however, seek advantage in this relationship. One of the most successful-to the disadvantage of the eukaryote-is the small (less than 1 μm diameter) and nearly spherical Staphylococcus aureus bacterium. For decades, successful clinical control of its infection has been accomplished using β-lactam antibiotics such as the penicillins and the cephalosporins. Over these same decades S. aureus has perfected resistance mechanisms against these antibiotics, which are then countered by new generations of β-lactam structure. This review addresses the current breadth of biochemical and microbiological efforts to preserve the future of the β-lactam antibiotics through a better understanding of how S. aureus protects the enzyme targets of the β-lactams, the penicillin-binding proteins. The penicillin-binding proteins are essential enzyme catalysts for the biosynthesis of the cell wall, and understanding how this cell wall is integrated into the protective cell envelope of the bacterium may identify new antibacterials and new adjuvants that preserve the efficacy of the β-lactams.

Exploring Mucin as Adjunct to Phage Therapy

Conventional phage therapy using bacteriophages (phages) for specific targeting of pathogenic bacteria is not always useful as a therapeutic for gastrointestinal (GI) dysfunction. Complex dysbiotic GI disorders such as small intestinal bowel overgrowth (SIBO), ulcerative colitis (UC), or Crohn’s disease (CD) are even more difficult to treat as these conditions have shifts in multiple populations of bacteria within the microbiome. Such community-level structural changes in the gut microbiota may require an alternative to conventional phage therapy such as fecal virome transfer or a phage cocktail capable of targeting multiple bacterial species. Additionally, manipulation of the GI microenvironment may enhance beneficial bacteria–phage interactions during treatment. Mucin, produced along the entire length of the GI tract to protect the underlying mucosa, is a prominent contributor to the GI microenvironment and may facilitate bacteria–phage interactions in multiple ways, potentially serving as an adjunct during phage therapy. In this review, we will describe what is known about the role of mucin within the GI tract and how its facilitation of bacteria–phage interactions should be considered in any effort directed at optimizing effectiveness of a phage therapy for gastrointestinal dysbiosis.

Bacteriophage Treatment: Critical Evaluation of Its Application on World Health Organization Priority Pathogens

Bacteriophages represent an effective, natural, and safe strategy against bacterial infections. Multiple studies have assessed phage therapy’s efficacy and safety as an alternative approach to combat the emergence of multi drug-resistant pathogens. This systematic review critically evaluates and summarizes published articles on phages as a treatment option for Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Enterococcus faecalis infection models. It also illustrates appropriate phage selection criteria, as well as recommendations for successful therapy. Published studies included in this review were identified through EMBASE, PubMed, and Web of Science databases and were published in the years between 2010 to 2020. Among 1082 identified articles, 29 studies were selected using specific inclusion and exclusion criteria and evaluated. Most studies (93.1%) showed high efficacy and safety for the tested phages, and a few studies also examined the effect of phage therapy combined with antibiotics (17.2%) and resistance development (27.6%). Further clinical studies, phage host identification, and regulatory processes are required to evaluate phage therapy’s safety and efficacy and advance their clinical use.

Structure of the putative long tail fiber receptor-binding tip of a novel temperate bacteriophage from the Antarctic bacterium Bizionia argentinensis JUB59

Tailed bacteriophages are one of the most widespread biological entities on Earth. Their singular structures, such as spikes or fibers are of special interest given their potential use in a wide range of biotechnological applications. In particular, the long fibers present at the termini of the T4 phage tail have been studied in detail and are important for host recognition and adsorption. Although significant progress has been made in elucidating structural mechanisms of model phages, the high-resolution structural description of the vast population of marine phages is still unexplored. In this context, we present here the crystal structure of C24, a putative receptor-binding tip-like protein from Bizionia argentinensis JUB59, a psychrotolerant bacterium isolated from the marine surface waters of Potter Cove, Antarctica. The structure resembles the receptor-binding tip from the bacteriophage T4 long tail fiber yet showing marked differences in its domain organization, size, sequence identity and metal binding nature. We confirmed the viral origin of C24 by induction experiments using mitomycin C. Our results reveal the presence of a novel uncharacterized prophage in the genome of B. argentinensis JUB59, whose morphology is compatible with the order Caudovirales and that carries the nucleotide sequence of C24 in its genome. This work provides valuable information to expand our current knowledge on the viral machinery prevalent in the oceans.

Conserved and Diverse Traits of Adhesion Devices from Siphoviridae Recognizing Proteinaceous or Saccharidic Receptors

Bacteriophages can play beneficial roles in phage therapy and destruction of food pathogens. Conversely, they play negative roles as they infect bacteria involved in fermentation, resulting in serious industrial losses. Siphoviridae phages possess a long non-contractile tail and use a mechanism of infection whose first step is host recognition and binding. They have evolved adhesion devices at their tails' distal end, tuned to recognize specific proteinaceous or saccharidic receptors on the host's surface that span a large spectrum of shapes. In this review, we aimed to identify common patterns beyond this apparent diversity. To this end, we analyzed siphophage tail tips or baseplates, evaluating their known structures, where available, and uncovering patterns with bioinformatics tools when they were not. It was thereby identified that a triad formed by three proteins in complex, i.e., the tape measure protein (TMP), the distal tail protein (Dit), and the tail-associated lysozyme (Tal), is conserved in all phages. This common scaffold may harbor various functional extensions internally while it also serves as a platform for plug-in ancillary or receptor-binding proteins (RBPs). Finally, a group of siphophage baseplates involved in saccharidic receptor recognition exhibits an activation mechanism reminiscent of that observed in Myoviridae.

Structure of the host cell recognition and penetration machinery of a Staphylococcus aureus bacteriophage

Staphylococcus aureus is a common cause of infections in humans. The emergence of virulent, antibiotic-resistant strains of S. aureus is a significant public health concern. Most virulence and resistance factors in S. aureus are encoded by mobile genetic elements, and transduction by bacteriophages represents the main mechanism for horizontal gene transfer. The baseplate is a specialized structure at the tip of bacteriophage tails that plays key roles in host recognition, cell wall penetration, and DNA ejection. We have used high-resolution cryo-electron microscopy to determine the structure of the S. aureus bacteriophage 80α baseplate at 3.75 Å resolution, allowing atomic models to be built for most of the major tail and baseplate proteins, including two tail fibers, the receptor binding protein, and part of the tape measure protein. Our structure provides a structural basis for understanding host recognition, cell wall penetration and DNA ejection in viruses infecting Gram-positive bacteria. Comparison to other phages demonstrates the modular design of baseplate proteins, and the adaptations to the host that take place during the evolution of staphylococci and other pathogens.

Unusual features and molecular pathways of Staphylococcus aureus L-form bacteria

Staphylococcus aureus can be converted to cell wall-deficient L-form bacteria in specific environment which is associated with recurrent and persistent infections. The biophysical properties and molecular basis involved in S. aureus L-form formation are poorly understood. Here, S. aureus unstable L-form model was established not only in Newman strain, but also in ATCC 25923 and five different antibiotic-resistant clinical strains, and the morphology and mechanical properties of Newman strain L-forms were characterized by using atomic force microscopy. Meanwhile, zeta potential, growth and proliferation properties, and hemolysis of L-forms were determined. Gene expression changes involved in transition from S. aureus wild type into L-forms were identified. Our studies showed that L-form S. aureus presented pleomorphism, rough surface, and higher elasticity modulus. L-forms were characterized by less surface charge and had higher hemolysis than the walled form. The S. aureus L-form "fried egg" colony was derived from a single bacterium rather than from aggregation of different bacterial cells. Transcriptomics analysis revealed that several pathways involved in energy metabolism, stress response, protein synthesis, RNA metabolism, and virulence were involved in L-form formation in S. aureus. Our results shed new light on the biological properties and mechanisms underlying L-form formation in S. aureus. These findings will not only be useful for understanding the unique properties and mechanisms of L-form bacteria, but also provide therapeutic targets for developing more effective treatments for S. aureus L-forms.

Structure and genome ejection mechanism of Staphylococcus aureus phage P68

Phages infecting Staphylococcus aureus can be used as therapeutics against antibiotic-resistant bacterial infections. However, there is limited information about the mechanism of genome delivery of phages that infect Gram-positive bacteria. Here, we present the structures of native S. aureus phage P68, genome ejection intermediate, and empty particle. The P68 head contains 72 subunits of inner core protein, 15 of which bind to and alter the structure of adjacent major capsid proteins and thus specify attachment sites for head fibers. Unlike in the previously studied phages, the head fibers of P68 enable its virion to position itself at the cell surface for genome delivery. The unique interaction of one end of P68 DNA with one of the 12 portal protein subunits is disrupted before the genome ejection. The inner core proteins are released together with the DNA and enable the translocation of phage genome across the bacterial membrane into the cytoplasm.

The "fighting wisdom and bravery" of tailed phage and host in the process of adsorption

In the process of bacteriophage and bacteria struggle, adsorption is the key factor to determine who is the winner. In this paper, the molecular mechanism of tailed bacteriophage recognition and adsorption to host and the strategy of "fighting wisdom and courage" between them are reviewed.

A unique ferrous iron binding mode is associated with large conformational changes for the transport protein FpvC of Pseudomonas aeruginosa

Pseudomonas aeruginosa secretes pyoverdine, a major siderophore to get access to iron, an essential nutrient. Pyoverdine scavenges ferric iron in the bacterial environment with the resulting complex internalized by bacteria. Releasing of iron from pyoverdine in the periplasm involves an iron reduction by an inner membrane reductase and two solute-binding proteins (SBPs) FpvC and FpvF in association with their ABC transporter. FpvC and FpvF belong to two different subgroups of SBPs within the structural cluster A: FpvC and FpvF were proposed to be a metal-binding protein and a ferrisiderophore-binding protein respectively. Here, we report the redox state and the binding mode of iron to FpvC. We first solved the crystal structure of FpvC bound to a fortuitous Ni²⁺ by single anomalous dispersion method. Using a different protein purification strategy, we determined the structure of FpvC with manganese and iron, which binds to FpvC in a ferrous state as demonstrated by electron paramagnetic resonance. FpvC is the first example of a hexahistidine metal site among SBPs in which the Fe²⁺ redox state is stabilized under aerobic conditions. Using biophysics methods, we showed that FpvC reversibly bind to a broad range of divalent ions. The structure of a mutant mimicking the apo FpvC reveals a protein in an open state with large conformational changes when compared with the metal-bound FpvC. These results highlight that the canonical metal site in FpvC is distinct from those yet described in SBPs and they provide new insights into the mechanism of PVD-Fe dissociation in P. aeruginosa.

Structure and transformation of bacteriophage A511 baseplate and tail upon infection of Listeria cells

Contractile injection systems (bacteriophage tails, type VI secretions system, R-type pyocins, etc.) utilize a rigid tube/contractile sheath assembly for breaching the envelope of bacterial and eukaryotic cells. Among contractile injection systems, bacteriophages that infect Gram-positive bacteria represent the least understood members. Here, we describe the structure of Listeria bacteriophage A511 tail in its pre- and post-host attachment states (extended and contracted, respectively) using cryo-electron microscopy, cryo-electron tomography, and X-ray crystallography. We show that the structure of the tube-baseplate complex of A511 is similar to that of phage T4, but the A511 baseplate is decorated with different receptor-binding proteins, which undergo a large structural transformation upon host attachment and switch the symmetry of the baseplate-tail fiber assembly from threefold to sixfold. For the first time under native conditions, we show that contraction of the phage tail sheath assembly starts at the baseplate and propagates through the sheath in a domino-like motion.

Structure and Analysis of R1 and R2 Pyocin Receptor-Binding Fibers

The R-type pyocins are high-molecular weight bacteriocins produced by some strains of Pseudomonas aeruginosa to specifically kill other strains of the same species. Structurally, the R-type pyocins are similar to "simple" contractile tails, such as those of phage P2 and Mu. The pyocin recognizes and binds to its target with the help of fibers that emanate from the baseplate structure at one end of the particle. Subsequently, the pyocin contracts its sheath and drives the rigid tube through the host cell envelope. This causes depolarization of the cytoplasmic membrane and cell death. The host cell surface-binding fiber is ~340 Å-long and is attached to the baseplate with its N-terminal domain. Here, we report the crystal structures of C-terminal fragments of the R1 and R2 pyocin fibers that comprise the distal, receptor-binding part of the protein. Both proteins are ~240 Å-long homotrimers in which slender rod-like domains are interspersed with more globular domains-two tandem knob domains in the N-terminal part of the fragment and a lectin-like domain at its C-terminus. The putative substrate binding sites are separated by about 100 Å, suggesting that binding of the fiber to the cell surface causes the fiber to adopt a certain orientation relative to the baseplate and this then triggers sheath contraction.

Molecular Basis of Bacterial Host Interactions by Gram-Positive Targeting Bacteriophages

The inherent ability of bacteriophages (phages) to infect specific bacterial hosts makes them ideal candidates to develop into antimicrobial agents for pathogen-specific remediation in food processing, biotechnology, and medicine (e.g., phage therapy). Conversely, phage contaminations of fermentation processes are a major concern to dairy and bioprocessing industries. The first stage of any successful phage infection is adsorption to a bacterial host cell, mediated by receptor-binding proteins (RBPs). As the first point of contact, the binding specificity of phage RBPs is the primary determinant of bacterial host range, and thus defines the remediative potential of a phage for a given bacterium. Co-evolution of RBPs and their bacterial receptors has forced endless adaptation cycles of phage-host interactions, which in turn has created a diverse array of phage adsorption mechanisms utilizing an assortment of RBPs. Over the last decade, these intricate mechanisms have been studied intensely using electron microscopy and X-ray crystallography, providing atomic-level details of this fundamental stage in the phage infection cycle. This review summarizes current knowledge surrounding the molecular basis of host interaction for various socioeconomically important Gram-positive targeting phage RBPs to their protein- and saccharide-based receptors. Special attention is paid to the abundant and best-characterized Siphoviridae family of tailed phages. Unravelling these complex phage-host dynamics is essential to harness the full potential of phage-based technologies, or for generating novel strategies to combat industrial phage contaminations.

A Decade of Streptococcus thermophilus Phage Evolution in an Irish Dairy Plant

Phages of Streptococcus thermophilus present a major threat to the production of many fermented dairy products. To date, only a few studies have assessed the biodiversity of S. thermophilus phages in dairy fermentations. In order to develop strategies to limit phage predation in this important industrial environment, it is imperative that such studies are undertaken and that phage-host interactions of this species are better defined. The present study investigated the biodiversity and evolution of phages within an Irish dairy fermentation facility over an 11-year period. This resulted in the isolation of 17 genetically distinct phages, all of which belong to the so-called cos group. The evolution of phages within the factory appears to be influenced by phages from other dairy plants introduced into the factory for whey protein powder production. Modular exchange, primarily within the regions encoding lysogeny and replication functions, was the major observation among the phages isolated between 2006 and 2016. Furthermore, the genotype of the first isolate in 2006 was observed continuously across the following decade, highlighting the ability of these phages to prevail in the factory setting for extended periods of time. The proteins responsible for host recognition were analyzed, and carbohydrate-binding domains (CBDs) were identified in the distal tail (Dit), the baseplate proteins, and the Tail-associated lysin (Tal) variable regions (VR1 and VR2) of many isolates. This supports the notion that S. thermophilus phages recognize a carbohydrate receptor on the cell surface of their host.IMPORTANCE Dairy fermentations are consistently threatened by the presence of bacterial viruses (bacteriophages or phages), which may lead to a reduction in acidification rates or even complete loss of the fermentate. These phages may persist in factories for long periods of time. The objective of the current study was to monitor the progression of phages infecting the dairy bacterium Streptococcus thermophilus over a period of 11 years in an Irish dairy plant so as to understand how these phages evolve. A focused analysis of the genomic region that encodes host recognition functions highlighted that the associated proteins harbor a variety of carbohydrate-binding domains, which corroborates the notion that phages of S. thermophilus recognize carbohydrate receptors at the initial stages of the phage cycle.

In Vitro Characteristics of Phages to Guide 'Real Life' Phage Therapy Suitability

The increasing problem of antibiotic-resistant pathogens has put enormous pressure on healthcare providers to reduce the application of antibiotics and to identify alternative therapies. Phages represent such an alternative with significant application potential, either on their own or in combination with antibiotics to enhance the effectiveness of traditional therapies. However, while phage therapy may offer exciting therapeutic opportunities, its evaluation for safe and appropriate use in humans needs to be guided initially by reliable and appropriate assessment techniques at the laboratory level. Here, we review the process of phage isolation and the application of individual pathogens or reference collections for the development of specific or "off-the-shelf" preparations. Furthermore, we evaluate current characterization approaches to assess the in vitro therapeutic potential of a phage including its spectrum of activity, genome characteristics, storage and administration requirements and effectiveness against biofilms. Lytic characteristics and the ability to overcome anti-phage systems are also covered. These attributes direct phage selection for their ultimate application as antimicrobial agents. We also discuss current pitfalls in this research area and propose that priority should be given to unify current phage characterization approaches.

Surfaceome and Proteosurfaceome in Parietal Monoderm Bacteria: Focus on Protein Cell-Surface Display

The cell envelope of parietal monoderm bacteria (archetypal Gram-positive bacteria) is formed of a cytoplasmic membrane (CM) and a cell wall (CW). While the CM is composed of phospholipids, the CW is composed at least of peptidoglycan (PG) covalently linked to other biopolymers, such as teichoic acids, polysaccharides, and/or polyglutamate. Considering the CW is a porous structure with low selective permeability contrary to the CM, the bacterial cell surface hugs the molecular figure of the CW components as a well of the external side of the CM. While the surfaceome corresponds to the totality of the molecules found at the bacterial cell surface, the proteinaceous complement of the surfaceome is the proteosurfaceome. Once translocated across the CM, secreted proteins can either be released in the extracellular milieu or exposed at the cell surface by associating to the CM or the CW. Following the gene ontology (GO) for cellular components, cell-surface proteins at the CM can either be integral (GO: 0031226), i.e., the integral membrane proteins, or anchored to the membrane (GO: 0046658), i.e., the lipoproteins. At the CW (GO: 0009275), cell-surface proteins can be covalently bound, i.e., the LPXTG-proteins, or bound through weak interactions to the PG or wall polysaccharides, i.e., the cell wall binding proteins. Besides monopolypeptides, some proteins can associate to each other to form supramolecular protein structures of high molecular weight, namely the S-layer, pili, flagella, and cellulosomes. After reviewing the cell envelope components and the different molecular mechanisms involved in protein attachment to the cell envelope, perspectives in investigating the proteosurfaceome in parietal monoderm bacteria are further discussed.

Review of the nature, diversity and structure of bacteriophage receptor binding proteins that target Gram-positive bacteria

As the importance of bacteriophages as novel antimicrobials and potential diagnostics comes increasingly into focus, there is a heightened interest in understanding the mechanisms of how they interact with their bacterial hosts. The first step of a bacteriophage (phage) infection is the recognition of specific moieties on the bacterial cell surface as determined by their phage receptor binding proteins (RBPs). Knowledge of RBPs and how they interact with bacteria has been driven by studies of model phages and of industrially important phages, such as those that impact the dairy industry. Therefore, data from these phage groups constitute the majority of this review. We start with a brief introduction to phages, their life cycles and known receptors. We then review the state-of-the-art knowledge of phage RBPs of Gram-positive bacteria in the context of the better understood Gram-negative bacterial RBPs. In general, more is known about the RBPs of siphoviruses than myoviruses, which is reflected here, but for both virus families, where possible, we show what RBPs are, how they are arranged within phage genomes and what is known about their structures. As RBPs are the key determinant of phage specificity, studying and characterising them is important, for downstream applications such as diagnostic and therapeutic purposes.

Bacteriophage T4 long tail fiber domains

Bacteriophage T4 initially recognizes its host cells using its long tail fibers. Long tail fibers consist of a phage-proximal and a phage-distal rod, each around 80 nm long and attached to each other at a slight angle. The phage-proximal rod is formed by a homo-trimer of gene product 34 (gp34) and is attached to the phage-distal rod by a monomer of gp35. The phage-distal rod consists of two protein trimers: a trimer of gp36, attached to gp35, although most of the phage-distal rod, including the receptor-binding domain, is formed by a trimer of gp37. In this review, we discuss what is known about the detailed structure and function of the different long tail fiber domains. Partial crystal structures of gp34 and gp37 have revealed the presence of new protein folds, some of which are present in several repeats, while others are apparently unique. Gp38, a phage chaperone protein necessary for folding of gp37, is thought to act on an α-helical coiled-coil region in gp37. Future studies should reveal the remaining structure of the long tail fibers, how they assemble into a functional unit, and how the long tail fibers trigger the infection process after successful recognition of a suitable host bacterium.

Genetic engineering of a temperate phage-based delivery system for CRISPR/Cas9 antimicrobials against Staphylococcus aureus

Discovery of clustered, regularly interspaced, short palindromic repeats and the Cas9 RNA-guided nuclease (CRISPR/Cas9) system provides a new opportunity to create programmable gene-specific antimicrobials that are far less likely to drive resistance than conventional antibiotics. However, the practical therapeutic use of CRISPR/Cas9 is still questionable due to current shortcomings in phage-based delivery systems such as inefficient delivery, narrow host range, and potential transfer of virulence genes by generalized transduction. In this study, we demonstrate genetic engineering strategies to overcome these shortcomings by integrating CRISPR/Cas9 system into a temperate phage genome, removing major virulence genes from the host chromosome, and expanding host specificity of the phage by complementing tail fiber protein. This significantly improved the efficacy and safety of CRISPR/Cas9 antimicrobials to therapeutic levels in both in vitro and in vivo assays. The genetic engineering tools and resources established in this study are expected to provide an efficacious and safe CRISPR/Cas9 antimicrobial, broadly applicable to Staphylococcus aureus.