There are many more than two things that characterize bacteria.Perhaps tiny size and lack of a nucleus would be just abeginning.As newborns, we encounter our first microbes as we pass throughthe birth canal. Until that moment, we are 100 percent human.Thereafter, we are, numerically speaking, 10 percent human, and90 percent bacteria. Our microbiome contains at least 150 timesmore genes, collectively, than our human genome.Think of it as a hulking instruction manual compared to a singlepage to-do list.As we mature, we pick up more microbes from breast milk, food,water, animals, soil, and other people. Sometime in childhood, thebustling community of between 500 and 1,000 species stabilizes.Some species are native only to humans, and may have been passeddown within the family like heirlooms.Others are generalists - maybe they've hopped aboard from pets,livestock, and other animal sources. Most of our microbes inhabitthe colon, the final loop of intestine, where they help us breakdown fibers, harvest calories, and protect us frommicro-marauders.But they also do much, much more. Animals raised withoutmicrobes essentially lack a functioning immune system.Entire repertoires of white blood cells remain dormant; theirintestines don't develop the proper creases and crypts; theirhearts are shrunken; genes in the brain that should be in the "off"position remain stuck "on."Without their microbes, animals aren't really "normal."What do we do for our microbes in return? Some scientists arguethat mammals are really just mobile digestion chambers forbacteria.After all, your stool is roughly half living bacteria by weight.Every day, food goes in one end andmicrobes come out the other. The human gut is roughly 26 feet inlength. Hammered flat, it would have a surface area of a tenniscourt.