Warm wet and dark, I know mushrooms love to grow in that environment, in fact, I think there's lots of stuff that likes to grow in that situation.
The bacteria would need a consistent source of food, however, so unless they are getting fresh water in decent amounts on a regular basis, they may not be able to survive. What's more, if this was the case, would it really matter? How would the bacteria get back out to get you?
My concern would be a tub that continues to rust and rust out unseen under the fiberglass. Then we have a whole new set of issues to deal with.
Personally, I'd replace the tub and start over. If the tub is that old, chances are the whole bathroom is ready for rebirth...