The bacterium Propionibacterium acnes (P. acnes) plays an important role in the inflammatory types of acne; however, it lives in the depths of the hair follicle where it is not easily accessible, even by close contact. This bacterium is not typically active except under certain conditions such as a blocking off of the opening of the pore from the surface of the skin, which cuts off the oxygen supply. This makes the chances of spreading the bacterium even less likely because there is less access to the outside world.
By the time you see a pustule, it means that your body has reacted to the infection and the pus is mostly a mixture of a lot of dead bacteria, white blood cells, and skin cells, along with other very complicated but minute factors that create inflammation.
P. acnes can also colonize the nose, where it can then be responsible for local spread of acne to the face and back through contact, such as from touching the nose and then touching the face or back. However, acne is not known to spread from person to person and is not considered contagious.
More Answers from Doris Day, MD