66 SKIN MICROBIOME


they are enriched in sebum that is composed of triglycerides, amongst other compounds. The triglycerides can be hydrolysed by Cutibacterium acne’s lipases, leading to the consumption of the formed free fatty acids, aiding to the colonisation of the sebaceous glands by these bacteria3 fungi.10


and Malassezia Besides these, gram-negative bacteria,


we also find mites and viruses. Understanding the content of microbiome


of different body regions is the first step to comprehending the imbalance of microbiome due to various skin alterations and consequently selecting the correct approach to help restore skin to its “normal” status.


Main issues associated with microbiome disbalance People with other than normal skin types more often tend to experience microbiome imbalance at some point in their life.3,11


For


example, a higher amount of sebum produced by enlarged sebaceous glands is associated with acne and also related to abnormal levels of C. acnes. Interestingly, people with both higher amounts of sebum and higher presence of C. acnes bacteria also have sensitive scalp.12


This


might be due to alterations of the hydrolipidic balance of the skin in both face and scalp which sends first signals before other, more serious complications, like seborrheic dermatitis.13 Seborrheic dermatitis is another type of skin inflammation linked to excess sebum production and is associated with the presence of Malassezia. The increase in this fungus leads to increased release of lipases that break the triglycerides into free fatty acids. Oleic acid (a free fatty acid) alone or as a byproduct of triglycerides hydrolysis is accompanied by rapid inflammation and excessive renewal of the skin that we observe as dandruff.14 Dry skin, which is a result of impaired skin barrier function, is often associated with atopic dermatitis and an increased presence of Staphylococcus aureus (S. aureus).3,11


Currently,


it is hypothesised that when S. aureus intensely colonises the skin with poor skin barrier, it releases proteases which further damages the skin barrier, and the skin becomes infiltrated by environmental irritants and allergens. Besides that, S. aureus releases toxins that further exacerbate the inflammation.15 The perturbations of the scalp microbiome


were also observed in people with hair growth issues. Androgenetic alopecia is characterised by the shortening of the anagen cycle that results in premature hair fall and progressive miniaturisation of the hair follicle. Studies found an increased content of porphyrins in the follicles. This hints at the involvement of Cutibacterium (especially C. acnes) because it is typical of their metabolism product.14


,16 It is


also hypothesised that UV-activated porphyrins are involved in forming reactive oxygen species that lead to oxidative stress that can contribute to inflammation in the hair follicle.17, 18, 19 Although facial and scalp problems are


highlighted due to their visibility, other body regions suffer also from microbiome imbalance. The armpit is a region associated with malodour, which is a noticeable and extremely


PERSONAL CARE September 2021


uncomfortable problem due to apocrine glands that are present in both the armpits and genital areas. They secrete a mixture of lipids, proteins, and steroids, which are odourless until the degradation by the resident bacteria.12 The most predominant bacteria in the armpit region are Staphylococcus, Corynebacterium, Cutibacterium and Micrococcus., although only the Corynebacterium. is described as responsible for the strong malodour.20 Interestingly, the fluctuation of levels of Corynebacterium also correlates with pH fluctuation in armpits.21


While the pH of these


skins range from 4.5 to 5.5, the armpit region’s average pH is ~6. Although odour is produced in a pH range of 5 to 8, pH 6 is associated with the maximal odour production. The good news is that when the disease


is not too advanced, the microbiome creates its own balancing system. Staphylococcus epidermidis can limit the over colonisation of C. acnes and the inflammatory response associated to its presence. C. acnes can limit the increase of S. aureus and Staphylococcus pyogenes in the skin, maintaining the pilosebaceous follicle with an acidic pH.20 C. acnes can ferment glycerol naturally present on the skin into short-chain fatty acids, which penetrate S. aureus walls, reduce its internal pH and inhibit its growth.21


Could we also help our


microbiome by selecting gentle conventional products and using specific products that are designed to balance the microbiome?


The influence of regular cosmetic ingredients on microbiome Research suggests that skin conditions can be modulated with the application of cosmetic products. Consequently, it seems possible to modulate the skin microbiome with the use of cosmetic products and help in the treatment of skin alterations. Although research is in its infancy, already interesting trends have been observed. The 3D skin maps of microorganisms and specific chemicals present in different skin regions suggest that our microbiome is affected by daily routine and hygiene practices.22, 23 authors correlated the presence of particular


The


microbial species with expected metabolites. The study also found residues of surfactants, sunscreen compounds and some polymeric substances from beauty products. Additionally, it was found that (i) half-life of cosmetic residues on the skin vary between half a week to almost two weeks, and (ii) the modulation of the microbiome by the same ingredients varies depending on body site.22


Although a


deeper relationship between all the residual cosmetic ingredients found on the body and the microbiome was not investigated, this work could serve as a foundation for interrogating spatial relationships between cosmetic ingredients and skin microbiome. Other research also suggests that cosmetic ingredients have an effect on both microbiome diversity and population size.24 The first things that come to mind of a


regular consumer, are the preservatives and their effect on microbiome. In vitro research on isolated skin strains suggests that the most sensitive skin resident species is S. epidermidis.26


It was shown that the following


cosmetic preserving mixtures are the harshest to this microorganism: (i) sodium benzoate and 1,2-hexanediol], C8 [1,2-hexanediol, o-cymen- 5-ol and ppg-3 benzyl ether myristate, (ii) 1,2-hexanediol, caprylyl glycol, tropolone, levulinic acid, sodium levulinate, glycerin. Other studies revealed that wearing makeup


(foundation and powder) is significantly more beneficial for improving skin microbiome diversity than merely keeping the skin clean and well moisturised.27


Another study showed


a positive shift in skin microbiome diversity after prolonged use of moisturising products in both dry and normal skin. However, microbiome diversity of dry skin did not become similar to that of normal skin, despite restoration of the skin hydration level.25 The armpit microbiome diversity might be


modulated using cosmetic treatments as well. It appears that deodorants are able to increase biodiversity in respective locations, which were partially preserved even after product use was discontinued.23 Clearly, within the selection of products


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