II. 1. Introduction and the Brief History of Microbiota Diagnostics

Microbiome diagnostics is dazzling technology, but a test only matters when its result actually changes the treatment decision.

A microbiome test. A few hundred thousand forints. But does it tell us anything useful?

Microbiome diagnostics is a rapidly expanding field – rich in marketing, but still fragmentary in clinical interpretation [12], [13]. Tests costing hundreds of thousands of forints are available, summarising a patient's gut microbiota composition in colourful PDF reports – marketed as "next-generation sequencing", "whole metagenomic analysis", or "personalised microbiome profiling". The question every clinician and patient should ask is not whether the technology is impressive – because it is. The question is whether the test has a therapeutic consequence. Because a diagnostic procedure only makes clinical and financial sense if its result influences the treatment decision.

This chapter is not a rejection of microbiome diagnostics. On the contrary: it aims to give an accurate picture of what these procedures measure, what they do not measure, where their value lies, and where – given the current state of knowledge – they produce misleading or at least misinterpretable results [12], [13].

A brief history of microbiome diagnostics – between impressive technology and clinical reality

The scientific roots of microbiome investigation reach back to the 19th century. Following the work of Louis Pasteur and Robert Koch, microbiology initially relied on culture-based methods: bacteria isolated from the gut were grown on artificial growth media and identified. This approach ran into a fundamental limitation: the gut bacteria — once estimated at 70–80% — were thought to be uncultivable under laboratory conditions. The culturomics revolution (Lagier et al. 2016, 2018; Almeida et al. 2019; Nayfach et al. 2021) has reduced this figure to approximately 25–50% over the past decade, although culture-based methods alone still cannot describe the ecosystem comprehensively. Much of the entire ecosystem was simply invisible.

The breakthrough came with molecular biology. In the late 1980s, building on the work of Carl Woese, sequence analysis of the 16S ribosomal RNA^[G] gene made it possible to identify bacteria without culturing, directly from their DNA [3]. This was the forerunner of amplicon sequencing – 16S rRNA-based analysis. The first comprehensive human gut microbiome studies appeared around the turn of the millennium, and the technique has developed rapidly over the past two decades.

From the mid-2000s, shotgun metagenomics^[G] – sequencing the entire DNA content without targeted amplification – became available for research purposes. The first phase of the Human Microbiome Project (HMP1, 2008–2013), followed by its integrated successor (iHMP, 2014–2018), and the European MetaHIT consortium (2008–2012) generated large reference datasets mapping the healthy human microbiome. In parallel, commercial ventures began offering tests for consumer and clinical use – initially primarily in the United States (uBiome, Viome, DayTwo), then in Europe and Hungary as well.

However, technological development and commercial expansion outpaced clinical validation. The literature behind these tests is largely observational; the number of studies describing associations between microbiome composition and disease has grown exponentially, while interventional randomised controlled trials – which would confirm clinical utility – have remained far rarer [12], [13]. The result: a diagnostic industry collecting the most exciting biological data of our era, but whose clinical interpretive framework is still under construction.

References

[3] Turnbaugh PJ, Ley RE, Hamady M, Fraser-Liggett CM, Knight R, Gordon JI. The Human Microbiome Project. Nature. 2007. Link

Strategic framework outlining the Human Microbiome Project's approach to characterizing the microbial components of the human genetic and metabolic landscape. The initiative aims to establish how microbiota contribute to normal physiology and predisposition to disease. Serves as the foundational programmatic statement for large-scale population-level microbiome research.

[12] Zmora N, Zilberman-Schapira G, Suez J et al. Personalized Gut Mucosal Colonization Resistance to Empiric Probiotics Is Associated with Unique Host and Microbiome Features. Cell. 2018. Link

Sequential invasive multi-omics profiling of the mucosal-associated gastrointestinal microbiome in mice and humans during consumption of an 11-strain probiotic versus placebo showed that probiotics remained viable through gastrointestinal passage but encountered marked mucosal colonization resistance in colonized hosts. Humans displayed person-, region- and strain-specific mucosal colonization patterns predictable from baseline host and microbiome features, while stool probiotic presence was uninformative. Stool microbiome correlated only partially with mucosal microbiome. The findings challenge the empiric use of probiotics in healthy individuals.

[13] Sonnenburg JL, Gardner E. Microbiome tests: Ignore the hype. Science. 2016. Link

Sonnenburg and Gardner's Science commentary cautions against the marketing hype around direct-to-consumer microbiome tests in 2016. They argue that while gut microbiota research is advancing rapidly, commercial 16S rRNA profiling cannot yet deliver clinically actionable personalised advice because reference 'healthy' microbiomes are not defined, longitudinal data are sparse, and causal links between taxa and outcomes are largely unproven. The authors emphasise inter-individual variability, methodological differences between platforms, and the gap between association and intervention evidence. They recommend that clinicians treat such reports with skepticism and call for regulatory oversight, standardised methodology, and longitudinal cohort studies before personalised microbiome diagnostics enter routine care.