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I define "foreign microorganism" as a microorganism which is not produced by the human body (not antibodies or leukocytes) including bacteria, viruses, fungi, biofilm aggregates or small lifeforms independent if they are benign or hostile.

Let's say I have currently a weight of exactly 100 kg. How much of it consists of organisms not belonging to me?

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    $\begingroup$ Maybe a good definition of "foreign microorganism" could be: a microorganism with a different DNA from the human host? $\endgroup$ – Emanuele Paolini Oct 10 '15 at 9:00
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Edit: Matters Arising

In this Nature News article, Scientists bust myth that our bodies have more bacteria than human cells, and in the bioRxiv pre-print article, Revised estimates for the number of human and bacteria cells in the body, a new estimate of the ratio of microbial cells on the human body to human cells that make up the body has been revised down to close to 1-to-1, with there being slightly more microbial cells to human cells.

It should be noted that near 90% of the human cells taken into account were red blood cells and platelets.

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- Revised estimates for the number of human and bacteria cells in the body, Sender et.al. bioRxiv preprint.
doi: http://dx.doi.org/10.1101/036103

Note: Preprint articles have not been refereed. The information in them is subject to change, may not pass review, and in the end my not be accepted for publication.

Readers should therefore be aware that articles on bioRxiv have not been finalized by authors, might contain errors, and report information that has not yet been accepted or endorsed in any way by the scientific or medical community.
- bioRxiv.org: What is an unrefereed preprint?


Original Post

The human body contains trillions of microorganisms — outnumbering human cells by 10 to 1. Because of their small size, however, microorganisms make up only about 1 to 3 percent of the body's mass (in a 200-pound adult, that’s 2 to 6 pounds of bacteria), but play a vital role in human health.

From: NIH Human Microbiome Project defines normal bacterial makeup of the body

Additional Information on Microbiome and Obesity

From Cell

Altering the Intestinal Microbiota during a Critical Developmental Window Has Lasting Metabolic Consequences

From The American Journal of Gastroenterology Supplements (a Nature Press Journal)

Disclaimer

This article was published as part of a supplement sponsored by the Gi Health Foundation, a nonprofit 501(c)(3) educational organization dedicated to increasing awareness of the effect of gastrointestinal disorders in the United States. The foundation's goal is to provide health professionals with the most current education and information on gastrointestinal health.

Impact of the Gut Microbiota on the Development of Obesity: Current Concepts

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    $\begingroup$ That not only is surprising, but gives also more interesting information. That they are outnumbering our cells by a factor of 10 is astonishing to say the least, so by looking how that is possible I was surprised how diverse the size of human cells is. $\endgroup$ – user10094 Oct 9 '15 at 20:51
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    $\begingroup$ Yep... It makes you start to question whether humans actually are just transportation units for the microorganisms... Look at some of the studies that have looked at how the microbiome affects thought processes. $\endgroup$ – AMR Oct 9 '15 at 20:55
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    $\begingroup$ Nuts! And there for a brief moment I had a perfect scapegoat! "It's not ME, Doc - it's my microorganisms! THEY'RE why I'm overweight!!". Yeah...if only... :-( $\endgroup$ – Bob Jarvis Oct 10 '15 at 0:36
  • $\begingroup$ @BobJarvis, They may still give you an out. nature.com/ajgsup/journal/v1/n1/full/ajgsup20125a.html $\endgroup$ – AMR Oct 10 '15 at 2:55
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    $\begingroup$ The paper on cell ratios has now been published in peer-reviewed form: journals.plos.org/plosbiology/article?id=10.1371/… $\endgroup$ – Jack Aidley Apr 3 '17 at 13:57

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