Understanding Acne

Part One: How does an acne pimple work?

Understanding how acne pimples form will be key to understanding the best treatments for acne.

Pimples form in something called the pilosebaceous unit, which includes the hair follicle and the sebaceous gland.

The sebaceous glands are filled with sebocytes, cells that make sebum. Sebum is an oily lubricant that helps cells move around and contributes to the protective acid mantle of the skin. These sebocytes are made in the sebaceous gland, and then travel up to the hair follicle, where they release the sebum to lubricate the movement of keratinocytes (skin cells) and growth of the hair follicle.

Keratinocytes make up the internal shaft, forming a roughly cylindrical shape through which the hair grows. Normally, these keratinocytes simply flatten and become hard as they move through their “life cycle.”

When they become flat and hard at the end of their life cycle, they break off and move up the shaft to release onto the skin surface, lubricated by the sebum. However, when the keratinocytes grow too fast, they can “stick together” and not be removed to the skin surface properly. This can create a “plug” in the shaft, which means that sebum and dead keratinocytes will build up in the shaft. This is what causes a whitehead or blackhead pimple.

When all this gunk builds up under the surface, it creates anaerobic conditions – conditions where there is no oxygen. This is really good for some opportunistic bacteria that are found on the skin, called Cutibacterium acnes. They multiply rapidly in the pimple, which causes an immune response – i.e. inflammation.

Acne can affect anyone

All people have a community of bacteria and other microbes inhabiting their skin and gut. These communities are called the “microbiome”, and they are composed of organisms that are commensal – they don’t have any negative effects on us. In fact, many of them have beneficial effects, such as producing antimicrobials that act to kill pathogenic (disease-causing) bacteria. When the microbiome is balanced, many different species of bacteria successfully live on the skin’s surface and contribute to healthy skin.

Of particular interest in acne cases are the three bacterial species Staphylococcus aureus (S. aureus), Cutibacterium acnes (C. acnes), and Staphylococcus epidermis (S. epidermis). Each of these bacteria is a commensal organism that usually has no negative impact on us, but when they grow in excess they can become pathogenic (bad for us).

For example, S. aureus is the species responsible for most impetigo (school sores) infections, while C. acnes is the bacteria found in acne lesions. S. epidermis is a bacteria that is also found on all skin, and it seems to live alongside C. acnes and keep its population in check through producing a particular acid (succinic acid).

In turn, C. acnes and S. epidermis together control S. aureus. There is also some evidence suggesting that particular strains of C. acnes are especially inflammatory, as they are often found in acne patients but not in the skin of unaffected people.

However, there is no evidence that C. acnes infection is the cause of acne; significantly larger populations of the bacteria are not found in acne sufferers – it’s just that an excess population of C. acnes, attributed to the build-up of sebum in an oxygenless environment, stimulates an immune response (inflammation).

C.acnes has specific effects on the immune system. Molecules produced by C. acnes, and the bacteria itself stimulates production of inflammatory molecules by the keratinocytes, which causes a disproportionate immune response and inflammation (redness).

This is bad enough when the effects are confined to the pilosebaceous unit, but when the pressure builds up and the bacteria-filled sebum leaks into the epidermis, it triggers an even harsher immune response, causing pain and redness. This is what causes papules or pustules.

This picture shows the different types of pimple mentioned.

To find out why some people are affected by acne while others aren’t, watch our for our next post: Why are some people more prone to acne?

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