- Amplicon sequencing is a widely used molecular sequencing approach for studying specific genetic regions within microbial communities. Instead of sequencing all of the DNA present in a sample, amplicon sequencing selectively targets one or more genetic marker regions that can provide information about the microorganisms present. Because it focuses on predefined regions, the approach is generally less data-intensive and less expensive than shotgun metagenomic sequencing, making it an important method for microbial community profiling.
- The basic principle of amplicon sequencing is based on the selective amplification of a particular DNA region using polymerase chain reaction (PCR). Primers are designed to bind to conserved sequences surrounding a variable region of interest. PCR then produces many copies of the targeted region, known as an amplicon. These amplified DNA fragments are subsequently prepared for sequencing, and the resulting sequences are analyzed to determine the composition and diversity of the microbial community.
- Amplicon sequencing is particularly important in microbial community analysis because microorganisms often contain genetic marker genes or regions that provide useful information for classification. Researchers can use differences in these sequences to distinguish among microbial groups and estimate their relative representation within a sample. The specific marker selected depends on the organisms being studied and the biological question being addressed.
- One of the most commonly used markers for bacterial and archaeal community studies is the 16S rRNA gene. The 16S rRNA gene contains conserved regions that are useful for designing broad-range primers as well as variable regions that contain sequence differences among microorganisms. Sequencing selected portions of this gene can therefore provide valuable information about bacterial and archaeal community composition.
- For fungal community studies, researchers frequently use the internal transcribed spacer (ITS) region. ITS regions can provide useful taxonomic resolution for fungi and are therefore widely used in fungal community profiling. Other organisms may require different genetic markers, depending on their evolutionary characteristics and the objectives of the study.
- The choice of marker region is one of the most important decisions in an amplicon-sequencing experiment. Different marker regions can provide different levels of taxonomic resolution, and no single marker is universally optimal for every microbial community. Researchers must consider the organisms of interest, the expected diversity of the sample, available reference databases, primer compatibility, sequencing technology, and the desired level of taxonomic identification.
- Another important consideration is primer design. Primers determine which DNA molecules are preferentially amplified during PCR. Ideally, primers should amplify the target organisms broadly while minimizing amplification of unwanted sequences. However, differences between microbial genomes can cause some organisms to be amplified more efficiently than others. Consequently, primer choice can introduce PCR bias into the final community profile.
- PCR amplification itself can introduce additional sources of variation. Differences in template concentration, amplification efficiency, cycle number, polymerase characteristics, and reaction conditions can influence the number and composition of amplicons generated. Excessive PCR amplification may also increase the risk of artifacts. Careful optimization and appropriate controls are therefore important parts of the experimental workflow.
- The typical amplicon sequencing workflow begins with sample collection, followed by DNA extraction. The extracted DNA is then subjected to PCR using primers targeting the selected marker region. The amplified products are evaluated for quality and quantity before being prepared for sequencing. After sequencing, the resulting reads undergo quality control and bioinformatics processing before taxonomic and diversity analyses are performed.
- DNA extraction is an important part of the workflow even though amplicon sequencing focuses on a specific genetic marker. The DNA extraction procedure can influence which organisms’ DNA is recovered efficiently. Microorganisms differ in cell-wall composition and resistance to physical or chemical disruption, so extraction methods may affect the representation of organisms in the final sequencing dataset.
- After PCR, researchers generally perform amplicon library preparation. This process prepares the amplified DNA fragments for sequencing and may involve the addition of sequencing adapters and sample-specific index sequences. Indexing allows multiple samples to be sequenced together in a process known as multiplexing, after which computational methods can separate the reads according to their sample-specific identifiers.
- Sequencing platforms used for amplicon studies can differ in read length, accuracy, throughput, and technical characteristics. Short-read sequencing platforms are commonly used because many commonly targeted marker regions are relatively short. The selected sequencing platform should be compatible with the length and characteristics of the amplified region as well as the requirements of the downstream analysis.
- Once sequencing is complete, researchers obtain a collection of raw amplicon reads. These reads must undergo quality assessment before biological interpretation. Quality control can involve examining base quality scores, identifying adapter sequences, removing poor-quality reads, trimming low-quality regions, and filtering reads that do not meet predetermined criteria.
- An important computational step is demultiplexing, which assigns sequencing reads to their original samples using sample-specific barcode or index information. Accurate demultiplexing is essential when many samples have been sequenced together. Incorrect assignment can lead to cross-sample contamination in the computational dataset and may affect downstream conclusions.
- Following quality control, researchers can process the reads using a bioinformatics pipeline. Depending on the analytical approach, processing may include sequence filtering, denoising, error correction, chimera detection, sequence clustering or inference, taxonomic classification, diversity analysis, and statistical comparison between sample groups.
- Traditional amplicon studies frequently grouped similar sequences into operational taxonomic units (OTUs). OTUs were commonly defined using a specified sequence-similarity threshold and were used as approximate representations of microbial groups. More recent approaches often use amplicon sequence variants (ASVs), which attempt to distinguish unique biological sequence variants after accounting for sequencing errors.
- Amplicon sequence variants can provide greater resolution and reproducibility than traditional OTU-based approaches in many applications. Instead of clustering sequences at a predetermined similarity threshold, ASV methods attempt to infer exact biological sequence variants from the sequencing data. Tools based on denoising and error modeling have therefore become important components of modern amplicon-sequencing analysis.
- After sequence processing, taxonomic classification is performed to determine which microbial groups correspond to the observed sequences. Classification generally involves comparison against a curated reference database containing known marker sequences. Depending on the marker, database, sequence quality, and evolutionary relationships, identification may be possible at different taxonomic levels.
- The accuracy of taxonomic classification depends partly on the quality and completeness of reference databases. Well-characterized organisms may be identified with relatively high confidence, whereas organisms that have not been previously characterized may remain at a higher taxonomic level. Closely related organisms can also possess highly similar marker sequences, making species-level identification difficult or impossible using a particular marker region.
- One of the major applications of amplicon sequencing is the measurement of microbial diversity. Researchers commonly distinguish between alpha diversity and beta diversity. Alpha diversity describes diversity within individual samples, while beta diversity describes differences in community composition between samples. These concepts allow researchers to compare microbial communities under different environmental, clinical, experimental, or geographical conditions.
- Researchers can also investigate microbial community composition by examining the relative abundance of detected taxonomic groups. Such analyses can reveal which microbial groups dominate a particular environment and how their relative representation changes between conditions. However, relative abundance should be interpreted carefully because sequencing data are inherently compositional.
- Amplicon sequencing has become particularly valuable in microbiome research. It can be used to characterize microbial communities associated with the human gut, oral cavity, skin, respiratory tract, and other body sites. Similar approaches are widely used to investigate microbial communities associated with animals, plants, soil, water, food, and other environments.
- In environmental microbiology, amplicon sequencing can help researchers characterize microbial communities in soil, freshwater, marine environments, sediments, wastewater, and extreme environments. By comparing samples across locations or environmental conditions, researchers can investigate patterns of microbial diversity and community turnover.
- In agricultural microbiology, amplicon sequencing can be used to study soil microbiomes and plant-associated microbial communities. Researchers may investigate how farming practices, soil properties, plant species, fertilizers, environmental conditions, or disease influence microbial community composition.
- Food microbiology is another important application. Amplicon sequencing can help characterize microorganisms associated with fermented foods, raw materials, food-processing environments, and food products. It can provide information about microbial succession, fermentation communities, spoilage organisms, and changes associated with food-processing conditions.
- Amplicon sequencing is also useful for investigating microbial ecology. Because microbial communities interact with their surrounding environments, researchers can combine sequencing results with environmental metadata such as temperature, pH, nutrient concentrations, salinity, moisture, or geographical information. Statistical approaches can then be used to explore associations between microbial communities and environmental factors.
- One of the principal advantages of amplicon sequencing is its cost efficiency. Because only selected genetic regions are sequenced, considerably less sequencing data may be required than for shotgun metagenomics. This makes amplicon sequencing attractive for studies involving large numbers of samples, particularly when the primary objective is to compare microbial community composition rather than characterize the complete genetic potential of the community.
- Another advantage is its relatively straightforward experimental design. Researchers can select an established marker region, use validated primers, sequence the resulting amplicons, and apply established computational workflows. This has contributed to the widespread adoption of amplicon sequencing in microbial ecology and microbiome research.
- However, amplicon sequencing also has important limitations. Because only selected genetic regions are examined, the method provides limited information about the broader genomic content of the microorganisms. It generally cannot provide the same level of functional information as shotgun metagenomics. The presence of a particular taxon does not directly reveal which genes or metabolic pathways are present in its genome.
- PCR bias is another important limitation. Some organisms may be amplified more efficiently than others because of differences in primer matching, DNA concentration, genome characteristics, or PCR efficiency. As a result, the relative abundance of sequences in an amplicon dataset should not automatically be interpreted as an exact measurement of the relative abundance of organisms in the original sample.
- Primer bias can be particularly significant when studying highly diverse microbial communities. A primer set may fail to amplify some organisms efficiently if the primer-binding region contains sequence differences. Consequently, organisms that are genuinely present in the sample may be underrepresented or completely missed in the sequencing results.
- Another challenge is chimera formation. During PCR, fragments from different DNA templates can occasionally combine to form artificial sequences. These chimeric sequences may be incorrectly interpreted as genuine biological variants unless they are identified and removed during bioinformatics processing.
- The level of taxonomic resolution is also an important limitation. A particular marker may distinguish organisms effectively at the genus level but provide insufficient information for reliable species-level identification. Closely related microorganisms can share highly similar marker sequences, making more precise identification difficult without additional genomic information.
- Amplicon sequencing is therefore best understood as a powerful method for answering particular types of biological questions rather than as a replacement for all other sequencing approaches. When the main objective is to characterize and compare microbial community composition across many samples, it can be highly effective. When researchers need information about genes, metabolic pathways, antimicrobial-resistance determinants, viruses, or individual microbial genomes, shotgun metagenomics may provide a broader perspective.
- The relationship between amplicon sequencing and shotgun metagenomics is therefore complementary. Amplicon sequencing offers a focused and cost-effective view of selected microbial marker regions, while shotgun metagenomics examines a much broader collection of DNA sequences. The appropriate method depends on the research question, sample characteristics, budget, sequencing resources, and desired level of taxonomic and functional resolution.
- The future of amplicon sequencing is likely to involve improved primer design, longer sequencing reads, better error correction, more comprehensive reference databases, improved ASV-based methods, and integration with other molecular and computational techniques. Combining amplicon data with shotgun metagenomics, metatranscriptomics, metaproteomics, and metabolomics can provide increasingly comprehensive views of microbial ecosystems.
- Overall, amplicon sequencing remains one of the most widely used approaches for investigating microbial communities. From marker selection, primer design, PCR amplification, library preparation, and sequencing to quality control, denoising, ASV analysis, taxonomic classification, and diversity analysis, each stage contributes to the reliability of the final results. Its relatively accessible workflow, scalability, and cost efficiency make it an important tool for microbial ecology, microbiome research, environmental science, agriculture, food science, and many other areas of biological research.
