Renewable Energy from Engineered Microbes Business

Microbes are outstandingly powerful and have the potential to be engineered to do almost anything. If I had more time, I would love to look more into the idea of engineering a bacteria that could efficiently convert a renewable resource into a usable fuel. Obviously, many microbes already exist that can do this, and so I would like to do more research into why this is not an option in common practice yet, and what strategies could be developed to make it possible here and now. Is the problem that the fuels they are making are incompatible with our current infrastructure (e.g. the microbes make ethanol, and most cars run on gas)? Or is the problem that bugs that can make usable fuel do so inefficiently, and thus need to be engineered to produce sufficient quantities in a cost-effective manner and/or on a reasonable time scale? In short, what problem(s) need to be solved to utilize microbes as fuel sources?