Objectives of using the biological method
The biological diagnostic method is used when in vitro capabilities (in a test tube) are insufficient. The main goals of experimental animal infection include:
- Pathogen isolation. This is the main advantage of the method. It is indispensable when working with microorganisms that fundamentally do not grow on artificial nutrient media.
- Assessment of pathogenicity. The method allows for accurate determination of the virulence degree of the isolated microbe and identification of the type of toxin it produces.
- Infection modeling. In a living organism, it is possible to reproduce the clinical picture of a disease, study the stages of pathogenesis in detail, and analyze the complex interaction within the "microorganism – macroorganism" system.
- Immunology and pharmacology. The bioassay is necessary for studying the immune response of the macroorganism, as well as for evaluating the actual efficacy and activity of new antimicrobial drugs and vaccines.
Methods of administering the test material
To ensure that the infectious process develops as adequately as possible, the material is administered via various routes depending on the tropism of the pathogen. The choice of animal (mice, rats, guinea pigs, or rabbits) and the route of administration are dictated by the biological characteristics of the infectious agent.
- Enteral: administration of material through the mouth (orally).
- Respiratory: delivery of the infectious agent directly into the respiratory tract.
- Parenteral: injection techniques bypassing the digestive tract. These include intraperitoneal, intravenous, intramuscular, subcutaneous, and intradermal administration.
- Into special loci: complex manipulations for specific infections. For example, injection into the anterior chamber of the eye, subdurally (requires prior trepanation of the skull), or suboccipitally (injection into the cisterna magna of the brain).
Advantages and disadvantages of the method
Like any diagnostic approach, in vivo research has its strengths and weaknesses.
Advantages:
- Extremely high sensitivity. A living organism reacts even to minimal doses of a pathogen that may not be detected by other methods.
- Ability to isolate fastidious pathogens. Classic examples are the causative agents of leprosy and syphilis, which cannot be grown on standard nutrient media in the laboratory.
Disadvantages:
- Species insusceptibility. Laboratory animals are genetically insusceptible to most anthroponotic infections — diseases that affect humans exclusively.
- High economic costs. The method requires significant financial investment for the purchase of purebred animals, their proper nutrition, as well as the construction and regular maintenance of a specialized vivarium.
Briefly: answers to topical questions
In what units and how exactly is the degree of a microorganism's virulence assessed during a bioassay?
During an in vivo bioassay, the degree of virulence is evaluated in laboratory animals by determining lethal doses. The infectious dose depends on the portal of entry of the infection: 1–2 cells may be sufficient for respiratory infection, whereas thousands of cells are required for the oral infection of the same microorganism. Units of measurement for the degree of virulence are not specified in the cited source.
How is bacteriological examination of the organs of a deceased laboratory animal performed?
For sectional material, including the spleen, liver, lymph nodes, and other organs, bacteriological examination includes microscopy of smears of pathological material with Gram staining and capsule-staining methods, as well as isolation of a pure culture by plating onto a meat-peptone agar plate and into a test tube with meat-peptone broth or Hottinger medium.
Class and exam questions
If the microorganism is fastidious and unable to multiply on artificial nutrient media (e.g., causative agents of leprosy or syphilis).
In such situations, infecting a susceptible animal is the only way to obtain a culture for study.
How is a laboratory animal chosen for a bioassay?
The selection criterion depends entirely on the biological characteristics of the specific pathogen — the animal must be susceptible to the given infection.
Why are laboratory animals unsuitable for studying all infections?
Animals possess natural insusceptibility to the causative agents of anthroponotic infections that affect humans only, making it impossible to reproduce the clinical picture of such diseases in them.
What else to study on this topic
- Criteria and methods for assessing microbial virulence
- Methods for in vivo typing of bacterial toxins
- Features of the pathogenesis of leprosy and syphilis
- Rules of organization and operation of a vivarium
- Preclinical evaluation of vaccine activity in animals
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