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Skin: Normal Microbiota
Author: Rose D.
Skin is considered to part of the first line of defense and intact skin is the largest organ on the human body in terms of surface area and weight. The other parts of the first line of defense include normal microbiota and mucous membranes along with their secretions.
Normal microbiota on skin is considered a defense mechanism because competition for resources can impede the growth of potentially harmful pathogens, disease causing pathogens.
The cells of microorganisms that live on and in the body occupy 10x more than body cells. The Skin is composed of two parts- the dermis and epidermis. The epidermis’ outer layer, the stratum corneum, is dead and contains a protective protein called keratin, part of enucleated keratinocytes, also known as squames.  Squames consist of keratin fibrils and crosslinked, cornified envelopes embedded in lipid bilayers, forming the epidermis. The skin's squames are constantly shedding off the top layer removing the microbes from the surface about every 4 weeks.
Normal Microbiota or Normal Flora are established in utero, in the womb or in the uterus. In preparation for birth a mother’s vagina will proliferate lactobacilli. This is the newborn’s first contact with microorganisms. These lactobacilli will become the predominant organism in the newborn’s intestine. Gradually more microorganisms are introduced from the environment through breathing and feeding.
E. coli is introduced through food and begins to inherit the large intestine. Colonized microbiota holds permanent resident and under normal conditions do not cause disease. Microorganisms are not found throughout the body; many reside in specific areas such as mucosal epithelium inside the nose, the lining of the stomach, and within the large intestine. There are many factors that contribute to proliferation, distribution, and composition of normal microbiota Figure 1.
Microorganisms can only colonize and proliferate under favorable conditions and appropriate available nutrients. Some Physical/Chemical factors that affect microorganism growth include temperature, pH, oxygen, salinity, and sunlight.
While mechanical factors that affect microorganism growth include chewing (which may dislodge microbes contained on mucosal), GI tract (dispels microbes through saliva and digestive secretions), the flushing action of urine, and overall mucous, which clear microbes by propelling towards the throat through cilia and are then coughed out. Conditions that support or expel microbes can vary from person to person.
Emergent research around microbial technology, brought on by sequencing technology, has yielded further insight into the characterization, cataloging, and categorization of the microbiome in and on the human body.
Initiatives like the Human Microbiome Project strive to do just that. Research around particular microbial functions has been limited, microbial cataloging helps to identify and distinguish microbial traits between microbes living in similar and different environments. The information acquired from this research has already assisted with understanding human diseases.
In addition, more research suggests that microbes are engaged in communication with the immune system T-Cells, through cutaneous innate and adaptive immune responses. This communication can modulate and alter the skin microbiota.
The development of molecular methods to identify microorganisms has shown that resident skin bacteria as highly diverse and variable. 
As a result, the understanding of organisms and the environment, especially those that reside on human skin has been insightful in addressing many issues from nosocomial (occurring in a hospital) acquired infections and thus infection control in a hospital, surgical or community settings.
Normal microbiota that live on the skin are usually harmless, however, any changes that divert from the norm, which is individual, can alter microbial activity and quickly change from harmless to harmful.
Other factors that can alter normal microbiota are:
(Depression may affect personal care)
Normal microbiota provides immune protection and the characterization, activity, and categorization of microorganisms in different areas of the body; have allowed for advances, to be made in addressing beneficial vs detrimental microbial growth. These advances have provided valuable insight and even strategies to address skin conditions or skin related diseases along with the development of pathogen specific diseases.
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