Leptin-deficient obese mice have significantly more Firmicutes and fewer Bacteriodetes species compared to their wild-type and heterozygous littermates.1 In stressed mice, there seems to be a correlation between their Firmicutes levels and their responses in stress tests.2 Mice that by knockout are made unable to produce the active form of vitamin D or the vitamin D receptor have fewer Firmicutes bacteria in their feces compared with wild-type mice.3 On the other hand, the families Lactobacillaceae and Lachnospiraceae show increased abundance in feces from these knockout mice.3