Microbes Rule!

The Human Integumentary System

When I have asked my students, what is the largest organ of the human body? Inevitably, I will get two main answers, liver and intestines. Students often forget that their skin is an organ; in fact it is the largest organ of the human body. Our skin is comprised of different tissues working together with accessory organs to create the Integumentary system. Some of the accessory organs include hair, (sweat) glands, and sebaceous glands (oil glands) which support the three layers of the skin. Skin is actually a cutaneous membrane comprised of three interlocking layers: the epidermis, dermis, and hypodermis.

Epidermis

What we see as skin is actually epithelial tissue (a group of cells which line a lumen or duct). The epithelial tissue is comprised of joined epithelial cells of varying shapes (flat or squamous, square or cuboidal, rectangular or columnar). Layers of cells create different strata. Hence, various thicknesses of tissues can be attributed to epithelial tissue that is stratified. Depending on the type of epithelia that comprise the strata the tissue may be described as stratified squamous, stratified cuboidal, stratified columnar, or pseudostratified. The epidermis consists of 5 distinct layers with the top being the stratum corneum and the bottom being the stratum basale which is the source of mitotically dividing epithelia. ⁴

The outer epidermal layer consists of several layers of keratinized epithelial cells which include pigment producing cells known as melanocytes. Because of the protein keratin, the epidermis is impermeable to water and provides the thin layer with strength and flexibility. ⁵ A major function of this wall of keratin cells is to provide a barrier to heat, chemicals, and microbes. 90% of epidermal cells are keratinocytes. ⁶ The process of pushing old keratinized cells upwards to be shed is known as keratinization. Keratinization is an important concept to understand since keratinization is a major factor in maintaining the skin's homeostatic balance between old cells and new cells.

The second most abundant skin cells are melanocytes. Melanocytes are the source of melanin production. The amount of melanin accounts for skin pigmentation along with genetic, environmental, and dietary factors. It is also the lack of melanin that increases one's susceptibility to ultraviolet rays and is the source of a type of skin cancer. Merkel and Langerhans cells also exist in the skin but are few. They are important because they play roles in immune response, sensory perception, and perhaps even the biodiversity of microbes of the skin. ⁷

Dermis

Beneath the epidermal layer lie the dermal papillae, the barrier separating the upper epidermis and the lower dermis. Once the wall of the epidermis is passed, the machine factory of the dermal accessory organs becomes visible. Here are the hair follicles, sweat glands, sebaceous glands, blood vessels, and receptor organs. Ducts traverse both epidermal and dermal layers providing exits for sweat, salt, and oil through tiny pores. This layer is thicker than the epidermal layer and is hub for many skin functions. These include production of sweat and the oily moisturizer called sebum secreted by the sebaceous gland. Feeding the needs of the machine are dermal blood vessels that capture carbon dioxide waste products and deliver oxygen necessary for cellular respiration, the energy source for aerobic cells.

Embedded in the dermis are hair follicles, which work with sebaceous glands to promote hair growth. Attached to each hair root is a collection of small muscle fibers known as the arrector pili muscle that reacts to changes in temperature, pressure, and even changes in emotion. The bottom layer, the hypodermis or subcutaneous layer is composed of primarily adipose tissue (fat tissue) and contains lymphatic and vascular tissue. ⁸

The Human Skin – An Ideal Microbiome

Beyond the study of Anton van Leeuwenhoek, Antoni Redi, and Louis Pasteur in Biology, very little time during high school science classes is actually spent on microbiology and biodiversity. Though students are still taught the hierarchy of organisms, the debunking of spontaneous generation, and Darwin's theory of natural selection, they spend little time outside of Biology learning about the biodiversity of microbes, their potential for disease and decay, their beneficial roles in nitrogen fixation and photosynthesis, or their useful role in both the food and pharmaceutical industry. This unit attempts to fill some of the gaps that exist in high school microbiology education. Because the unit is intended for Physiology students who are primarily in their third or fourth year of science, students are expected to have some fundamental knowledge of bacterial and viral life cycles, which will not be a focus of this unit. Most studies of bacteria and viruses in relation to the human integumentary system focus attention on pathogenic microbes, when, in fact, benign viruses, fungi, arthropods, and bacteria all call the human skin "home". ⁹

The skin contains many different types of "environmental niches" The various anatomical locations of the skin allow for a mini-planet of different ecosystems. Like earth, the skin has it desert regions which are dry and arid, its tropical regions which are moist and warm, its deciduous forest regions which are hotter at some times of the year and cooler at others, and the skin even has savannah regions which are vast areas of the skin that are subjected to wind and other elements. Depending on the location of the skin (scalp, armpit, head and neck), the surface may be cooler, warmer, drier, wetter, acidic, or basic creating environments in which a variety of microbes may thrive. These different areas allow for development of 19 different phyla and more than 1000 bacterial species. ¹⁰ In addition, the different microenvironments allow for different strains of bacteria to develop and thrive. However, different microenvironments are not the only factors that contribute to microbial diversity. Other factors include a person's gender, age, and preferred diet, which factor into the types of microbes that appear in the skin. ¹¹

It is well known that the diversity of microbes has allowed for their presence in all areas of earth including the most environmentally extreme areas like salt flats, hot geysers, and frozen tundras. ¹² Schommer and Gallo present the reason for microbe success on human hosts in their article, "Structure and function of the human skin microbiome." They claim that humans and microbes have co-evolved and are co-dependent upon each other. This evolution has allowed for microbes to adapt to new environmental conditions. The article, "Skin Microbiota: A source of disease or defense," further asserts that healthy skin is dependent on a balance between host cells and microbial presence. ¹³ If an imbalance occurs, it would cause the host to be ill or weakened reducing the microbes' own chances for survival. This is a striking argument, which one could conclude is in part the reason why humans can be found in just about every corner of the earth. Could microbes really be the reason why humans are such a successful species? Or are microbes driving and allowing humans to develop and adapt so that they can use humans as a vehicle to prevent themselves from dying?

Unfortunately, despite how perfect the skin might be for cultivation of microbes, imbalances of the skin-host equilibrium are major contributing factors to disease. The host factors that contribute to dysbiosis or imbalance of the host-microbe equilibrium include one's "age, sex, use of medication, lifestyle, and hygiene." ¹⁴ Factors like the pH, thickness of the skin, the rate of sebum, sweat, and hormone production add to reasons for dysbiosis. How often one cleanses their skin, the manner in which they do, and the cosmetic products they use or avoid also contribute to the potential imbalance. Schommer and Gallo also indicate that where we live on earth, how we live, the uses of modern conveniences, and our choices of birth delivery drive differences in our microbiome and of course health. ¹⁵

The Skin's Defenses against Microbial Pathogens

Michael Wilson in Microbial inhabitants of humans: ecology and role in health and disease asserts that the skin's first line of defense against an environment teeming with microbes is actually airflow over the surface of the skin. This flow acts as barrier from airborne microbes which cannot settle so easily on the skin. It is similar to a piece of paper in a room with a rotating fan; as long as there is airflow and little resistance, the paper will float as it is blown by a fan until there is no airflow. Because the upper layers of the stratum corneum are covered with products of the sweat and sebaceous glands, tiny little cracks and gaps between the keratinized cells are filled to create a tough wall not easily breached by microbes. ¹⁶ Thus, epidermal layers of the skin are tough walls that act as a physical barrier to water loss, mechanical damage, and assault by UV rays. While the lower layers are acidic, making the environments much less hospitable to some microbes that are resistant to pH changes. All the while chemicals are released on the skin's surface that further inhibit the implantation or prevent the proliferation of some microbial organisms. ¹⁷ Wilson cites that chemicals such as free fatty acids, lauric, and myristic acids are "the most effective antimicrobials and have a wide spectrum of activity against members of the resident microbes." ¹⁸ Microbes such as S. aureus, P. acnes, and S. pneumonia can be inhibited by such chemicals contributing to the skin's defense mechanisms.

Furthermore, the production of sweat and deposit of sodium chloride on the epidermis also deters some microbes. The increased osmolarity of the epidermis creates an inhospitable environment against gram negative bacteria like Acinetobacter species. In "Antimicrobial Defence Mechanisms of the Skin", Wilson includes evidence that sweat is a natural antibiotic by the presence of dermicidin which acts against strains of Escherichia coli, Staphylococcus aureus, and Candida albicans preventing their growth in sudoriferous glands. ¹⁹ The implication of this statement is an intriguing discussion for students. Would the public be open to sampling their armpits, for example, to find new strains of bacteria that might prove to be a better antibiotic than those that are synthetically developed?