Roommate doesn't wash his hands after using the bathroom

I heard my housemate leave the bathroom but I didn't hear the faucet run after so I went outside and asked if he washed his hands. Housemate said he only washes his hands sometimes or usually after using the bathroom and I told him to do it always now so we don't get sick (which is overdramatic maybe but I said it to make a point and didn't want to say he's gross to be a jerk and all) and he said how will we get sick or something when I said that's the case cuz he doesn't touch stuff and all.

He hasn't done so again I asked in the groupchat and he tried to turn it around on me saying "You're listening to people in on the bathroom" and I told him that's not the case as I am right next door and can't help but overhear. He then claimed he washes after he touches something dirty but I've seen him leave the bathroom numerous times and not wash

Its basic hygiene and super gross as we share common areas so what should I do in this situation as they touch knobs and the fridge door.