Moselio Schaechter

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« The Uncultured Bacteria | Main | Microbiology in the Andes: Ancient and Unexpected »

July 15, 2010

Power of Ten


Tenfold Power. A gorilla can lift 10 times its own
weight, it is said. It can also sit wherever it wants.

by Elio & Stanley

How often have you heard it said, or seen it stated in writing, that we carry ten times more microbial cells than cells of our own? We don't dispute this figure, at least not as a ballpark estimate. But we were curious to find out where it came from. The paper that seems to be quoted most often in this regard is Microbial Ecology of the Gastrointestinal Tract by Dwayne Savage in the Annual Review of Microbiology, 1977, 31:107-33. Savage was an eminent intestinal microbiologist and those of us who knew him believe that he was a stickler for details. So he should have known. He stated: The various body surfaces and the gastrointestinal canals of humans may be colonized by as many as 1014 indigenous prokaryotic and eukaryotic microbial cells (70).

So now let’s go to reference # 70. It is to a paper by T.D. Luckey in the American Journal of Clinical Nutrition. 1970, (11) 1430-1432. Now we’re getting somewhere. Luckey writes: Assume one viable microbial mutation each 108 cells, a microbial count of 1011/g intestinal contents and 103 g intestinal contents, then at a given time each of us harbors about 1 million newly mutated microbes (1011 x 103/108 = 106).

Just how authoritative is that? References to experimental work? None are given. Surely, the number of bacteria in feces must have been determined countless times through the years. A search of older literature is called for (but who’s got the time!). Moreover, is the number of human cells based on more solid ground? We have no idea.

Nowadays, a quantitative PCR using universal bacterial primers should give a reasonable estimate of the total intestinal bacterial DNA, a figure that can be converted with some assurance to the number of bacteria present. For good measure, add a few percent to account for eukaryotic microbes. Note, however, that we take leave from a good portion of our intestinal microbiome every day, the lucky among us with satisfying regularity. So, the ratio fluctuates daily. No big deal, but let’s qualify the ten-to-one mantra by saying “estimated.” To say the least.

Stanley Maloy is Dean of Sciences and Associate Director of the Center for Microbial Sciences at San Diego State University.


An homage to the Powers of Ten by IBM

Elio comments:

Road makes the key point that the number of human cells is not easy to estimate. We should have make this point more forcefully.

Many thanks, Benoit, for the most useful references. Yes, there are a lot of data about the microbial content of feces, but less so about what's in the gut, which is of course harder to analyze.

I agree, Nathan, with regard to mitochondria: they have been winnowed down to a shell of their former prokaryotic glory (grin). But when remains interesting to me is the continuing role that microbes play, interactively, with the eukaryotic-only portion of our selves. Pretty much every week, I learn more about how microbes are not simply "listening" to our eukaryotic components, but "telling" those components things as well.

But as always, the dead hand of Darwin is upon us all.

Macfarlane & Macfarlane
Bacterial diversity in the human gut
Adv. Appl. Microbiol. 54 261-289 (2004)

« Bacteria comprise 40–45% of fecal material on a dry weight basis (Stephen and Cummings, 1980), which equates to about 18 grams of bacterial dry matter, or a total bacterial mass in the colon of about 90 grams (Macfarlane and Cummings, 1991). The large bowel is the main area of permanent microbial colonization of the human gastrointestinal tract, and several hundred bacterial strains and species have been isolated from this complex ecosystem, where viable counts in feces typically reach 10^11–10^12 per gram (Finegold et al., 1983; Moore and Holdeman, 1974)... »

Stephen & Cummings
The microbial contribution to human faecal mass
J. Med. Microbiol. 13 45-56 (1980)

Macfarlane, G. T., and Cummings, J. H. (1991). The colonic flora, fermentation and large bowel digestive function. In ‘‘The Large Intestine: Physiology, Pathophysiology, and disease’’ (S. F. Phillips, J. H. Pemberton, and R. R. Shorter, eds.), pp. 51–92. Raven Press, New York.

Finegold, S. M., Sutter, V. L., and Mathisen, G. E. (1983). Normal indigenous intestinal flora. In ‘‘Human Intestinal Microflora in Health and Disease’’ (D. J. Hentges, ed.), pp. 3–31. Academic Press, New York.

Moore, W. E. C., and Holdeman, L. V. (1974). Human fecal flora: The normal flora of 20 Japanese-Hawaiians. Appl. Microbiol. 27, 961–979.

By Mark's standard, we're all prokaryotic, in the same sense that birds are dinosaurs and we and dinosaurs are both fish. At some point in the past we engulfed something better at managing its DNA than we were -- or, depending on what you identify with, were engulfed by something that wasn't -- and went eukaryotic. After that everything is just details.

I think the bacterial count is probably pretty accurate, and can be calculated without much difficulty. However, I've never seen a convincing derivation for estimates of the number of human cells in the human body... anyone?

Not to mention we are not factoring in the enslaved prokaryotes we call "mitochondria." Honestly, we are mostly prokaryotic, even though many folks no longer like that term. We are "anthology organisms," made up of many partners.

Again, hats off to Elio and Merry (and Stan Maloy, who turns out to be from my hometown!), for giving educators like myself some "discussion points" for our microbiology courses. There is a pedagogical balance teetering in the classroom nowadays: information we need to impart to students, versus the ability to analyze and critically review that information. This particular "paper chase" to find out where the common quote originated (and it's reliability) is useful. "Conventional wisdom" is neither, sometimes.

Textbooks can't do this kind of thing, because of the publication delay. Blogs like this one can, with the thoughts of well known authorities in microbiology so clearly stated.

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