If there is space, put a layer on top of the sponges. Filter wool should be last in the direction of water flow through the filter to trap the tiny particles which go through sponges. it'll probably need changing frequently as it gets clogged and does not wash very well - it goes into holes. Because there's only a layer and it's changed regularly, there won't be time to grow many bacteria, and it's small part of the filter media so even if it was full of bacteria the numbers would only be a small fraction of the total number.
Edited to add the very important word 'not' in "does not wash well"