Not if you engineer your mirror bacterium to eat normal nutrients to get them started. Or they scavenge enough non-chiral molecules to eat that they get the opportunity to evolve the capability on their own.
building from non chiral molecules is prototrophy, not scavenging. anyways, existing lifeform already scavenge mirror molecules (especially d-ala since thats used in cell walls) so they are already competing in a quite saturated environment with highly adapted organisms.