Glossary · Bioinformatics
Relative abundance
In one sentence
Relative abundance is the share of a microorganism in the total detected in a sample, usually given as a percentage. It reflects its weight within the community, not its absolute quantity.
Also known as: relative abundances · taxon relative abundance · bacterial relative abundance
What is relative abundance?
In a sequencing-based microbiota analysis, the instrument does not count bacteria. It produces millions of DNA reads, which are then assigned to taxa. Relative abundance expresses how many of those reads belong to each microorganism as a share of the total. It is the most common way of presenting community composition in a report.
How it is calculated
In simple terms:
relative abundance of a taxon = reads assigned to that taxon ÷ total assigned reads × 100
All taxa add up to 100%. Several technical choices sit behind the figure:
- Technique: in 16S rRNA sequencing, gene copy number per bacterium and amplification bias both play a part; in shotgun metagenomics, genome size and the reference database matter.
- Taxonomic level: the percentage for a phylum is not comparable with that for a species.
- DNA extraction: some thick-walled bacteria lyse less readily and end up under-represented.
Limitations: compositional data
Because every percentage is part of a fixed 100%, the data are compositional: if one bacterium expands, the others shrink in percentage terms even if their actual numbers stay the same. Gloor and colleagues made the point bluntly in Frontiers in Microbiology in 2017, in a paper titled “Microbiome Datasets Are Compositional: And This Is Not Optional”. In practice:
- A relative increase does not prove that an organism has grown in absolute terms.
- Correlations between taxa calculated on percentages can be misleading.
- Measuring true load requires absolute quantification, for example qPCR or spike-in standards.
Relative abundance also underpins the Shannon index and beta diversity distances such as Bray-Curtis.
Example
A dietitian reviews a report in which Bacteroides accounts for 40% of reads and Faecalibacterium prausnitzii for 3%. A few months after a change in diet, Bacteroides is at 25% and F. prausnitzii at 6%. The dietitian recognises that the rise could reflect real growth of F. prausnitzii, a fall in other groups, or both, and reads it cautiously alongside the patient’s symptoms.
Relative abundance at Xenogene
According to its product page, the Essential Microbiota test reports relative abundance as the percentage of each of the 150 key species analysed, alongside alpha and beta diversity, dysbiosis indices and functional profiles. The page stresses that results should be interpreted together with the clinical history, which is the role of the requesting clinician.
Frequently asked questions
What does relative abundance mean in a gut microbiome test?
It is the percentage each microorganism represents out of all the reads assigned in the sample. A result of 5% for a bacterium means that 5 in every 100 identified reads belong to it.
What is the difference between relative and absolute abundance?
Relative abundance is a proportion; absolute abundance estimates the actual amount, such as cells or gene copies per gram of stool. If one group of bacteria falls, others can rise in percentage terms even though their real numbers have not changed.
Does a high relative abundance of a pathogen mean infection?
Not necessarily. A sequencing signal does not equal viable organisms and does not prove causation. Interpretation depends on the clinical picture, the sample type and other tests, and rests with the clinician.
Related terms
- Alpha and beta diversity
Alpha diversity measures the variety of microorganisms within a single sample (how many types and how evenly spread); beta diversity measures how different two or more samples are from each other.
- Shannon index
The Shannon index is an alpha diversity measure that combines how many types of microorganism are present in a sample and how evenly they are distributed: the higher the value, the more diverse the community.
- OTU and ASV
OTUs (operational taxonomic units) group similar DNA sequences, usually at 97% identity; ASVs (amplicon sequence variants) are exact sequences that can be told apart by a single nucleotide.
- Shotgun metagenomics
Shotgun metagenomics sequences all the DNA in a sample in an untargeted way, without amplifying a single marker gene. It can identify bacteria, DNA viruses, fungi and parasites, often to species level, and reveal their functional and resistance genes.
- Microbiota
The microbiota is the community of living microorganisms (bacteria, archaea, fungi, protozoa) that inhabits a specific site of the body, such as the gut, mouth, skin or vagina.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
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