Acne, microbial basis of

Acne is a condition that affects the hair follicles. A hair follicle consists of a pore the opens to the surface of the skin. The pore leads inward to a cavity that is connected to oil glands. The glands, which are called sebaceous glands, produce oil (sebum) that lubricates the skin and the hair that grows out of the cavity. As the hair grows the oil leaves the cavity and spreads out over the surface of the skin, were it forms a protective coating. However, in conditions such as acne, the oil becomes trapped in the cavities of the hair follicles. This accumulation of oil is irritating and so causes an inflammation . One consequence of the inflammation is an unsightly, scabby appearing crust on the surface of the skin over the inflamed follicles. This surface condition is acne.

Acne is associated with the maturation of young adults, particularly boys. Part of the maturation process involves the production or altered expression of hormones. In adolescence certain hormones called androgens are produced. Androgens stimulate the enlargement of the sebaceous glands and the resulting production of more oil, to facilitate the manufacture of more facial hair. In girls, androgen production is greater around the time of menstruation. Acne often appears in young women at the time of their monthly menstrual period.

In this altered hormonal environment, bacteria play a role in the development of acne. The principal bacterial species associated with acne is Proprionibacterium acnes. This microorganism is a normal resident on the skin and inside hair follicles. Normally, the outward flow of oil will wash the bacteria to the surface and be removed when the face is washed. However, in the androgen-altered hair follicles, the cells lining the cavity shed more frequently, stick together, mix with the excess oil that is being produced, and pile up in clumps inside the cavity. The accumulated material is a ready nutrient source for the Proprionibacterium acnes in the cavity. The bacteria grow and multiply rapidly.

Two other bacterial species that live and grow on the surface of the skin can be associated with acne. These are Proprionibacterium granulosum and Staphylocccus epidermidis. Their significance is less than Proprionibacterium acnes, however.

As the numbers of bacteria increase, the by-products of their metabolic activities cause even more inflammation. Also, the bacteria contain enzymes that can degrade the oil from the oil glands into what are known as free fatty acids. These free fatty acids are very irritating to the skin. Various other bacterial enzymes contribute to inflammation, including proteases and phosphatases.

The immune system does react to the abnormal growth of the bacteria by trying to clear the bacteria. Death of bacteria combined with the immune response generates the material known as pus. A hallmark of acne is often the pus that is exuded from the crusty sores on the skin.

The altered environment within the hair follicle that facilitates the explosive growth of Proprionibacterium acnes can be stimulated by factors other than the altered hormone production of puberty. The external environment, particularly a warm and moist one, is one factor.

The damage caused by bacteria in acne ranges from mild to severe. In a mild case of acne, only a so-called blackheads or whiteheads are evident on the skin. More severe cases are associated with more blackheads, whiteheads and pimples, and also with inflammation. The most severe form, called cystic acne, may produce marked inflammation over the entire upper body, and requires a physician's attention to reduce the bacterial populations.

Reduction in the bacterial number involves slowing down the secretion of the oil from the oil glands and making the follicle pore more open, so that the normal outward flow can occur. Oil production can be slowed in the presence of 12-cis-retinoic acid (Accutane). Use of this medication is reserved for severe cases of acne, as the retinoic acid can have significant adverse side effects. Antibacterial agents can also be useful. For example, many antibacterial creams and face washes contain the compound called benzoyl peroxide, which is very active against Proprionibacterium acnes.

Because the bacteria active in acne are normal residents of the skin, there is no "cure" for acne. Rather, the condition is lessened until biochemical or lifestyle changes in the individual lessen or eliminate the conditions that promote bacterial overgrowth.

See also Microbial flora of the skin; Skin infections