Evaluation of Paramagnetic Beads Coated with RecombinantListeriaPhage Endolysin–Derived Cell-Wall-Binding Domain Proteins for Separation ofListeria monocytogenesfrom Raw Milk in Combination with Culture-Based and Real-Time Polymerase Chain Reaction–Based Quantification

Split fluorescent protein-mediated multimerization of cell wall binding domain for highly sensitive and selective bacterial detection

Cell wall peptidoglycan binding domains (CBDs) of cell lytic enzymes, including bacteriocins, autolysins and bacteriophage endolysins, enable highly selective bacterial binding, and thus, have potential as biorecognition molecules for nondestructive bacterial detection. Here, a novel design for a self-complementing split fluorescent protein (FP) complex is proposed, where a multimeric FP chain fused with specific CBDs ((FP-CBD)n) is assembled inside the cell, to improve sensitivity by enhancing the signal generated upon Staphylococcus aureus or Bacillus anthracis binding. Flow cytometry shows enhanced fluorescence on the cell surface with increasing FP stoichiometry and surface plasmon resonance reveals nanomolar binding affinity to isolated peptidoglycan. The breadth of function of these complexes is demonstrated through the use of CBD modularity and the ability to attach enzymatic detection modalities. Horseradish peroxidase-coupled (FP-CBD)n complexes generate a catalytic amplification, with the degree of amplification increasing as a function of FP length, reaching a limit of detection (LOD) of 103 cells/droplet (approximately 0.1 ng S. aureus or B. anthracis) within 15 min on a polystyrene surface. These fusion proteins can be multiplexed for simultaneous detection. Multimeric split FP-CBD fusions enable use as a biorecognition molecule with enhanced signal for use in bacterial biosensing platforms.

Design of a novel affinity probe using the cell wall-binding domain of a Listeria monocytogenes autolysin for pathogen detection

IMPORTANCE

Human listeriosis is caused by consuming foods contaminated with the bacterial pathogen Listeria monocytogenes, leading to the development of a severe and life-threatening foodborne illness. Detection of L. monocytogenes present in food and food processing environments is crucial for preventing Listeria infection. The L. monocytogenes peptidoglycan hydrolase IspC anchors non-covalently to the bacterial surface through its C-terminal cell wall-binding domain (CWBD), CWBDIspC. This study explored the surface binding property of CWBDIspC to design, construct, characterize, and assess an affinity molecular probe for detecting L. monocytogenes. CWBDIspC recognized a cell wall ligand lipoteichoic acid that remains evenly displayed and mostly unoccupied on the bacterial surface for interaction with the exogenously added CWBDIspC. CWBDIspC, when fused to the enhanced green fluorescent protein reporter or covalently conjugated onto magnetic beads, exhibited the functionality as an antibody alternative for rapid detection and efficient separation of the pathogen.

Recent Advances in the Detection of Listeria monocytogenes

Listeria monocytogenes is the third-most severe pathogen causing a yearly outbreak of food poisoning in the world that proliferates widely in the environment. Infants, pregnant mothers, and immuno-compromised people are at high risk. Its ability to grow in both biotic and abiotic environments leads to epidemics that infect 5 out of 10 people annually. Because of the epithelial adhesion (by E-cadherin binding), it can suppress immune cells and thrive in the gastrointestinal tract till the brain through blood flow (E-cadherin). Microbial culture is still used as a gold standard, but takes a long time and often yields false positive results due to incompetence and temperature variations. Therefore, in order to treat it rather than using broad spectrum antibiotics, a standardized time-saving and highly specific technology for early detection is very important. It has been observed that the production of a particular antibody is delaying (so does the detection process) as a result of the inadequate understanding of the pathophysiology of the bacteria. This book chapter provides a brief summary of a pathogen as well as the scientific advances that led to its identification more easily.

Potential of bacteriophage proteins as recognition molecules for pathogen detection

Bacterial pathogens are leading causes of infections with high mortality worldwide having a great impact on healthcare systems and the food industry. Gold standard methods for bacterial detection mainly rely on culture-based technologies and biochemical tests which are laborious and time-consuming. Regardless of several developments in existing methods, the goal of achieving high sensitivity and specificity, as well as a low detection limit, remains unaccomplished. In past years, various biorecognition elements, such as antibodies, enzymes, aptamers, or nucleic acids, have been widely used, being crucial for the pathogens detection in different complex matrices. However, these molecules are usually associated with high detection limits, demand laborious and costly production, and usually present cross-reactivity. (Bacterio)phage-encoded proteins, especially the receptor binding proteins (RBPs) and cell-wall binding domains (CBDs) of endolysins, are responsible for the phage binding to the bacterial surface receptors in different stages of the phage lytic cycle. Due to their remarkable properties, such as high specificity, sensitivity, stability, and ability to be easily engineered, they are appointed as excellent candidates to replace conventional recognition molecules, thereby contributing to the improvement of the detection methods. Moreover, they offer several possibilities of application in a variety of detection systems, such as magnetic, optical, and electrochemical. Herein we provide a review of phage-derived bacterial binding proteins, namely the RBPs and CBDs, with the prospect to be employed as recognition elements for bacteria. Moreover, we summarize and discuss the various existing methods based on these proteins for the detection of nosocomial and foodborne pathogens.

Bacteriophage Tail Proteins as a Tool for Bacterial Pathogen Recognition-A Literature Review

In recent years, a number of bacterial detection methods have been developed to replace time-consuming culture methods. One interesting approach is to mobilize the ability of phage tail proteins to recognize and bind to bacterial hosts. In this paper, the authors provide an overview of the current methodologies in which phage proteins play major roles in detecting pathogenic bacteria. Authors focus on proteins capable of recognizing highly pathogenic strains, such as Acinetobacter baumannii, Campylobacter spp., Yersinia pestis, Pseudomonas aeruginosa, Listeria monocytogenes, Staphylococcus aureus, Enterococcus spp., Salmonella spp., and Shigella. These pathogens may be diagnosed by capture-based detection methods involving the use of phage protein-coated nanoparticles, ELISA (enzyme-linked immunosorbent assay)-based methods, or biosensors. The reviewed studies show that phage proteins are becoming an important diagnostic tool due to the discovery of new phages and the increasing knowledge of understanding the specificity and functions of phage tail proteins.

Exploitation of a Klebsiella Bacteriophage Receptor-Binding Protein as a Superior Biorecognition Molecule

Klebsiella pneumoniae is a Gram-negative bacterium that has become one of the leading causes of life-threatening healthcare-associated infections (HAIs), including pneumonia and sepsis. Moreover, due to its increasingly antibiotic resistance, K. pneumoniae has been declared a global top priority concern. The problem of K. pneumoniae infections is due, in part, to the inability to detect this pathogen rapidly and accurately and thus to treat patients within the early stages of infections. The success in bacterial detection is greatly dictated by the biorecognition molecule used, with the current diagnostic tools relying on expensive probes often lacking specificity and/or sensitivity. (Bacterio)phage receptor-binding proteins (RBPs) are responsible for the recognition and adsorption of phages to specific bacterial host receptors and thus present high potential as biorecognition molecules. In this study, we report the identification and characterization of a novel RBP from the K. pneumoniae phage KpnM6E1 that presents high specificity against the target bacteria and high sensitivity (80%) to recognize K. pneumoniae strains. Moreover, adsorption studies validated the role of gp86 in the attachment to bacterial receptors, as it highly inhibits (86%) phage adsorption to its Klebsiella host. Overall, in this study, we unravel the role and potential of a novel Klebsiella phage RBP as a powerful tool to be used coupled with analytical techniques or biosensing platforms for the diagnosis of K. pneumoniae infections.

Rapid and multiplex detection of nosocomial pathogens on a phage-based magnetoresistive lab-on-chip platform

Nosocomial or hospital-acquired infections (HAIs) have a major impact on mortality worldwide. Enterococcus and Staphylococcus are among the leading causes of HAIs and thus are important pathogens to control mainly due to their increased antibiotic resistance. The gold-standard diagnostic methods for HAIs are time-consuming, which hinders timely and adequate treatment. Therefore, the development of fast and accurate diagnostic tools is an urgent demand. In this study, we combined the sensitivity of magnetoresistive (MR) sensors, the portability of a lab-on-chip platform, and the specificity of phage receptor binding proteins (RBPs) as probes for the rapid and multiplex detection of Enterococcus and Staphylococcus. For this, bacterial cells were firstly labelled with magnetic nanoparticles (MNPs) functionalized with RBPs and then measured on the MR sensors. The results indicate that the RBP-MNPS provided a specific individual and simultaneous capture of more than 70% of Enterococcus and Staphylococcus cells. Moreover, high signals from the MR sensors were obtained for these samples, providing the detection of both pathogens at low concentrations (10 CFU/ml) in less than 2 h. Overall, the lab-on-chip MR platform herein presented holds great potential to be used as a point-of-care for the rapid, sensitive and specific multiplex diagnosis of bacterial infections.

Wall teichoic acids: physiology and applications

Wall teichoic acids (WTAs) are charged glycopolymers containing phosphodiester-linked polyol units and represent one of the major components of Gram-positive cell envelope. WTAs have important physiological functions in cell division, gene transfer, surface adhesion, drug resistance and biofilm formation, and are critical virulence factors and vital determinants in mediating cell interaction with and tolerance to environmental factors. Here, we first briefly introduce WTA structure, biosynthesis and its regulation, and then summarize in detail four major physiological roles played by WTAs, i.e. WTA-mediated resistance to antimicrobials, virulence to mammalian cells, interaction with bacteriolytic enzymes and regulation of cell metabolism. We also review the applications of WTAs in these fields that are closely related to the human society, including antibacterial drug discovery targeting WTA biosynthesis, development of vaccines and antibodies regarding WTA-mediated pathogenicity, specific and sensitive detection of pathogens in food using WTAs as a surface epitope and regulation of WTA-related pathways for efficient microbial production of useful compounds. We also point out major problems remaining in these fields, and discuss some possible directions in the future exploration of WTA physiology and applications.

The Application of Bacteriophage Diagnostics for Bacterial Pathogens in the Agricultural Supply Chain: From Farm-to-Fork

Bacteriophages (phages) have great potential not only as therapeutics but as diagnostics. Indeed, they have been developed and used to diagnose and detect bacterial infections, primarily in human clinical settings. The ability to rapidly detect and control bacterial pathogens in agriculture is of primary importance to maintain food security, improve animal health, and prevent the passage of zoonotic pathogens into the human population. Culture-based detection methods are often labor-intensive, and require further confirmatory tests, increasing costs and processing times needed for diagnostics. Molecular detection methods such as polymerase chain reaction are commonly used to determine the safety of food, however, a major drawback is their inability to differentiate between viable and nonviable bacterial pathogens in food. Phage diagnostics have been proven to be rapid, capable of identifying viable pathogens and do not require cultivation to detect bacteria. Phage detection takes advantage of the specificity of interaction between phage and their hosts. Furthermore, phage detection is cost effective, which is vitally important in agricultural supply chains where there is a drive to keep costs down to ensure that the cost of food does not increase. The full potential of phage detection/diagnostics is not wholly realized or commercialized. This review explores the current use and potential future scope of phage diagnostics and their application to various bacterial pathogens across agriculture and food supply chains.

Lysin cell-binding domain-functionalized magnetic beads for detection of Staphylococcus aureus via inhibition of fluorescence of Amplex Red/hydrogen peroxide assay by intracellular catalase

Accurate and rapid identification of Staphylococcus aureus (S. aureus) is of great significance for controlling the food poisoning and infectious diseases caused by S. aureus. In this study, a novel strategy that combines lysin cell-binding domain (CBD)-based magnetic separation with fluorescence detection was developed for the specific and sensitive quantification of S. aureus in authentic samples. The S. aureus cells were separated from the sample matrix by lysin CBD-functionalized magnetic beads. Following lysis by lysostaphin, intracellular catalase was released from S. aureus cells and detected by a fluorometric system composed of horseradish peroxidase (HRP), hydrogen peroxide (H₂O₂), and Amplex Red. S. aureus was quantified via the inhibitory effect of the released intracellular catalase on the fluorometric system since the catalase could decompose the H₂O₂. Optimized conditions afforded a calibration curve for S. aureus ranging from 1.0 × 10² to 1.0 × 10⁷ CFU mL^-1. The detection limit was as low as 78 CFU mL^-1 in phosphate-buffered saline (PBS), and the total detection process could be completed in less than 50 min. Other bacteria associated with common food-borne and nosocomial infections negligibly interfered with S. aureus detection, except for Staphylococcus epidermidis, which may have slightly interfered. Moreover, the potential of this proposed method for practical applications has been demonstrated by detection assays of sterilized milk and human serum. Graphical abstract.

Point-of-Need DNA Testing for Detection of Foodborne Pathogenic Bacteria

Foodborne pathogenic bacteria present a crucial food safety issue. Conventional diagnostic methods are time-consuming and can be only performed on previously produced food. The advancing field of point-of-need diagnostic devices integrating molecular methods, biosensors, microfluidics, and nanomaterials offers new avenues for swift, low-cost detection of pathogens with high sensitivity and specificity. These analyses and screening of food items can be performed during all phases of production. This review presents major developments achieved in recent years in point-of-need diagnostics in land-based sector and sheds light on current challenges in achieving wider acceptance of portable devices in the food industry. Particular emphasis is placed on methods for testing nucleic acids, protocols for portable nucleic acid extraction and amplification, as well as on the means for low-cost detection and read-out signal amplification.

Ultrasensitive and Fast Diagnostics of Viable Listeria Cells by CBD Magnetic Separation Combined with A511::luxAB Detection

The genus Listeria includes foodborne pathogens that cause life-threatening infections in those at risk, and sensitive and specific methods for detection of these bacteria are needed. Based on their unrivaled host specificity and ability to discriminate viable cells, bacteriophages represent an ideal toolbox for the development of such methods. Here, the authors describe an ultrasensitive diagnostic protocol for Listeria by combining two phage-based strategies: (1) specific capture and concentration of target cells by magnetic separation, harnessing cell wall-binding domains from Listeria phage endolysins (CBD-MS); and (2) highly sensitive detection using an adaptation of the A511::luxAB bioluminescent reporter phage assay in a microwell plate format. The combined assay enabled direct detection of approximately 100 bacteria per ml of pure culture with genus-level specificity in less than 6 h. For contaminated foods, the procedure included a 16 h selective enrichment step, followed by CBD-MS separation and A511::luxAB detection. It was able to consistently detect extremely low numbers (0.1 to 1.0 cfu/g) of viable Listeria cells, in a total assay time of less than 22 h. These results demonstrate the superiority of this phage-based assay to standard culture-based diagnostic protocols for the detection of viable bacteria, with respect to both sensitivity and speed.

Development of a specific fluorescent phage endolysin for in situ detection of Clostridium species associated with cheese spoilage

Late blowing defect (LBD) is a major cause of spoilage in cheeses, caused by the growth of Clostridium spp. in the cheese matrix. We investigated the application of CTP1L, a bacteriophage endolysin active against Clostridium tyrobutyricum, and its enzymatically active and cell wall-binding domains (EAD and CBD) attached to green fluorescent protein (GFP) to detect dairy-related Clostridium species by fluorescence microscopy. GFP-CTP1L and GFP-CBD demonstrated specificity for Clostridium spp. by labelling 15 and 17 of 20 Clostridium strains, respectively, but neither bound to other members of the cheese microbiota. However, GFP-EAD did not label any Clostridium strain tested. Unexpectedly, GFP-CTP1L and GFP-CBD were also able to bind to clostridial spores. In addition, GFP-CBD allowed us to visualize the vegetative cells of C. tyrobutyricum directly in the matrix of a LBD cheese. Site-directed mutants of GFP-CTP1L and GFP-CBD were made to examine the amino acids involved in binding and oligomer formation. Oligomerization was not essential for binding, but specific mutations in the CBD which affected oligomer formation also affected binding and lytic activity. We conclude that GFP-CTP1L and GFP-CBD could be good biomarkers for rapid detection of Clostridium spores in milk, so measures can be taken for the prevention of LBD in cheese, and also provide effective tools to study the development of Clostridium populations during cheese ripening.

Modified Bacteriophage S16 Long Tail Fiber Proteins for Rapid and Specific Immobilization and Detection of Salmonella Cells

Bacteriophage-based assays and biosensors rival traditional antibody-based immunoassays for detection of low-level Salmonella contaminations. In this study, we harnessed the binding specificity of the long tail fiber (LTF) from bacteriophage S16 as an affinity molecule for the immobilization, enrichment, and detection of Salmonella We demonstrate that paramagnetic beads (MBs) coated with recombinant gp37-gp38 LTF complexes (LTF-MBs) are highly effective tools for rapid affinity magnetic separation and enrichment of Salmonella Within 45 min, the LTF-MBs consistently captured over 95% of Salmonella enterica serovar Typhimurium cells from suspensions containing from 10 to 105 CFU · ml-1, and they yielded equivalent recovery rates (93% ± 5%, n = 10) for other Salmonella strains tested. LTF-MBs also captured Salmonella cells from various food sample preenrichments, allowing the detection of initial contaminations of 1 to 10 CFU per 25 g or ml. While plating of bead-captured cells allowed ultrasensitive but time-consuming detection, the integration of LTF-based enrichment into a sandwich assay with horseradish peroxidase-conjugated LTF (HRP-LTF) as a detection probe produced a rapid and easy-to-use Salmonella detection assay. The novel enzyme-linked LTF assay (ELLTA) uses HRP-LTF to label bead-captured Salmonella cells for subsequent identification by HRP-catalyzed conversion of chromogenic 3,3',5,5'-tetramethylbenzidine substrate. The color development was proportional for Salmonella concentrations between 102 and 107 CFU · ml-1 as determined by spectrophotometric quantification. The ELLTA assay took 2 h to complete and detected as few as 102 CFU · ml-1S Typhimurium cells. It positively identified 21 different Salmonella strains, with no cross-reactivity for other bacteria. In conclusion, the phage-based ELLTA represents a rapid, sensitive, and specific diagnostic assay that appears to be superior to other currently available tests.IMPORTANCE The incidence of foodborne diseases has increased over the years, resulting in major global public health issues. Conventional methods for pathogen detection can be laborious and expensive, and they require lengthy preenrichment steps. Rapid enrichment-based diagnostic assays, such as immunomagnetic separation, can reduce detection times while also remaining sensitive and specific. A critical component in these tests is implementing affinity molecules that retain the ability to specifically capture target pathogens over a wide range of in situ applications. The protein complex that forms the distal tip of the bacteriophage S16 long tail fiber is shown here to represent a highly sensitive affinity molecule for the specific enrichment and detection of Salmonella Phage-encoded long tail fibers have huge potential for development as novel affinity molecules for robust and specific diagnostics of a vast spectrum of bacteria.

Lateral flow assay-based bacterial detection using engineered cell wall binding domains of a phage endolysin

The development of a cost-effective and efficient bacterial detection assay is essential for diagnostic fields, particularly in resource-poor settings. Although antibodies have been widely used for bacterial capture, the production of soluble antibodies is still expensive and time-consuming. Here, we developed a nitrocellulose-based lateral flow assay using cell wall binding domains (CBDs) from phage as a recognition element and colloidal gold nanoparticles as a colorimetric signal for the detection of a model pathogenic bacterium, Bacillus cereus (B. cereus). To improve conjugation efficiency and detection sensitivity, cysteine-glutathione-S-transferase-tagged CBDs and maltose-binding protein-tagged CBDs were produced in Escherichia coli (E. coli) and incorporated in our assays. The sensitivity of the strip to detect B. cereus was 1×10⁴ CFU/mL and the overall assay time was 20min. The assay showed superior results compared to the antibody-based approach, and did not show any significant cross-reactivity. This proof of concept study indicates that the lateral flow assay using engineered CBDs hold considerable promise as simple, rapid, and cost-effective biosensors for whole cell detection.

Bacteriophages and phage-derived proteins--application approaches

Currently, the bacterial resistance, especially to most commonly used antibiotics has proved to be a severe therapeutic problem. Nosocomial and community-acquired infections are usually caused by multidrug resistant strains. Therefore, we are forced to develop an alternative or supportive treatment for successful cure of life-threatening infections. The idea of using natural bacterial pathogens such as bacteriophages is already well known. Many papers have been published proving the high antibacterial efficacy of lytic phages tested in animal models as well as in the clinic. Researchers have also investigated the application of non-lytic phages and temperate phages, with promising results. Moreover, the development of molecular biology and novel generation methods of sequencing has opened up new possibilities in the design of engineered phages and recombinant phage-derived proteins. Encouraging performances were noted especially for phage enzymes involved in the first step of viral infection responsible for bacterial envelope degradation, named depolymerases. There are at least five major groups of such enzymes – peptidoglycan hydrolases, endosialidases, endorhamnosidases, alginate lyases and hyaluronate lyases – that have application potential. There is also much interest in proteins encoded by lysis cassette genes (holins, endolysins, spanins) responsible for progeny release during the phage lytic cycle. In this review, we discuss several issues of phage and phage-derived protein application approaches in therapy, diagnostics and biotechnology in general.

Bacteriophage endolysins and their applications

Endolysins (lysins) are bacteriophage-encoded enzymes that have evolved to degrade specific bonds within the bacterial cell wall. These enzymes represent a novel class of antibacterial agents against infectious pathogens, especially in light of multidrug-resistant bacteria, which have made antibiotic therapy increasingly redundant. Lysins have been used successfully to eliminate/control bacterial pathogens in various anatomical locations in mouse and other animal models. Engineering tactics have also been successfully applied to improve lysin function. This review discusses the structure and function of lysins. It highlights protein-engineering tactics utilised to improve lysin activity. It also reviews the applications of lysins towards food biopreservation, therapeutics, biofilm elimination and diagnostics.

Bacteriophage endolysins: applications for food safety

Bacteriophage endolysins (peptidoglycan hydrolases) have emerged as a new class of antimicrobial agents useful for controlling bacterial infection or other unwanted contaminations in various fields, particularly in the light of the worldwide increasing frequency of drug-resistant pathogens. This review summarizes and discusses recent developments regarding the use of endolysins for food safety. Besides the use of native and engineered endolysins for controlling bacterial contamination at different points within the food production chain, this also includes the application of high-affinity endolysin-derived cell wall binding domains for rapid detection of pathogenic bacteria. Novel approaches to extend the lytic action of endolysins towards Gram-negative cells will also be highlighted.

The challenge of enumerating Listeria monocytogenes in food

Listeria monocytogenes is recognised as a serious foodborne pathogen in humans. However, food products are usually contaminated at low levels (i.e. <100 CFU/g) and there is still no adequate enumeration method for testing food. Much research has been carried out to improve Listeria enumeration methods, leading to several proposed alternative methods such as the most probable number technique, molecular-based methods and bacterial cell concentration techniques. Here, we catalogue the current knowledge concerning L. monocytogenes enumeration, with a particular focus on the problem of enumerating low level contamination.

Prevention of bacterial foodborne disease using nanobiotechnology

Foodborne disease is an important source of expense, morbidity, and mortality for society. Detection and control constitute significant components of the overall management of foodborne bacterial pathogens, and this review focuses on the use of nanosized biological entities and molecules to achieve these goals. There is an emphasis on the use of organisms called bacteriophages (phages: viruses that infect bacteria), which are increasingly being used in pathogen detection and biocontrol applications. Detection of pathogens in foods by conventional techniques is time-consuming and expensive, although it can also be sensitive and accurate. Nanobiotechnology is being used to decrease detection times and cost through the development of biosensors, exploiting specific cell-recognition properties of antibodies and phage proteins. Although sensitivity per test can be excellent (eg, the detection of one cell), the very small volumes tested mean that sensitivity per sample is less compelling. An ideal detection method needs to be inexpensive, sensitive, and accurate, but no approach yet achieves all three. For nanobiotechnology to displace existing methods (culture-based, antibody-based rapid methods, or those that detect amplified nucleic acid) it will need to focus on improving sensitivity. Although manufactured nonbiological nanoparticles have been used to kill bacterial cells, nanosized organisms called phages are increasingly finding favor in food safety applications. Phages are amenable to protein and nucleic acid labeling, and can be very specific, and the typical large "burst size" resulting from phage amplification can be harnessed to produce a rapid increase in signal to facilitate detection. There are now several commercially available phages for pathogen control, and many reports in the literature demonstrate efficacy against a number of foodborne pathogens on diverse foods. As a method for control of pathogens, nanobiotechnology is therefore flourishing.

Phages of Listeria offer novel tools for diagnostics and biocontrol

Historically, bacteriophages infecting their hosts have perhaps been best known and even notorious for being a nuisance in dairy-fermentation processes. However, with the rapid progress in molecular microbiology and microbial ecology, a new dawn has risen for phages. This review will provide an overview on possible uses and applications of Listeria phages, including phage-typing, reporter phage for bacterial diagnostics, and use of phage as biocontrol agents for food safety. The use of phage-encoded enzymes such as endolysins for the detection and as antimicrobial agent will also be addressed. Desirable properties of candidate phages for biocontrol will be discussed. While emphasizing the enormous future potential for applications, we will also consider some of the intrinsic limitations dictated by both phage and bacterial ecology.

Application of bacteriophages for detection of foodborne pathogens

Bacterial contamination of food products presents a challenge for the food industry and poses a high risk for the consumer. Despite increasing awareness and improved hygiene measures, foodborne pathogens remain a threat for public health, and novel methods for detection of these organisms are needed. Bacteriophages represent ideal tools for diagnostic assays because of their high target cell specificity, inherent signal-amplifying properties, easy and inexpensive production, and robustness. Every stage of the phage lytic multiplication cycle, from the initial recognition of the host cell to the final lysis event, may be harnessed in several ways for the purpose of bacterial detection. Besides intact phage particles, phage-derived affinity molecules such as cell wall binding domains and receptor binding proteins can serve for this purpose. This review provides an overview of existing phage-based technologies for detection of foodborne pathogens, and highlights the most recent developments in this field, with particular emphasis on phage-based biosensors.

Use of bacteriophage cell wall-binding proteins for rapid diagnostics of Listeria

Diagnostic protocols for food-borne bacterial pathogens such as Listeria need to be sensitive, specific, rapid, and inexpensive. Conventional culture methods are hampered by lengthy enrichment and incubation steps. Bacteriophage-derived high-affinity binding molecules (cell wall-binding domains, CBDs) specific for Listeria cells have recently been introduced as tools for detection and differentiation of this pathogen in foods. When coupled with magnetic separation, these proteins offer advantages in sensitivity and speed compared to the standard diagnostic methods. Furthermore, fusion of CBDs to differently colored fluorescent reporter proteins enables differentiation of Listeria strains in mixed cultures. This chapter provides protocols for detection of Listeria in food by CBD-based magnetic separation and subsequent multiplexed identification of strains of different serotypes with reporter-CBD fusion proteins.

Listeria phages: Genomes, evolution, and application

Listeria is an important foodborne pathogen and the causative agent of Listeriosis, a potentially fatal infection. Several hundred Listeria bacteriophages have been described over the past decades, but only few have actually been characterized in some detail, and genome sequences are available for less than twenty of them. We here present an overview of what is currently known about Listeria phage genomics, their role in host evolution and pathogenicity, and their various applications in biotechnology and diagnostics.

Bacteriophage endolysins as novel antimicrobials

Endolysins are enzymes used by bacteriophages at the end of their replication cycle to degrade the peptidoglycan of the bacterial host from within, resulting in cell lysis and release of progeny virions. Due to the absence of an outer membrane in the Gram-positive bacterial cell wall, endolysins can access the peptidoglycan and destroy these organisms when applied externally, making them interesting antimicrobial candidates, particularly in light of increasing bacterial drug resistance. This article reviews the modular structure of these enzymes, in which cell wall binding and catalytic functions are separated, as well as their mechanism of action, lytic activity and potential as antimicrobials. It particularly focuses on molecular engineering as a means of optimizing endolysins for specific applications, highlights new developments that may render these proteins active against Gram-negative and intracellular pathogens and summarizes the most recent applications of endolysins in the fields of medicine, food safety, agriculture and biotechnology.

Highly specific fiber optic immunosensor coupled with immunomagnetic separation for detection of low levels of Listeria monocytogenes and L. ivanovii

BACKGROUND

Immunomagnetic separation (IMS) and immunoassays are widely used for pathogen detection. However, novel technology platforms with highly selective antibodies are essential to improve detection sensitivity, specificity and performance. In this study, monoclonal antibodies (MAbs) against Internalin A (InlA) and p30 were generated and used on paramagnetic beads of varying diameters for concentration, as well as on fiber-optic sensor for detection.

RESULTS

Anti-InlA MAb-2D12 (IgG2a subclass) was specific for Listeria monocytogenes and L. ivanovii, and p30-specific MAb-3F8 (IgM) was specific for the genus Listeria. At all bacterial concentrations (10³-10⁸ CFU/mL) tested in the IMS assay; the 1-μm diameter MyOne beads had significantly higher capture efficiency (P < 0.05) than the 2.8-μm diameter M-280 beads with both antibodies. The highest capture efficiency for MyOne-2D12 (49.2% for 10⁵ CFU/mL) was significantly higher (P < 0.05) than that of MyOne-3F8 (16.6 %) and Dynabeads anti-Listeria antibody (9 %). Furthermore, capture efficiency for MyOne-2D12 was highly specific for L. monocytogenes and L. ivanovii. Subsequently, we captured L. monocytogenes by MyOne-2D12 and MyOne-3F8 from hotdogs inoculated with mono- or co-cultures of L. monocytogenes and L. innocua (10-40 CFU/g), enriched for 18 h and detected by fiber-optic sensor and confirmed by plating, light-scattering, and qPCR assays. The detection limit for L. monocytogenes and L. ivanovii by the fiber-optic immunosensor was 3 × 10² CFU/mL using MAb-2D12 as capture and reporter antibody. Selective media plating, light-scattering, and qPCR assays confirmed the IMS and fiber-optic results.

CONCLUSIONS

IMS coupled with a fiber-optic sensor using anti-InlA MAb is highly specific for L. monocytogenes and L. ivanovii and enabled detection of these pathogens at low levels from buffer or food.

Endolysins as antimicrobials

Peptidoglycan (PG) is the major structural component of the bacterial cell wall. Bacteria have autolytic PG hydrolases that allow the cell to grow and divide. A well-studied group of PG hydrolase enzymes are the bacteriophage endolysins. Endolysins are PG-degrading proteins that allow the phage to escape from the bacterial cell during the phage lytic cycle. The endolysins, when purified and exposed to PG externally, can cause "lysis from without." Numerous publications have described how this phenomenon can be used therapeutically as an effective antimicrobial against certain pathogens. Endolysins have a characteristic modular structure, often with multiple lytic and/or cell wall-binding domains (CBDs). They degrade the PG with glycosidase, amidase, endopeptidase, or lytic transglycosylase activities and have been shown to be synergistic with fellow PG hydrolases or a range of other antimicrobials. Due to the coevolution of phage and host, it is thought they are much less likely to invoke resistance. Endolysin engineering has opened a range of new applications for these proteins from food safety to environmental decontamination to more effective antimicrobials that are believed refractory to resistance development. To put phage endolysin work in a broader context, this chapter includes relevant studies of other well-characterized PG hydrolase antimicrobials.

Lytic activity of LysH5 endolysin secreted by Lactococcus lactis using the secretion signal sequence of bacteriocin Lcn972

Bacteriophage endolysins have an interesting potential as antimicrobials. The endolysin LysH5, encoded by Staphylococcus aureus phage vB_SauS-phi-IPLA88, was expressed and secreted in Lactococcus lactis using the signal peptide of bacteriocin lactococcin 972 and lactococcal constitutive and inducible promoters. Up to 80 U/mg of extracellular active endolysin was detected in culture supernatants, but most of the protein (up to 323 U/mg) remained in the cell extracts.

Methods used for the detection and subtyping of Listeria monocytogenes

Listeria monocytogenes is an important foodborne pathogen responsible for non-invasive and invasive diseases in the elderly, pregnant women, neonates and immunocompromised populations. This bacterium has many similarities with other non-pathogenic Listeria species which makes its detection from food and environmental samples challenging. Subtyping of L. monocytogenes strains can prove to be crucial in epidemiological investigations, source tracking contamination from food processing plants and determining evolutionary relationships between different strains. In recent years there has been a shift towards the use of molecular subtyping. This has led to the development of new subtyping techniques such as multi-locus variable number tandem repeat analysis (MLVA) and multi-locus sequence based typing (MLST). This review focuses on the available methods for Listeria detection including immuno-based techniques and the more recently developed molecular methods and analytical techniques such as matrix-assisted laser desorption/ionisation time-of-flight based mass spectrometry (MALDI-TOF MS). It also includes a comparison and critical analysis of the available phenotypic and genotypic subtyping techniques that have been investigated for L. monocytogenes.

Domain shuffling and module engineering of Listeria phage endolysins for enhanced lytic activity and binding affinity

Bacteriophage endolysins are peptidoglycan hydrolases employed by the virus to lyse the host at the end of its multiplication phase. They have found many uses in biotechnology; not only as antimicrobials, but also for the development of novel diagnostic tools for rapid detection of pathogenic bacteria. These enzymes generally show a modular organization, consisting of N‐terminal enzymatically active domains (EADs) and C‐terminal cell wall‐binding domains (CBDs) which specifically target the enzymes to their substrate in the bacterial cell envelope. In this work, we used individual functional modules of Listeria phage endolysins to create fusion proteins with novel and optimized properties for labelling and lysis of Listeria cells. Chimaeras consisting of individual EAD and CBD modules from PlyPSA and Ply118 endolysins with different binding specificity and catalytic activity showed swapped properties. EAD118–CBDPSA fusion proteins exhibited up to threefold higher lytic activity than the parental endolysins. Recombineering different CBD domains targeting various Listeria cell surfaces into novel heterologous tandem proteins provided them with extended recognition and binding properties, as demonstrated by fluorescent GFP‐tagged CBD fusions. It was also possible to combine the binding specificities of different single CBDs in heterologous tandem CBD constructs such as CBD500‐P35 and CBDP35‐500, which were then able to recognize the majority of Listeria strains. Duplication of CBD500 increased the equilibrium cell wall binding affinity by approximately 50‐fold, and the enzyme featuring tandem CBD modules showed increased activity at higher ionic strength. Our results demonstrate that modular engineering of endolysins is a powerful approach for the rational design and optimization of desired functional properties of these proteins.

Food applications of bacterial cell wall hydrolases

Bacterial cell wall hydrolases (BCWHs) display a remarkable structural and functional diversity that offers perspectives for novel food applications, reaching beyond those of the archetype BCWH and established biopreservative hen egg white lysozyme. Insights in BCWHs from bacteriophages to animals have provided concepts for tailoring BCWHs to target specific pathogens or spoilage bacteria, or, conversely, to expand their working range to Gram-negative bacteria. Genetically modified foods expressing BCWHs in situ showed successful, but face regulatory and ethical concerns. An interesting spin-off development is the use of cell wall binding domains of bacteriophage BCWHs for detection and removal of foodborne pathogens. Besides for improving food safety or stability, BCWHs may also find use as functional food ingredients with specific health effects.