Study of the human-hosted microbiome led to the realization that the human body is a physical ecosystem, an entire universe of organisms, arrangements and processes working in harmony. Since we are at the tip of the iceberg in this area of research, this knowledge may be associated with unrealistic fear to the effect of these organisms on the human body. The tools currently used may not fully capture this universe; in fact, we are currently facing challenging limitations in detection, mapping and, most importantly, understanding the relationships between these systems, their purpose and role in life health and disease. Simply put, when comes to relationships between the human microbiome and the human innate biology, we have yet to connect the dots. Questions to be answered include: what is the complete map of the human microbiome? Where and how pathogens emerge? Why is the microbiome so diverse between individuals? How and how often is it changing over the course of our life and between generations? How does the environment affect it? What are the relationships between the host (human body) and the inhabitants (collection of microbiome)? These are basic questions. How can we attempt at controlling these complicated systems before we understand the baseline?


Epidemics of Choice


The hygiene theory, also termed “biome depletion theory” or “lost friends theory” is a theory in medicine related to physiological development and adaptation. It stems from the idea that, in developed countries, parents raise their children in an overly clean environment preventing interaction with bacteria, viruses and parasites that should be educating the immune system at an early stage of life and equipping it with means of protection from physiological challenge. The parallel between emotional and physical strength is clear. The diminished exposure to infectious agents and symbiotic microorganisms is suspected to cause immune intolerance and allergic diseases.


In their paper “Fifty years of allergy: 1965-2015,” Campbell and Mehr note: “the last 50 years of allergy could almost be considered the first 50 years, calling it the “allergic epidemic.” Since the original suggestion of the “hygiene theory” in the late 1980s, science expanded its research to the implications of lower incidence of infection in early childhood to diseases other than “the atopic march” that includes asthma, atopic dermatitis and rhinitis to a variety of other chronic diseases such as type 1 diabetes, multiple sclerosis and even specific types of depression and cancer. Some researchers claim that although limited biota exposure may contribute to deprived immunity, this reason alone cannot explain the increased prevalence of asthma. In their 2005 paper, Platts-Mills TA et al. note that in North American inner cities, asthma is increasing among children who live in very poor houses which they assume are less clean. They offer a broader view of a Western lifestyle such as diet and exposure to allergens in foods and decreased physical activity.


A more recent paper by Platts-Mills, from 2015, reviews the allergy epidemics from 1870 to 2010. The authors elucidate that the best explanation for the appearance and increase in allergy is the combination of hygiene and increased pollen secondary to changes in agriculture. Asthma did not start to increase in children until 1960, and by 1990 it had clearly grown to epidemic numbers in all countries where children adopted an indoor lifestyle. Atopic dermatitis, also termed “asthma of the skin” had grown parallel to asthma and is considered a non-treatable skin disease of babies and toddlers.


Interestingly, a suggested cause for the rise in atopic dermatitis is linked to reduced exposure to UVB radiation. In their publication from late 2015, Thyssen and co-authors suggest that, in addition to decreased exposure to pathogens in early life, lack of sufficient UVB exposure could be contributing to the rapid increase in the incidence of atopic dermatitis in developed countries. Recent generations who are trained to use sunscreens extensively may be deprived of such exposure.


The reportedly overly clean environment in Western society stems from the importance and public awareness of refraining from spreading infections. In his presentation to the Institute of Medicine Forum (IOM) of the National Academies in 2008, Prof. Stephen Mores from Columbia University points toward important aspects of the current status of human infectious disease. He notes the following:

• In most of the world, the major cause of death remains infections;
• The types of infections can be “emerging” in nature, meaning such that were not previously appeared, or “re-emerging,” meaning those that were forgotten. The re-emerging diseases were never fully eradicated and are left at the margins of society rising from time to time in the general population mainly in areas where immunization is lacking or when hygiene practices are not correctly exercised;
• The following lifestyle changes are thought to contribute to emergence of the infections epidemics: animal feed industry and animal-human interactions, globalization and extensive travel, growth in industrialized foods, ecosystems affected by waste management, climate changes and deforestation; and
• Zoonotic pathogens, meaning pathogens harbored by non-human animals are responsible for most of the recent pandemics such as HIV/AIDS, severe acute respiratory syndrome and pandemic influenza.

Mores notes that the main infection source is interspecies transfer but when humans are infected, the spread is facilitated by human-to-human interactions. He further points toward years of satisfactory handling of infectious diseases through use of antibiotics, immunization and improved public health measures. These extended lifespans, but also increased the prevalence of chronic diseases. While this may be the case in the Western world infectious diseases remain a major cause of mortality in much of the world. Globalization and intensity of travel complicates the situation exposing the general population to infections.


The balance between appropriate hygiene practice to prevent spreading infections and over cleaning that contributes to deprived immunity and a chronic disorder is the challenge that both regulators and industry should be jointly addressing.


FDA Ruling


The FDA latest ruling directs the public to skip using antibacterial soaps. On its website, the FDA states: “there isn’t enough science to show that over the counter (OTC) antibacterial soaps are better at preventing illness than washing with plain soap and water. To date, the benefits of using antibacterial hand soap haven’t been proven.”


If a product or a practice one uses with the belief that it brings certain value is not delivering such benefit, it may actually be harmful. Providing a false sense of protection may lead to actions not conducted otherwise and certainly presents an ethical problem.


The FDA adds: “the wide use of these products over a long time has raised the question of potential negative effect to health.”


One challenge we face as a society that experiences waves of enormous change is the inability in the evaluation of chronic cumulative effect of exposure to environmental setting. In toxicology, most of our assessments are short-term and extreme, involving a single variable of assessment while, in real life, we are exposed to multiple variables at different timing and for lifespan. We have yet to establish tools to harmonized data collection on epidemiology or studies to gain insights to the effects of chronic exposure.


The FDA further states: “manufacturers haven’t proven that those ingredients are safe for daily use over a long period of time.”


The FDA’s ruling raises the inevitable question: if indeed the general population should be washing hands with plain soap and water is there science to support that such practice is beneficial and is the public sufficiently educated to do so in an efficient manner?


The Centers for Disease Control (CDC) outlines the following instructions on its website:


“Keeping hands clean is one of the most important steps we can take to avoid getting sick and spreading germs to others. Many diseases and conditions are spread by not washing hands with soap and clean, running water. CDC recommends cleaning hands in a specific way to avoid getting sick and spreading germs to others. The guidance for effective hand washing and use of hand sanitizer was developed based on data from a number of studies.

• Wet your hands with clean, running water (warm or cold), turn off the tap, and apply soap. 
• Lather your hands by rubbing them together with the soap. Be sure to lather the backs of your hands, between your fingers, and under your nails. 
• Scrub your hands for at least 20 seconds.
• Rinse your hands well under clean, running water.
• Dry your hands using a clean towel or air dry them.”

CDC detailed guidance for hand washing; but while hand sanitizer is mentioned, no specific instructions for its use are mentioned. In addition, what measures one is to take when no running water is available? A study published in 2010 to review the efficacy of waterless hand hygiene in comparison with hand washing with soap points toward the fact that more than three billion people do not have access to piped water. Even in modern society, people find themselves in situations where such access is absent or limited. The research compared the antimicrobial efficacy of alcohol-based hand sanitizer with soap and water in field conditions. Researchers found that hand sanitizer use by mothers resulted in 0.66 and 0.64 log reductions per hand of Escherichia coli and fecal streptococci, respectively. In comparison, hand washing with soap resulted in 0.50 and 0.25 log reductions per hand of E. coli and fecal streptococci, respectively. This study concludes therefore that hand sanitizer was significantly better than hand washing with respect to reduction in levels of fecal streptococci (P = 0.01).


It should be highlighted that while the FDA guidance is for the general population, it does not apply to healthcare professionals. FDA further states on its website: “The FDA’s final rule covers only consumer antibacterial soaps and body washes that are used with water. It does not apply to hand sanitizers or hand wipes. It also does not apply to antibacterial soaps that are used in health care settings, such as hospitals and nursing homes.”


A study published in 2002 reviews the efficacy of hand rubbing with alcohol-based solution versus standard hand washing with antiseptic soap. It compares the efficacy of hand rubbing with an alcohol-based solution versus conventional hand washing with antiseptic soap in reducing hand contamination during routine patient care by nurses. The study endpoint being bacterial reduction of hand contamination. Study findings are that with hand rubbing, the median percentage reduction in bacterial contamination was significantly higher than with hand washing (83% v 58%, P=0.012), and the conclusion therefore is that during routine patient care, hand rubbing with an alcohol-based solution is significantly more efficient in reducing hand contamination than hand washing with antiseptic soap.


Antiseptic vs. Regular Soap


Simple soaps composed of detergents, by their chemical nature, form micelles when emulsified with water, and remove oily and particulate matter. Washing hands with soap and water removes lipophilic as well as hydrophilic residues from the skin surface. Since regular soaps are not bactericidal or bacteriostatic in nature they will not eradicate bacteria but will physically remove it as well as bacteria nutrients that are bound to lipophilic and hydrophilic skin residue. Washing with soap and water, if not extensive, should allow the commensal skin biota to return rapidly to its natural balance. Antibacterial soaps contain bacteriostatic and bactericidal compounds. They will therefore eliminate skin residing bacteria not differentiating between protective beneficial bacteria to that which is harmful. This may be a challenge to skin’s innate immunity since beneficial bacteria may be replaced with harmful species if innate immunity is weak or if hand-cleaning regimen is intense and chronic.


A recent Korean publication reported on the effects of triclosan, a widely used antimicrobial agent in soap and now banned for use by the FDA. Here, in vitro and in vivo studies compared exposure to plain and antibacterial soap in a base formulation. The only difference between products was the inclusion of 0.3% triclosan in the antibacterial soap. For the in vivo study, hands of participating volunteers were inoculated with Serratia marcescens, a human pathogen known to be involved in hospital acquired infections, and assessed after washing with both products. The study results revealed no significant difference in bactericidal activity between plain soap and antibacterial soap after short term but only after nine hours. Researchers concluded that although triclosan-containing soap does have antibacterial activity, the effects are not apparent during the short time required for hand washing under “real-life” conditions.


Another publication by the Department of Epidemiology and Center for Social Epidemiology and Population Health at the University of Michigan describes a systematic literature review of studies that examined the efficacy of products containing triclosan, compared with plain soap as well as aspects related to the emergence of antiseptic and antibiotic resistance bacteria. Here the PubMed database was searched and evaluated for articles published between 1980 and 2006. The evaluation revealed that soaps containing triclosan were no more effective when compared to use of plain soap at preventing infectious illness symptoms and reducing bacterial levels on the hands. Several laboratory studies demonstrated evidence of triclosan-adapted cross-resistance to antibiotics among different species of bacteria. Researchers concluded that the lack of an additional health benefit associated with the use of triclosan-containing consumer soaps over regular soap, coupled with demonstration of potential risk for drug resistance, requires further evaluation regarding antibacterial product claims and advertising. In 2002 the Council on Scientific Affairs, American Medical Association published a paper titled: “use of antimicrobial agents in consumer products.” Their review method included data sourcing of Medline database from 1966 to 2001. Publishers note that the effect of use of antimicrobial ingredients in consumer products has not been studied extensively and that no data support the efficacy or necessity of antimicrobial agents in such products. Furthermore, a growing number of studies suggest increasing acquired bacterial resistance to them. Studies also suggest that acquired resistance to the antimicrobial agents used in consumer products may predispose bacteria to resistance against therapeutic antibiotics, but further research is needed. It is concluded that the use of common antimicrobials for which acquired bacterial resistance has been demonstrated should be discontinued in consumer products unless data emerge to conclusively show that such resistance has no effect on public health and that such products are effective at preventing infection.


A study by School of Nursing, Mailman School of Public Health at Columbia University reviewed skin reactions to hand hygiene products. The paper evaluated the World Health Organization’s (WHO) World Alliance for Patient Safety and noted that health care professionals have a higher prevalence of skin irritation than the general population because of the necessity for frequent hand hygiene during patient care. Ways to minimize adverse effects of hand hygiene include selecting less irritating products, using skin moisturizers and modifying certain hand hygiene practices such as unnecessary washing.


Not all products are created equal. Profound differences in benefit vs. harm exist between skin cleansers. Moreover, the target population, general public or occupation related cleansing, skin condition and the specific circumstances; i.e., access to running water, are critical to the overall assessment of benefit vs. risk.  As FDA noted, while the efficacy of antiseptic soaps that require the use of water for cleansing has not been fully demonstrated to be more efficacious when compared to hand washing with regular soap, the use of alcohol based sanitizers is still very relevant. Although alcohol-based hand sanitizers may not be effective against all bacteria types and viruses and are not adequate in conditions where hands have visible soil or other visible particulate dirt that needs to be washed, there is strong evidence to support the use of alcohol-based hand gels in the fight against health care-acquired infection in both the hospital and for the general population. Significant research supports their efficacy, but more research is needed due to the emerging knowledge of the skin microbiome and the tools developed to understand the interaction between the microbiome and human immunity. Aspects to be addressed include long-term chronic use, formulation changes to boost efficacy and maintain a strong barrier both physically and biologically.


Despite research limitations, it appears alcohol hand gels offer a rapid, efficacious means for decontaminating hands. Special care should be paid to the condition of the skin as a weak barrier can be more permeable to bacteria and viruses, and cleansing products may contribute to weakened barrier. If product use weakens the skin over time it, even if effective against bacteria, may cause the skin to be more susceptible to infections.

Gojo Industries researchers recently published a study describing the effect of repeat exposure of ethanol to skin. In this exploratory study, the effects of ethanol, isopropanol and n-propanol were assessed in vitro and in vivo. Significant differences were observed between the three types of alcohols with ethanol exhibiting the mildest, least barrier damaging of the three. In vitro, ethanol had the mildest effect on keratinocyte metabolism and cytokine secretion. In vivo, panelists applied the alcohols 20 and 100 times a day for 14 days. No significant differences were observed between the alcohols at the high application frequency. However, at the low application frequency, n-propanol and isopropanol generated greater barrier impairment when compared to ethanol as measured by TEWL and capacitance. The team concluded that ethanol-based sanitizers are better tolerated by skin. While this study is missing the effect on skin microbiome and the time of observation is limited to 14 days, it points toward a potential correlation between skin condition and relevant skin barrier biomarkers. Moreover, ethanol, while a strong antimicrobial agent, evaporates rapidly from the skin surface, and therefore may exhibit a short-term effect that allows restoration of barrier homeostasis. Industry should conduct more of these studies to unravel the effect of skin cleansing regimens.


Healthy Barrier


Skin cleansing practice should be studied with consideration of skin barrier condition. The barrier should be assessed not only physically by measuring transepidermal water loss (TEWL), but also by other means that allow broader evaluation of its condition such as pH, a rapid recovery after stress, microbiome population, sebum secretion, and the presence of antimicrobial lipids and peptides. These underlying skin conditions may not be visible clinically but are critical in the assessment of benefit vs. harm in the overall evaluation of skin cleaning practice. Today’s measuring devices can evaluate endpoints and provide direction to improve products and create new ones. Elements of a healthy barrier include:

• Sufficient skin layers. A healthy barrier contains several layers of corneocytes structured as “brick and mortar,” with the corneocyte’s keratin being the “bricks” and the lamellar intercellular lipids as the “mortar.”
• Proper ratio. The ratio of intercellular lipids/skin intercellular lipids exhibits a unique organization of lamellae in a thermodynamic phase that is rather rigid. The main components of these lipids are ceramides, followed by cholesterol, saturated long chain fatty acids and cholesteryl esters.
• Acidic pH. The pH of healthy skin is around 5-5.5 and is maintained by presence of fatty acids on the skin surface. Such pH is essential to support a healthy skin microbiome.
• Relatively low levels of water. The upper layer of the skin, the stratum corneum contains about 10% water. It is dry for a reason. Elevating water contents in the skin artificially by either occlusion or extensive use of moisturizers may support the environment for harmful biota.
• Sebum secretion. Human sebum is composed mainly of triglycerides and contains key elements of antimicrobial lipids that balance harmful and commensal skin biota.
• Low transepidermal water loss (TEWL) as a measure of healthy barrier. The lower the TEWL, the more intact the barrier.

A 2014 publication by Bouwstra’s research team explains the importance of stratum corneum lipids in maintaining skin barrier. Extensive use of products that deplete lipid barrier components can lead to barrier impairment and changes that may facilitate skin diffusion of pollutants and allergens. Another publication, by Foweler and co-authors from the University of Louisville, explains that skin cleansing products can damage the stratum corneum, leading to barrier dysfunction. Any regimen that generates a constant chronic imbalance to a healthy skin barrier, whether it includes an antiseptic agent or not, can trigger a skin disease and further exposure to harmful agents from the environment. A healthy skin needs no extra means of cleansing or moisturization. Over-cleansing and over-protection can generate a new state of imbalance that the skin must “correct.” In recent decades there is documented growth in population that reports to have “sensitive skin,” which may be due to adaptation of a Western lifestyle that includes changes in nutrition, emotional stress, pollution and overall extensive use of skin care products that are complex in their contents. A healthy barrier should not require extensive treatment and support an innate ability to bounce back to normal homeostasis after being challenged.


Wanted: Dead or Alive?


In “The Skin Microbiome,” a publication co-authored by Julia Segre from the NIH and Elizabeth Grice from the University of Pennsylvania, the authors note: “Colonization is driven by the ecology of the skin surface, which is highly variable depending on topographical location, endogenous host factors and exogenous environmental factors. The cutaneous innate and adaptive immune responses can modulate the skin microbiota, but the microbiota also functions in educating the immune system.”


They further explain that many of these microorganisms are harmless and in some cases are essential at maintaining healthy skin protection. Skin microbiome genome sequencing reveals:

• A high diversity in biota population between body sites, ages, lifestyle, individual, gender and other variables.
• Genomic sequencing, while providing new information, does not lead to in-depth understanding. It may yield inaccuracies in identification, mass, unknown species, unknown functions and cannot assess vitality, death or dormancy of the detected species.
• Merely mapping the microbiome is insufficient. Unless its activity and effect on human cells and functions are understood, we cannot utilize the information.
• Bacteria can be beneficial or harmful depending on its environment.
• Bacteria genome identity can change fairly rapidly if challenged.

Scientists from Harvard Medical School and the Technion-Israel Institute of Technology designed a simple way to follow bacteria’s movement in its journey to acquire antibiotic resistant characteristics. This experiment is a demonstration of the rapid adaptation and enormous flexibility of biota to environmental conditions. On a large petri dish the team seeded E. coli and exposed it to increasing doses of antibiotics, the higher dose being 1000 times higher compared to the lower dose. For two weeks, they followed bacterial movement, death, survival, adaptation, evolution and proliferation. The research demonstrated that bacteria spread until reaching a certain concentration of antibiotic dose it could no longer tolerate. However, in each position a small group of bacteria adapted, survived, multiplied and strived to high numbers of multiplication. The sequential process brought the understanding that at threatening conditions, low resistance mutants give rise to moderately resistant mutants eventually leading to highly resistant bacteria. This simple, yet ingenious experiment clearly reveled what we already know. There is no point fighting bacteria. We might as well “join” it. First, when pathogenic bacteria attack, we can lower its levels with antibiotics but we cannot create a sterile environment. If the human immune system is weak, the bacteria will take over. Second, if we study how to support the beneficial bacteria rather than harm the overall biota population, we may be able to achieve better results in overall health and strength of immunity. Understanding the skin immunity response, not eradicating biota, should be the focus.


Consumers may be confused with contradicting advertisements. While one is encouraged to consume probiotics to boost immunity and improve overall health, at the same time he is instructed to eliminate bacteria on skin to reduce infections and disease. But “good” and “bad” are human terms. In nature, organisms only try to survive, thrive and reproduce—bacteria has no desire to gain power, money or respect. Bacteria can be “harmful” or “beneficial” in respect to humans, but even those lines blur because the same bacteria can act in both manners depending on its environment.


What Is Next?


Product development is complicated. We want clean skin, but what is “clean skin?” Is it the removal of dirt only or is it the removal of harmful biota as well? Is there a way to remove harmful biota and maintain beneficial species? If so, how would one define “harmful” when, in fact, biota can transform to be harmful under specific condition?


FDA’s removal of antimicrobial agents from product formulas raises many questions: Is the public sufficiently educated to wash hands in a manner that’s been scientifically demonstrated to lead to clean skin? When the industry is instructed to gain additional data on the efficacy and safety of such products, how are endpoints defined? With the clear differentiation between populations, shall the market be more segmented and products developed for sub populations? Should products for healthcare providers and those who work with animals be different from those designed for the general population? How do we assess the balance between maintaining public health and preventing the next epidemics while not abusing and over-cleansing the skin to a point where we harm the immune system irreversibly and create chronic disorders?


“Antimicrobial resistance poses a fundamental, long-term threat to human health, sustainable food production and development,” said United Nations Secretrary-General Ban Ki-moon. “It is not that it may happen in the future. It is a very present reality …in all parts of the world, in developing and developed countries; in rural and urban areas; in hospitals; on farms and in communities. We are losing our ability to protect both people and animals from life-threatening infections.”


Humanity never wins wars with nature since nature never declares war on humanity. It is clear that eliminating bacteria merely because it is bacteria creates even greater challenges for the future. Bacteria were here before humanity and will probably outlast us. Most biota we host in our body is essential to our health and survival. We must create an environment where biota can thrive. The host has limited space. If “seats” are taken by the beneficial species and behavior, there’s less room for harmful ones.


Strengthening the body’s immunity is an approach that requires a profound shift in medicine perspective since it suggests we study health more than we study disease.


More research is required by scientists, academia and regulators, but how do we overcome the challenge of biota diversity and its enormous capacity for adaptation? The fact that beneficial bacteria, under specific conditions become harmful mean that they have the genes to express harmful proteins that will only be expressed if the environment changes. This is also the key mechanism for developing biota resistances. These proteins share common destructive activities. Therefore, instead of studying taxonomy of biota identification, the scientific community must focus on pathogenicity. It is not important to give a name to the organism because the same species is evolving rapidly. It is more important to find common expressed proteins that may express harmful toxins.


The barrier, too, should be a consideration. Certain populations, such as the elderly and people with serious systemic conditions, may have a weak barrier both physiologically and biologically. Mapping their baseline immunity and barrier strength can allow for product development paths that will serve this population with reduced harm.


The skin care industry faces more challenges.  Active beneficial bacteria can be incorporated to refrigerated foods and supplements, but cannot be added to a skin care cream that requires a two years shelf life under ambient conditions. Topical creams and lotions contain preservatives to prevent microbial growth. The correct approach, therefore, should be prebiotic not probiotic, incorporating nutrients and generating conditions to support healthy skin’s innate immunity.


In summary, focusing on understanding the healthy relationship between the host and the symbiont and strengthening that relationship may be the key for future product development. If there is a lesson that humans can learn from biota’s behavior it is supporting each other and collaborating. The future should include collaboration between entities; industry, regulatory, academia and research organization. Epidemiology should be studied and statistics drawn to assist in broader understanding of history, present and future. 


References:

• FDA website: http://www.fda.gov/ForConsumers/ConsumerUpdates/ucm378393.htm September 17th 2016
• Campbell DE, Mehr S. Fifty years of allergy: 1965-2015. J Paediatr Child Health. 2015 Jan;51(1):91-3.
• Platts-Mills TA, Erwin E, Heymann P, Woodfolk J. Is the hygiene hypothesis still a viable explanation for the increased prevalence of asthma? Allergy. 2005;60 Suppl 79:25-31.
• Thyssen JP, Zirwas MJ, Elias PM Potential role of reduced environmental UV exposure as a driver of the current epidemic of atopic dermatitis. J Allergy Clin Immunol. 2015 Nov;136(5):1163-9.
• Kim SA, Moon H, Lee K, Rhee MS. Bactericidal effects of triclosan in soap both in vitro and in vivo. J Antimicrob Chemother. 2015 Dec;70(12):3345-52.
• Aiello AE, Larson EL, Levy SB. Consumer antibacterial soaps: effective or just risky? Clin Infect Dis. 2007 Sep 1;45 Suppl 2:S137-47.
• Tan L, Nielsen NH, Young DC, Trizna Z; Use of antimicrobial agents in consumer products.
• Arch Dermatol. 2002 Aug;138(8):1082-6.
• Larson E, Girard R, Pessoa-Silva CL, Boyce J, Donaldson L, Pittet D. Skin reactions related to hand hygiene and selection of hand hygiene products Am J Infect Control. 2006 Dec;34(10):627-35.
• van Smeden J, Janssens M, Gooris GS, Bouwstra JA. The important role of stratum corneum lipids for the cutaneous barrier function. Biochim Biophys Acta. 2014 Mar;1841(3):295-313.
• Fowler JF Jr, Eichenfield LF, Elias PM, Horowitz P, McLeod RP. The chemistry of skin cleansers: an overview for clinicians. Semin Cutan Med Surg. 2013 Jun;32(2 Suppl 2):S25-7.
• Elizabeth A. Grice and Julia A. Segre The skin microbiome Nat Rev Microbiol. 2011 Apr; 9(4): 244–253. 
• Ekaterina Peshera; Howard Gazette http://news.harvard.edu/gazette/story/2016/09/a-cinematic-approach-to-drug-resistance/
• Cartner T., Brand N., Tian K., Saud A., Carr T., Stapelton P., Lane M.E., Rawlings A.V. Effect of different alcohols on stratum corneum kallikrein 5 and phospholipase A2 together with epidermal keratinocytes and skin irritation. Int. J. Cosmet. Sci. (2016) 1-9.
• http://www.cdc.gov/handwashing/show-me-the-science-handwashing.html
• http://www.nationalacademies.org/hmd/~/media/92A4333B47FB4271992A21E25156E826.ashx
• Mores S.S., Mazet J.A., Woolhouse M., Parrish C.R., Carroll D., Karesh W.B., Zambrana-Torrelio C., Wlan L., Daszak P. Prediction and prevention of the next pandemic zoonosis. Lancet 380 (2012) 1956-65.
• Patel S. The efficacy of alcohol based hand disinfectant products. Nursing Times Journal 100(23)(2004) 32.
• Amy J. Pickering, Alexandria B. Boehm, Mathew Mwanjali, and Jennifer Davis. Efficacy of Waterless Hand Hygiene Compared with Handwashing with Soap: A Field Study in Dar es Salaam, Tanzania. Am J Trop Med Hyg. 2010 Feb; 82(2): 270–278.
• Emmanuelle Girou Sabrina Loyeau, Patrick Legrand Françoise Oppein, Christian Brun-Buisson, Efficacy of handrubbing with alcohol based solution versus standard handwashing with antiseptic soap: randomised clinical trial Published online 2010 Feb 5. doi:  10.4269/ajtmh.2010.09-0220

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Nava Dayan, PhD,  is the owner of Dr. Nava Dayan, LLC, a skin science and research consultancy, serving the pharmaceutical, cosmetic and personal care industries. She has more than 25 years of experence in the skin care segment and more than 150 publication credits. Tel: 201-206-7341; Email: [email protected]