Does a buildup of bacteria cause acne?

A buildup of bacteria can worsen acne by changing a whitehead or a blackhead into a pus-filled pimple. Acne occurs when oil produced by microscopic glands in the skin mixes with dead skin cells to clog pores, trapping bacteria inside. As bacteria multiply and the body's immune system fights back, pores can become red, swollen, and filled with pus. The result may be a few stray pimples or a widespread breakout.