Fecal Transplants: What Is Being Transferred?

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.

Fig 1. The growth of fecal transplants as reflected in references in PubMed and the estimated composition of human feces.

The charts show (A) the rapid rise in publications on fecal transplants in the National Library of Medicine's search service (PubMed), particularly between 2012 and 2015, and (B) the estimated upper concentration of the biological entity per gram of unprocessed human feces, as cited in the text. Estimates do not necessarily reflect the viable number of the biological entity, and the concentration of the archaea is estimated from a methanogen breath test that is not solely based on the presence of archaea. Concentrations of metabolites, protists, and other entities were not identified.