countermeasures against host innate immunity which include dampening immune
defences. Mechanisms of virulent strains that allow vibrios to evade host cellular
defences have been shown to be microbial species-specific (Rubio et al. 2019).
Among these mechanisms, resistance and tolerance to host antimicrobials compounds (ROS and AMPs) and heavy metals have been described in different Vibrio
pathogens including V. cholerae and V. tasmaniensis LGP32 (Mathur and Waldor
2004; Mathur et al. 2007; Duperthuy et al. 2011; Destoumieux-Garzón et al. 2014;
Vanhove et al. 2016). Mechanisms to escape from hemocyte control were also
identified in V. tasmaniensis and V. crassostreae (Vanhove et al. 2016; Piel et al.
2019; Rubio et al. 2019). For instance, hemocyte evasion has been shown to be
dependent on different molecular effectors, including the T6SS active against
eukaryotic cells in V. tasmaniensis, or the ancestral R5–7 virulence factor together
with a T6SS carried by a plasmid in V. crassostreae (Piel et al. 2019; Rubio et al.
2019). Besides, modifications of the LPS or the peptidoglycan have been linked in
some bacterial species with the ability to escape from host recognition by becoming
less immunogenic or by reducing the host immunity response (Charroux et al. 2018;
Destoumieux-Garzón et al. 2020), although these mechanisms are poorly studied in
Vibrio species. A recent study (Wegner et al. 2019) explored the molecular targets of
coevolutionary interactions between Pacific oysters and their local Vibrio communities, by analysing the genomic traits of nine Vibrio strains from the Splendidusclade with opposite virulence patterns. Those authors were able to identify two genes
that could explain the host specific virulence patterns that represent local adaptation
and may constitute targets for coevolutionary interactions, and which are involved in
functions very different from other previously described virulence genes.
Despite our increasing efforts to completely unravel the mechanisms underlying
the complex interactions and balance between vibrios and bivalves there is still a
long way to go before we fully understand this partnership.
24.5 Concluding Remarks and Future Trends
Permanent associations between a macrobe host and microbe symbionts are widespread in nature involving mutualistic, commensal or parasitic relationships. Assuming these complex associations as a unit, termed the holobiont, natural selection acts
by removing disadvantageous mutations in both partners and thereby promoting
successful coevolution of the combined entity.
These associations have been less studied in marine organisms than in terrestrial
animals, although representative examples do exist for marine environments. The
A. fischeri-squid system has proven to be an excellent opportunity to study the
principles of microbe-host interactions and the coevolution of these two organisms.
Selection of the more suitable symbiont, based upon the availability of a high
microbial diversity, is a crucial step to establish the permanent mutualistic
association.
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A. Lasa and J. L. Romalde
defences. Mechanisms of virulent strains that allow vibrios to evade host cellular
defences have been shown to be microbial species-specific (Rubio et al. 2019).
Among these mechanisms, resistance and tolerance to host antimicrobials compounds (ROS and AMPs) and heavy metals have been described in different Vibrio
pathogens including V. cholerae and V. tasmaniensis LGP32 (Mathur and Waldor
2004; Mathur et al. 2007; Duperthuy et al. 2011; Destoumieux-Garzón et al. 2014;
Vanhove et al. 2016). Mechanisms to escape from hemocyte control were also
identified in V. tasmaniensis and V. crassostreae (Vanhove et al. 2016; Piel et al.
2019; Rubio et al. 2019). For instance, hemocyte evasion has been shown to be
dependent on different molecular effectors, including the T6SS active against
eukaryotic cells in V. tasmaniensis, or the ancestral R5–7 virulence factor together
with a T6SS carried by a plasmid in V. crassostreae (Piel et al. 2019; Rubio et al.
2019). Besides, modifications of the LPS or the peptidoglycan have been linked in
some bacterial species with the ability to escape from host recognition by becoming
less immunogenic or by reducing the host immunity response (Charroux et al. 2018;
Destoumieux-Garzón et al. 2020), although these mechanisms are poorly studied in
Vibrio species. A recent study (Wegner et al. 2019) explored the molecular targets of
coevolutionary interactions between Pacific oysters and their local Vibrio communities, by analysing the genomic traits of nine Vibrio strains from the Splendidusclade with opposite virulence patterns. Those authors were able to identify two genes
that could explain the host specific virulence patterns that represent local adaptation
and may constitute targets for coevolutionary interactions, and which are involved in
functions very different from other previously described virulence genes.
Despite our increasing efforts to completely unravel the mechanisms underlying
the complex interactions and balance between vibrios and bivalves there is still a
long way to go before we fully understand this partnership.
24.5 Concluding Remarks and Future Trends
Permanent associations between a macrobe host and microbe symbionts are widespread in nature involving mutualistic, commensal or parasitic relationships. Assuming these complex associations as a unit, termed the holobiont, natural selection acts
by removing disadvantageous mutations in both partners and thereby promoting
successful coevolution of the combined entity.
These associations have been less studied in marine organisms than in terrestrial
animals, although representative examples do exist for marine environments. The
A. fischeri-squid system has proven to be an excellent opportunity to study the
principles of microbe-host interactions and the coevolution of these two organisms.
Selection of the more suitable symbiont, based upon the availability of a high
microbial diversity, is a crucial step to establish the permanent mutualistic
association.
520
A. Lasa and J. L. Romalde