Bacteroides fragilis or ‘B-frag’. In 2002, Sarkis Mazmanian showed that this particular microbe can fix some of the immune problems in germ-free mice. Specifically, its presence restores normal levels of ‘helper T cells’, a crucial class of immune cell that rallies and coordinates the rest of the ensemble.25 Mazmanian didn’t even need the entire microbe. He showed that a single sugar molecule in its coat, polysaccharide A (PSA), could boost the numbers of helper T cells on its own. This result was the first time anyone had shown that a single microbe – no, a single microbial molecule – could
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