Big numbers

I've been getting a bit mind boggled at the numbers quoted for bacteria that we live with.  For example, from the New York Times recently:

We may think of ourselves as just human, but we’re really a mass of microorganisms housed in a human shell. Every person alive is host to about 100 trillion bacterial cells. They outnumber human cells 10 to one and account for 99.9 percent of the unique genes in the body....

Our collection of microbiota, known as the microbiome, is the human equivalent of an environmental ecosystem. Although the bacteria together weigh a mere three pounds, their composition determines much about how the body functions and, alas, sometimes malfunctions.

I take it that our personal bacteria are really small then...

And this from a review of a book at the TLS:

 When pathogenic bacteria were discovered to cause disease in the nineteenth century, the body was assumed to be in a pristine state until invaded and rendered sick. The appreciation of healthy carriers shattered such illusions. Now we learn that a single gram of faeces contains 100 million archaea and 40 billion bacteria.

A gram of poo has 40 billion bacteria?   I thought it might be a typo, but no, I go to the journal Gut and find this at the start of an article about measuring bacteria from the colon:

Antigens, both of dietary and bacterial origin, are abundantly present in the colonic lumen. Bacterial antigens predominate as there are as many as 10^11  bacteria per gram contents while most dietary antigens are degraded. It is important to realise that over 99.9% of the colonic microflora are part of a relatively stable ecosystem consisting of possibly as many as 400 different indigenous species as well as a few recently arrived species. Most of these anaerobic bacteria are not infectious and each person has a characteristic combination of these micro-organisms.

 Yeah, that's 10 to the power of 11 per gram.

But wait - don't go thinking a nice salty dip in the ocean will help you:

A millilitre of seawater from the North Atlantic contains 15 million viruses. 

By way of comparison, I had forgotten how many sperm cells your average male orgasm may release.  The total, of course, varies by volume, but a very detailed article from the Journal of Andrology indicates that it can be up to (a rather surprising) 1.2 billion*.  But it also shows volume varying from .7 to 8.6 ml, so I assume the big numbers come from the high end of the volume scale.    But if a ml weighs about a gram, you're looking at an average of about 100 million cells, which again just goes to show how tiny our gut bacteria seem to be.  (Or is it just that they are packed in incredibly closely in the gut?) 

I'm not sure you should talk about this at work during today's coffee break, but feel free to quote me...

*  it seems that even allowing for a high proportion of dud sperm cells, it should only take a month or so for one male to populate a planet.