Acne (especially inflammatory acne) is usually the result of bacterial growth deep within the hair follicle. These bacteria produce molecules which incite an immune response, leading to inflammation and acne symptoms. The more bacterial growth there is, the more likely an individual will develop acne symptoms.
The bacteria primarily responsible for acne is Propionibacterium acnes (P. acnes), but other bacteria such as Staphylococcus aureus can also live on the skin and in hair follicles. Antibiotics are commonly used to control the growth of bacteria and can greatly improve symptoms for many acne sufferers.
There are many different strains of P. acnes bacteria, and many of these strains have developed resistance to one or many different antibiotics. As a result, some antibiotics, including erythromycin and tetracycline, are becoming less effective, because many people who suffer from acne carry strains of bacteria that are resistant. Fortunately, there are still many antibiotics available that do not have this shortcoming.