Intraspecific competition can be particularly intense, when
the partners involved have needs that are very close and
use comparable if not identical means for the resource
acquisition. Competition can also occur between
individuals belonging to different trophic levels, and
even different kingdoms, such as the competition for
nitrate between plant roots and microorganisms in soil.
Competition is sometimes assisted by the production
of toxic compounds in the environment, such as
bacteriocins or antibiotics. It corresponds to interference
competition*, which relies on chemical warfare
mechanisms, to be distinguished from competition by
exploitation. The distinction between the two types of
competition is sometimes difficult to establish. Another
difficulty lies in the distinction between interference
competition and another conflictual interaction,
amensalism*. Amensalism (synonym of antagonism) is
an interaction that has a negative effect on one partner
but no effect on the other. Amensalism assumes that the
two species do not compete significantly with each other.
Amensalism is based on a physical or chemical modification of the environment and, in the latter case, often
involves the release of toxic compounds. Thus, the production of antibiotics (i.e., antibiosis) may correspond to
interference competition or antagonism, depending on
the nature of the partner affected and on the energy
invested in the synthesis of these secondary metabolites.
Allelopathy, i.e., the synthesis by a higher organism of
compounds bioactive on another higher organism, is
conceptually close to antibiosis and deserves to be mentioned in this chapter because microorganisms can
metabolize some of these compounds and thus interfere
with the interaction.
Finally, there are two types of conflictual interactions
for which the interaction is negative for one partner but
beneficial for the other, i.e., predation and parasitism.
Predation* is of short duration (a cat that eats a mouse
or a protozoan that feeds on a bacterium) and usually leads
to destruction of the genetic information of the prey (in
this case the mouse or the bacterium). Parasitism* is
sustained in time (the parasite in its host) and the genetic
informations of the partners remain in intimate contact
over time; here we recognize the traditional distinction
between predator–prey and parasite–host relationships.
The predator is free, while the parasite is physically
associated with its host, at least for part of its life cycle.
It should be noted that the importance of signals is very
different in the two types of interactions. When a cat
chases a mouse, signals are visual, auditory, or olfactory,
and they last only the duration of the hunt. When a virus
or bacterium settles in a host, the exchange of molecular
signals can persist for months or years, for example,
involving the immune system of the host and processes
by which pathogens circumvent these defenses. One of the
most amazing aspects of these sustainable interactions is
manipulation: Toxoplasma are parasitic protozoa whose
complex life cycle requires them, when they are in a
rodent, to pass into a cat to perform their sexual reproduction; Toxoplasma settles in the nerve cells of the rodent
and induces in them a suicidal attraction to cats, as well as
altered social behavior. Although we do not know the
precise mechanisms involved, it is clear that this manipulation of rodent behavior must rely on biochemical
compounds produced by Toxoplasma. The more research
is done on it, the more manipulation appears to be widespread – the result of natural selection – in pathogen–host
interactions.
11.1.3 Beneficial Interactions
Some interactions, unlike the previous ones in which conflict
predominates, provide benefits to the different partners.
Among beneficial interactions, commensalism* is the only
one for which the positive effects are exerted on only one of
the two partners. This interaction is therefore the counterpart
of amensalism. Cooperation* is a mutually beneficial, facultative relationship, this facultative aspect distinguishing it
from symbiosis.
In the case of mutualism* or symbiosis (Box 11.1), both
species benefit from the interaction. It is interesting to note
that, as in conflictual interactions, genetic distance is not a
barrier to partnership: one can observe mutualistic
interactions between plants and animals, bacteria and
vertebrates, and fungi and plants; a complete list would be
endless. Mutualistic interactions have played a very important role in evolution (Fig. 11.1). Mitochondria and
chloroplasts – and certainly other structures – of eukaryotic
cells are nothing more than ancient bacteria. In fact, the
modern eukaryotic cell, the cornerstone of all multicellular
organisms, can be seen as a collection of bacteria. Such
interactions are also reversible: for example, the agent of
malaria, the Plasmodium protozoan, has lost its chloroplasts
(which its ancestors had, as indicated by recently discovered
remnants of these chloroplasts), the same way tapeworms
lost their gut.
11 Microorganisms and Biotic Interactions
397
the partners involved have needs that are very close and
use comparable if not identical means for the resource
acquisition. Competition can also occur between
individuals belonging to different trophic levels, and
even different kingdoms, such as the competition for
nitrate between plant roots and microorganisms in soil.
Competition is sometimes assisted by the production
of toxic compounds in the environment, such as
bacteriocins or antibiotics. It corresponds to interference
competition*, which relies on chemical warfare
mechanisms, to be distinguished from competition by
exploitation. The distinction between the two types of
competition is sometimes difficult to establish. Another
difficulty lies in the distinction between interference
competition and another conflictual interaction,
amensalism*. Amensalism (synonym of antagonism) is
an interaction that has a negative effect on one partner
but no effect on the other. Amensalism assumes that the
two species do not compete significantly with each other.
Amensalism is based on a physical or chemical modification of the environment and, in the latter case, often
involves the release of toxic compounds. Thus, the production of antibiotics (i.e., antibiosis) may correspond to
interference competition or antagonism, depending on
the nature of the partner affected and on the energy
invested in the synthesis of these secondary metabolites.
Allelopathy, i.e., the synthesis by a higher organism of
compounds bioactive on another higher organism, is
conceptually close to antibiosis and deserves to be mentioned in this chapter because microorganisms can
metabolize some of these compounds and thus interfere
with the interaction.
Finally, there are two types of conflictual interactions
for which the interaction is negative for one partner but
beneficial for the other, i.e., predation and parasitism.
Predation* is of short duration (a cat that eats a mouse
or a protozoan that feeds on a bacterium) and usually leads
to destruction of the genetic information of the prey (in
this case the mouse or the bacterium). Parasitism* is
sustained in time (the parasite in its host) and the genetic
informations of the partners remain in intimate contact
over time; here we recognize the traditional distinction
between predator–prey and parasite–host relationships.
The predator is free, while the parasite is physically
associated with its host, at least for part of its life cycle.
It should be noted that the importance of signals is very
different in the two types of interactions. When a cat
chases a mouse, signals are visual, auditory, or olfactory,
and they last only the duration of the hunt. When a virus
or bacterium settles in a host, the exchange of molecular
signals can persist for months or years, for example,
involving the immune system of the host and processes
by which pathogens circumvent these defenses. One of the
most amazing aspects of these sustainable interactions is
manipulation: Toxoplasma are parasitic protozoa whose
complex life cycle requires them, when they are in a
rodent, to pass into a cat to perform their sexual reproduction; Toxoplasma settles in the nerve cells of the rodent
and induces in them a suicidal attraction to cats, as well as
altered social behavior. Although we do not know the
precise mechanisms involved, it is clear that this manipulation of rodent behavior must rely on biochemical
compounds produced by Toxoplasma. The more research
is done on it, the more manipulation appears to be widespread – the result of natural selection – in pathogen–host
interactions.
11.1.3 Beneficial Interactions
Some interactions, unlike the previous ones in which conflict
predominates, provide benefits to the different partners.
Among beneficial interactions, commensalism* is the only
one for which the positive effects are exerted on only one of
the two partners. This interaction is therefore the counterpart
of amensalism. Cooperation* is a mutually beneficial, facultative relationship, this facultative aspect distinguishing it
from symbiosis.
In the case of mutualism* or symbiosis (Box 11.1), both
species benefit from the interaction. It is interesting to note
that, as in conflictual interactions, genetic distance is not a
barrier to partnership: one can observe mutualistic
interactions between plants and animals, bacteria and
vertebrates, and fungi and plants; a complete list would be
endless. Mutualistic interactions have played a very important role in evolution (Fig. 11.1). Mitochondria and
chloroplasts – and certainly other structures – of eukaryotic
cells are nothing more than ancient bacteria. In fact, the
modern eukaryotic cell, the cornerstone of all multicellular
organisms, can be seen as a collection of bacteria. Such
interactions are also reversible: for example, the agent of
malaria, the Plasmodium protozoan, has lost its chloroplasts
(which its ancestors had, as indicated by recently discovered
remnants of these chloroplasts), the same way tapeworms
lost their gut.
11 Microorganisms and Biotic Interactions
397