The problem with the dirt on mushies people have is that they are grown in manure. (Feed on s*** and kept in the dark lol)
So many people don't like it, they even peel the mushrooms!
Though honestly, just brushing it off or using a dry towel will get rid of the dirt, and microscopic bits can actually be good for you too, and personally, I don't think there is much of a risk of bacteria hurting you form the mushroom dirt.
As for the precut ones, I think the tools they are using and the hygeine of those preparing them is the big concern, but Coles/Safeway would be in a world of trouble if thye were found to have issues with that in their prep kitchens - and isn't worth it - they even have auto meat slicers in the delis now so the staff never have to touch the meat to prevent issues like this.
There is a time when you can over think things I think - I wouldn't worry at all.
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