Jiang Chunpeng

Suzhou Greenleaf Daily Commodity Co., Ltd., China

Abstract This paper discusses the researches on human skin microbial flora in the past 50 years, including research methods, characteristics of human skin microbial flora and key influencing factors, etc. The dynamic relationship between microbial flora and skin health is further discussed. Finally, the potential application of the research in the future is put forward. This paper also points out that the microbial flora may become the inevitable and necessary future trend of daily chemical products, maximizing the balance between human skin health and beauty.

Key words human microbial flora; micro flora diversity; gene detection; skin health

As the largest organ of the human body, the skin plays an irreplaceable barrier function to the external environment and resists the invasion of foreign substances. At the same time, it also protects the body from loss of water, heat and so on. An adult has an area of about 1.8 m2of skin, where there are a large number of microorganisms[1,2]bacteria, fungi,and viruses up to 107/cm2. These microbial floras are closely related to human skin and human health.Early research suggested that the number of bacteria was about ten times that of human cells.[3]According to the calculation of recent studies, the ratio of bacteria to human cells is approximately 3:1 to 1:1.[4]

Research progress on the skin microbial flora

Researchers have done a lot of researches on skin microbes over the last 50 years. The traditional method is to isolate and identify skin epidermis bacteria by using laboratory standard cultural method. Because some bacteria grow slowly in traditional nutrient solution while others (such as staphylococcus) may multiply quickly, the final count is difficult to re flect the real distribution of skin bacterial flora. What is important is that, like natural ecological environment, some microbes which are in a small proportion in fluence the diversity of skin bacterial flora and are closely associated with skin health.

Modern human microbiology research focuses on gene sequencing analysis, using small subunits(16s) shared by prokaryotic ribosomal RNA for PCR identification. The establishment of a ribosomal database can look up the identity of the microbe, like looking up in a dictionary. However, 16S rRNA is still costly to sequence and analyze rare microorganisms,and its efficiency is too low to quickly map out real biodiversity.[5-8]As a second-generation sequencing technique, pyrosequencing conducts medium and short-chain high-throughput gene sequencing and analysis by using reactions of nucleotide releasing pyrophosphate. Macrogene sequencing directly measures all sample genes in the sample group, so more microbial species can be found. The decrease in sequencing prices has made research more in-depth into microcosmic. Sequencing technology requires a sufficient amount of biological samples, while the sample size of skin bacterial flora has limited application of macrogene technology. In addition,16S rRNA technology cannot determine whether a microorganism is alive or in an inanimate state. The combination of traditional biological culture methods and modern various sequencing technologies has enabled the research of skin microbial flora to continue to expand, and promote the understanding of skin health and the dynamic relationship of microbial flora.

Based on the abundance, quantity and distribution characteristics of skin flora in healthy people, it brings different perspectives and ideas to the research and development of skin care products. For example,for oily facial skin with strong sebum secretion, the number of bacteria is large, while the variety is small.The flora of the skin can be controlled by quantity to prevent the mass propagation of Propionibacterium.Further researches have shown that at the same species level, the distribution of different Propionibacterium bacterial strains on normal skin and acne skin is different.[6]How to selectively control the propagation of Propionibacterium strains that are prone to acne is a very big challenge in practice. For dry areas, there is small number of bacteria whose abundance is high.These skin parts are similar to the fragile plateau ecosystem and are highly susceptible to damage. Skin care should focus on maintaining bacterial abundance and protecting the environment in which the native flora grows. For skin care of moist skin area, different solutions are provided according to different areas and different needs. Taking the hand, for example, the palm of the hand is a moist area, where there is no sebaceous gland and the sweat glands are strong. The number and abundance of the flora are high, and the contact between it and the external environment is also the highest. Appropriate control of the number of elemental flora can enhance competition for transient pathogenic bacteria and reduce the resident and growth of foreign bacteria.[7]

Also for the axilla, there are both strong sweat glands and many oil glands. Bacterial species include hygrophilous Corynebacterium and Staphylococcus,and oil-loving Propionibacterium. Mutual competition between microbial floras can affect the generation of body odor. Controlling the overall number of microorganisms, targeted control of the growth of Corynebacterium, and even the selective control of certain species of Staphylococcus genus level(S. hominis), is the most ideal technical solution to eliminate body odor.[8]

A recent metagenomic research founds that human skin microbial flora is fairly stable. Researchers conducted a metagenomic shotgun sequencing on 17 skin areas of 12 healthy individuals at three different time points (from one month to two years).Research has confirmed that bacteria, fungi, and virus communities exhibit strong habitat preferences at specific skin areas and function as microbial fingerprints. Despite frequent external disturbances,such as frequent contact with other people, clothing and the environment, the skin micro flora remains stable over time. The most stable bacterial and fungal communities appeared in the oilskin areas with the lowest diversity. The dry areas with the most diversity also exhibit relative stability. Relatively,the structure of micro flora in the moist areas shows the most instability. It is important that each person does not obtain the prevalent microbes from the environment, but instead, maintains their unique microbial characteristics.[9]

These important findings are critical to clearing the relationship between skin health and microbial flora structure. Researches have shown that healthy people's skin itself has the ability to maintain the microbial structure of the skin and can be an indicator of skin health. The above research can basically answer the first question of the microbial flora of healthy human skin, which is about the types of human skin flora and quantity varying with the body areas, gender, time, ethnicity and other factors.Investigations into various skin problems and diseases have helped to understand the dynamic relationship between skin micro flora diversity, the physiological characteristics of the skin and skin diseases.

Skin microbial flora biodiversity and key influencing factors

Diversity of skin microbial flora in different parts of the human body

The National Institutes of Health (NIH) launched a large-scale Human Microbiome Project (HMP)in 2008. In-depth basic research and clinical practice have been carried out on the molecular level of skin microbes. Global scientists are also constantly involved in this field. More and more dermatologists, microbiologists and personal care product professionals are beginning to regard the skin as a complete ecosystem. This ecosystem continues to evolve with an interplaying biophysical and chemical environment that lives on survival. The environment and biosystems are interrelated and interact to form a dynamic ecological balance. This new macrointeraction mechanism provides new ideas and possible pathways for medicine, cosmetics, food,health care products, etc. to improve skin health.For personal care products, including skin care,cleaning, and oral care products, it means getting rid of the original antibacterial limitation, balancing the ecological balance factors, thus effectively avoiding the negative effects caused by long-term use.

In fact, until now, many researchers still have doubts about the mechanism of action of the skin symbiotic flora. How does the symbiotic flora work directly to the skin, or how to prevent other harmful microbes from remaining on the skin? Before answering these questions, three issues need to be clarified: What kind of microbes are on the surface of the skin; how microbial diversity affects skin health and its relationship to disease states; how the various treatments and care of the skin affect the diversity of human microbes.[6]Human skin microbes can be simply divided into resident flora and transient bacteria.[10]The characteristics of human skin parts are different, so is the human microbial flora. Generally,human skin can be divided into an oily area, a moist area, and a dry area. The typical oily areas include the forehead, cheeks, nose wings, chest, back, and other parts; the moist area includes the armpit, navel, palm,foot, nostril, etc.; the dry area includes arms, legs,buttocks and etc. Generally, the oily skin areas of skin have the lowest diversity while the dry areas have the highest diversity.[11]

The dominant bacterium in the oily areas is Propionibacterium. Staphylococcus and Corynebacterium dominate the moist areas. According to the data measured by the cultural method, the number of bacteria in the dry areas, such as the arms and hips, is less than that of the moist skin parts.[12]Surprisingly,the diversity of the flora in the dry skin is highest,even higher than that of the intestinal flora. In addition, resident Gram-negative bacteria were also found in these areas. In the past, this was generally considered to be caused by contamination of the intestinal flora.[13]

There are various factors affecting the ecological structure of skin microbial flora, both environmental and intrinsic. Different skin areas of the human body actually determine different ecological environments,such as humidity, pH, oil, and skin roughness.Through lateral comparison of the microbial flora structure (β-diversity) of different people, it can be found that the physiological characteristics of the skin are more influential than other factors, such as age, gender and time.[14]For a single individual(α-diversity), physiological changes caused by changes in age, such as oil secretion during puberty,hormone levels during physiological periods, and differences in skin sweat glands/sebum glands between men and women, can cause differences in human skin microbial flora.[15.16]

Key influencing factors

H. Lambers et al. studied the pH of human skin and indicates that the pH of the skin should be below 5. Bath products can cause significant changes in skin pH. For example, after bathing with commercially available soap for 6 hours, the pH is 5.6, which is significantly higher than the initial value and higher than that treated with water or a pH neutral shower gel. Corresponding in vitro experiments revealed that the human original bacteria, Staphylococcus epidermidis, grow robustly under weak acidic conditions, and the growth of foreign pathogenic bacteria Staphylococcus aureus was slow. After the pH became neutral, the growth state of both was reversed, and Staphylococcus aureus became a strong flora. This study provides a unique and relatively easy solution to care products to better maintain skin health.[17]In fact, in 1990, H. Korting pointed out the relationship between skin pH and skin microbial growth in a crossover experiment. The forwardlooking research indicates that even small changes in pH may cause significant disturbances in the skin flora.[18]

The dynamic relationship between skin microbial flora and skin health

So far, there is no sufficient evidence showing a causal relationship between microbes and problem skin, and there is no direct evidence that skin problems are directly caused by microbes.The relationship between them is similar to the relationship of “egg” and “chicken”. Considering the complex physiological environment of the skin, this is more likely to be a two-way feedback and interactive symbiosis process: Changes in the physiological state of the skin are similar to changes in the ecological environment, inducing changes in the species and/or quantity of the microbial flora; Changes in microbial flora, vice versa, affect changes in the skin's microecological environment. The barrier function of the skin such as transepidermal water loss rate, stratum corneum structure, and sebum secretion are all changed with it.

Interactive symbiosis is considered possible in theory and practical observation in research. The sweat secreted by the small sweat glands (eccrine)on the skin surface provides moisture, salt and antimicrobial peptides (AMP), which provide moisture and essential mineral elements for microbial growth.The sebaceous gland secretion is rich in oil and fat, which forms a free fatty acid after degradation and provides nutrition for the microorganism. The sweat gland contains lactic acid and the fatty acid formed by decomposition of the oil component,which constitutes a weak acidic microenvironment of the skin. These epidermal structures and local physiological characteristics directly control the number and type of resident bacteria that affect the skin.[19]At the same time, certain resident bacteria,such as Staphylococcus epidermidis, can promote the synthesis of beta-defensins 2 and 3 defensins,increasing the resistance of foreign pathogenic bacteria such as Staphylococcus aureus. This constitutes a typical symbiotic relationship1.[20]P. Zeeuwen, J.Boekhorst focused on how microbial flora affects interaction when skin barrier function is impaired,and proposes both theoretical models of microbial community determinism and host biological determinism(Figure 1 and 2).[21]

Figure 2. Host biological determinism / pathology theory

In short, the skin microbial flora has considerable stability and exhibits different diversity and quantity according to the different physiological characteristics of the body parts. The balance and diversity of microbial flora are key indicators of human skin health. In the future development of daily chemical products, the balance of the flora should be fully considered, and the interaction between the microorganisms and the skin symbiotic environment should be utilized to provide better, gentler and healthier solutions.