Is it possible to produce bread using only microorganisms?
It looks like that yeast has been engineered to produce starch. 

First of all, the authors had to knock out the genes responsible for the glycogen metabolism pathway (both glycolysis and gluconeogenesis). Then they introduced eight genes known to belong to the amylopectin biosynthetic pathway. The authors succeeded in having starch granules in yeast.

You could start with this yeast strain to produce your bread. You'll probably need to at least dry them in order to reduce them to some sort of flour (probably a hard one, since the yeast would be rich in proteins); otherwise, some means to purify starch grains from yeast should be devised. Then you'd mix them with water and regular baker yeast and maybe you could obtain a bread-like food!