associations between higher organisms, including animals and plants. Interesting
examples of microbial symbionts associated with a specific higher organism are also
known. There, surprisingly significant roles are played by the relatively invisible
microbial partner providing insights into the significance of partnerships even
though the microbes often are presumed to be “lower” organisms.
In contrast to cases involving higher organisms, the details of interactions
between microorganisms have not yet been fully understood. This is simply because
the participants often are not visible to the naked eye. Furthermore, the manipulation
techniques typically are based on pure cultivation of the partnering species, which
prevents observing associations between different microorganisms. However, studies focusing on the physiology of syntrophic microorganisms, whose growth
depends on an interaction with other microbes, have progressed the understanding
of the mechanism underlying this mutualism.
A well-known instance of syntrophic interaction is the nutritional connection in
methanogenic microbial communities (Schink 1997). This complex consortium
consists of anaerobic bacteria digesting polysaccharides, oligosaccharides, and
organic acids, as well as methane-producing archaea. These constituents are nutritionally interlinked; hence, pure isolation of each constituent either cannot be
performed or requires highly specialized manipulation conditions.
Thus, each constituent of the methanogenic microbial community is physically
challenged. In addition, they are highly sensitive to oxygen. This makes it likely for
these weak organisms not to survive by themselves and would become extinct.
However, the community widely occurs in the natural environment. The versatility
of the complex community implies that the consortium constituents are intimately
linked not only by mutual dependence in terms of nutritional supply but some multidimensional mechanism that keeps the rigid community structure intact, ensuring
every constituent’s survival by active involvement in the organization of the
community.
25.2.2.3 Communication with Signals
The discovery of surprising social behavior in prokaryotic cells was triggered by the
identification of the N-acyl-homoserine lactone (AHL) auto-inducer molecule
(Horinouchi et al. 2010; Schuster et al. 2013). Originally, the compound was
identified in a culture of Vibrio spp., a group that contains bioluminescent marine
bacteria associated with marine animals, as a chemical factor essential for the
expression of its autoinducible bacterial luciferase that catalyzes light emission. It
became clear that the ability to synthesize and respond to this type of autoinducer is
widely distributed among gram-negative bacteria. The AHL receptor protein serves
as a transcriptional regulator that induces specific functions, including light emission, antibiotic production, and biofilm formation. Also, AHL is presumed to serve
as an indicator of cell density. Accordingly, an increase in the number of cells in a
specific bacterial group turns on the social behavior mode via elevated AHL
532
K. Ueda
examples of microbial symbionts associated with a specific higher organism are also
known. There, surprisingly significant roles are played by the relatively invisible
microbial partner providing insights into the significance of partnerships even
though the microbes often are presumed to be “lower” organisms.
In contrast to cases involving higher organisms, the details of interactions
between microorganisms have not yet been fully understood. This is simply because
the participants often are not visible to the naked eye. Furthermore, the manipulation
techniques typically are based on pure cultivation of the partnering species, which
prevents observing associations between different microorganisms. However, studies focusing on the physiology of syntrophic microorganisms, whose growth
depends on an interaction with other microbes, have progressed the understanding
of the mechanism underlying this mutualism.
A well-known instance of syntrophic interaction is the nutritional connection in
methanogenic microbial communities (Schink 1997). This complex consortium
consists of anaerobic bacteria digesting polysaccharides, oligosaccharides, and
organic acids, as well as methane-producing archaea. These constituents are nutritionally interlinked; hence, pure isolation of each constituent either cannot be
performed or requires highly specialized manipulation conditions.
Thus, each constituent of the methanogenic microbial community is physically
challenged. In addition, they are highly sensitive to oxygen. This makes it likely for
these weak organisms not to survive by themselves and would become extinct.
However, the community widely occurs in the natural environment. The versatility
of the complex community implies that the consortium constituents are intimately
linked not only by mutual dependence in terms of nutritional supply but some multidimensional mechanism that keeps the rigid community structure intact, ensuring
every constituent’s survival by active involvement in the organization of the
community.
25.2.2.3 Communication with Signals
The discovery of surprising social behavior in prokaryotic cells was triggered by the
identification of the N-acyl-homoserine lactone (AHL) auto-inducer molecule
(Horinouchi et al. 2010; Schuster et al. 2013). Originally, the compound was
identified in a culture of Vibrio spp., a group that contains bioluminescent marine
bacteria associated with marine animals, as a chemical factor essential for the
expression of its autoinducible bacterial luciferase that catalyzes light emission. It
became clear that the ability to synthesize and respond to this type of autoinducer is
widely distributed among gram-negative bacteria. The AHL receptor protein serves
as a transcriptional regulator that induces specific functions, including light emission, antibiotic production, and biofilm formation. Also, AHL is presumed to serve
as an indicator of cell density. Accordingly, an increase in the number of cells in a
specific bacterial group turns on the social behavior mode via elevated AHL
532
K. Ueda