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The human body routinely harbors about 1014 bacteria
– ten times more bacteria than there are cells.
Although they’ve developed quite the reputation for causing diseases, there are
actually many beneficial and essential bacteria that live both on our skin and
inside our bodies. In fact, there are less than 100 species of bacteria that cause
infectious diseases in humans, but more than several thousand species that exist
symbiotically in our digestive systems. These bacteria that live in and on
humans are called microflora. Alot of the time, the relationship between these
microflora and humans are mutualistic (two organisms of different species both
benefit from each other).
Let’s take a closer look at what these beneficial bacteria are, as well as what
they provide us with.

Bacteria on the Skin
bacteria that live on skin are usually either commensal (relationship where
one organism benefits without affecting the other) or mutualistic. The skin
flora’s main role for humans is to act as a line of defence. They usually prevent
pathogenic organisms (those that can cause disease) from colonizing the
skin surface by:

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Competing against pathogens for nutrients that are provided by the
Since commensal and mutualistic microflora compete with competing invaders for
the host’s limited resources, it makes it much more difficult for invading
microbes to survive and cause a disease.

Secreting or creating chemicals such as fatty acids, peroxides and
bacteriocins that kill invading microbes.  

Stimulating the skin’s immune system so that white blood
cells can destroy any pathogens on the skin.

One example
of a beneficial bacteria on our skin is Propionibacterium. It uses our body’s sebum to inhibit the
growth of invading pathogens.
It does this by converting the triglycerides found in sebum into free fatty
acids; hence lowering the pH of skin. This contributes to the skin’s acidic pH
of approximately 5, which limits the growth of many common bacteria that can
turn pathogenic like Staphylococcus aureus and Streptococcus pyogenes. 

Another similar example of a
mutualistic skin bacterium Pseudomonas
fluorescens. By producing the antimicrobial substance pseudomonic acid, it
not only prevents that bacterial infections such as impetigo, but also inhibits
the growth of fungus species such as Candida albicans – the fungus that can cause thrush.
In fact, this 

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