I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

The traditional view of the immune system is full of military metaphors and antagonistic lingo. We see it as a defence force that discriminates self (our own cells) from non-self (microbes and everything else), and eradicates the latter. But now we see that microbes craft and tune our immune system in the first place! Consider just one example: a common gut bacterium called 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 correct a specific immune problem. Mazmanian’s team later showed that PSA can prevent and cure inflammatory diseases like colitis (which affects the gut) and multiple sclerosis (which affects nerve cells), at least in mice.