Permanent residents are microorganisms that make up a part of the human body’s normal flora, such as in the small and large intestine, nasopharyngeal mucosa, skin, and genital area. These include bacteria, viruses, and a few fungi.
Inside the human body, microorganisms live in synergy and perform a wide variety of beneficial functions, including synthesis and secretion of vitamins K and B12, prevention of colonization by harmful pathogens by competing for attachment sites or essential nutrients, stimulation of development of specific tissues like the cecum and Payer’s patches in the GI tract, and amplification of immune system.
Some examples of microorganisms residing in the human body include Staphylococcus, Streptococcus, and E. coli.
Transient residents are organisms that colonize the human body for hours, days, or weeks but do not establish themselves permanently and move on or die off.
As permanent residents, transient organisms are not always harmful. However, when given an opportunity, such as immunosuppression or poor infection control, they tend to cause infection and disease. Some examples of transient pathogens are Clostridia and Acinetobacter.