Microbiological research methods

Classification of diagnostic methods

In modern medical practice, there are exactly seven main groups of methods that allow accurate identification of the disease pathogen:

1. Microscopic (bacterioscopic) — direct visual study of microorganisms under a microscope.
2. Cultural (bacteriological or virological) — inoculation of biomaterial onto artificial nutrient media to obtain a pure culture.
3. Serological (immunological) — aimed at searching for specific antibodies in blood serum.
4. Allergological — based on skin-allergy tests (e.g., tuberculin diagnostics).
5. Biological — a bioassay in which laboratory animals are infected with the material.
6. Molecular genetic — allows detecting microbial nucleic acids (most often using PCR).
7. Physicochemical — used to identify specific chemical components of the microbial cell.

Features of applying key methods

Each of the listed approaches has its own strict rules of application and indications that a physician must consider when ordering laboratory tests.

• Serological diagnostics often acts as the primary research method. To purposefully search for specific antibodies to the pathogen, reactions such as enzyme-linked immunosorbent assay (ELISA) and indirect immunofluorescence assay (indirect IFA) are used.
• Bacteriological method requires special attention to the timing of clinical material collection. For example, blood culture (obtaining hemoculture) on special nutrient media must be performed exclusively during the febrile period.
• Histological method is based on the microscopic examination of biopsies of affected tissues. Using classical Romanowsky–Giemsa staining, a microbiologist can visualize the pathogen directly in the tissue — for example, detecting characteristic bright red chains of bacteria of the genus Bartonella.
• Bacterioscopic method requires proper preparation of smears. Specific stains are used when examining native sputum. To detect the most important property — acid-fastness — Ziehl–Neelsen staining is used. Luminescent microscopy also shows high diagnostic value, for which smears are pre-treated with special fluorochrome dyes.

Rules for collecting biomaterial

The success of any of the seven methods directly depends on the quality of the studied material. Depending on the clinical picture and the suspected localization of the infectious process, a wide variety of substrates can be sent to the laboratory. The main types of studied material include:

• Sputum (especially important in the diagnosis of respiratory infections).
• Bronchial washings.
• Pleural fluid.
• Cerebrospinal fluid (CSF).
• Biopsies of various tissues.

Principles of laboratory specialization

Microbiological laboratories are not absolutely universal institutions. Their classification and structure are built on two main criteria: the object of study (i.e., a specific group of studied microbes) and the applied laboratory methods.

Specialization by studied groups of microorganisms is traditionally divided into main profiles and narrow specialization. The main profiles include:

• Bacteriological laboratories (study bacteria).
• Virological laboratories (work with viruses).
• Mycological laboratories (study pathogenic fungi).
• Protozoological laboratories (specialize in protozoa).

Narrow specialization implies the laboratory working only with specific taxa or groups of pathogens. In practice, there are separate rickettsial, chlamydial, tuberculosis, and leptospiral laboratories, as well as specialized units for studying anaerobic infections.

Specialization by research methods distinguishes laboratories that predominantly use:

• Classic cultural (microbiological) methods.
• Immunological methods (performing various serological reactions).
• Molecular genetic methods (PCR and other technologies).

It is important to note that in practice, a rational combination of methods is very common. For example, performing serological diagnostics can be successfully and fully carried out on the basis of a classical microbiological laboratory.

Class and exam questions

What else to study on this topic

• Technique of indirect IFA and ELISA
• Romanowsky–Giemsa staining: mechanism and application
• Features of cultivating anaerobic infections
• Tuberculin diagnostics: types of skin tests
• Luminescent microscopy with fluorochrome dyes

Other topics in the section "Microbiological diagnostics"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.