Adding four probiotic microbes or the naked DNA from probiotic bacteria to the mix substantially ratcheted down the response of immune cells, especially for people with allergies. The resulting cholic acid “is not a good fat emulsifier,” notes Nicholson, a coauthor of the study—and after the probiotic treatment there was a 50-fold higher ratio of cholic to taurocholic acid in the treated animals’ guts. Although the infectious agent in some cases was identical to the probiotic used, Boyle’s group notes that an indicted strain of microbe may sometimes also “be found in the internal microbiota of healthy humans, so the source of infection in these cases is not conclusively [due to probiotics].”