I think the OP's statement is perfectly fine. We don't really care which of those trillions of sewer bacteria carries the pre-existing genes that resist an ant

I think the OP's statement is perfectly fine. We don't really care which of those trillions of sewer bacteria carries the pre-existing genes that resist an anti-biotic; we care when a new strain starts becoming prevalent and decreasing the effectiveness. From our perspective, it's a new resistant strain.