Microscopic diagnostic method

General characteristics of the microscopic method

The essence of this approach lies in the direct visual examination of a collected sample using various magnifying equipment. Depending on the research objectives and the type of microorganism, different types of microscopy may be used:

• Light microscopy (the most common, routine option).
• Phase-contrast microscopy.
• Dark-field microscopy.
• Fluorescence microscopy.
• Electron microscopy.

The main tasks of the microscopic method are to establish the presence or complete absence of microorganisms in a clinical sample, as well as to determine in detail the morphological features of the identified pathogen.

This approach has obvious advantages. First and foremost, these are simplicity of execution and cost-effectiveness. In addition, the method is rapid, making it indispensable for rapid diagnostics (results can be obtained on the same day). However, there are also serious disadvantages. The causes of frequent diagnostic errors include the subjectivity of specimen evaluation by the laboratory physician and a high risk of false-negative results if the pathogen concentration in the sample is too low.

Bacterioscopic method

Bacterioscopy is aimed at diagnosing bacterial infections. Its essence lies in the preliminary staining of bacterial cells for the subsequent study of their morphological and tinctorial properties (i.e., the ability of cell structures to take up various stains).

During the study, the laboratory assistant evaluates the shape of the cells, their exact size, and their characteristic mutual arrangement. For example, bacteria can be arranged in chains (typical for streptococci), in the form of "grape-like clusters" (staphylococci), or in pairs (diplococci).

Specific staining methods are used to identify certain structures of the microorganism:

• Gram stain: the main method used to assess the properties of the cell wall.
• Burri-Gins stain: used to detect the presence of a capsule.
• Aujeszky stain: helps detect bacterial spores.
• Morozov silver impregnation and Löffler stain: designed for flagella visualization.
• Neisser stain: used to detect volutin granules.

Method limitations: Bacterioscopy has a high sensitivity threshold (requiring a concentration of 1,000 to 100,000 cells per milliliter) and low specificity (about 20–80%). Due to the morphological similarity of many bacteria, the result is often only indicative.

Nevertheless, the method is reliable if the microbe has a unique morphology and the patient presents with a characteristic clinical picture. Excellent examples of effective application include the diagnosis of tuberculosis (searching for acid-fast bacilli), gonorrhea (detecting diplococci inside neutrophils in purulent discharge), and leptospirosis (searching for spirochetes in a dark field in the early stages of the disease).

Virusoscopic method

Virusoscopy requires entirely different instrumentation. The main tool here is electron microscopy, while fluorescence microscopy is used much less frequently. Light microscopy is practically never used in routine virology because viruses are nanometer-sized and invisible under a conventional microscope.

There are few exceptions to this rule. Using a light microscope, one can detect:

1. Large viruses (larger than 200 nm in size). For example, the smallpox pathogen can be visualized using Morozov's silver impregnation method.
2. Indirect signs of viral damage. These include manifestations of the cytopathic effect on cells, as well as specific intracellular inclusions (intranuclear or intracytoplasmic) that form in infected tissues.

Microscopic diagnosis of parasites and fungi

Microscopy is considered the most effective and reliable method when parasitic diseases are suspected. The objects of study here are protozoa and various helminths.

Parasitoscopic method is based on the fact that these pathogens are large in size and have very characteristic morphology. This is the main way to confirm diagnoses such as malaria, amebiasis, giardiasis, and various helminthiases. Technically, the study includes the preparation of native smears, as well as smears followed by Romanowsky-Giemsa staining.

Mycology microscopy has its own features. Specimens are examined both in their pristine (native) and stained forms. For deep mycoses, the histological method is often used. Its goal is to find fungal elements (such as hyphae and spores) directly in the patient's affected tissues or organs. For this, thin or ultrathin sections are prepared from the obtained biopsy samples, subjected to special staining (using histochemical methods), and then examined under a light microscope.

Class and exam questions

What else to study on this topic

• Technique and biochemical mechanism of Gram staining
• Operating principles of phase-contrast and dark-field microscopes
• Types of cytopathic effects of viruses on cell cultures
• Features of histochemical staining of preparations in systemic mycoses
• Morphological classification of bacteria: cocci, bacilli, spiral forms

Other topics in the section "Microbiological diagnostics"

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