In what units and how is the degree of microbial 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: 1–2 cells may suffice for respiratory infection, whereas thousands of cells may be required for oral infection with the same microbe. Specific quantitative units of virulence measurement are not detailed in the source.

How is bacteriological examination of organs from a deceased laboratory animal performed?

For necropsy material—including the spleen, liver, lymph nodes, and other organs—bacteriological examination involves smear microscopy (using Gram staining and capsule-detection techniques) and isolation of a pure culture by plating on meat-peptone agar and inoculating meat-peptone broth or Hottinger medium.

When is the biological method chosen over standard culture techniques?

When a microorganism is fastidious and incapable of multiplying on artificial nutrient media (e.g., Mycobacterium leprae or Treponema pallidum). In such cases, inoculating a susceptible animal is the only way to obtain a culture for study.

How is the 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 that particular infection.

Why are laboratory animals unsuitable for studying all infections?

Animals possess natural resistance to pathogens of strict anthroponotic infections that affect humans only, making it impossible to reproduce the clinical picture of such diseases in animal models.

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Biological Diagnostic Method

In vivo

For medical students2 min readUpdated 2026-10-10

The biological diagnostic method (bioassay) is a classic microbiological technique based on the experimental infection of susceptible laboratory animals. It enables the isolation of pure pathogen cultures, the assessment of their pathogenicity, and the study of microbe-host interactions in a living organism.

Core PrincipleInoculation of susceptible animals (mice, rats, guinea pigs, rabbits)

Main AdvantageEnables the isolation of microorganisms that do not grow on standard culture media

Main LimitationAnimals are naturally resistant to strict anthroponotic infections (exclusive human diseases)

Animal SelectionStrictly depends on the biological characteristics of the specific pathogen

Purposes of the Biological Method

The biological diagnostic method is used when in vitro (test tube) capabilities are insufficient. The main goals of experimental animal inoculation include:

Pathogen Isolation. This is the primary advantage of the method. It is indispensable when working with microorganisms that fundamentally refuse to grow on artificial nutrient media.
Pathogenicity Assessment. The method allows for the precise determination of the virulence degree of an isolated microbe and the identification of the type of toxin it produces.
Infection Modeling. A living organism makes it possible to reproduce the clinical picture of a disease, study the stages of pathogenesis in detail, and analyze complex host-pathogen interactions.
Immunology and Pharmacology. The bioassay is essential for studying the host immune response, as well as for evaluating the actual efficacy and activity of new antimicrobial drugs and vaccines.

Routes of Inoculation

To ensure the infectious process develops adequately, 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 inoculation route are dictated by the biological features of the infectious agent.

Enteral: Administration of the material via the oral route.
Respiratory: Delivery of the infectious agent directly into the respiratory tract.
Parenteral: Injection techniques bypassing the gastrointestinal tract. These include intraperitoneal, intravenous, intramuscular, subcutaneous, and intradermal administration.
Specialized Loci: Complex manipulations for specific infections, such as injection into the anterior chamber of the eye, subdural injection (requires preliminary craniotomy), or suboccipital injection (into the cisterna magna).

Advantages and Disadvantages of the Method

Like any diagnostic approach, in vivo testing has its strengths and weaknesses.

Advantages:

Extreme Sensitivity: A living organism reacts even to minimal doses of a pathogen that might go undetected by other methods.
Isolation of Fastidious Pathogens: Classic examples include the causative agents of leprosy and syphilis, which cannot be cultured on standard laboratory media.

Disadvantages:

Species Resistance: Laboratory animals are genetically resistant to most anthroponotic infections—diseases that affect humans exclusively.
High Economic Cost: The method requires significant financial investment to purchase specialized strains of animals, provide adequate nutrition, and construct and maintain a specialized vivarium.

Frequently asked questions

In what units and how is the degree of microbial 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: 1–2 cells may suffice for respiratory infection, whereas thousands of cells may be required for oral infection with the same microbe. Specific quantitative units of virulence measurement are not detailed in the source.
How is bacteriological examination of organs from a deceased laboratory animal performed?: For necropsy material—including the spleen, liver, lymph nodes, and other organs—bacteriological examination involves smear microscopy (using Gram staining and capsule-detection techniques) and isolation of a pure culture by plating on meat-peptone agar and inoculating meat-peptone broth or Hottinger medium.
When is the biological method chosen over standard culture techniques?: When a microorganism is fastidious and incapable of multiplying on artificial nutrient media (e.g., Mycobacterium leprae or Treponema pallidum). In such cases, inoculating a susceptible animal is the only way to obtain a culture for study.
How is the 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 that particular infection.
Why are laboratory animals unsuitable for studying all infections?: Animals possess natural resistance to pathogens of strict anthroponotic infections that affect humans only, making it impossible to reproduce the clinical picture of such diseases in animal models.

Go deeper

Criteria and methods for assessing microbial virulence
In vivo methods for bacterial toxin typing
Pathogenesis features of leprosy and syphilis
Regulations and operation of a laboratory vivarium
Preclinical evaluation of vaccine activity in animals