The last ten years have witnessed a tsunami of research implicating an important role of the
gut microbiota in a wide range of diseases. At the same time, this research was all based upon
non-quantitative measures, allowing only the observation of relative microbiota shifts.
Preliminary evidence from the MiQuant consortium shows that strong inter-individual
variation in absolute microbial load exists in humans, with important clinical implications.
With this proposal, we will deepen these surprising observations and investigate, for the first
time, population-level *quantitative* microbiota variation and the factors driving this. The
causes and implications at ecological, cellular, and physiological level will be studied using a
combination of epidemiological and experimental (in vitro, ex-vivo human and in vivo murine
model) research lines, supported by high-throughput multi-omics. The research will be
performed by an interdisciplinary consortium of world experts in their relative domains.
Furthermore, the clinical relevance and translation of quantitative microbiomics will be
investigated by applying this approach in (i) a next-generation probiotic clinical trial in obesity
and cardiovascular health, and (ii) a multicenter fecal transplantation trial in Ulcerative Colitis.
The results of this work are expected to be of massive impact in the microbiome field and
human health research in general.