The internal ecosystem

Proper functioning of the gastrointestinal tract is crucial for a healthy life, and the gut flora plays an essential role in this. The gut flora—also called the microbiome—consists of a vast number of bacteria, amounting to roughly ten times the number of human cells. The microbiome is known as one of the most complex ecosystems in the world and exerts far-reaching physiological and metabolic influences and functions. These functions and their activities are comparable to those of an organ; hence the microbiome is often called “the forgotten organ.”

A healthy gut flora today comprises 1,000–1,150 bacterial species and accounts for about 1–3% of total body weight. The bacteria in the gut are 99% gram-positive anaerobes. Unlike aerobic bacteria, anaerobes do not need oxygen to live or grow. This distinction was introduced in the 19th century by Louis Pasteur—the discoverer of pasteurization—from the negating Greek a(n)- + aerobic.

Because the local environment differs along the gastrointestinal tract, each segment harbors its own bacteria, with large differences in species and numbers. When the equilibrium of the gut flora is disturbed, this has a “downstream effect” on other systems in the body. Humans can survive without gut flora, but not without consequences.

Despite the complexity of this ecosystem, it is tightly orchestrated. Immediately after birth, colonizing bacteria enter the gut in an organized sequence. Diversity is very low at first but increases over time. An adult’s microbiome is individually unique, constantly changing, and context-dependent. To hint at the complexity mentioned above: even just the more than 400 million different variables—species, counts, locations, etc.—are telling.

The microbiome is a direct result of the symbiosis between host and gut flora and a fundamental part of maintaining homeostasis in a healthy individual. Bacteria provide the host with essential nutrients such as vitamins, but also compounds like endocannabinoids and serotonin, a neurotransmitter. In return, the host supplies nutrients and a stable environment. Bacteria and humans are attuned to each other to gain the benefits of a shared existence—this is a form of microevolution.

History

At the end of the 17th century, the Dutch microbiologist Antoni van Leeuwenhoek likely became the first person to glimpse a new world of tiny cells through his self-made microscope while examining his own dental plaque. He could hardly have predicted that, a few centuries later, the billions of microbes we share our lives with would become one of the most fascinating topics in (micro)biology.

Nobel laureate Élie Metchnikoff observed in 1907 that regular consumption of lactic acid bacteria from fermented dairy products affected the lives of people in Bulgarian villages. His conclusion: “the dependence of the intestinal microbes on the food makes it possible to adopt measures to modify the flora in our bodies and to replace the harmful microbes by useful microbes.” In retrospect, that is entirely correct.

The term probiotics was first mentioned in 1965 and cited again in publications in 1971 and 1974. In 1989, Fuller revised the original definition to what is essentially today’s usage.

Only in 2001 did Nobel laureate Joshua Lederberg coin the term “microbiome” to denote the billions of microorganisms in our bodies as a collective. Since then, the trend has continued and knowledge about probiotics keeps growing. A simple Google search shows that the actions of our gut bacteria are linked to a wide range of topics—sometimes rightly, sometimes not. Depending on the situation, they can be friend or foe!

Commensalism

Today, the term “commensal flora” also appears frequently in the literature—another label for gut flora or microbiome. In 1876, Pierre-Joseph van Beneden coined “commensalism,” derived from the Latin “commensal,” meaning “to eat at the same table.”

In biology, commensalism is defined as a relationship between two species in which one benefits without harming or positively affecting the other—a non-harmful coexistence.

Based on the information above, however, this is not an accurate description of the human–microbiome relationship. There is indeed positive—and sometimes harmful—interaction. “Mutualism” is therefore more appropriate, as it emphasizes interactive cooperation. Mutualism is a form of symbiosis, a long-term association between different species.

Prebiotics

Foods containing live bacteria that pass through the gut have not only probiotic but often also prebiotic effects. Prebiotics are special, non-digestible fibers that feed gut bacteria. Marcel Roberfroid first described these fibers in 1995 and in 2007 defined them as: “A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and in the activity of the gastrointestinal microbiota, thus conferring benefits upon host well-being and health.”

Prebiotics are converted into short-chain fatty acids (SCFAs). Production of these fatty acids by the gut flora is another interesting aspect of this complex human–bacteria interaction. With many biological functions in the gut, butyrate, acetate, and propionate are the best-known and most common end products.

Well-known forms of prebiotics include oligosaccharides such as galactooligosaccharides (GOS), fructooligosaccharides (FOS), mannan-oligosaccharides (MOS), and chito-oligosaccharides (COS). Potato starch, nuts, and unripe bananas are also excellent sources (resistant starch).

The appendix—an organ without a function?

For generations, the appendix was thought to be functionless—merely a useless evolutionary remnant—partly because one can live perfectly well without it. This worm-shaped part of the gut is often surgically removed in cases of potentially fatal appendicitis.

More recently, however, new insights have linked the appendix to an important function. It appears to serve as a refuge for gut flora when they die off—for example, during serious infections such as cholera or dysentery.

The role of the appendix is thus that of a reservoir situated just outside the main intestinal flow. Beneficial bacteria can reside there to recolonize the gut later. It may also act as a “bacteria factory,” where helpful microbes are formed. These functions likely went unnoticed for so long because they are less needed in today’s world. In densely populated areas, person-to-person bacterial exchange helps recolonize the gut after microbiome die-off.

In the past, recolonizing the gut during epidemics was not so straightforward—then the appendix came in handy. In some countries and remote regions, it is quite plausible that the appendix still serves this role.