Over the many years that I taught microbiology to medical students, perhaps my least favorite lecture was the one on the “normal flora,” what we now call the microbiome. What used to make me grumpy was that I could talk only in vague generalities, rambling about the intestinal flora being an immunostimulant, a contributor to the host’s nutrition (Vitamin K, choline), a guard against invaders, maybe a source of carcinogens, and who knows what else. The reason for my dissatisfaction was not that the information wasn’t believable or that the experiments were at fault. A lot of good work had gone into it. In fact, some very fine science was done, some going back to the early days of our science. And yet….
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