We still can't do nano-engineering at the molecular level, so even if we knew exactly how a microbe was put together, we still couldn't duplicate one, much less design a new one.

But we could create virtual life on the computer that would metabolize, interact with its virtual surroundings, reproduce and even evolve. And there is no reason we couldn't create models or simulations of entire ecosystems of these virtual critters. These organisms would by every definition of biology be "alive". They would just not be made of matter.

Has anyone actually tried this?

It took about three billion years for multicellular life to evolve from the simplest microbes on this planet, but it was limited by the relatively slow pace of chemical reactions and the distances between the creatures involved. In the computer, we could create an environment stripped of all extraneous inessentials, and computation is faster than molecular chemistry. And we wouldn't need to interfere very much, evolution does a wonderful job all by itself