Skin is a natural barrier for the body, and it functions as a connection between the body and the environment. The microecosystem plays an important role in protecting the health of the skin. 1 Probiotics are "live microorganisms, which, when administered in adequate amounts, confer a health benefit to the host." 2 Probiotics supplement nutrition, suppress diarrhoea, prevent intestinal inflammation and exhibit anticolorectal cancer activities. [3][4][5][6] Recent intriguing studies reported that probiotics also exert skin health-promoting effects. Oral probiotics may be effective in treating topical skin conditions, such as psoriasis, acne vulgaris and atopic dermatitis. 7,8 Atopic dermatitis (AD) is a genetically predisposed, recurring allergic skin disease that is characterized by the destruction of the epidermal barrier. 9 Oxidative stress is also one cause of AD 10 . Therefore, strengthening the skin barrier by reducing the level of oxidative stress may reduce the incidence and severity of AD. Notably, topical probiotic formulations to alleviate skin conditions have become a relatively new field of study. Topically applied probiotics in patients with AD reduced skin erythema, scaling and pruritus and improved skin barriers. 11 The Bifidobacteria species is a probiotic and the dominant bacterial species in the human intestinal tract. 12 Oral Bifidobacterium longum (B. longum) preparation effectively reduced the SCORAD

Sensitive skin (SS) is a common cutaneous condition that seriously affects people’s quality of life, but studies of sensitive skin pathogenesis are unclear, the exploration are ongoing, and the biophysical properties of sensitive skin disagree with the study results. In this paper, we summarize the noninvasive biophysical and imaging instrumental methods used for sensitive skin and provide support for the classification of sensitive skin subtypes to prescribe precise treatment. PubMed and Web of Science databases were searched according to PRISMA guidelines for articles from January 1971 to May 2022 that used noninvasive biophysical or imaging methods to monitor adult subjects with sensitive skin. The quality of the included articles was determined based on 22 items of the STrengthening the Reporting of OBservational studies in Epidemiology (STROBE) statement. A total of 55 studies were included, representing 8 biophysical and 5 imaging methods and their applications in treatment efficacy evaluation studies. The biophysical parameter and cutaneous morphological property changes in sensitive skin subjects were observed. The quality of the studies was relatively low, and there was high variability in results between studies. Several parameters have shown tremendous potential in exploring the pathogenesis with different sensitive skin subtypes: type I may be detected with higher transepidermal water loss and lower stratum corneum hydration values, as well as with thinner epidermis with a shallower and more irregular honeycomb structure; Type II and III are more prone to higher blood flow, lower current perception threshold than normal skin. This systematic review identifies key reasons for the lack of uniform trends in noninvasive measurements and recommends the use of effective selection instruments or relevant parameters to explore the pathogenesis of sensitive skin, and to differentiate the subtypes of sensitive skin for achieving the precise treatment.