Mirror image microorganisms process and produce molecules of opposite normal chirality as a result of their metabolism. Since there's no a priori reason why life should prefer either left handed or right handed molecules, the way we got here is the result of the first proto-metabolic processes billions of years ago just happening to choose what we use today. If we introduced mirror image microorganisms into the ecosystem, the danger is they could outcompete existing organisms while simultaneously contaminating the environment with their mirror-image waste products.