organ that will develop as the ‘light organ’ of the squid. Selection upon the microbial
community maintains partner fidelity and prevents non-bioluminescent bacteria
from colonising the organ (Mandel and Dunn 2016). The microbial population
interact with the ciliated epithelial fields, an organ coated by a mucus layer (peptidoglycan derivatives), which facilitates bacterial aggregation (Nyholm et al. 2000;
Stubbendieck et al. 2019). Then, nitric oxide (NO) production by the eukaryotic cells
appears to be a specificity determinant in the establishment and maintenance of the
host-microbe association (Wang and Ruby 2011). Several studies have shown that
the ability to sense and detoxify NO is important for symbiotic specificity that also
acts as a temporal signal to modulate bacterial gene expression promoting successful
colonisation (Wang et al. 2010; Tucker et al. 2010; Mandel and Dunn 2016). Only
when the selected symbiont has colonised the ciliated appendages, does morphogenesis of the host light organ start (apoptosis, hemocyte infiltration and tissue
regression) and recruiting of bacteria occur in the appendages. This process is
promoted by the production of tracheal cytotoxin by A. fischeri, which, at the
same time, diminishes the production of NO by the squid (Altura et al. 2011).
Finally, the bacteria proceed to migrate through the pores in response to a gradient
of chitibiose produced in the duct and into the deep crypts of the light organ (Mandel
et al. 2012). In the organ crypts, proliferation and reprogramming of the bacteroid
occurs which results in the loss of the flagella (Ruby and Asato 1993). A major
determinant of squid colonisation specificity is biofilm formation of A. fischeri (Yip
et al. 2005; Mandel et al. 2009), which is facilitated by the squid through specialised
mucosal structures and generating environmental conditions to select A. fischeri over
non-symbiotic bacteria (Nyholm et al. 2002; Nyholm and McFall-Ngai 2003).
An important aspect on the symbiosis is the selection of the appropriate partner,
and the ability of individual bacteria to produce light seems to be important in
successful persistence of the interaction. Several studies (Visick et al. 2000; Tong
et al. 2009; Koch et al. 2013) have supported the idea that the host is detecting light
production by bacterial cells and possibly altering physiological conditions to
sanction the non-luminiscent strains. The host may even eject those light deficient
strains allowing future recolonisation. The fact is that the squid influences the local
A. fischeri populations by enriching the planktonic community with those strains that
are more capable at symbiosis (Lee and Ruby 1992). The squid subjects its symbiotic
partners to daily cycles of expulsion (‘venting’) and regrowth of nearly 95% of light
organ populations to >10
5 bacteria, thus increasing the relative abundance of their
light organ inhabitants in the surrounding environment (Lee and Ruby 1992). As a
free-living bacteria, evolution while away from the host is likely to occur, which has
the potential to impact microbial-host interaction (Soto et al. 2019). Increased
microbial motility and biofilm capacity have been shown to be beneficial traits for
host colonisation (Stabb and Visick 2013). In marine habitats, an increased biofilm
capacity can facilitate attachment to suspended particulate matter or sediment, where
nutrients might be concentrated (Thompson and Polz 2006; McDougald and
Kjelleberg 2006). Thus, environmental adaptive evolution acting on these traits
might have positive impact on host colonisation (Soto et al. 2019).
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A. Lasa and J. L. Romalde
community maintains partner fidelity and prevents non-bioluminescent bacteria
from colonising the organ (Mandel and Dunn 2016). The microbial population
interact with the ciliated epithelial fields, an organ coated by a mucus layer (peptidoglycan derivatives), which facilitates bacterial aggregation (Nyholm et al. 2000;
Stubbendieck et al. 2019). Then, nitric oxide (NO) production by the eukaryotic cells
appears to be a specificity determinant in the establishment and maintenance of the
host-microbe association (Wang and Ruby 2011). Several studies have shown that
the ability to sense and detoxify NO is important for symbiotic specificity that also
acts as a temporal signal to modulate bacterial gene expression promoting successful
colonisation (Wang et al. 2010; Tucker et al. 2010; Mandel and Dunn 2016). Only
when the selected symbiont has colonised the ciliated appendages, does morphogenesis of the host light organ start (apoptosis, hemocyte infiltration and tissue
regression) and recruiting of bacteria occur in the appendages. This process is
promoted by the production of tracheal cytotoxin by A. fischeri, which, at the
same time, diminishes the production of NO by the squid (Altura et al. 2011).
Finally, the bacteria proceed to migrate through the pores in response to a gradient
of chitibiose produced in the duct and into the deep crypts of the light organ (Mandel
et al. 2012). In the organ crypts, proliferation and reprogramming of the bacteroid
occurs which results in the loss of the flagella (Ruby and Asato 1993). A major
determinant of squid colonisation specificity is biofilm formation of A. fischeri (Yip
et al. 2005; Mandel et al. 2009), which is facilitated by the squid through specialised
mucosal structures and generating environmental conditions to select A. fischeri over
non-symbiotic bacteria (Nyholm et al. 2002; Nyholm and McFall-Ngai 2003).
An important aspect on the symbiosis is the selection of the appropriate partner,
and the ability of individual bacteria to produce light seems to be important in
successful persistence of the interaction. Several studies (Visick et al. 2000; Tong
et al. 2009; Koch et al. 2013) have supported the idea that the host is detecting light
production by bacterial cells and possibly altering physiological conditions to
sanction the non-luminiscent strains. The host may even eject those light deficient
strains allowing future recolonisation. The fact is that the squid influences the local
A. fischeri populations by enriching the planktonic community with those strains that
are more capable at symbiosis (Lee and Ruby 1992). The squid subjects its symbiotic
partners to daily cycles of expulsion (‘venting’) and regrowth of nearly 95% of light
organ populations to >10
5 bacteria, thus increasing the relative abundance of their
light organ inhabitants in the surrounding environment (Lee and Ruby 1992). As a
free-living bacteria, evolution while away from the host is likely to occur, which has
the potential to impact microbial-host interaction (Soto et al. 2019). Increased
microbial motility and biofilm capacity have been shown to be beneficial traits for
host colonisation (Stabb and Visick 2013). In marine habitats, an increased biofilm
capacity can facilitate attachment to suspended particulate matter or sediment, where
nutrients might be concentrated (Thompson and Polz 2006; McDougald and
Kjelleberg 2006). Thus, environmental adaptive evolution acting on these traits
might have positive impact on host colonisation (Soto et al. 2019).
516
A. Lasa and J. L. Romalde