Monday, June 18, 2018

Propionibacterium - the importance of diversity

Cutibacterium acnes (Propionibacterium acnes) and acne vulgaris: a brief look at the latest updates

Among the multiple commensal microorganisms present in the healthy skin flora, Propionibacterium is a ubiquitous gram positive anaerobic bacterium belonging to the Actinobacteria phylum, that predominantly resides deep within the sebaceous follicle in contact with keratinocytes. Conversely, at the skin surface Propionibacteria are less represented (<2% of all bacteria), in favour of Staphylococci, especially Staphylococcus epidermidis (S. epidermidis), which dominate with >27% of the total bacteria population. Specific metabolic features allow Propionibacterium to colonise the lipid-rich sebaceous follicle environment and protect skin from other harmful pathogens to preserve the stability of resident skin microbiota. In particular, it can degrade triglycerides present in sebum to generate short-chain fatty acids, including propionic acid, which accumulation participates in the maintenance of an acid skin pH.
However this organism can also act as an opportunistic pathogen.The latest findings on Propionibacterium shed light on the critical role of a tight equilibrium between its phylogenetic groups.  

While Propionibacterium is present on the skin surface at a low level, it is the dominant resident bacterial species in the sebaceous follicles. Contrary to what was previously thought, acne vulgaris is not the result of a greater proliferation of all Propionibacterium strains, as patients with acne do not harbour more Propionibacterium in follicles than normal individuals. Instead, acne might be triggered by the selection of a subset of Propionibacterium strains, including the acne-associated phylotype IA1, probably enhanced by a hyperseborrheic environment. 

In addition, biofilm formation and differences in virulence and inflammatory potential of Propionibacterium strains might enhance their pathogenicity. 

It now appears that in healthy skin, an equilibrium state exists within the skin microbiota and between the different Propionibacterium subtypes. Furthermore,  S. epidermidis and Propionibacterium interact together and are critical in the regulation of skin homeostasis. S. epidermidis is known to inhibit Propionibacterium growth.

Changes in physiological conditions may lead to an imbalance between the different skin community members, called  dysbiosis, and eventually to the selection of more pathogenic Propionibacterium strains. Disruption of the equilibrium within the skin microbiota and intrinsic properties of Propionibacterium might therefore be conducive to the activation of innate immunity, resulting in  inflammation.

This review underscores the importance of Propionibacterium phylotype IA1 in acne and suggests the implication of other members of the human cutaneous microbiome in this skin condition.

Improved understanding of the genetic and phenotypic diversity of Propionibacterium strains as well as the involvement of other bacterial species could be applied in the development of alternative and personalised therapies addressing the pathogenic strains only and leaving the commensal strains intact.

Comment: These findings emphasize the importance of strain diversity in maintaining healthy skin and may provide insight into why some shoulders are more at risk for Propionibacterium-related arthroplasty failure than others.
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