Understanding Bacteriophages: Detection Methods and Applications
News 18 8 月, 2024
Bacteriophages, often referred to as phages, are viruses that specifically infect bacteria. They have distinctive viral morphology and structure and exhibit high specificity towards their bacterial hosts. This host specificity is useful for strain identification but can vary. Phages attach to specific receptor sites on the bacterial cell surface using their tail sheath, inject RNA into the bacterium, replicate inside, and ultimately lyse the host cell to release progeny phages. They cannot grow or reproduce in metabolically inactive hosts.
Detection of Bacteriophages
Phages can be isolated using methods such as the plate assay or broth culture lysis. Currently, the double-layer agar plate method is widely used for phage detection. In these methods, the host bacteria are metabolically active, and phage growth ceases if the host’s metabolism stops. Phage growth typically occurs at 37°C. The efficiency of plating, a common method for detecting phage particles, measures the proportion of phage particles that form plaques on a host cell lawn. Factors influencing plating efficiency include the presence of dead bacteria and the composition of the medium.
Morphology and Structure of Bacteriophages
Phages cannot be observed under an optical microscope, necessitating the use of an electron microscope to study their morphology and structure. Phages, or virus particles, come in three shapes: tadpole-like, spherical, and filamentous.
Most known phages are tadpole-shaped, consisting of a head and a tail. The tail, a complex structure, is responsible for infection, adsorption, and invasion of the host cell. This structural specialization differentiates phages from bacteriocins and bdellovibrios. Tadpole phages can be categorized into two groups based on tail morphology: those with long, contractile tails (e.g., T2, T4, and T6) and those with non-contractile, flexible tails (e.g., T1, T3, T5, T7, and λ phage).
For instance, T2 phage has a hexagonal head measuring 95nm long and 65nm wide, composed of a protein shell made of 2000 protein subunits, each with a molecular weight of 80kD. The head is elastic, facilitating the injection of nucleic acids into the host bacterium. The tail includes structures such as the tail sheath, core, base plate, tail fibers, and tail pins.
Spherical phages are smaller (20-60nm), with an icosahedral structure visible under an electron microscope, lacking tails or protrusions. For example, φ174 has a knot structure at each vertex of its icosahedron. The protein shell encloses the nucleic acid.
Filamentous phages are simple, flexible filaments, 600-800nm long, that can directly penetrate the host’s cell wall without attachment organs.
Applications of Bacteriophages
Phage typing techniques are widely adopted for bacterial identification. In 1938, Chun-Hui Yan first reported the Vi phage typing method for Salmonella typhi, which became a model for phage-based bacterial typing. Phage typing is particularly useful in epidemiological tracking of infection sources, such as for Vibrio, Salmonella, Staphylococcus aureus, Corynebacterium diphtheriae, and Pseudomonas aeruginosa.
Due to their strong specificity for host cells, phages have therapeutic applications for certain infectious diseases. For example, Pseudomonas aeruginosa phages can effectively inhibit infections caused by P. aeruginosa. The most effective treatment is topical application for surface infections, followed by cavity spraying, with oral treatment being the least effective. Recently, Xiao-Qing He and colleagues successfully used phages to identify Enterobacteriaceae bacteria, significantly facilitating Salmonella detection.
Phages are also widely used as biological materials in research, such as Escherichia coli phages, demonstrating their versatility and importance in various biological studies.