Bacteriological method (primary)
This approach is recognized as the "gold standard" in the diagnostics of infections caused by opportunistic microflora. It provides the clinician with the most complete data necessary for prescribing etiotropic treatment.
Key capabilities of the method:
- Precise identification: the isolated microorganism can be classified down to the species, subspecies, and even specific strain level.
- Quantitative assessment: performing a quantitative culture makes it possible to determine the exact concentration of bacteria in the test material.
- Susceptibility testing: the method allows testing the susceptibility of the isolated pathogen in vitro to various therapeutic agents, including antibiotics, bacteriophages, and antiseptics.
Microscopic method (indicative)
Despite its rapid turnaround time, microscopy is purely indicative in nature. The main limitation of the method is low specificity. This is due to the fact that many opportunistic microorganisms have extremely similar morphology (appearance), making an accurate diagnosis impossible by merely looking through the eyepiece.
Objectives of microscopy:
- Initial verification of the mere presence of bacterial cells in the clinical specimen.
- Assessment of basic parameters: shape of microorganisms and their staining properties (ability to take up stains, which directly depends on the thickness of the cell wall, for example, in Gram staining).
Serological method (low information value)
In purulent-inflammatory processes caused by opportunistic flora, serological diagnostics is significantly inferior in its information value compared to studies used in classical infections.
Reasons for low efficiency:
- Delayed immune response: antibodies in the patient's body accumulate too slowly for prompt diagnosis.
- Weak reaction: titers of specific antibodies in such diseases remain relatively low, making their reliable detection and interpretation difficult.
Allergological method (specialized)
This is a highly specialized tool used strictly according to indications. The main goal is to identify the presence of body hypersensitivity to a specific microbe, provided it is suspected to play a major role in pathogenesis.
Indications for use: Chronic diseases in whose clinical course an allergic component can be clearly traced. These include recurrent pathologies of the respiratory tract, gastrointestinal tract, kidneys, joints, and skin.
Execution technique: Based on skin allergy tests. The patient is injected with standardized allergen preparations derived from various pathogens, such as:
- Staphylococcus
- Streptococcus
- Escherichia coli
- Proteus
- Pseudomonas aeruginosa and others.
Briefly: answers to topical questions
What stages does the standard bacteriological examination scheme in the laboratory include?
The standard bacteriological examination scheme includes sequential steps:
- Primary inoculation — applying clinical material to a set of nutrient media.
- Growth assessment — quantitative characterization of grown colonies: single, moderate, or abundant growth.
- Isolation of pure culture.
- Pathogen identification.
What methods for rapid identification of microorganisms exist?
Automated systems, molecular-genetic, and physicochemical methods are used for rapid identification of microorganisms.
- Automated systems (such as Bactec, MB/BacT) — use fluorimetric registration of metabolism (CO₂ accumulation or O₂ consumption).
- PCR — a molecular method for subsequent precise identification.
- Gas-liquid chromatography (GLC) — detection of specific volatile metabolites (e.g., propionic or butyric acid).
- Rapid culture tests — evaluation of specific growth (e.g., at 43 °C on litmus milk).
- Direct DFA — direct fluorescent antibody assay for rapid antigen detection.
How is the susceptibility of microorganisms to antibiotics assessed using the disk diffusion method?
Assessment of microorganism susceptibility to antibiotics by the disk diffusion method is a qualitative test that determines whether a microbe is susceptible or resistant.
- Preparation and inoculation — an inoculum at a concentration of 1.5×10⁸ CFU/mL according to the McFarland standard is inoculated onto a solid nutrient medium (agar).
- Agent application — special disks with antibiotics are placed on the surface of the medium.
- Incubation — cultures are incubated, and results are read after 24 hours.
- Result evaluation — inhibition zones around the disks are measured; the presence or absence of these zones is evaluated.
Which nutrient media are used for the primary inoculation of clinical material?
The choice of nutrient media for primary inoculation depends on the suspected pathogen and the goals of the study.
- Blood agar (including with the addition of sheep blood) — a solid medium for isolating pure culture and assessing hemolysis.
- Chocolate agar — used for cultivating fastidious bacteria (e.g., Haemophilus species).
- Thioglycolate broth / Kitt-Tarozzi medium — a liquid enrichment medium for primary inoculation of anaerobes.
- Sugar agar — used for deep stab inoculation of anaerobes.
- Combined media (Ploskirev agar, bismuth sulfite agar) — selective-differential media for isolating pathogenic enteric bacteria.
Class and exam questions
The bacteriological method is the gold standard of diagnosis in this case, as it allows for the precise determination of the microbe type, its quantity, and its susceptibility to drugs.
The bacteriological method is the gold standard of diagnosis in this case, as it allows for the precise determination of the microbe type, its quantity, and its susceptibility to drugs.
What is the main drawback of the microscopic method?
Its main drawback is low specificity. Many opportunistic bacteria look identical under the microscope, so the method provides only tentative information about the presence and shape of the pathogen.
Why is serology rarely used for infections caused by opportunistic flora?
Due to the slow accumulation of antibodies and their relatively low titers in such purulent-inflammatory diseases. Unlike the diagnostics of classical infections, serology has low information value here.
What else to study on this topic
- Methods of performing skin allergy tests
- Tinctorial properties of bacteria and Gram staining
- Methods for determining bacterial susceptibility to antibiotics and bacteriophages
- Guidelines for specimen collection for bacteriological culture
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