can havecooperative interactions, which benefit the populations, orcompetitive interactions, in which one
population competes with another for resources. The study of these interactions between populations is called
microbial ecology.
Any interaction between different species within a community is calledsymbiosis. Such interactions fall along a
continuum between opposition and cooperation. Interactions in a symbiotic relationship may be beneficial or harmful,
or have no effect on one or both of the species involved.Table 4.1summarizes the main types of symbiotic
interactions among prokaryotes.
Types of Symbiotic Relationships
Type Population A Population B
Mutualism Benefitted Benefitted
Amensalism Harmed Unaffected
Commensalism Benefitted Unaffected
Neutralism Unaffected Unaffected
Parasitism Benefitted HarmedTable 4.1When two species benefit from each other, the symbiosis is calledmutualism(or syntropy, or crossfeeding). For
example, humans have a mutualistic relationship with the bacteriumBacteroides thetaiotetraiotamicron, which lives
in the intestinal tract.B. thetaiotetraiotamicrondigests complex polysaccharide plant materials that human digestive
enzymes cannot break down, converting them into monosaccharides that can be absorbed by human cells. Humans
also have a mutualistic relationship with certain strains ofEscherichia coli, another bacterium found in the gut.E.
colirelies on intestinal contents for nutrients, and humans derive certain vitamins fromE. coli,particularly vitamin
K, which is required for the formation of blood clotting factors. (This is only true for some strains ofE. coli, however.
Other strains are pathogenic and do not have a mutualistic relationship with humans.)
A type of symbiosis in which one population harms another but remains unaffected itself is calledamensalism.
In the case of bacteria, some amensalist species produce bactericidal substances that kill other species of bacteria.
For example, the bacteriumLucilia sericataproduces a protein that destroysStaphylococcus aureus, a bacterium
commonly found on the surface of the human skin. Too much handwashing can affect this relationship and lead toS.
aureusdiseases and transmission.
In another type of symbiosis, calledcommensalism, one organism benefits while the other is unaffected. This occurs
when the bacteriumStaphylococcus epidermidisuses the dead cells of the human skin as nutrients. Billions of these
bacteria live on our skin, but in most cases (especially when our immune system is healthy), we do not react to them
in any way.
If neither of the symbiotic organisms is affected in any way, we call this type of symbiosisneutralism. An example
of neutralism is the coexistence of metabolically active (vegetating) bacteria and endospores (dormant, metabolically
passive bacteria). For example, the bacteriumBacillus anthracistypically forms endospores in soil when conditions
are unfavorable. If the soil is warmed and enriched with nutrients, some endospores germinate and remain in
symbiosis with other endospores that have not germinated.
A type of symbiosis in which one organism benefits while harming the other is calledparasitism. The relationship
between humans and many pathogenic prokaryotes can be characterized as parasitic because these organisms invade
the body, producing toxic substances or infectious diseases that cause harm. Diseases such as tetanus, diphtheria,
pertussis, tuberculosis, and leprosy all arise from interactions between bacteria and humans.
Scientists have coined the termmicrobiometo refer to all prokaryotic and eukaryotic microorganisms that are
associated with a certain organism. Within the human microbiome, there areresident microbiotaandtransient
Chapter 4 | Prokaryotic Diversity 143