Our skin is populated by billions of diverse bacteria. As the skin and outer tissues are in constant contact with the environment, microbes have easy access to colonize these areas of the body. Most of the bacteria that reside on skin and hair are either commensalistic (beneficial to the bacteria but do not help or harm the host) or mutualistic (beneficial to both the bacteria and the host).
Some skin bacteria even protect against pathogenic bacteria by secreting substances that prevent harmful microbes from taking up residence. Others protect against pathogens by alerting immune system cells and inducing an immune response.
- The vast majority of bacteria that inhabit our skin are commensalistic or mutualistic.
- Commensalistic bacteria are bacteria that neither help or harm us, but that themselves benefit from the relationship. Mutualistic bacteria help us and benefit from the relationship.
- The bacteria we find on our skin are categorized by the environment in which they thrive: oily skin, moist skin, or dry skin.
While most strains of bacteria on the skin are harmless, others can pose serious health problems. These bacteria can cause everything from mild infections (boils, abscesses, and cellulitis) to serious infections of the blood, meningitis, and food poisoning.
Skin bacteria are characterized by the type of environment in which they thrive: sebaceous or oily areas (head, neck, and trunk); moist areas (creases of the elbow and between the toes); and dry areas (broad surfaces of the arms and legs).
Propionibacterium acnesPropionibacterium acnes bacteria are found deep in the hair follicles and pores of the skin, where they usually cause no problems.
Science Photo Library/Getty Images
Propionibacterium acnes thrive on the oily surfaces of the skin and hair follicles. These bacteria contribute to the development of acne as they proliferate due to excess oil production and clogged pores. Propionibacterium acnes bacteria use the sebum produced by sebaceous glands as fuel for growth. Sebum is a lipid consisting of fats, cholesterol, and a mixture of other lipid substances and it is necessary for proper skin health, moisturizing and protecting hair and skin. Abnormal production levels of sebum, however, contribute to acne as it can clog pores, lead to excess growth of Propionibacterium acnes bacteria, and induce a white blood cell response that causes inflammation.
CorynebacteriumCorynebacterium diphteriae bacteria produce toxins that cause the disease diptheria.
BSIP/UIG/Universal Images Group/Getty Images
The genus Corynebacterium includes both pathogenic and non-pathogenic bacteria species. Corynebacterium diphteriae bacteria produce toxins that cause the disease diphtheria. Diphtheria is an infection that typically affects the throat and mucous membranes of the nose. It is also characterized by skin lesions that develop as the bacteria colonize previously damaged skin. Diphtheria is a serious disease and in severe cases can cause damage to the kidneys, heart, and nervous system. Even non-diphtherial corynebacteria have been found to be pathogenic in individuals with suppressed immune systems. Severe non-diphtherial infections are associated with surgical implant devices and can cause meningitis and urinary tract infections.
Staphylococcus epidermidisStaphylococcus epidermidis bacteria are part of the normal flora found in the body and on the skin.
Janice Haney Carr/ CDC
Staphylococcus epidermidis bacteria are typically harmless inhabitants of the skin that rarely cause disease in healthy individuals. These bacteria form a thick biofilm barrier (a slimy substance that protects bacteria from antibiotics, chemicals, and other substances or conditions that are hazardous) that can adhere to polymer surfaces. As such, S. epidermidis commonly cause infections associated with implanted medical devices such as catheters, prostheses, pacemakers, and artificial valves. S. epidermidis has also become one of the leading causes of hospital-acquired blood infection and is becoming increasingly resistant to antibiotics.
Staphylococcus aureusStaphylococcus aureus bacteria are found on the skin and mucous membranes of humans and many animals. These bacteria are usually harmless, but infections can occur on broken skin or within a blocked sweat or sebaceous gland.
SCIENCE PHOTO LIBRARY/Science Photo Library/Getty Images
Staphylococcus aureus is a common type of skin bacterium that may be found in areas such as the skin, nasal cavities, and respiratory tract. While some staph strains are harmless, others such as methicillin-resistant Staphylococcus aureus (MRSA), can cause serious health issues. S. aureus is typically spread through physical contact and must breach the skin, through a cut, for example, to cause an infection. MRSA is most commonly acquired as a result of hospital stays. S. aureus bacteria are able to adhere to surfaces due to the presence of cell adhesion molecules located just outside of the bacterial cell wall. They can adhere to various types of surfaces, including medical equipment. If these bacteria gain access to internal body systems and cause infection, the consequences can be fatal.
Streptococcus pyogenesStreptococcus pyogenes bacteria cause skin infections (impetigo), abscesses, bronchio-pulmonary infections, and a bacterial form of strep throat that can lead to complications such a acute articular rheumatism.
BSIP/UIG/Universal Images Group/Getty Images
Streptococcus pyogenes bacteria typically colonize the skin and throat areas of the body. S. pyogenes reside in these areas without causing issues in most cases. However, S. pyogenes can become pathogenic in individuals with compromised immune systems. This species is responsible for a number of diseases that range from mild infections to life-threatening illnesses. Some of these diseases include strep throat, scarlet fever, impetigo, necrotizing fasciitis, toxic shock syndrome, septicemia, and acute rheumatic fever. S. pyogenes produce toxins that destroy body cells, specifically red blood cells and white blood cells. S. pyogenes are more popularly known as "flesh-eating bacteria" because they destroy infected tissue causing what is known as necrotizing fasciitis.
- Todar, Kenneth. “The Normal Bacterial Flora of Humans.” Online Textbook of Bacteriology,
- “Microbes of the Skin.” The Scientist Magazine, .2014.
- Otto, Michael. "Staphylococcus Epidermidis the 'accidental' Pathogen." Nature Reviews. Microbiology 7.8 (2009):555-567.
- “Antimicrobial (Drug) Resistance.” National Institute of Allergy and Infectious Diseases, U.S. Department of Health and Human Services, 2016.
- “GAS Frequently Asked Questions. Group A Streptococcal (GAS) Disease." Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 2016,