Makeup brushes can be a breeding ground for bacteria and fungus, so it’s best to wash your makeup brushes at least once a week. I prefer to clean my brush after each use.
Every time you use one on your face, it picks up dirt, bacteria, oils, and dead skin cells (not to mention spit and tears). After a while, dirty brushes get stiff, the bristles separate, and your makeup streaks when you apply it. This is not ideal versus using a clean brush, which is softer on the skin and can distribute makeup more evenly.
Clean brushes will give your make up a smoother application. To clean my brushes I use Pink Soap (you can purchase at Michaels craft store for $7.99) along with olive oil to pull the make-up from your brush, a foaming cleanser like Olay Foaming face wash (helps dissolve make-up and oils). DO NOT use bar soap, laundry detergent , or especially dishwashing liquid on you brushes, as this can weaken your bristles and leave residue that can break you out.
Side note: when the bristles start falling out, it’s time to replace that brush.