Fecal Transplants: What Is Being Transferred?

TLDR

Fecal transplants are increasingly used to treat recurrent Clostridium difficile infections and as a research tool in animal models, with recipient changes typically attributed to the ~10¹¹ bacterial cells per gram of donor stool. The study aims to review literature on fecal matter composition to highlight its multipart nature. The authors summarize findings from existing studies on fecal matter composition. In addition to viable bacteria, fecal matter contains colonocytes (~10⁷ per gram), archaea (~10⁸), viruses (~10⁸), fungi (~10⁶), protists, and metabolites, and these nonbacterial components could contribute to recipient biology, warranting further reductionist studies to isolate their effects.

Abstract

Fecal transplants are increasingly utilized for treatment of recurrent infections (i.e., Clostridium difficile) in the human gut and as a general research tool for gain-of-function experiments (i.e., gavage of fecal pellets) in animal models. Changes observed in the recipient's biology are routinely attributed to bacterial cells in the donor feces (~1011 per gram of human wet stool). Here, we examine the literature and summarize findings on the composition of fecal matter in order to raise cautiously the profile of its multipart nature. In addition to viable bacteria, which may make up a small fraction of total fecal matter, other components in unprocessed human feces include colonocytes (~107 per gram of wet stool), archaea (~108 per gram of wet stool), viruses (~108 per gram of wet stool), fungi (~106 per gram of wet stool), protists, and metabolites. Thus, while speculative at this point and contingent on the transplant procedure and study system, nonbacterial matter could contribute to changes in the recipient's biology. There is a cautious need for continued reductionism to separate out the effects and interactions of each component.