Almost every organic molecule has a mirrored counterpart, like a normal screw and a left-handed screw.

Almost none of them occur in the nature.

So we have the technology to synthesize them now, and synthesize a bacteria out of them.

But if you do that, and the bacteria escapes, all your existing medicine will be useless, so you need to re-synthesize all your antibiotics in left-hand configuration.

That typically does not happen with regular bacteria experiments, because most of what you can synthesize in the lab will be a descendant of some other well-known bacteria, which already have an appropriate medicine to treat it, and in most cases it will be effective against your new strain.