Discovery and Visualization
The term derives from the Greek word phagos meaning 'devouring'. Spontaneous destruction (lysis) of anthrax bacilli was first observed by Russian scientist N.F. Gamaleya in 1898. Later, in 1915, the English investigator F. Twort described the ability of a filtrate to dissolve staphylococcal cultures.
A key contribution was made by the French-Canadian scientist F. d'Hérelle (1917): he isolated a lytic agent from the stool of dysentery patients, proved its viral nature, and coined the term 'bacteriophage'.
Macroscopically, phage action appears very characteristic:
- In liquid media: a turbid bacterial culture becomes completely transparent (cleared).
- On solid media: when a phage is applied to a confluent bacterial 'lawn', round sterile spots form, termed negative colonies or 'plaques'.
Morphology and Structure of Complex Phages
Based on shape, phages are divided into small cubic, filamentous, and tailed (spermatozoan) forms. Large tailed phages (such as T-series coliphages) are the most thoroughly studied. Their structure includes three main parts:
- Head (Capsid). Features an icosahedral (cubic) type of symmetry, with a size of 65–100 nm. It consists of a protein capsid tightly packing a nucleic acid molecule (most commonly double-stranded circular DNA). A histone-like protein is also present to aid in DNA supercoiling.
- Tail. Measures over 100 nm in length. It contains a hollow central core tube surrounded by a contractile sheath (containing ATP and calcium ions).
- Baseplate. The distal part featuring a hexagonal baseplate, spikes, and extended long tail fibers (fibrils)—all essential for attachment to the bacterium. It also houses the enzyme lysozyme for local dissolution of the cell wall.
Life Cycle of Virulent Bacteriophages
Virulent phages cause a productive infection that inevitably ends in bacterial death. The process, taking 20–40 minutes, is divided into several stages:
- Adsorption. The phage attaches via its tail fibers to specific cell surface receptors (lipopolysaccharides, teichoic acids, pili).
- Injection (Penetration). The tail sheath contracts, lysozyme digests a patch of the cell wall, and the hollow core punctures the membranes. Through this, the phage genome is injected into the cytoplasm, leaving the protein 'ghost' (capsid) outside.
- Replication. The injected DNA or RNA halts the synthesis of host macromolecules. The bacterium is repurposed to manufacture viral components.
- Assembly (Maturation). Empty capsids are packed with nucleic acid, after which tail structures are attached.
- Release. Driven by osmotic changes and phage lysozyme activity, the bacterial cell wall ruptures, releasing 200–300 new virions.
Specificity and Medical Applications
The interaction between a phage and a bacterium is strictly specific. Based on this, phages are classified as polyvalent (infecting related species), monovalent (infecting a single species), and type-specific (specific variants within a single species).
High specificity has found application in three medical directions:
- Diagnostics (Phage typing): allows precise identification of a pathogen species or strain based on its susceptibility to standard diagnostic phages.
- Therapy: oral or topical administration of phage preparations to treat bacterial infections (e.g., in suppurative surgery, cholera, dysentery). Therapeutic efficacy is generally rated as moderate.
- Prophylaxis: deployment during epidemic outbreaks to block the spread of infection.
Phage preparations are produced by co-culturing bacteria with phages followed by isolation of the filtrate, the activity of which is titrated on sensitive cultures.