Habitat heterogeneity of the human skin microbiome: A comparison of the dorsal & ventral forearms
Introduction
The human skin is the largest organ of the human body and provides a microbiome ecosystem for bacteria. Human skin is composed of 1.8 m2 diverse habitats made up of creases, folds, and specialized niches that support various microorganisms (Grice and Segre 2011). Its variety of regions offers a wide array of habitats for microbial bacteria to reside. These various habitats make up environments with different chemical and physical properties. The chemical and physical make-up of the human body also dictates the type of bacteria residing in these habitats. Physiochemical skin features select for different sets of microbial communities whose niches are specialized to these habitats (Grice and Segre 2011). Biogeography of the human body also determines the composition of bacterial communities (Costello et al. 2009). Human skin is divided into biogeographical habitats via the effects of temperature, moisture, and density of hair follicles.
As the human skin provides a diverse combination of regions, the habitat complexity of these “geographic” areas plays a significant role in the bacterial composition. Since the skin varies topographically amongst regions, differing habitats are recognized to support distinct assemblages of microorganisms (Grice and Segre 2011). The topography of human skin is based on skin thickness and density of hair follicles and glands (Grice and Segre 2011). As skin thickness and the density of hair follicles and glands vary throughout the human body, there is a distinction between habitat heterogeneity. Hair density shapes habitat heterogeneity, as it may provide bacteria with more coverage from elements like the sun or temperature variation (Busse et al. 2018). The combination of hair follicles and sebaceous glands affects habitat structure as the gland sits at the base of the follicle and secretes oils that alter moisture and pH levels (Grice and Segre 2011). The density of these two structures ultimately influences the differentiation between habitats as regions with a high density of glands are moister and harbor more bacteria, whereas regions with more hair follicles shelter a wider variety of species (Grice and Segre 2011). These characteristics are important to consider when studying the biogeography of the skin microbiome because they differentiate between areas like the face and the arm. Ultimately, the variation in habitat creates niche-specific bacteria with physiological differences that affect community diversity, composition, and biomass (Oh et al. 2014). We must also consider habitat complexity and its effect on niche specification, and therefore species richness, to accurately map the habitat heterogeneity of the human body.
Comparable to our planet’s diversity rules, the human body also has diversity rules that make up our skin microbiome which we can use to predict areas of diversity hotspots. Recent studies have discovered a link between habitat complexity and species richness, and we can use these findings as grounds to formulate a hypothesis on the human skin microbiome. Regions of high topographic complexity like the tropics and the benthos of marine environments contain high species diversity because they harbor challenging abiotic factors that lead to speciation. Speciation is driven by the partitioning of niches via factors like temperature, precipitation, and variability. Topographic complexity drives the increase in niche space, allowing more species to coexist and therefore more richness within the habitat (Allouche et al. 2012). A recent study on the benthos of marine environments provides evidence for this trend since they discovered that coral reefs harbored high species diversity and primary productivity because these areas were more topographically complex (Zawada et al. 2010). Taking these biodiversity rules into account plays a significant role in the composition of human microbial communities as regions of topographic complexity contain biodiversity hotspots and shape species traits, biotic interactions, and range distributions (Badgley et al. 2017). Therefore, the connection between habitat complexity and species richness will assist in the prediction of microbial diversity.
To study the diversity and richness of microbial communities on the human skin, we will compare topographically varying environments throughout the body. A recent study provides evidence for this comparison as they displayed that the human skin offers an opportunity to study the taxonomic and functional compositions of our microbial communities (Oh et al. 2014). As this composition differs across regions, looking at the density of hair follicles and sebaceous glands creates topographically varying regions as dissimilar as rainforests and deserts. (Oh et al. 2014). To display this compositional relationship between habitats, we will study the dorsal and ventral forearms as they offer differing temporal and topographic environments (Grice et al. 2009). The dorsal and ventral forearms are ideal for the study of varied bacterial composition between regions as they offer different spectrums of habitat heterogeneity. The dorsal forearm offers more heterogeneity as there tends to be a higher density of hair follicles and sebaceous glands (NYU Medical Center and School of Medicine 2007). The dorsal forearm also experiences greater exposure to sun and temporal variability. The ventral forearm harbors less topographic complexity and hair density along with decreased environmental exposure (NYU Medical Center and School of Medicine 2007). As these regions vary drastically in habitat heterogeneity, they are the ideal subject for the purposes of this study.
We aim to characterize the relationship of habitat complexity and the level of species richness on the human body. We hypothesize that topographically complex environments produce greater species richness. Using human forearms, we predict that more hairy forearms are more topographically complex environments than less hairy forearms. We also predict that the hairier environments produce more bacteria growth (richness) than less hairy environments.