Microbial Ecology of Human Skin and Wounds

Abstract

Human skin is a complex organ that provides protection and regulates our interaction with the outside environment. The skin is composed of three layers, which include the epidermis, dermis, and hypodermis. Skin appendages include hair follicles, sebaceous glands, and sweat glands. These appendages are unevenly distributed on the skin. The stratum corneum is the outer protective layer of the epidermis and is composed of dead cells that are regularly shed from the surface. The outer layers of the epidermis are inhabited by microorganisms considered permanent skin residents as well as transient microorganisms that do not normally grow and multiply on the skin. The number and types of microorganisms inhabiting the skin are influenced by skin conditions, including the density and activity of sebaceous and sweat glands. The secretions of these glands provide nutrients and selective conditions that influence the composition of the resident microflora. This community is composed primarily of Gram-positive bacteria, including staphylococci, micrococci, and corynebacteria as well as lipophilic yeasts (Malassezia). This resident microflora is believed to help prevent skin colonization by pathogenic microorganisms. However, under certain conditions, skin disease can be caused by members of the resident flora. Skin infections are most often the result of injury to the skin. Cutaneous wounds enable access of microorganisms to normally sterile tissue and provide a much different niche for microbial growth than does intact skin. In the case of acute wounds, the healing process, including the immune response, is capable of stemming invasion by microbes and repairing the wound. However, in some cases wounds become chronic and fail to heal within a reasonable time frame. Most often chronic wounds afflict the ill and elderly with underlying disorders (e.g., diabetes) or weakened immune systems. Large bacterial populations in wounds have been correlated with delayed healing, and control of microbial infection is recognized as an important aspect of wound care. However, the role of specific microorganisms in preventing wound healing remains unclear. Similar types of microorganisms have been isolated from both acute and chronic wounds, although the latter tend to harbor more anaerobic bacteria. Growth of microorganisms as biofilms in wounds may also contribute to the delayed healing and poor response to treatment of chronic wounds. Overall, human skin and cutaneous wounds are complex ecosystems harboring diverse communities of microorganisms. A better understanding of these ecosystems may lead to improvements in human health.