| Commensalism |
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.
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
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
Mycobacteria
Propionibacterium
Corynebacteri
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
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.
| 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
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
Mycobacteria
Propionibacterium
Corynebacteri
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
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.
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,
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.
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
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References
http://www.cehs.siu.edu/fix/medmicro/normal.htmhttp://www.bmb.leeds.ac.uk/mbiology/ug/ugteach/icu8/flora/cases.html http://www.psychology.odu.edu/bio/fachomes/asg100f/humanflora.htm
http://www.bact.wisc.edu/Bact303/Bact303normalflora
http://cwx.prenhall.com/bookbind/pubbooks/brock/chapter19/deluxe.html