4 · Atopic dermatitis
Atopic dermatitis, sometimes called “atopic eczema”, is a chronic, allergic, inflammatory disease of the skin. Its prevalence is increasing and it is the most common skin disease. Although it affects 15 to 20% of babies and usually disappears before the age of four years, it can persist to adolescence, even to adult age in one out of ten cases. The skin is very dry and sensitive, with red plaques, lesions, itching: these are the main symptoms of this disease which manifests in flares. Available treatments aim to eliminate the symptoms and to improve quality of life for those affected. Its cause has not been fully identified but there is a significant genetic component which incorporates other allergies such as asthma and allergic rhinitis. As with asthma and food allergies, excessive hygiene –and therefore reduced exposure of the organism to bacteria–could contribute to an overreaction by the immune system. The first element to be affected is the cutaneous microbiota, whose diversity is impoverished by the disease and whose balance must be restored to optimize treatment.
Emollients and atopic dermatitis: proven benefits
Emollients (substances that promote relaxation and softening of the skin) are already one of the essential treatments for atopic dermatitis. Employed to restore the cutaneous barrier of the skin, they are also used as a preventive measure in infants at risk. It is not known precisely how they act on the cutaneous microbiota and it needs to be clarified to improve the therapeutic solutions.
Atopic dermatitis is a chronic, allergic, inflammatory skin disease which manifests mainly during the first six months of life32. Not all newborns are equally susceptible: babies whose parents themselves suffer from atopic dermatitis or an allergic disease such as asthma or allergic rhinitis have in fact a two–even three–times greater risk Emollients and atopic dermatitis: proven benefits of developing the disease than babies with no family history33.
Staphylococcus aureus: an identified enemy
The skin is permanently colonized by the microorganisms which form the cutaneous microbiota. But in atopic dermatitis, its barrier function is impaired and no longer provides the same level of protection against external attacks. This condition weakens the skin, makes it more vulnerable and subject to colonization by undesirable microbes34. In this case, Staphylococcus aureus plays a crucial role: analyses have shown that, on the skin of infants suffering from atopic dermatitis with lesions, the bacterium was present in more than 90% of cases This figure falls to 78% for dermatitis without lesions, but drops to 10% if the skin is healthy35. The density of Staphylococcus aureus is therefore thought to be correlated with disease severity. Since this is not however systematic, the researchers consider that the disease displays heterogeneity between individuals. Other methods of analysis have enabled better characterization of the bacterial communities of skin affected by this disease: of course the proportion of Staphylococcus aureus increases during the flare phases, but significant changes have been observed in other bacteria (classic or more rare) during and after treatment36.
The role of emollients better understood
Effective and widely approved in the treatment of atopic dermatitis in infants, emollient creams rehydrate and repair the damaged skin by regenerating injured tissue, and reduce disease severity37. Their regular application over the first six months of life to the skin of babies with a high probability of developing the disease might prevent its manifestations, although results diverge from one individual to the next38. Some researchers have studied these emollients, in order to better understand their preventive action on the cutaneous flora. They compared different skin parameters, monitored the development of atopic dermatitis and genetically analyzed skin samples taken from different places on the body, depending on whether the infants (all at risk) had received emollient treatment or not39.
Result: the skin of infants treated beforehand had a lower pH than that of the control group (healthy skin has a slightly acidic pH, which helps it to function correctly, while that of skin with atopic dermatitis is higher40, favouring colonization by Staphylococcus aureus). Modulation of pH might then be a means of rebalancing the cutaneous microbiota. Generally, the Streptococcus genus also clearly contributed to the differences observed in the samples from the two groups. The skin of the infants who received treatment contained a richer and more diverse bacterial population, which resembled a restoration of the balance of the cutaneous microbiota. In particular, Streptococcus salivarius bacteria were present in greater numbers. The latter colonize the organism (especially the mouth) from the first moments of life and seem to have a protective role: they are present in higher levels in infants who do not have atopic dermatitis than in those who have the condition. This result is consistent with those of other studies conducted in older children which showed that the higher the proportion of Streptococcus salivarius, the milder the atopic dermatitis.
These data confirm the benefits of the long-term use of emollients as a preventive measure in infants at high risk. They provide new information on the way in which emollients act, although the mechanisms involved and the impact of localized changes in the cutaneous microbiota on the development of the disease in these infants remain to be identified.
32 Bieber T. Atopic dermatitis. N Engl J Med. 2008; 358(14):1483±94
33 Bohme M, Wickman M, Lennart Nordvall S, Svartengren M, Wahlgren CF. Family history and risk of atopic dermatitis in children up to 4 years. Clin Exp Allergy. 2003; 33(9):1226±31
34 Ong PY, Leung DY. The infectious aspects of atopic dermatitis. Immunol Allergy Clin North Am. 2010; 30(3):309±21
35 Matsui K, Nishikawa A, Suto H, Tsuboi R, Ogawa H. Comparative study of Staphylococcus aureus isolated from lesional and non-lesional skin of atopic dermatitis patients. Microbiol Immunol. 2000; 44 (11):945±7
36 Kong HH, Oh J, Deming C, Conlan S, Grice EA, Beatson MA, et al. Temporal shifts in the skin microbiome associated with disease flares and treatment in children with atopic dermatitis. Genome research. 2012; 22(5):850±9
37 Evangelista MT, Abad-Casintahan F, Lopez-Villafuerte L. The effect of topical virgin coconut oil on SCORAD index, transepidermal water loss, and skin capacitance in mild to moderate pediatric atopic dermatitis: a randomized, double-blind, clinical trial. Int J Dermatol. 2014; 53(1):100±8
38 Horimukai K, Morita K, Narita M, Kondo M, Kitazawa H, Nozaki M, et al. Application of moisturizer toneonates prevents development of atopic dermatitis. The Journal of allergy and clinical immunology.2014; 134(4):824±30 e6
39 Glatz M, Jo J-H, Kennedy EA, Polley EC, Segre JA, Simpson EL, et al. (2018) Emollient use alters skin barrier and microbes in infants at risk for developing atopic dermatitis. PLoS ONE 13(2): e0192443
40 Rippke F, Schreiner V, Doering T, Maibach HI. Stratum corneum pH in atopic dermatitis: impact on skin barrier function and colonization with Staphylococcus Aureus. Am J Clin Dermatol. 2004; 5(4):217±23