mucus-loving bacterium, Akkermansia muciniphila, whose presence correlates with a lower risk of obesity and malnutrition. There’s Bacteroides fragilis, which stokes the anti-inflammatory side of the immune system. There’s Faecalibacterium prausnitzii, another anti-inflammatory bug, which is conspicuously rare in the guts of people with IBD, and whose arrival can reverse the symptoms of that disease in mice.