[frevd]

Given that you can run the same program on a billion bacteria should quite improve the odds of getting the job done.

Surely, also one has to account for possible other "jobs" that get done due to interference.

However, if your goal is to automate processes rather than develop cures to "run" in the human body then this is a very interesting alternative to using silicon, the parallel pipeline potential is enormous.

EDIT: Would it be possible to develop a biological CPU this way? I.e. having "instruction sensors" and a touring-machine-like DNA-robot that can execute externally supplied instructions? Putting that into a bacteria that can clone itself would surely cut down on costs of computing.

[dnautics]

>Would it be possible to develop a biological CPU this way? I.e. having "instruction sensors" and a touring-machine-like DNA-robot that can execute externally supplied instructions?

No, it is not possible (not this way). Tl;Dr how do you plan on storing information on the Turing machine tape? If you're happy doing computation with a relatively high stochastic failure rate things look better, but I wouldn't count on it.