We know that the human body is host to a fairly impressive collection
of bacteria, some benign and some unwelcome.  What I'm curious about
tonight is what percentage of an average person's body weight is
accounted for by bacteria.  Put another way, if there were no bacteria
in our bodies (and we were still alive), how big would we be?

Thanks,
Apteryx

Hello again, apteryx! What an interesting (if somewhat
queasiness-inspiring) question!

It appears that the average human bacterial load is approximately 2 to
9 pounds, depending upon which reference source is consulted. Below
you'll find a variety of sources from which to choose.

"Within every human being is a flourishing, living colony of
approximately four pounds of bacteria. Most of these bacteria reside
in the human digestive tract although some are found elsewhere (i.e.
the oral cavity, throat, etc.)"

Vital Probiotics
http://www.vital-probiotics.com/what-is-probiotics.html

"There are between 2-4 pounds of bacteria in one's intestinal tract.
Scientists estimate that there are more bacterial cells in your
gastrointestinal tract than the entire number of cells in the rest of
your body. There are two types of bacteria in our intestinal tracts:
there is beneficial bacteria and pathogenic or harmful bacteria. The
ratio of beneficial to harmful bacteria is perhaps the most crucial
element in our health today. Researchers estimate that the average
intestinal tract of a healthy individual should be approximately 85%
beneficial and 15% pathogenic. While that 15% pathogenic is still in
your body, it is benign when it is in the presence of the 85%
beneficial bacteria. The real problem here is that the average
individual, consuming the standard American diet, has the exact
opposite ratio, which is 15% beneficial and 85% pathogenic."

Crohns.net
http://www.crohns.net/Miva/education/glossary.htm 

"A normal human being hosts about 1.2 kilograms of bacteria (8). The
bulk are in the gut lumen, and the rest in the skin, oro-pharynx, and
genitalia."

What-Is-Cancer.com
http://www.what-is-cancer.com/papers/HowToDiminishResistance.html 

"Bacteria are all around us even though we cannot see any of them.
They affect our everyday life, and in fact we could not live without
them. Bacteria perform necessary functions for humans; the most
important is the breakdown of food in our intestines. The average
human actually has around 1.2 kilograms of bacteria in their body."

http://www.lifesci.utexas.edu/courses/mcmurry/fall97/13/mattpage.html

"A normal human being hosts about 1.2 kilograms of bacteria. The bulk
are in the gut lumen, and the rest in the skin, oro-pharynx, and
genitalia. Some are potential killers and yet they thrive without
harming us."

La Tribune de la Gauche Républicaine
http://www.tribunes.com/tribune/edito/6-2z.htm

There are other estimates on the Web ranging from 2 to 9 pounds. Here
are two of the larger estimates:

"At any time we carry about seven pounds of bacteria, all busily
making gas."

Dewan Kraft Systems 
http://www.primeindia.com/dewankraft/pollution.html

"The average human female carries nine pounds of bacteria on her skin
or within her intestines."

Flashed MRG Radio
http://24.138.11.236/nutrition.html

Obviously, the amount and weight of a human's microbial contents will
vary, depending upon the size of the human and the sort of diet that
has been consumed.

You may be interested in an answer given by my friend and colleague
tehuti-ga to a similar question:

Google Answers: Microorganisms intake 
http://answers.google.com/answers/main?cmd=threadview&id=174301

Finding the proper search terms to unearth this information was a
challenge. These were the combinations of keywords that gave me the
best results:

Google Web Search: "pounds of bacteria" + "human"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22pounds+of+bacteria%22+human

Google Web Search: "kilograms of bacteria" + "human"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22kilograms+of+bacteria%22+human

Thanks for providing a fascinating quest. If any clarification is
needed, please ask.

Best,
Pink

Why, hello, Pink--how nice to hear from you!  That's what I wanted to
know, all right.  Turns out to be a lot less weight than I thought. 
<Sigh> Guess diet and exercise are still the way to go.

I haven't got around to contributing to your temple yet, so I'll just
offer you this historical tidbit instead:

http://www.stmarysbythesea.org/history/GuildOff.htm

Best regards,
Apteryx

Greetings Apteryx:

I wasn't able to locate your answer but I wanted to share this bit of
trivia I found at http://www.puzzlegrid.com/facts.cgi?groups=Human&startvalue=0:

"The skin of the armpits can harbor up to 516,000 bacteria per square
inch, while drier areas, such as the forearm, have only about 13,000
bacteria per square inch."


Best regards,
journalist-ga

Thank you, journalist <shudder>.  I think I'll go take another shower now.

Apteryx