In 2009, students at Cambridge University genetically engineered E. coli bacteria, adding DNA sequences in order to create colors visible to the naked eye (standard E. coli does not have a pigment). The students called this new strain E. chromi, and it has limited uses today, primarily related to detecting pollution or other chemicals -- if the bacteria detects a chemical, it changes color.
That's all well and good, but how do we take this and go nuts with it, creating a Terminator-like dystopian horrorscape? I'm so glad you asked. In the short film below (which is largely fictional), an example is given in which an advanced form of this bacteria could be ingested by humans, and it could then identify illnesses within the GI tract...which you'd learn about after pooping a rainbow of bacteria-laden diagnostic material. (Oh, were you eating? I should have warned you.)
I should emphasize: this is a speculative art project based on some limited real-world science. But who among us does not dream of a poop-rainbow-filled tomorrow? I, for one, welcome our new bacterial overlords.