The early detection of pathogenic microorganisms is essential to prevent serious public health issues. Bacteriophage-based biosensors have been developed to detect pathogenic contaminants in food and water. These biosensors are more beneficial than conventional biosensors for sensitivity, specificity, resistance to extreme environmental factors, and reduced assay times. This article focuses on different types of bacteriophage-based biosensors that ensure food and water safety.
What is a Bacteriophage?
Bacteriophages are also referred to as phages that can infect other microorganisms, including bacteria, fungi, and actinomycetes. These are the most abundant and ubiquitous biological agents present in the environment. Phages are extremely diverse in morphology, size, and genomic organization. Most bacteriophages are linked to double-stranded DNA viruses and are referred to as tailed bacteriophages. These bacteriophages are divided into three families, namely, Myoviridae, Podoviridae, and Siphoviridae.
All bacteriophages contain a nucleic acid genome encapsulated in a shell of phage-encoded capsid proteins. The primary functions of the proteinaceous shell are to protect the genetic material and promote its delivery to the next host cell. The morphology of a typical bacteriophage studied under an electron microscope revealed the presence of heads, legs, and tail-like structures.
Phages are non-motile and depend on Brownian motion to reach their targets. Like all viruses, bacteriophages are extremely species-specific. This means that phages can only infect a single bacterial species or specific strains within a species.
During infection, bacteriophage binds to a susceptible host and promotes one of the two replication strategies, namely, the lysogenic or lytic replication cycle. In a lytic cycle, bacteriophage attaches to a host bacterium and introduces its genome to the host cell cytoplasm. Subsequently, it uses the host's ribosome to synthesize its proteins and produce multiple copies of the original phage. After the host cell dies, it either actively or passively lyses to release the new bacteriophages, which infect other host cells.
In the lysogenic cycle, bacteriophages infect the target bacterium and introduce its genome into the cytoplasm of the host cell. The phage genome is integrated into the bacterial cell chromosome, and it is replicated and passed onto the daughter bacterial cells.
Bacteriophage Biosensors for Food and Water Safety
Several microorganisms have been identified, including Listeria monocytogenes, Cryptosporidium, Salmonella, Staphylococcus aureus, Escherichia coli, Clostridium perfringens, and Campylobacter that cause serious food poisoning. Fruits, vegetables, ready-made supermarket foods, and takeaways can be contaminated with harmful pathogens.
Wastewater discarded from different sources, such as livestock, industry, and municipal waste, contains many toxic chemicals and pathogenic microorganisms. Therefore, it is essential to treat wastewater before disposal. If these are not properly processed before being discarded into the environment, they could reach the groundwater and affect thousands of people. Every year, many people die across the world due to water contamination.
Biosensors are devices that rapidly and accurately detect pathogenic contaminants in food and water. Bacteriophages are used to control pathogenic bacteria in food and water as their components are not harmful to the human body. In addition, phages do not change the texture, color, or taste of food.
Bacteriophage-based Biosensors to Detect Pathogens in Food and Water
Different types of phage-based biosensors, including optical, electrochemical, and micromechanical, have been developed. These are discussed in brief below:
Phage-Based Optical Biosensors
Optical biosensors are commonly used to detect pathogenic bacterial contaminants in food and water with high specificity and sensitivity. The phage-based optical biosensors are developed based on various optical techniques, including fluorescence spectrometry, chemo/bioluminescence, and surface plasmon resonance (SPR).
Bacteriophage-based SPR biosensors have been developed for real-time and quantitative detection of binding agents or molecules without labeling. In this biosensor, the tail spike protein of an engineered phage was immobilized onto a gold surface for fast and accurate detection of Salmonella.
Bacteriophage-based bioluminescence sensors have been developed using luciferase enzymes. A NanoLuc luciferase (NLuc) phage-based assay has been designed to detect Listeria. NLuc-CBM has also been developed to detect E. coli in water. A bioluminescence sensor based on an engineered phage, SPC32H-CDABE, has also been developed to detect Salmonella in milk, lettuce, and sliced pork. Phage-based fluorescent biosensors have been developed to identify enteric bacteria, such as E. coli and S. Typhimurium, in water.
Phage-Based Electrochemical Biosensors
Phages can act as transducers for electrochemical sensors. An electric current is applied from an external source in a phage-based electrochemical biosensor. Phage-based electrochemical biosensors can be categorized based on potentiometric and amperometric measurements.
Phage-based amperometric biosensors have recently gained popularity due to their simplicity, high specificity and sensitivity, and suitability for field testing. This biosensor uses enzymes, such as glucose oxidase, horseradish peroxidase (HRP), and alkaline phosphatase (AP), as bio-receptors.
Several phage-based amperometric biosensors have been devised to detect foodborne bacterial pathogens, such as E. coli, and Bacillus cereus. Furthermore, phage-based electrochemical impedance spectroscopy (EIS) biosensors have been used to detect E. coli and Staphylococcus arlettae.
Phage-based Micromechanical Biosensors
Phage-based quartz crystal microbalance sensor has been created by immobilizing phages on the surface of sensors made up of quartz crystal. This type of sensor has been developed for the fast detection of S. typhimurium.
Phage-Based Magnetoelastic Biosensor
Phage-based magnetoelastic biosensors are a simple, mass-sensitive, wireless technique used to rapidly detect various biological analytes, including Salmonella, S. Typhimurium B. anthracis spores, and E. coli cells on food surfaces. This biosensor contains a magnetoelastic resonator immobilized with phages that function as bio-probes to detect specific microorganisms. This phage-based biosensor detects pathogens based on the changes in the resonant frequency.
Commercially Available Bacteriophage-based Biosensors
Bacteriophage-based biosensors have rapidly attracted the limelight due to their high accuracy, specificity, rapid detection process, and cost-effective technique. Several bacteriophage-based biosensors are commercially available. For instance, in 2006, Listex™P100 received approval from the U.S. Food and Drug Administration to be used as Generally Recognized as Safe (GRAS)-grade additives to control Listeria in food. The SPREETA biosensor and the BIACORE 3000 biosensor are two more commercially available phage-based optical biosensors used to detect L. monocytogenes, S.enteritidis, S. typhimurium and E. coli O157:H7.
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