Levels of organization: we are an ecosystem within ecosystems

Graham Lawton (2020) writes in NEW SCIENTIST:

More profoundly for our definition of self, we are also holobionts: we aren’t individuals, but collectives. Every bit of our body is teeming with microbial life: bacteria, fungi, protists, archaea and viruses. They live on us and in us, on our skin, inside every orifice, and above all in our gut. We are even surrounded by an invisible cloud of them, a bit like Pig-Pen from the Peanuts cartoons.  //  These microbes outnumber our own cells, though not by 10:1 as is often claimed. An average human body is made up of about 30 trillion human cells and 38 trillion microbial ones. By mass, we absolutely dwarf our companions: a 70-kilogram human contains just 200 grams of microbe. //  But they punch well above their weight. The microbiome is different from parasitic freeloaders like lice and intestinal worms: it is an active and vital participant in our lives. Our gut microbiota, for example, do huge amounts of work digesting food that the products of our human genome can’t break down on their own. They are, in fact, the principal determinant of how we respond to food. Our microbiome influences our health in many other ways, contributing to mental well-being and modulating our emotions and cognition, and helping determine how our immune systems function.” (Read more:  

The microbiome is a major new frontier in medicine, but is providing important new insights in ethology: read about the microbiome in animal behavior  and the “psychobiome“–both in the journal SCIENCE [if you are not a member of AAAS the librarian can help you get this article]


For example, can gut microbes regulate appetite and body temperature?  –YES!  Bacterial cell wall molecules that travel to the brain could trigger a host of behaviors:


“With more microbes than cells in our body, it’s not surprising that bacteria and other invisible “guests” influence our metabolism, immune system, and even our behavior. Now, researchers studying mice have worked out how bacteria in the mammalian gut can ping the brain to regulate an animal’s appetite and body temperature—and it involves the same molecular pathway the immune system uses to detect bacterial pathogens.

“It’s quite an important finding,” says Antoine Adamantidis, a neuroscientist at the University of Bern who was not involved with the work. “Our life depends on food intake, and this is one more [thing] that bacteria can [influence].”

Over the past 20 years, researchers have uncovered connections between the human gut and the rest of the body. They have linked certain intestinal microbes to conditions such as depression, multiple sclerosis, and immune system disorders; they have also documented nervous system connections between the gut and the brain. But researchers have been hard pressed to understand exactly how gut microbes—or the molecules they make—influence the brain.” 

(read entre news item reported by ELIZABETH PENNISI (14 April 2022) in SCIENCE




Humans have many distinct and interconnecting microbial populations that exert systemic effects throughout the body. Understanding the ways these communities interact provides insight into how the collective microbiome shapes health and disease.

The healthy human body is home to an abundance of bacteria, viruses, and fungi that are acquired in early postnatal life. The bulk of the microbiome is in the gut, but distinct microbial communities exist on the skin and in the mouth, nose, lungs, and genital tract. The human microbiome has important roles in maintaining homeostasis, and disruption of microbial colonization of an infant has systemic effects that may influence health later in life, potentially promoting the development of autoimmunity, allergies, metabolic diseases, and even cancer.

Studies of microbial niches in the skin have revealed that they are integral to the skin’s multifaceted barrier function. At the same time, there is growing appreciation for how cross-talk between the skin microbiome and the immune system influences (and is influenced by) distant organs such as the gut, lungs, and brain. The microbiome in the oral cavity may also exacerbate diseases in distal sites, such as colorectal cancer, asthma, rheumatoid arthritis, and cardiovascular disease.

Given their abundance, interactions among diverse microbial communities will likely influence host physiology. If a microbial population is disrupted by antibiotics or a low-fiber diet, then pathogens can gain advantage, resulting in systemic infection and inflammation. Some emerging clinical evidence indicates that the “soup” of metabolites and signaling molecules produced by the gut microbiota could influence many diseases, not to mention behavioral states, but there are few examples of verified mechanisms.”

(excerpted from “A multiplicity of microbiomes” by Priscilla Kelly, et al. (2022) SCIENCE 26 May 2022