If you can figure out a way to stop acne at the beginning of the chain of events (e.g. hormones...lipid profile...), then a little extra bacteria on your face from old makeup won't be much of an issue. I think about more primitive cultures that rarely bathe...or use antibacterial soap...and are probably exposed to more bacteria and fungi that we can dream of. If things internally are working out, then some bacteria in your makeup isn't going to make a bit of difference.
So to answer your question, when I still had acne, I was very very conscientious of any exposure to bacteria. Threw out makeup. Had a clean pillowcase every single night. Never touched my face. Carried around hand sanitizer. etc.
I have been using up two bottles of MAC liquid foundations that are approaching four years old. Yes, I know it's gross. Whatever. It's not giving me any skin problems and the foundation hasn't oxidized so the pigment is the same. I'm not throwing out $80 of foundation when it's functionally the same as when I bought it.
Hope I grossed you out enough
There's probably more bacteria on my keyboard than in that foundation anyhow.
Edited by Green Gables, 06 November 2013 - 08:57 PM.