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Hi Reddit! We are Simone Li (/u/simone_s_li), Sebastian Schmidt (/u/TSBSchm), Nicolai Karcher (/u/YummyYam123) and Daniel Podlesny (/u/DanielPodlesny). We are lead authors on three independent, recently published studies on microbiome dynamics following fecal microbiota transplantation (FMT, aka stool transplants). Ask Us Anything!

An FMT is the transfer of stool from a donor to a recipient, usually to improve the recipient's health. FMTs are an increasingly popular intervention in different diseases, ranging from recurrent infection with C. difficile (where clinical success rates are >90%) all the way to autism. Yet while FMTs seem to "work" well in some people and diseases, clinical effects are meagre in others and the reasons for this remain very incompletely understood. For a broader introduction to FMT, check out wikipedia: https://en.wikipedia.org/wiki/Fecal_microbiota_transplant.

As FMT targets the gut microbiome, it is generally thought that clinical success depends on the successful engraftment of "good" microbes from the donor and decolonization of "bad" microbes from the recipient. However, what really happens to the microbiome following an FMT, and whether outcomes can be predicted in advance (for example, to pick suitable donors for every recipient) has remained unclear. We represent three independent research teams who tackled this problem by analysing data from several independent trials where FMTs were conducted for different diseases: we used metagenomic data (i.e. DNA sequences directly from stool samples) to track microbes between donors and recipients. We developed models to predict whether donor microbes would colonize or recipient microbes persist after the intervention, and we used this information to pinpoint the factors that determine these outcomes. Broadly speaking, all three teams made similar observations: microbiome dynamics after FMT were somewhat predictable, and there is a limited list of factors that drive outcomes - most of them are on the recipient's side, meaning that choice of a "matching" donor seems less relevant than previously thought.

You can freely access all three studies online:

• Podlesny et al, Cell Reports Medicine (2022): https://www.cell.com/cell-reports-medicine/fulltext/S2666-3791(22)00254-3

• Ianiro, Puncochar, Karcher, et al, Nature Medicine (2022): https://www.nature.com/articles/s41591-022-01964-3

• Schmidt, Li, et al, Nature Medicine (2022): https://www.nature.com/articles/s41591-022-01913-0

For less formal introductions, check the press releases by the lead institutions University of Hohenheim, Germany (in German: https://idw-online.de/en/news799487), University of Trento, Italy (https://www.eurekalert.org/news-releases/964850) or EMBL Heidelberg, Germany (https://www.embl.org/news/science/when-microbiomes-collide/).

We will be on at noon Eastern (16 UT) and we are looking forward to your questions!

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