You might expect that microbes are all bad - but in fact they're not; amazingly, there are more 'good' types of microorganisms than 'bad' ones ('bad' ones are microbes that harm us).
One way in which different sorts of microbes are amazingly helpful is in the decay and decomposition (rotting) of things that have died, like a branch from a tree.
Basically, these microbes are nature's RECYCLERS, turning what was simply the remains of a living organism back into substances that can be used again and again. They're brilliant!