AND I THOUGHT THE SCIENCE WAS SETTLED: 40 Trillion Bacteria on and in Us? Fewer Than We Thought. “There are a lot of bacteria on us and in us — our microbiome, it is called. Calculating exactly how many microbes each of us carries is hard, and the most common number cited, both in popular and scientific literature, is almost certainly wrong.” It’s interesting how much settled science turns out to be somebody’s guess that just got repeated over and over in the literature.