Every teenager knows the horrors of Propionibacterium acnes: the rod-shaped bacteria lodge deep in skin pores and give rise to whiteheads. What few people realize is that P. acnes are crucial for healthy skin. They secrete enzymes that help break down fats in sebum, the oily substance made in sebaceous glands. Thanks to their efforts, sebum is a natural moisturizer, rather than an oil slick on your face. P. acnesalso increases the acidity of the skin, which keeps far more dangerous bacteria, such as Staphylococcus aureus and Streptococcus pyogenes, at bay.