It has become well-accepted that microbiota of the gut (a.k.a. gut bacteria) consume calories that are ingested and can have significant effects on energy metabolism in humans. For example, if you take the feces from mice that have had a gastric bypass and transplant those into the colon/GI tract of obese mice, then the obese mice lose weight (See here for article about this study).
Are there any known estimates on how many calories the typical gut microbiota in an adult human consumes? That is, about how many calories your gut microbiota use per day?
For more information there is a freely available source on Pubmed, "The Human Microbiome, Diet and Health Workshop Summary."