Principle of operation and main types
Molecular-genetic diagnostics is based on searching for unique genetic markers of an infectious agent. Unlike classical culture methods, microbial viability is not required here, as the target is directly the pathogen's genetic material (DNA or RNA).
The main types of molecular-genetic analysis include:
- Polymerase chain reaction (PCR) — the most demanded and widespread variant.
- Reverse transcription PCR (RT-PCR) — used for RNA-containing agents.
- Restriction fragment length polymorphism (RFLP) analysis.
- Sequencing — deciphering the nucleotide sequence.
- Southern blotting.
- Determination of the plasmid profile of the pathogen.
Characteristics of the polymerase chain reaction (PCR)
PCR is the foundation of modern molecular diagnostics. The essence of this assay lies in the targeted search for a small, strictly specific DNA fragment that is unique to a particular species or strain of the pathogen.
The method has outstanding differentiating capability. With its help, a microbiologist can classify the detected microorganism at several levels:
- At the genus level.
- At the species level.
- At the serotype level.
- Distinguishing pathogenic strains from non-pathogenic ones within the same species.
Advantages and capabilities of the method
The introduction of genetic technologies has radically changed approaches to laboratory pathogen detection due to three key advantages:
- Multiplexing. The technology allows simultaneous detection of the genetic material of several different pathogens in a single biological sample, which significantly saves time.
- Highest sensitivity. The method is capable of detecting even unculturable forms of pathogens — those microorganisms that cannot be grown on standard microbiological nutrient media in principle.
- Real-time mode. Modern modifications (real-time PCR) make it possible not only to establish the presence of a microbe, but also to quantitatively assess the viral or bacterial load, as well as perform dynamic monitoring of the infectious process.
Applications and limitations
Molecular-genetic tests are actively used for primary screening, monitoring treatment efficacy, and are combined with other laboratory approaches in diagnosing viral, bacterial, and parasitic invasions.
However, the method has significant limitations that define its place in modern medicine:
- When searching for absolute pathogens: despite all obvious advantages, the reliability of existing test systems is still considered insufficient to completely replace classical (culture and microscopic) microbiological techniques.
- In opportunistic infections: at the current stage of laboratory development, the clinical significance of molecular-genetic methods in diagnosing diseases caused by opportunistic flora is assessed as extremely low.
Briefly: answers to topical questions
What temperature stages make up a single cycle of classical polymerase chain reaction?
- Denaturation — heating the mixture to 92–95 °C to separate DNA strands.
- Annealing — cooling the mixture to 37–60 °C for complementary primer binding.
- Elongation — heating the mixture to 63–75 °C for the synthesis of a new strand using thermostable DNA polymerase.
What components must be included in the PCR reaction mixture?
Polymerase chain reaction requires the following mandatory components:
- DNA template — the target DNA region of the pathogen being studied.
- Primers — short oligonucleotides complementary to the 3' ends of the target gene.
- Thermostable DNA polymerase — an enzyme for synthesizing the new strand.
- Free nucleotides — building blocks for constructing the strands.
How does classical PCR methodologically differ from real-time PCR?
The key methodological difference is that in real-time PCR, amplification and detection occur simultaneously rather than sequentially. In addition, a special molecular probe is added to the reaction mixture in the real-time variant, which generates a fluorescent signal upon binding to the amplified strand.
Which enzymes are used to perform reverse transcription PCR (RT-PCR)?
Based on the provided sources, the following enzymes are confirmed for reverse transcription PCR:
- Reverse transcriptase (revertase) — synthesizes DNA on an RNA template.
- DNA polymerase — used for amplifying the DNA region in PCR.
Why is the clinical significance of PCR in opportunistic infections assessed as low?
At the current stage, the significance of molecular-genetic methods in diagnosing opportunistic infections is assessed as low and is not recognized as clinically significant.
For quick memorization of key PCR advantages, use the rule "S-M-R": Sensitivity (finds unculturable forms), Multiplexing (several pathogens in one tube), Real-time (quantitative monitoring).
Class and exam questions
What exactly does PCR look for in a sample?
The reaction aims to find a small specific DNA fragment that is a unique target for a specific pathogen.
Can we completely abandon cultures in favor of molecular diagnostics?
No. Despite high accuracy, the reliability of genetic test systems is still insufficient to completely replace classical techniques when searching for absolute pathogens.
How effective is PCR for infections caused by opportunistic microflora?
To date, the significance of molecular-genetic methods in diagnosing opportunistic infections is not recognized as significant and is assessed as low.
What else to study on this topic
- Specifics of performing reverse transcription PCR (RT-PCR)
- Principle and stages of restriction fragment length polymorphism analysis
- Sequencing: deciphering nucleotide sequences
- Southern blotting hybridization technique
- Diagnostic value of plasmid profile determination
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