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.
Conclusion
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.
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