Like many biological concepts, engraftment can be examined through the lens of ecological principles. A new species introduced into an established ecosystem, whether a seed into an area of dirt, a bird into a forest, a fish into a lake, or a probiotic bacterium in a human body, enters the landscape looking for a way to thrive. Imagine a probiotic bacterium as a pioneering species, a newcomer entering an mature ecological community — the human gut — in search of a permanent home, or a niche. This journey begins at the mouth — the gateway to the gastrointestinal tract. The microbe, whether delivered in food or a capsule, navigates varied landscapes: the highly acidic stomach, a formidable barrier; the hydrated and bile-rich environment of the small intestine, a bustling river of activity; and finally, the teeming, microbially diverse large intestine. Along the way, the bacterium is seeking just the right conditions for metabolizing a food source and eventually growing and dividing, while also experiencing competition and a novel environment that can make this a challenge.
The term engraftment describes how a new bacterial species, one not typically found in a person’s stable microbiome, might integrate and become a permanent member of that ecosystem. For instance, if a person’s existing microbiome is a flourishing garden composed of familiar plants (bacteria A, B, and C), engraftment would be the successful establishment of a new plant (bacteria D) not just for a season, but to take hold and thrive season after season. The bacteria is akin to a seed borne on the wind seeking fertile ground in an already cultivated landscape. The seed knows the conditions they need — rich soil, adequate sunlight, protection from predators — but the specific local conditions are unknown, making the search for the ideal spot a difficult endeavor, and the field is already crowded. This is why the vast majority of probiotics do not engraft, but rather deliver time-bound benefits as they travel down the gastrointestinal tract.
Thus, for most bacteria, including almost all studied probiotics, true engraftment — a permanent residence — is rarely established. Instead, these microbial visitors temporarily impart their benefits–production of beneficial metabolites, cross-feeding with resident microbiota, or interaction with the gut lining and immune system– before being outcompeted by the resident microbiome’s natural circadian rhythms or simply moved along with the next meal. The stability of our baseline microbiome is an important part of human ecology; we don’t want it easily disturbed, as this could open the door to both beneficial and harmful newcomers. The gentle and temporary introduction of bacteria through probiotics is often long enough to produce desired benefits, but short enough to avoid fundamentally altering the established, resilient microbiome. Regardless of whether a microbe finds a permanent foothold or is merely a transient visitor, its ability to support your gut depends on more factors than just its presence in the community. This is why rotation of probiotics and the exposure of the microbiome to new strains at different concentrations can offer an advantage to a traditional probiotic, as each one may interact differently in their temporary home in the gastrointestinal tract. Because these microbes are transient, diversity of exposure may be more significant than permanence. Rotation ensures continual novelty in microbial inputs, potentially expanding the range of interactions within the gut. Even inert bacteria can exert a positive outcome, as the immune system can recognize their cellular components. At Good Bacteria, our focus is benefit, not engraftment, working with your stable microbiome and our rotating strains to deliver daily support to your gut.
CITATIONS
Jens Walter et al., “To Engraft or Not to Engraft: An Ecological Framework for Gut Microbiome Modulation with Live Microbes,” Current Opinion in Biotechnology 49 (2018), https://doi.org/10.1016/j.copbio.2017.08.008.
Muriel Derrien and Jan E. van Hylckama Vlieg, “Fate, Activity, and Impact of Ingested Bacteria within the Human Gut Microbiota,” Trends in Microbiology 23, no. 6 (2015): 354–66.