Power of Ten

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).

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

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.