Intraspecies identification: phage typing method
In laboratory diagnostics, the most crucial tool is phage typing—a method that allows for the identification of a specific pathogen variant within a single bacterial species. The main principle of this method lies in the strict specificity of bacterial virus action: a specific phage is capable of destroying (lysing) exclusively its own, strictly defined strains of microorganisms.
The procedure is carried out in several stages:
- A pure culture of the studied pathogen is seeded in a continuous layer (a so-called "lawn") onto a Petri dish with solid nutrient medium.
- Drops of various diagnostic type-specific phages are applied on top of the inoculation, strictly following a pattern or pre-marked squares.
- The inoculations are sent for incubation in a thermostat.
- After incubation is complete, the result is evaluated. Zones of lysis form at the sites where the drops were applied and the phage proved active against the bacteria. Visually, this appears as transparent "sterile spots", "plaques", or "negative colonies" against the background of turbid bacterial growth.
The combination of phages that successfully induced culture lysis determines its phagovar (or phage type).
Epidemiological and sanitary analysis
Determining the phagovar is of critical importance for epidemiological analysis and marking. The logic of investigations is extremely simple: if bacteria of an absolutely identical phagovar are isolated from several different patients, it directly indicates a common source of infection and a single route of transmission. This method is indispensable when searching for the causes of infectious disease outbreaks.
Furthermore, bacteriophages are actively used in sanitary and microbiological control. For example, the detection of specific phages in environmental objects (in particular, in water) serves as a reliable indicator that corresponding pathogenic bacteria are present there.
Phage therapy and phage prophylaxis
Bacterial viruses are used not only for diagnostics, but also for the direct prevention and treatment of bacterial infections. The pharmaceutical industry produces preparations in a wide variety of dosage forms: liquid solutions, tablets, suppositories, and aerosols. They can be administered orally (per os), parenterally, as well as applied topically or externally.
The spectrum of preparations is divided into two main groups:
- Monovalent: aimed at combating a single pathogen species. These include typhoid, salmonella, dysentery, Pseudomonas aeruginosa, staphylococcal, and streptococcal phages.
- Combined: mixtures of various phages. For example, coliprotei preparation or pyobacteriophages, which work effectively against a complex of pyogenic cocci and enterobacteria.
Biotechnology and genetic engineering
A separate yet extremely important area of practical application for bacteriophages is modern genetic engineering and biotechnology. Due to their ability to introduce nucleic acid into a cell, phages perform a vector function. They are used as carriers (vectors) of genetic material, allowing scientists to create new recombinant DNA molecules.
Briefly: answers to topical questions
What nutrient media are used for staphylococcal phage typing?
Solid nutrient media are used to perform phage typing. The specific names of the media for the actual phage application stage on staphylococci are not indicated in the text, but differential diagnostic media are used for the preliminary isolation of a pure culture:
- Egg-salt agar — detection of lecithinovitellase (an aggressive enzyme).
- Blood agar — determination of hemolytic activity.
Liquid nutrient media may also be used to assess phage activity via the titration method.
What contraindications and side effects exist in phage therapy?
During phage therapy, side effects and contraindications are completely absent. Preparations are prescribed to patients with a precisely established diagnosis and after confirming the pathogen's sensitivity to the phage. Dosage forms include liquid forms, tablets, suppositories, and aerosols, which can be administered orally, parenterally, or topically.
The "LTSA" rule helps remember product dosage forms — Liquid, Tablets, Suppositories, Aerosols.
Class and exam questions
What are "sterile spots" in phage typing?
These are transparent zones on a continuous bacterial "lawn" formed on solid nutrient media. They appear where a specific bacteriophage has destroyed (lysed) bacterial cells.
Why is it necessary to determine the phagovar in epidemiological analysis?
If bacteria of the exact same phagovar are detected in different patients, it proves they were infected from a common source. This allows for the rapid identification of an outbreak's cause and the cessation of infection transmission.
How do monovalent preparations differ from combined ones?
Monovalent phages (such as Pseudomonas aeruginosa or dysentery phages) act against only one specific pathogen. Combined preparations (such as pyobacteriophages) contain a mixture of viruses and are active against several groups of bacteria simultaneously, such as pyogenic cocci and enterobacteria.
What else to study on this topic
- The mechanism of "sterile spot" (negative colony) formation on solid nutrient media.
- Features of using combined pyobacteriophages in clinical practice.
- The use of phages as vectors in the construction of recombinant DNA molecules.
- Methods for the sanitary and microbiological detection of phages in drinking water.
- Comparison of the efficacy of oral and parenteral routes of administration for phage preparations.
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