Commensalism is a symbiotic relationship where the symbiont benefits but the host is neither harmed nor helped.  Out of all types of symbiotic bacteria, commesalists are the least studied.  The reason for this is simple; if a bacteria is causing not causing harm or benefit to is host there is simply not many reasons to be interested in studying it.  As with all types of symbiotic relationships, it is important to define the host organism that you are referring to.  A species of bacteria could be a commensal organism on one host species but a deadly pathogen on the next.  So for the purpose of this website we will consider the human commensal bacteria.  

 Human Commensal Flora

The human commensal flora is a term given to all the natural bacteria that live on and in a healthy person. There are about 10^4 bacteria living in the human gut alone, this is about 10 times as many human cells in the entire body.  Although they are all considered part of the human commensal flora,  many of these bacteria are not true commensalist .  Many of these bacteria have mutualistic or even potential pathogenic relationships with their human hosts.  The major areas of colonization are the skin, oral cavity, upper respertory tract, lower GI  tract, the urogenital tract.  

 Organisms That Inhabit The Skin--  Most normal skin bacteria are gram + areobes, however                              Image: Skin Flora Gram Stain         Propionibacteria are anarerobic and live inside hair follicles. Bacteria grow best in places where it is moist like the armpit and groin .Whether these organisms are strict commensals or not is debatable. They do not provide any direct benefit to the host, but they do out compete potentially harmful bacteria and prevent them from inhabiting the skin surface.  Some can be opertunistic pathogens. Most common skin commensals are:
Staphylococcus epidermidis

Culture of skin flora showing Gram + Bacteria

Organisms That Inhabit The Oral Cavity--  The oral cavity is a complex environment that contains both aerobic and anaerobic conditions.  If is hard to classify oral bacteria as commensals because many species can aid in the formation of plaque and dental carries, cause periodontal disease, or become potential pathogens as they invade other body tissue. Some also present mutualistic benefits such as vitamin synthesis and the suppression of no indigenous potential pathogens.   Species of Streptococcus, Neisseria, Lactobacillus, and Staphylococcus make up just a few of the many bacteria present in the oral cavity.  

Organisms That Inhabit The Upper Respertory Tract--  The upper respertory tract consists of the nostrils, nasal cavities Image: Nasal Flora Culture and throat.  The most common commensalistic inhabitants are:  Neisseria sp.,  Staphylococcus epidermidis, Micrococcus,  avirulant strains of Streptococcus pneumonea, and Corynebacteria.  Many other species such as Staphylococcus aureus inhabit the upper respitory tract but are potential pathgens.  The lower respitory system is virtually free of bacteria in a healthy person.

Culture of nasal flora containing Staphylococcus aureus and Corynebacterium on blood agar

Organisms That Inhabit The Lower GI Tract
--   More organisms inhabit the lower GI tract than anywhere else on or in the human body.   Bifidobacterium, Bacteroides,  Lactobacillus, Enterococcus, Escherichia coli and other anaerobic bacteria dominate this habitat. Populations in the small intestine can reach 10^11 cells per ml feces. As in other cases many of these organisms cannot be classified as pure commensalists. Many of them provide nutritional benefits such as vitaman production,  the break down of non-digestible materials, and competitive exclusion of pathogens.  Some intestinal inhabitants such as  Bacteroides can become opportunistic pathogens.  
Image: bacteria in GI tract


Common Symbionts on the Digestive System

Organisms That Inhabit Urogenital Tract--   The urogenital tract is in the most part is sterile in a healthy person except for the vagina and the urethra.  The urethra is colonized by skin bacteria such as Staphylococci, Streptococci,Image: lactobacillus and Diphtheroids which  are considered to be in a commensalistic relationship. The vagina is colonized mostly by lactobacillus. The lactobacillus produce lactic acid which lowers the pH and inhibits the colonization of other bacteria.  

                                                                                                                                                                                                     Lactobacillus acidophilus