Acne-causing bacteria, Cutibacterium acnes (C.acnes) is part of a healthy skin microbiome. Most people have it on their skin, and in majority of the time, it doesn’t cause any trouble.
The C. acnes bacteria can produce active enzymes and compounds that are recognized as “inflammation signals” by our skin - and this is how we get inflammed lesions.
While C. acnes plays a role in acne, it is not an infectious disease (hence you can’t contract it from someone with acne). Growth of the C. acnes colony on skin is a necessary precondition for developing acne, but much more factors need to come together for these skin inhabitants to cause the problem.
The main factors that trigger C. acnes to cause trouble are:
Excessive sebum production stimulated by androgen hormones;
Clogged pores (can be caused by slow skin cell turnover);
High skin’s responsiveness to inflammatory stimuli (this is why harsh irritating ingredients are a big no-no in acne - they can make skin more prone to inflammation).
An interesting fact about C. acnes: it is an anaerobic bacteria, meaning that it thrives in an environment without oxygen. And this is why it loves clogged pores. It means that it’s unlikely to transfer C. acnes from dirty make up brushes on your skin - it just doesn’t survive in presence of too much oxygen. (Still, clean your makeup brushes, there are a lot of other bacteria that can develop in those that you don’t want to have on your skin!).