Are the bristles biodegradable?
No, they are nylon, as on almost every bamboo brush. Remove them with pliers before composting the handle.
How do I stop the handle going mouldy?
Let it dry between uses, bristles-up in an open holder rather than in a closed pot.
How often should I replace it?
Every three months, or sooner if the bristles splay. That applies to any toothbrush.
Why has the handle darkened?
That is the bamboo ageing with use and water. It is not a fault.