Meh, they came from the soil. It's always been here, just never seen by human eyes. That's true of lots and lots of bacteria though - we find new species pretty much every single time we take a stomach sample from someone, let alone random forest soil.

Many bacteria have commensal lifestyles —- scientists don’t feel in control if they can’t culture bacteria in isolation but in nature many bacteria aren’t metabolically complete and son’s live in isolation.