Microbiome basics
HelloTummy uses full-length long-read 16S sequencing to identify bacteria in a stool sample and estimate how prominent each one is in the bacterial community.78
The gut microbiome is the community of bacteria and other microorganisms that live in the digestive tract. A stool sample contains some of their DNA, giving us a practical window into which bacteria are present and how they are distributed within your gut.
Reading the result
When your report says a species makes up 12% of your microbiome, the accurate reading is:
About 12% of the bacterial 16S sequences identified in this sample were assigned to that species.
That is a useful estimate of how prominent the bacterium is in the community. It is not a literal claim that 12 out of every 100 bacterial cells in your gut belong to that species.13
A stool sample can contain hundreds of species and billions of cells, many of which cannot practically be grown and counted one by one. Sequencing makes that otherwise invisible community measurable, although every measurement method introduces some bias.1
The key point: a measurement does not have to be perfectly unbiased to be highly informative.1
Repeatability
Imagine a scale that always reads five pounds high. You weigh 180; it says 185. In absolute terms, the scale is wrong.
Use it next month, see 195, and it still tells you something real: your weight increased by about ten pounds. A consistent offset can support comparison; random error cannot.
Studies using laboratory-built bacterial communities of known composition show that much of a protocol's bias is systematic: it may read one organism somewhat high and another somewhat low, but often does so consistently.1
That is why HelloTummy standardizes DNA extraction, primers, PCR conditions, sequencing, and data analysis. Results produced with the same workflow are more useful for following patterns over time than percentages produced by different laboratories using different methods.16
Measurement limits
Two important features separate the recovered 16S signal from a literal headcount of bacterial cells.
Primers are short DNA sequences that find shared regions of the 16S gene so it can be copied. Those regions are not perfectly identical across all bacteria, so a primer may fit one species more efficiently than another. Primer choice and validation therefore affect how faithfully a sample is represented.2
Some bacteria have one 16S gene copy per genome; others have several. Two species present at the same cell count can begin the assay with different numbers of 16S targets. Routine computational correction is limited because copy number is not known for every bacterium and estimates can introduce additional uncertainty.3
For that reason, HelloTummy reports the measurement for what it represents: the relative distribution of the bacterial 16S signal recovered by the assay.
Assay design
PCR copies DNA repeatedly. Small differences in copying efficiency can compound with every cycle, so fewer cycles at the bacteria-specific amplification stage reduce the opportunity for distortion to accumulate.14
This deliberately simplified example starts two species at 50:50. Species A copies at 100% efficiency per cycle and Species B at 95%. The example is not a model of every bacterium; it shows why cycle count matters.
| Stage | Species A | Species B |
|---|---|---|
| Starting material | 50.0% | 50.0% |
| After 14 cycles | 58.8% | 41.2% |
| After 25 cycles | 65.3% | 34.7% |
| After 30 cycles | 68.1% | 31.9% |
PCR 1 · 14 cycles
Every sample uses the same bacteria-specific primers. Standard attachment sites are added, but sample-specific barcodes are kept out of this stage.
PCR 2
Barcode primers recognize the standard attachment sites instead of bacterial DNA. This identifies each sample without changing the bacteria-specific primers used in PCR 1.
Separating bacterial amplification from barcoding keeps the step most likely to alter the community profile short and consistent across samples.4
Two different questions
Suppose a bacterium is 10% of one sample and 20% a month later. Did its cell count double? Not necessarily.
The first sample could contain 10 units of Species A and 90 units of everything else. If Species A stays at 10 units while everything else falls to 40, Species A becomes 20% without growing at all. Its share of the community changed.5
Absolute abundance asks how many bacterial cells are present in a given amount of stool. That requires a separate estimate of total microbial load, such as flow cytometry or quantitative PCR, and can itself vary with stool water content, transit time, and total microbial load.56
Identification
The 16S gene is roughly 1,500 bases long. Many tests read only a short region; long-read sequencing reads essentially the full gene. The additional information improves the ability to distinguish many closely related bacteria, although some species remain too similar to separate reliably.78
Shotgun metagenomics sequences all DNA in a sample and can provide gene and strain-level information, but it introduces its own extraction, preparation, database, and analytical biases. Targeted full-length 16S concentrates sequencing effort on the question HelloTummy is designed to answer: which bacteria are present, and how prominent are they?910
Putting it together
If your report says Bacteroides xylanisolvens — 18.4%, read it as:
About 18.4% of the bacterial 16S sequences identified in this sample were assigned to Bacteroides xylanisolvens.
It does not mean that 18.4% of every bacterial cell throughout your intestinal tract belongs to that species. The relative-abundance reading is the one that matches what the assay measured.135
HelloTummy uses full-length long-read 16S sequencing, a low-cycle two-stage PCR, standardized laboratory procedures, and a consistent analytical workflow to make bacterial community measurements useful and repeatable over time.168 The report supports wellness education and personal tracking; it does not diagnose, treat, prevent, or rule out a health condition.
The most useful questions are: what is there, what is unusual, and how is your microbiome changing?
