Bacteria digest food around them and produce waste products. Sometimes these waste products are dangerous. Such is the case of the bacteria that live on the skin of the rough-skinned newt. These bacteria produce a neuro toxin that can stun or paralyze a predator. This is the same sort of toxin in poison frogs and puffer fish and if concentrated—could kill a person or two! Natural predators like snakes seem to build-up an immunity to the toxin, so the bacterial defenders may be how the newt continues to defend themselves.
See the pictures and read more at: https://www.sciencenews.org/article/toxin-producing-skin-bacteria-can-make-newt-deadly-poisonous