I'm no plumber ... but ... we had an issue with the "silver" round covers for the outlets in our bathrooms - they kept getting very dirty. And we got an occasional smell.
Spoke to a plumber/friend and he suggested we get these new trap thingos - bit like a longish sock with a hole at either end. Apparently they touch - enter - the water at the bottom and stop any bacteria/crap building - thus stop any smell and bacteria getting onto our covers.
I'd been blamed for getting the covers dirty - apparently my sandals were destroying them ...
Forgot to add - you add a few drops of pine'o'clean to them every month or so.
This guy suggested it was the bacteria getting onto the metal. His gadgets prevent this.
Will be interesting to see how it works out.
A cheapish solution if it works.
I'm talking about that drain/trap you have in bathroom/toilet floors.