Suburban Skeptic - How human are we?

[Although they're not as ubiquitous or influential as urban legends, suburban legends offer two distinct debunking advantages: First - they have a way of finding me, and Second - they present a significantly less demanding challenge, the combination of which qualities generally allows my meager level of attention and effort to equal the task.]

"I heard that we're actually mostly other than human!"
"Hunh?"
"I heard that technically we're more bacteria than human, or something like that."

I overheard this chunk of conversation recently. It dovetails with similar claims I've encountered, including - "Our body mass is actually made up more or parasites than our own tissue," and "most of the cells making up your body are not your own cells."

I always thought both claims were overblown. As it turns out. I'm only half right.

It is estimated that 5000 to 10000 different species of bacteria live in the human body (Sears, 2005). Bacterial cells are much smaller than human cells, and there are about ten times as many bacteria as human cells in the body (1000 trillion (10¹⁵) versus 100 trillion (10¹⁴); Sears, 2005).

The above, which clearly show me to wrong about the "number of cells" issue, is from Wikipedia.

Now it seems that estimates as to the number of cells in the human body vary quite widely, from Sears' 100 trillion to about 60 trillion to other more moderate guesses in the ten trillion range. Most sources seem to agree on the 100 trillion estimate.

As well, most sources I've looked at agree that there are about ten times (one suggested 20x) more symbiotic microbes hanging around in your body (mostly bacteria, and mostly in the gut) than there are human cells making up your body. I offer this distinction between "hanging around in" and "making up" because I was at least right about one thing: most of the human body is not symbiotic cells. By number, bacteria et al are greater, but these cells are so much smaller than most human cells that it is incorrect to suggest we are mostly microbes, especially when considered by percentage of mass. The only source I found that gave numbers on this indicated that bacteria in the human body could mass up to 2 kilos. This is about 3% of body weight for an average sized man (150 lbs.)

And here's another interesting tidbit from Dr. George Weinstock, co-director of the Human Sequencing Center at the Baylor College of Medicine,

"Even though a microbial genome is one-thousandth the size of the human genome, the total number of microbial genes in [the human] body is much greater than human genes because you have so many different species."

Even your humble skeptical servant learns something now and then (though he hates to admit it).