General Characteristics of Microscopy
The core of this approach is the direct visual examination of a collected sample using magnification equipment. Depending on the aims of the study and the type of microorganism, various types of microscopy can 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 detail the morphological features of the identified pathogen.
This approach has clear advantages. First and foremost, it is simple to perform and cost-effective. Additionally, the method is rapid, making it indispensable for emergency diagnostics (results can be obtained on the same day).
However, it also has serious limitations. Causes of frequent diagnostic errors include the subjectivity of the laboratory technician's evaluation of the slide and a high risk of false-negative results if the pathogen concentration in the sample is too low.
Bacterioscopic Method
Bacterioscopy aims to diagnose bacterial infections. Its core principle involves pre-staining bacterial cells to subsequently study their morphological and tinctorial properties (i.e., the ability of cellular structures to take up various dyes).
During the examination, the laboratory technician evaluates cell shape, exact size, and characteristic spatial arrangement. For example, bacteria may be arranged in chains (typical of streptococci), clusters ("grape-like," staphylococci), or pairs (diplococci).
Specific staining methods are used to reveal particular structures of microorganisms:
- Gram stain: The primary method for evaluating cell wall properties.
- Burry-Ginns stain: Used to detect capsules.
- Aujeszky stain: Helps identify bacterial spores.
- Morozov silver impregnation and Loeffler stain: Designed to visualize flagella.
- 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, results are often merely suggestive.
Nevertheless, the method is reliable if the microbe has 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 intracellular diplococci within neutrophils in purulent discharge), and leptospirosis (searching for spirochetes in a dark field during early stages of the disease).
Viruscopic Method
Viruscopy requires entirely different instrumentation. The primary tool here is electron microscopy, while fluorescence microscopy is used much less frequently. Light microscopy is practically unused in routine virology because viruses are nanometer-sized and invisible under a standard microscope.
There are few exceptions to this rule. A light microscope can detect:
- Large viruses (larger than 200 nm). For example, the smallpox pathogen can be visualized using Morozov's silver impregnation method.
- Indirect signs of viral damage. These include manifestations of cytopathic effects 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.
The parasitoscopic method is based on the fact that these pathogens are large and possess very characteristic morphology. This is the primary way to confirm diagnoses such as malaria, amebiasis, giardiasis, and various helminthiases. Technically, the study involves preparing native wet mounts as well as smears followed by Romanowsky-Giemsa staining.
Fungal microscopy has its own characteristics. Specimens are examined in both native (unstained) and stained states. For deep mycoses, the histological method is often used. Its goal is to find fungal elements (such as hyphae and spores) directly in the affected tissues or organs of the patient. For this purpose, thin or ultrathin sections are prepared from biopsy samples, subjected to special staining (using histochemical methods), and then examined under a light microscope.