Microbes are those little creatures that come in all shapes and types, living everywhere and covering everything. I mean everywhere: dirt, air, the underground, the door handle, your dog’s hair, the ocean where you swim, the sand where you might walk on, on your phone, the food you eat and even inside you. Yes, even inside your mouth, gut, or brain! These little creatures are capable of many things, including affecting your health- both in bad and good ways. A whole ecosystem of microbes can affect how you feel, how you act, and even who you are!
The human microbiome is the collective genome of the microbes (bacteria, bacteriophage, fungi, protozoa and viruses) that live inside and on the human body (1). It’s shocking to learn that there are about 10 times as many microbial cells as human cells in our body. Fortunately they are so small that they only make up 3% of our body weight. The thought that 3 kg of a 100 kg person is likely to be their microbiome is pretty daunting!
The microbiome, and particularly the gut microbiome, is often considered a virtual organ (2-4). The concept emerges from the fact that it can influence distant organs and systems, such as the nervous, immune, or endocrine system. It does that through the production and regulation of multiple compounds that reach those systems. The microbiome is also considered a key regulator of stress, supporting the idea that it could actually be considered a system within the organism, with many functions, connections and influences (4,5).
The microbiome inside you really has a mind of its own. It can communicate with the rest of the organism, and its influence on human health is a subject of great interest in the scientific community. Microbial cells are capable of transmitting short and long distance signals through electrochemical signaling, including signaling through ion channels like signaling among neurons in a human brain (6). This signaling can also affect the host, which can send a feedback signal in response that could affect the microbial community state. The almost 100 trillion cells that form the microbiota (i.e. the aggregate of microorganisms that reside in an organism) develop and establish extremely intricate and complex communication and biofeedback networks not only with other microbes but also with the host cells. As a result, it is believed that they can influence many processes within the body. In fact, an ever-increasing number of scientific publications are linking the microbiome with a great diversity of health issues, including autism, depression, allergies, liver health, stress, or multiple sclerosis, among others.
Gnotobiotic (i.e. germ-free) organisms have been used to understand the interactions of the microbiome with the host organism, such as effects on behavior, the immune and endocrine systems, and stress response (reviewed in 2). For instance, studies with germ-free organisms identified a linkage between microbiome and anxiety-like behavior in rat and mice (7,8). There are many studies linking the gut microbiome with obesity. Some pioneer research using gnotobiotic mice (9) provided important evidence on the potential effects of the microbiome on obesity. The study showed how transplanting gut microbiota from normal to germ-free mice produced a 60% increase in body fat content and insulin resistance within 14 days, even when their food intake was reduced. Their findings suggested that the gut microbiota was actually affecting energy harvest and storage in the host. So, bottom line, your microbiome might be affecting your weight loss goals, now… who would have thought!
Exciting research shows how the immune system develops alongside the gut microbiome. For instance, researchers found that infants living with pets had a more diverse gut microbiome. As it has also been shown that children growing with dogs have lower rates of asthma, it is suggested that there is a link between gut microbiome development and allergic disease, likely due to exposure to a larger diversity of microbes, such as those present on dog fur (10,11). Children in farms do even better, with a 50% reduction on asthma risk, likely due to exposure to dirt and other farm animals. If you would like to learn more about the subject, there’s a book by Rob Knight and colleagues, pioneers on exploring the effects of the microbiome on health. Easy, informative, and entertaining read!
As often happens in scientific literature, many studies provide different or contradictory results, likely due to the many variables involved, which are practically impossible to control. Scientists often turn to citizen science in order to increase knowledge. A very interesting project, the American Gut, led by Dr Knight, aims to sequence the microbiome of as many people as possible and link it to diet, lifestyle, and health. It is a crowd-sourcing study that will send you your results completely analyzed for a fee. As they are also exploring the relationships between human microbiota and that of our non-human companions, they will even sequence your dog microbiome! In my world, that is pretty darn cool (I would not mind figuring out if Keanu’s poop can tell me something abut his way too sudden mood swings!).
They have already published some of their results (12), demonstrating an unexpected range of diversity in human microbiomes. The amount of data they are collecting could be crucial in understanding the role of the microbiome and the factors that might affect it. I haven’t signed up yet, but I’m seriously considering it. And of course, I’d like to analyze at least one of our dogs! I find the microbial ecology of the microbiome fascinating, and am curious to know what it can tell me about myself.
And now that you know the benefits of having a furry friend, please consider adopting one that needs a home!
1) Human microbiome Project. https://hmpdacc.org/
(2) Garcia-Reyero N. The clandestine organs of the endocrine system. Gen Comp Endocrinol. 2018 Feb 1;257:264-271.
(3) Clarke G, Stilling RM, Kennedy PJ, Stanton C, Cryan JF, Dinan TG. Minireview: Gut microbiota: the neglected endocrine organ. Mol Endocrinol. 2014 Aug;28(8):1221-38.
(4) Evans JM, Morris LS, Marchesi JR. The gut microbiome: the role of a virtual organ in the endocrinology of the host. J Endocrinol. 2013 Aug 28;218(3):R37-47.
(5) Rea K, Dinan TG, Cryan JF. The microbiome: A key regulator of stress and neuroinflammation. Neurobiol Stress. 2016 Mar 4;4:23-33.
(6) Prindle A, Liu J, Asally M, Ly S, Garcia-Ojalvo J, Süel GM. Ion channels enable electrical communication in bacterial communities. Nature. 2015 Nov 5;527(7576):59-63.
(7) Crumeyrolle-Arias M, Jaglin M, Bruneau A, Vancassel S, Cardona A, Daugé V, Naudon L, Rabot S. Absence of the gut microbiota enhances anxiety-like behavior and neuroendocrine response to acute stress in rats. Psychoneuroendocrinology. 2014 Apr;42:207-17.
(8) Neufeld KM, Kang N, Bienenstock J, Foster JA. Reduced anxiety-like behavior and central neurochemical change in germ-free mice. Neurogastroenterol Motil. 2011 Mar;23(3):255-64, e119.
(9) Bäckhed F, Ding H, Wang T, Hooper LV, Koh GY, Nagy A, Semenkovich CF, Gordon JI. The gut microbiota as an environmental factor that regulates fat storage. Proc Natl Acad Sci U S A. 2004 Nov 2;101(44):15718-23.
(10) Azad MB, Konya T, Maughan H, Guttman DS, Field CJ, Sears MR, Becker AB, Scott JA, Kozyrskyj AL. Infant gut microbiota and the hygiene hypothesis of allergic disease: impact of household pets and siblings on microbiota composition and diversity. Allergy Asthma Clin Immunol. 2013 Apr 22;9(1):15.
(11) Gupta S. Microbiome: Puppy power. Nature. 2017 Mar 29;543(7647):S48-S49.
(12) McDonald D, Hyde E, Debelius JW, Morton JT, et al. American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems. 2018 May 15;3(3). pii: e00031-18.