Only 1 bacteria ever arrives. It’s the probability wave that interferes with itself.
With the Young’s double slit experiment, if you fire a single photon, you get a single photon arriving. It looks just like how a cannon ball flies. It’s only when you let hundreds go (either collectively or individually) that the interference pattern appears.

The end pattern is the probability that the photon (or bacteria) arrives at any given point on the receiver screen.












The bacteria don’t need to be identical.
Think of it like rolling a dice. Any given roll can only have a single number. However patterns can be detected by combining multiple rolls. E.g. a biased dice.
As for larger things. It’s possible, but the speed required goes up with mass, and not linearly. In theory a person could go through. They would be moving a significant fraction of the speed of light however. Catching them alive on the other side would be… difficult.