The gut microbiota are mainly composed of strict anaerobes, which outnumber the facultative anaerobes— organisms able to grow both aerobically and anaerobically— and the aerobes by up to 100-fold.1 Although the presence of more than 50 bacterial phyla has been detected in the human gut to date,2 the microbiota are dominated by only 2 phyla: the Bacteroidetes and the Firmicutes. Estimates of the number of bacterial species present in the human gut vary widely among studies, but it is generally accepted that individuals harbor more than 1000 microbial, species-level phylotypes.3–5