A comparative guide to expression systems for phage lysin production.

Phage lysins, bacteriophage-encoded enzymes tasked with degrading their host's cell wall, are increasingly investigated and engineered as novel antibacterials across diverse applications. Their rapid action, tuneable specificity, and low likelihood of resistance development make them particularly interesting. Despite numerous application-focused lysin studies, the art of their recombinant production remains relatively undiscussed. Here, we provide an overview of the available expression systems for phage lysin production and discuss key considerations guiding the choice of a suitable recombinant host. We systematically surveyed recent literature to evaluate the hosts used in the lysin field and cover various recombinant systems, including the well-known bacterial host Escherichia coli or yeast Saccharomyces cerevisiae, as well as plant, mammalian, and cell-free systems. Careful analysis of the limited studies expressing lysins in various hosts suggests a host-dependent effect on activity. Nonetheless, the multitude of available expression systems should be further leveraged to accommodate the growing interest in phage lysins and their expanding range of applications.

Phage lysins for intestinal microbiome modulation: current challenges and enabling techniques.

The importance of the microbiota in the intestinal tract for human health has been increasingly recognized. In this perspective, microbiome modulation, a targeted alteration of the microbial composition, has gained interest. Phage lysins, peptidoglycan-degrading enzymes encoded by bacteriophages, are a promising new class of antibiotics currently under clinical development for treating bacterial infections. Due to their high specificity, lysins are considered microbiome-friendly. This review explores the opportunities and challenges of using lysins as microbiome modulators. First, the high specificity of endolysins, which can be further modulated using protein engineering or targeted delivery methods, is discussed. Next, obstacles and possible solutions to assess the microbiome-friendliness of lysins are considered. Finally, lysin delivery to the intestinal tract is discussed, including possible delivery methods such as particle-based and probiotic vehicles. Mapping the hurdles to developing lysins as microbiome modulators and identifying possible ways to overcome these hurdles can help in their development. In this way, the application of these innovative antimicrobial agents can be expanded, thereby taking full advantage of their characteristics.

DepoScope: Accurate phage depolymerase annotation and domain delineation using large language models.

Bacteriophages (phages) are viruses that infect bacteria. Many of them produce specific enzymes called depolymerases to break down external polysaccharide structures. Accurate annotation and domain identification of these depolymerases are challenging due to their inherent sequence diversity. Hence, we present DepoScope, a machine learning tool that combines a fine-tuned ESM-2 model with a convolutional neural network to identify depolymerase sequences and their enzymatic domains precisely. To accomplish this, we curated a dataset from the INPHARED phage genome database, created a polysaccharide-degrading domain database, and applied sequential filters to construct a high-quality dataset, which is subsequently used to train DepoScope. Our work is the first approach that combines sequence-level predictions with amino-acid-level predictions for accurate depolymerase detection and functional domain identification. In that way, we believe that DepoScope can greatly enhance our understanding of phage-host interactions at the level of depolymerases.

You get what you test for: The killing effect of phage lysins is highly dependent on buffer tonicity and ionic strength.

The phage lysin field has done nothing but grow in the last decades. As a result, many different research groups around the world are contributing to the field, often with certain methodological differences that pose a challenge to the interpretation and comparison of results. In this work, we present the case study of three Acinetobacter baumannii-targeting phage lysins (wild-type endolysin LysMK34 plus engineered lysins eLysMK34 and 1D10) plus one lysin with broad activity against Gram-positive bacteria (PlySs2) to provide exemplary evidence on the risks of generalization when using one of the most common lysin evaluation assays: the killing assay with resting cells. To that end, we performed killing assays with the aforementioned lysins using hypo-, iso- and hypertonic buffers plus human serum either as the reaction or the dilution medium in a systematic manner. Our findings stress the perils of creating hypotonic conditions or a hypotonic shock during a killing assay, suggesting that hypotonic buffers should be avoided as a test environment or as diluents before plating to avoid overestimation of the killing effect in the assayed conditions. As a conclusion, we suggest that the nature of both the incubation and the dilution buffers should be always clearly identified when reporting killing activity data, and that for experimental consistency the same incubation buffer should be used as a diluent for posterior serial dilution and plating unless explicitly required by the experimental design. In addition, the most appropriate buffer mimicking the final application must be chosen to obtain relevant results.

The New SH3b_T Domain Increases the Structural and Functional Variability Among SH3b-Like CBDs from Staphylococcal Phage Endolysins.

Endolysins, proteins encoded by phages to lyse their hosts and release their progeny, have evolved to adapt to the structural features of each host. The endolysins from Staphylococcus-infecting phages typically feature complex architectures with two enzymatically active domains (EADs) and one cell wall-binding domain (CBD) belonging to the bacterial SH3 (SH3b) superfamily. This study focuses on three SH3b-like CBDs from representative staphylococcal phage endolysins (LysRODI, LysC1C and LysIPLA5) that were structurally and functionally characterized. While RODI_CBD and C1C_CBD were assigned to the well-known SH3_5 family, a new family, SH3b_T (PF24246), was identified using the CBD from LysIPLA5 as a model. GFP-fused CBDs were created to assess their differential binding to a collection of staphylococcal strains. IPLA5_CBD showed enhanced binding to Staphylococcus epidermidis, while RODI_CBD and C1C_CBD exhibited distinct binding profiles, with RODI_CBD targeting Staphylococcus aureus specifically and C1C_CBD displaying broad binding. Sequence comparisons suggested that a few differences in key amino acids could be responsible for the latter binding difference. The CBDs modulated the activity spectrum of synthetic EAD-CBD combinations in accordance with the previous binding profiles, but in a manner that was also dependent on the EAD present in the fusion protein. These results serve as a context for the diversity and versatility of SH3b domains in staphylococcal endolysins, providing insights on how (i) the CBDs from this superfamily have diverged to adapt to diverse bacterial ligands in spite of sharing a common fold; and (ii) the evolution of specificity relies on the EAD-CBD combination rather than solely the CBD.

Prediction of Klebsiella phage-host specificity at the strain level.

Phages are increasingly considered promising alternatives to target drug-resistant bacterial pathogens. However, their often-narrow host range can make it challenging to find matching phages against bacteria of interest. Current computational tools do not accurately predict interactions at the strain level in a way that is relevant and properly evaluated for practical use. We present PhageHostLearn, a machine learning system that predicts strain-level interactions between receptor-binding proteins and bacterial receptors for Klebsiella phage-bacteria pairs. We evaluate this system both in silico and in the laboratory, in the clinically relevant setting of finding matching phages against bacterial strains. PhageHostLearn reaches a cross-validated ROC AUC of up to 81.8% in silico and maintains this performance in laboratory validation. Our approach provides a framework for developing and evaluating phage-host prediction methods that are useful in practice, which we believe to be a meaningful contribution to the machine-learning-guided development of phage therapeutics and diagnostics.

Meeting Report of the Second Symposium of the Belgian Society for Viruses of Microbes and Launch of the Phage Valley.

The second symposium of the Belgian Society for Viruses of Microbes (BSVoM) took place on 8 September 2023 at the University of Liège with 141 participants from 10 countries. The meeting program covered three thematic sessions opened by international keynote speakers: two sessions were devoted to "Fundamental research in phage ecology and biology" and the third one to the "Present and future applications of phages". During this one day symposium, four invited keynote lectures, nine selected talks and eight student pitches were given along with thirty presented posters. The president of the Belgian Society for Viruses of Microbes, Prof. Yves Briers, took advantage of this symposium to launch the Phage Valley concept that will put the spotlight on the exceptionally high density of researchers investigating viruses of microbes as well as the successful triple helix approach between academia, industry and government in Belgium.

Antibacterial synergy between a phage endolysin and citric acid against the Gram-negative kiwifruit pathogen Pseudomonas syringae pv. actinidiae.

Horticultural diseases caused by bacterial pathogens provide an obstacle to crop production globally. Management of the infection of kiwifruit by the Gram-negative phytopathogen Pseudomonas syringae pv. actinidiae (Psa) currently includes copper and antibiotics. However, the emergence of bacterial resistance and a changing regulatory landscape are providing the impetus to develop environmentally sustainable antimicrobials. One potential strategy is the use of bacteriophage endolysins, which degrade peptidoglycan during normal phage replication, causing cell lysis and the release of new viral progeny. Exogenous use of endolysins as antimicrobials is impaired by the outer membrane of Gram-negative bacteria that provides an impermeable barrier and prevents endolysins from accessing their target peptidoglycan. Here, we describe the synergy between citric acid and a phage endolysin, which results in a reduction of viable Psa below detection. We show that citric acid drives the destabilization of the outer membrane via acidification and sequestration of divalent cations from the lipopolysaccharide, which is followed by the degradation of the peptidoglycan by the endolysin. Scanning electron microscopy revealed clear morphological differences, indicating cell lysis following the endolysin-citric acid treatment. These results show the potential for citric acid-endolysin combinations as a possible antimicrobial approach in agricultural applications. IMPORTANCE: The phytopathogen Pseudomonas syringae pv. actinidiae (Psa) causes major impacts to kiwifruit horticulture, and the current control strategies are heavily reliant on copper and antibiotics. The environmental impact and increasing resistance to these agrichemicals are driving interest in alternative antimicrobials including bacteriophage-derived therapies. In this study, we characterize the endolysin from the Otagovirus Psa374 which infects Psa. When combined with citric acid, this endolysin displays an impressive antibacterial synergy to reduce viable Psa below the limit of detection. The use of citric acid as a synergistic agent with endolysins has not been extensively studied and has never been evaluated against a plant pathogen. We determined that the synergy involved a combination of the chelation activity of citric acid, acidic pH, and the specific activity of the ΦPsa374 endolysin. Our study highlights an exciting opportunity for alternative antimicrobials in agriculture.

Endolysin NC5 improves early cloxacillin treatment in a mouse model of Streptococcus uberis mastitis.

Streptococcus uberis frequently causes bovine mastitis, an infectious udder disease with significant economic implications for dairy cows. Conventional antibiotics, such as cloxacillin, sometimes have limited success in eliminating S. uberis as a stand-alone therapy. To address this challenge, the study objective was to investigate the VersaTile engineered endolysin NC5 as a supplemental therapy to cloxacillin in a mouse model of bovine S. uberis mastitis. NC5 was previously selected based on its intracellular killing and biofilm eradicating activity. To deliver preclinical proof-of-concept of this supplemental strategy, lactating mice were intramammarily infected with a bovine S. uberis field isolate and subsequently treated with cloxacillin (30.0 µg) combined with either a low (23.5 µg) or high (235.0 µg) dose of NC5. An antibiotic monotherapy group, as well as placebo treatment, was included as controls. Two types of responders were identified: fast (n = 17), showing response after 4-h treatment, and slow (n = 10), exhibiting no clear response at 4 h post-treatment across all groups. The high-dose combination therapy in comparison with placebo treatment impacted the hallmarks of mastitis in the fast responders by reducing (i) the bacterial load 13,000-fold (4.11 ± 0.78 Δlog10; p < 0.001), (ii) neutrophil infiltration 5.7-fold (p > 0.05), and (iii) the key pro-inflammatory chemokine IL-8 13-fold (p < 0.01). These mastitis hallmarks typically followed a dose response dependent on the amount of endolysin added. The current in vivo study complements our in vitro data and provides preclinical proof-of-concept of NC5 as an adjunct to intramammary cloxacillin treatment. KEY POINTS: • Engineered endolysin NC5 was preclinically evaluated as add-on to cloxacillin treatment. • Two types of mice (slow and fast responding) were observed. • The add-on treatment decreased bacterial load, neutrophil influx, and pro-inflammatory mediators.

Distinct mode of action of a highly stable, engineered phage lysin killing Gram-negative bacteria.

IMPORTANCE: Engineered lysins are considered as highly promising alternatives for antibiotics. Our previous screening study using VersaTile technology identified 1D10 as a possible lead compound with activity against Acinetobacter baumannii strains under elevated human serum concentrations. In this manuscript, we reveal an unexpected mode of action and exceptional thermoresistance for lysin 1D10. Our findings shed new light on the development of engineered lysins, providing valuable insights for future research in this field.

Development of engineered endolysins with in vitro intracellular activity against streptococcal bovine mastitis-causing pathogens.

Bacteriophage-derived endolysins are a novel class of antimicrobials known to rapidly kill bacteria, including antibiotic-resistant strains. We here engineered endolysins against the bovine mastitis pathogens Streptococcus uberis, Streptococcus agalactiae and Streptococcus dysgalactiae, also targeting intracellular survival and biofilm formation. For this purpose, high-throughput DNA assembly was used to create a library with >80,000 theoretical endolysin variants for screening of their bacteriolytic activity against Gram-positive isolates from (sub)clinically affected cows. This lytic activity was evaluated by turbidity reduction and time-kill assays in phosphate-buffered saline and pasteurized whole cow's milk to allow a rank up of the most potent leading candidates. A top candidate was selected with a 4.0 log killing efficacy against S. uberis, also showing similar activity against S. agalactiae and S. dysgalactiae. This top candidate eradicated S. uberis biofilm and showed intracellular activity in two bovine mammary epithelial cell lines as was confirmed by confocal microscopy. A potentiating effect on cloxacillin, a beta-lactam penicillin used to intramammarily treat bovine Gram-positive mastitis, was observed for this top candidate endolysin in raw cow's milk from (sub)clinically infected udders. Our in vitro results indicate that engineered endolysins may have a future role as add-on in the treatment of bovine streptococcal mastitis.

What's in a Name? An Overview of the Proliferating Nomenclature in the Field of Phage Lysins.

In the last few years, the volume of research produced on phage lysins has grown spectacularly due to the interest in using them as alternative antimicrobials. As a result, a plethora of naming customs has sprouted among the different research groups devoted to them. While the naming diversity accounts for the vitality of the topic, on too many occasions it also creates some confusion and lack of comparability between different works. This article aims at clarifying the ambiguities found among names referring to phage lysins. We do so by tackling the naming customs historically, framing their original adoption, and employing a semantic classification to facilitate their discussion. We propose a periodization of phage lysin research that begins at the discovery era, in the early 20th century, enriches with a strong molecular biology period, and grows into a current time of markedly applied research. During these different periods, names referring to the general concepts surrounding lysins have been created and adopted, as well as other more specific terms related to their structure and function or, finally, names that have been coined for the antimicrobial application and engineering of phage lysins. Thus, this article means to serve as an invitation to the global lysin community to take action and discuss a widely supported, standardized nomenclature.

Phage tailspike modularity and horizontal gene transfer reveals specificity towards E. coli O-antigen serogroups.

BACKGROUND: The interaction between bacteriophages and their hosts is intricate and highly specific. Receptor-binding proteins (RBPs) of phages such as tail fibers and tailspikes initiate the infection process. These RBPs bind to diverse outer membrane structures, including the O-antigen, which is a serogroup-specific sugar-based component of the outer lipopolysaccharide layer of Gram-negative bacteria. Among the most virulent Escherichia coli strains is the Shiga toxin-producing E. coli (STEC) pathotype dominated by a subset of O-antigen serogroups. METHODS: Extensive phylogenetic and structural analyses were used to identify and validate specificity correlations between phage RBP subtypes and STEC O-antigen serogroups, relying on the principle of horizontal gene transfer as main driver for RBP evolution. RESULTS: We identified O-antigen specific RBP subtypes for seven out of nine most prevalent STEC serogroups (O26, O45, O103, O104, O111, O145 and O157) and seven additional E. coli serogroups (O2, O8, O16, O18, 4s/O22, O77 and O78). Eight phage genera (Gamaleya-, Justusliebig-, Kaguna-, Kayfuna-, Kutter-, Lederberg-, Nouzilly- and Uetakeviruses) emerged for their high proportion of serogroup-specific RBPs. Additionally, we reveal sequence motifs in the RBP region, potentially serving as recombination hotspots between lytic phages. CONCLUSION: The results contribute to a better understanding of mosaicism of phage RBPs, but also demonstrate a method to identify and validate new RBP subtypes for current and future emerging serogroups.

Foundation of the Belgian Society for Viruses of Microbes and Meeting Report of Its Inaugural Symposium.

The Belgian Society for Viruses of Microbes (BSVoM) was founded on 9 June 2022 to capture and enhance the collaborative spirit among the expanding community of microbial virus researchers in Belgium. The sixteen founders are affiliated to fourteen different research entities across academia, industry and government. Its inaugural symposium was held on 23 September 2022 in the Thermotechnical Institute at KU Leuven. The meeting program covered three thematic sessions launched by international keynote speakers: (1) virus-host interactions, (2) viral ecology, evolution and diversity and (3) present and future applications. During the one-day symposium, four invited keynote lectures, ten selected talks and eight student pitches were given along with 41 presented posters. The meeting hosted 155 participants from twelve countries.

CkP1 bacteriophage, a S16-like myovirus that recognizes Citrobacter koseri lipopolysaccharide through its long tail fibers.

Citrobacter koseri is an emerging Gram-negative bacterial pathogen, which causes urinary tract infections. We isolated and characterized a novel S16-like myovirus CKP1 (vB_CkoM_CkP1), infecting C. koseri. CkP1 has a host range covering the whole C. koseri species, i.e., all strains that were tested, but does not infect other species. Its linear 168,463-bp genome contains 291 coding sequences, sharing sequence similarity with the Salmonella phage S16. Based on surface plasmon resonance and recombinant green florescence protein fusions, the tail fiber (gp267) was shown to decorate C. koseri cells, binding with a nanomolar affinity, without the need of accessory proteins. Both phage and the tail fiber specifically bind to bacterial cells by the lipopolysaccharide polymer. We further demonstrate that CkP1 is highly stable towards different environmental conditions of pH and temperatures and is able to control C. koseri cells in urine samples. Altogether, CkP1 features optimal in vitro characteristics to be used both as a control and detection agent towards drug-resistant C. koseri infections. KEY POINTS: • CkP1 infects all C. koseri strains tested • CkP1 recognizes C. koseri lipopolysaccharide through its long tail fiber • Both phage CkP1 and its tail fiber can be used to treat or detect C. koseri pathogens.

Phages and engineered lysins as an effective tool to combat Gram-negative foodborne pathogens.

One of the biggest challenges faced by food producers is ensuring microbiological safety. Despite strict criteria for food products, foodborne diseases are a global problem and pose a real risk to consumers. Therefore, it is necessary to identify new and more effective methods for eliminating pathogens from food and the food processing environment. According to the European Food Safety Authority (EFSA), the most common foodborne diseases are caused by Campylobacter, Salmonella, Yersinia, Escherichia coli, and Listeria. Out of the five listed, four are Gram-negative bacteria. Our review focuses on the use of bacteriophages, which are ubiquitous bacterial viruses, and bacteriophage endolysins to eliminate Gram-negative pathogens. Endolysins cleave specific bonds within the peptidoglycan (PG) of the bacterial cell, causing the cell to burst. Single phages or phage cocktails, which are, in some instances, commercially available products, eliminate pathogenic bacteria in livestock and various food matrices. Endolysins have matured as the most advanced class of antibacterial agents in the clinical sector, but their use in food protection is highly unexplored. Advanced molecular engineering techniques, different formulations, protein encapsulation, and the addition of outer membrane (OM) permeabilization agents enhance the activity of lysins against Gram-negative pathogens. This creates space for groundbreaking research on the use of lysins in the food sector.

Current challenges in designer cellulosome engineering.

Designer cellulosomes (DCs) are engineered multi-enzyme complexes, comprising carbohydrate-active enzymes attached to a common backbone, the scaffoldin, via high-affinity cohesin-dockerin interactions. The use of DCs in the degradation of renewable biomass polymers is a promising approach for biorefineries. Indeed, DCs have shown significant hydrolytic activities due to the enhanced enzyme-substrate proximity and inter-enzyme synergies, but technical hurdles in DC engineering have hindered further progress towards industrial application. The challenge in DC engineering lies in the large diversity of possible building blocks and architectures, resulting in a multivariate and immense design space. Simultaneously, the precise DC composition affects many relevant parameters such as activity, stability, and manufacturability. Since protein engineers face a lack of high-throughput approaches to explore this vast design space, DC engineering may result in an unsatisfying outcome. This review provides a roadmap to guide researchers through the process of DC engineering. Each step, starting from concept to evaluation, is described and provided with its challenges, along with possible solutions, both for DCs that are assembled in vitro or are displayed on the yeast cell surface. KEY POINTS: • Construction of designer cellulosomes is a multi-step process. • Designer cellulosome research deals with multivariate construction challenges. • Boosting designer cellulosome efficiency requires exploring a vast design space.

A Bioluminescence-Based Ex Vivo Burn Wound Model for Real-Time Assessment of Novel Phage-Inspired Enzybiotics.

The silent pandemic of antibiotic resistance is thriving, prompting the urgent need for the development of new antibacterial drugs. However, within the preclinical pipeline, in vitro screening conditions can differ significantly from the final in vivo settings. To bridge the gap between in vitro and in vivo assays, we developed a pig-skin-based bioluminescent ex vivo burn wound infection model, enabling real-time assessment of antibacterials in a longitudinal, non-destructive manner. We provide a proof-of-concept for A. baumannii NCTC13423, a multidrug-resistant clinical isolate, which was equipped with the luxCDABE operon as a reporter using a Tn7-based tagging system. This bioluminescence model provided a linear correlation between the number of bacteria and a broad dynamic range (104 to 109 CFU). This longitudinal model was subsequently validated using a fast-acting enzybiotic, 1D10. Since this model combines a realistic, clinically relevant yet strictly controlled environment with real-time measurement of bacterial burden, we put forward this ex vivo model as a valuable tool to assess the preclinical potential of novel phage-inspired enzybiotics.

High-resolution reconstruction of a Jumbo-bacteriophage infecting capsulated bacteria using hyperbranched tail fibers.

The Klebsiella jumbo myophage ϕKp24 displays an unusually complex arrangement of tail fibers interacting with a host cell. In this study, we combine cryo-electron microscopy methods, protein structure prediction methods, molecular simulations, microbiological and machine learning approaches to explore the capsid, tail, and tail fibers of ϕKp24. We determine the structure of the capsid and tail at 4.1 Šand 3.0 Šresolution. We observe the tail fibers are branched and rearranged dramatically upon cell surface attachment. This complex configuration involves fourteen putative tail fibers with depolymerase activity that provide ϕKp24 with the ability to infect a broad panel of capsular polysaccharide (CPS) types of Klebsiella pneumoniae. Our study provides structural and functional insight into how ϕKp24 adapts to the variable surfaces of capsulated bacterial pathogens, which is useful for the development of phage therapy approaches against pan-drug resistant K. pneumoniae strains.

Conversion of the free Cellvibrio japonicus xyloglucan degradation system to the cellulosomal mode.

Cellulosomes are multi-enzyme complexes produced by specialised micro-organisms. The spatial proximity of synergistically acting enzymes incorporated in these naturally occurring complexes supports the efficient hydrolysis of lignocellulosic biomass. Several functional designer cellulosomes, incorporating naturally non-cellulosomal cellulases, have been constructed and can be used for cellulose saccharification. However, in lignocellulosic biomass, cellulose is tightly intertwined with several hemicelluloses and lignin. One of the most abundant hemicelluloses interacting with cellulose microfibrils is xyloglucan, and degradation of these polymers is crucial for complete saccharification. Yet, designer cellulosome studies focusing on the incorporation of hemicellulases have been limited. Here, we report the conversion of the free Cellvibrio japonicus xyloglucan degradation system to the cellulosomal mode. Therefore, we constructed multiple docking enzyme variants of C. japonicus endoxyloglucanase, ß-1,2-galactosidase, α-1,6 xylosidase and ß-1,4-glucosidase, using the combinatorial VersaTile technique dedicated to the design and optimisation of modular proteins. We individually optimised the docking enzymes to degrade the xyloglucan backbone and side chains. Finally, we show that a purified designer xyloglucanosome comprising these docking enzymes was able to release xyloglucan oligosaccharides, galactose, xylose and glucose from tamarind xyloglucan. KEY POINTS: • Construction of xyloglucan-degrading designer cellulosome. • Conversion of free Cellvibrio japonicus enzymes to cellulosomal mode. • Type of linker inserted between dockerin and enzyme module affects docking enzyme activity.