Bacteria still rouse an “ick” factor among many people. Each day we assault them before and after our bathroom trips, during showers, by using hand sanitizers, by doing dishes, and the list continues. However, most are familiar with yogurt or other probiotic products that are supposedly filled with “good” bacteria. To better understand the good and the bad of bacteria, let’s explore the self-inviting critters living on our skin.


Our skin is a dynamic ecosystem…

The skin is our largest organ. It is a dynamic shield full of nerve endings that allow us to sense the world, while protecting us from extreme temperature, UV rays, toxins, and pathogens while keeping the water and nutrients within. There are three layers: epidermis (outer), dermis, and the subcutis (base).

The epidermis consist of cells known as keratinocytes, which form varying layers of thickness depending on the skin location. It is also where Langerhan cells are located, which help to survey pathogens and regulate our immune system. The epidermis is also the formidable defense for us to the outside world.

The dermis serves as the gummy portion with its collagen and elastin-giving our skin elasticity and strength. Nerve receptors also reside here to relay information back to the brain on touch, temperature, and pain. In addition, many secreting glands can also be found here. There are three main types of secreting glands: eccrine, apocrine, and sebaceous. Eccrine glands are found abundantly in all skin surfaces, and are responsible for regulating the body temperature by sweating. They also discourage microbial growth by acidifying your skin. Apocrine glands, on the other hand, are found in areas with more hair follicles, such as your armpits, nipple, and genital regions. They secrete milky fluids commonly associated with psychological activation, such as stress or sexual arousal. When bacteria meets the apocrine gland secretions, people often reach for deodorants. Lastly, sebaceous glands secrete an oily waxy matter called sebum, which coats and protects our skin and hair.

Lastly, the base layer is composed of fatty and connective tissues to insulate and cushion us while serving as backup fuel reserves.         


What is your skin flora?

The skin flora, or skin microbiota, refers to small microbes that reside on your skin. This includes bacteria, viruses, and fungi (to learn about the difference between the three groups, visit here). Mites also take residence on our skin. Together, your skin flora can act as a barrier from other invading bacteria that may or may not cause diseases. Sometimes, your skin flora is also a source of infection.


What differentiates between a “good” vs. “bad” microbes?

Much literature tries to distinguish between the “good” and “bad” microbes. For example, “bad” bacteria may have DNA coding traits to bypass our immune system, pump out antibiotics, or alter where the antibiotics would bind so they no longer can. However, my personal favorite explanation thus far is “depending on the location and timing” which I learned from an undergraduate Microbiology course. Bacteria, much like many living organisms, aims to survive and replicate. Like a true Darwinian species, it adapts to its environment, which ranges from deep sea vents, hot springs, your iPhone, and more-“bad” bacteria merely adapt to living on us in ways which cause unfavorable consequences. Likewise, “good” bacteria adapt in ways which we see beneficial to us. To bacteria, we are just petri dishes in many shapes.


What lives on our skin?

Generally described as cool, acidic, and dry habitat, our skin also has variations which lead to different flora depending on the number of hair follicles, skin folds, and skin thickness. Most skin bacteria found will fall into four main categories/“phyla” called Actinobacteria, Firmicutes, Bacteroides, and Proteobacteria.

  • In moist and warm places where your skin is partially covered (groin, armpits, toe webs), bacteria that like the high humidity includes Corynebacterium, Staphylococcus aureus, and Gram-negative bacilli*. Most likely, our sweat smell is associated with corynebacteria and staphylococci feasting on the apocrine gland secretions.
  • On our face, chest, and back, where there is more concentrated sebaceous glands, bacteria that feed off the sebum includes Propionibacterium and fungi Malassezia. There is also the lowest bacterial diversity in these regions. Demodex mites also reside in these regions.
  • On drier sites such as our arms and legs, there are lower microbial density due to large fluctuations in temperature. These are also sites with the highest skin diversity with representations from all four phyla previously mentioned.
  • Babies born with C-section tend to have skin associated flora, while babies delivered vaginally tend to be colonized with mothers’ vaginal flora. These skin floras will change as babies interact with their environment and grow.

Other factors that influence the skin flora:

  • Host factors such as age and gender, as well as underlying conditions and immune susceptibilities
  • Environmental factors such as climate and geographical location
  • Lifestyle choices, such as jobs, hygiene practices, antibiotic use, handedness, clothing, UV exposure, cosmetic uses, and more.

*Bacteria are traditionally classified as either Gram-positive or Gram-negative depending on their cell wall structures which causes them to color differently when stained. Gram-negative bacilli appear as pink rods, and were previously thought to rarely colonize our skin.  


What are some protective roles that microbes play on our skin?

The bacteria on our skin are constantly in a balancing act with our immune system, and this is accomplished by lots of crosstalk involving secreted molecules and complex immune pathways. They help to regulate our immune systems by “training” our immune cells to differentiate between pathogens and residents. Moreover, some skin flora bacteria will secrete antibacterial substances to discourage others from living on our skin, and can sometimes work together with our immune system to hasten the killing of the pathogens.


What harms can microbes do on our skin?

As there is constant crosstalk between the skin microbes and our immune system, diseases are suspected when the chi is interrupted. For example:

  • Seborrhoeic dermatitis is an itchy, scaly skin condition/dandruff that presents mainly on the scalp. Malassezia fungi is thought to participate in this process, and often times patients will see improvement with antifungal treatments.
  • Open wounds and sores are entries for our skin flora into our bodies. Staphylococcus epidermidis, for example, are frequently associated with infected medical devices such as catheters and heart valves.
  • Propiobacterium acnes is thought to be the cause for bad acne during puberty when our sebaceous glands mature. The hypothesis is that increased sebaceous glands means increased sebum, which attracts and selects for P. acnes in excess. This leads to inflammation of the pilosebaceous glands as the bacterium cross talks with the immune system, yielding the unsightly pimples. However, more study needs to be done regarding this association.  



Our skin microbiota is an exciting and dynamic field  with researchers continuing to find better methods and dissect the vast amount of data we have. It is important to keep in mind that correlation is not causation, especially regarding our skin flora (i.e. just because a bug is found to be commonly associated with a malady, does not mean the bug caused the condition). The best thing you can do to keep your skin healthy is to keep educating yourself with up to date literature while keeping an open mind.



Website Sources:

Journal Citations:

Cogen AL, Nizet V, Gallo RL. Skin microbiota: a source of disease or defence?. Br J Dermatol. 2008 Mar;158(3):442-55. doi: 10.1111/j.1365-2133.2008.08437.x.

Grice E, Segre J. The skin microbiome. Nature Reviews Microbiology. 2011 Apr;9: 244-53. doi:10.1038/nrmicro2537.

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