How many calories do bacteria consume?

Karen - About 10% of our energy comes from bacteria, but probably, the important point of that is that the energy is in the form of these short chain fatty acids that are actually very helpful to us. The bacteria produces short chain fatty acids that we then absorb and they're used by the gut epithelial cells to grow. So they're not actually going to be deposited in fat cells in our body or anything, but they're used to regenerate the lining of the gut.

Chris - I did also find one quite nice reference - Royal Society's proceedings B, Anastasia Makarieva in 2005 asked, "Do bacteria breathe at the same rate as whales?" and they found that bacterial cells do have the same metabolic rate as whale cells do. Twenty two watts per kilol. It's an interesting sideline that the human body has got about 1013 cells in it, so bacteria outnumber us 10:1 with 1014 and since the bacterial cell weighs 10 -12 grams per cell, that means that in the average person is 100 grams of bacteria which means that your bug load weighs almost as much as your kidneys!