The role of P. acnes in the pathogenesis of acne vulgaris

Acne vulgaris continues to burden every generation, and despite the multitude of the treatments and products on the market, there is still no magic formula that can guarantee a long-term benefit to all acne patients. Antibiotics are most common treatment. However they cannot be given long term. Furthermore, widespread use of antibiotics is thought to be a factor in the rise of antibiotic-resistant bacteria, possibly leading to greater harm than benefit. Vitamin A derivatives (isotretinoins) represent the second widely used treatment option. Oral isotretinoin suppresses sebaceous gland activity and so indirectly reduces inflammatory lesions; nevertheless, isotretinoin is not a curative drug and discontinuation of the treatment is frequently followed by recurrence in the absence of appropriate maintenance treatment.1 Besides, due to potentially serious side effects (birth defects, vision problems, mental health problems, hair loss, vision problems, etc.), isotretinoin is not routinely prescribed to patients. Therefore, acne patients would welcome an alternative, safer, and long-term effective treatment. Devising an alternative prophylactic and curative treatment may soon be possible, thanks to the recent research findings that have uncovered the pathogenic role of the long-suspected culprit, a bacterium named Propionibacterium acnes (P. acnes).

Acne vulgaris affects >85% of teenagers and >10% of adults and has recently been re-defined as a complex chronic disease associated with P. acnes, which takes advantage of immune susceptible host.2–5 In the past, the pathogenic role of P. acnes has been debated due to the fact that this bacterium is found not only on the skin of acne patients, but also on the skin of healthy individuals. The theory that P. acnes does not cause the acne per se, but rather contributes to inflammation once the acne lesion has been formed, has been challenged by the newest epidemiological studies using highly sensitive, improved methods for genetic profiling.

These and other studies have uncovered important new facts about acne formation:

  • Significant differences between the P. acnes strains isolated from the patients with severe acne compared to healthy controls;4,6
  • Striking functional differences among strains from different phylogenetic clusters.7,8
  • P. acnes secretion of a glycocalyx biofilm that acts as an “adhesive glue” for the corneocytes, thereby inducing comedonal acne.9,10
  • Bacterial biofilms acting as a protective barrier to antibactericidal agents used in acne therapy.11,12

P. acnes resides on the surface of our skin and in the most individuals it poses no problem. In fact, it is even considered the “gate keeper”, protecting our skin from colonization by pathogens that can cause more severe infections. However, not all P. acnes strains are harmless skin protectors. Each of us carries a distinct population of P. acnes strains, and if we happen to be colonized by a more virulent type, the strain may take the advantage of the skin’s unique environment, and in the absence of effective immune defenses, spread to the deeper skin layers (dermis) where it will induce inflammation.

What factors facilitate the P. acnes invasion of deeper skin layers?
P. acnes colonizes and grows optimally in the presence of lipids and in the absence of oxygen. Sebum secretion is intensified during puberty and other hormonally active periods (e.g. premenstrual period or pregnancy). Increased sebum production provides an ideal environment for P. acnes and attracts the bacteria into the hair follicles, which are located deeper within the skin (Figure 1). Additional genetic factors may be also at play. However, regardless of the signal that attracts them, our immune system must be able to recognize the bacteria and rapidly eliminate them, in order to prevent the formation of acne lesions.

If the immune system is not effective enough, it can be trained to respond more efficiently through vaccination.

A vaccine against P. acnes could be designed to induce a stronger immune response against specifically targeted virulent P. acnes strains. Such a vaccine could provide the benefits that are not available from other types of therapies including the following:

  • Rapid and effective cure
  • Long-term protective effect
  • Preservation of normal bacterial skin flora
  • Does not contribute to antibiotic resistance of bacteria
  • Avoids unwanted side-effects, since the vaccine would induce the immune response against foreign, and not host cells
  • Prophylactic application would be also possible, before acne scars are formed
The role of P. acnes in the pathogenesis of acne vulgaris
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Figure 1. The role of P. acnes in acne pathogenesis.

  1. Sebum secretion and clogged pores limit access to oxygen.
  2. High lipid content and low oxygen concentration creates optimal growth environment for P. acnes.
  3. P. acnes residing on the skin surface is attracted to lipid-rich, optimal environment inside the hair follicles. Here, the bacteria rapidly multiply inducing a local inflammatory response. If the immune system is not able to efficiently kill and remove the bacteria, the inflammatory reaction persists leading to the creation of cysts and pustules, ultimately leading to the creation of scars.


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