positively associated with oyster tissues, including V. crassostreae and V. splendidus
populations. A recent study based on 16S rRNA gene-based analysis (Vezzulli et al.
2018) investigated the different susceptibility of oysters and mussels to Vibrio
colonisation. The authors showed that in hemolymph and the digestive gland of
Crassostreae gigas, vibrios accounted the larger fraction of the microbiota compared
to Mytilus galloprovincialis, suggesting that oysters may provide better conditions
for their survival.
Indeed, a study characterising the structural variation of the microbiota of
M. galloprovincialis at the tissue scale (Musella et al. 2020) showed specific
alterations in the different tissues (gill, digestive gland, foot, stomach and hemolymph) analysed. The dominant microbial families were different in each tissue
reflecting an adaption to the respective tissue. For instance, the digestive gland was
characterised by the presence of bacteria that ferment complex polysaccharides of
dietary origin (Ruminococcacea and Lachnospiraceae), stomach and foot were
dominated by anaerobic microorganisms (Spirochaetaceae and Mycoplasmataceae,
respectively), while gills and hemolymph were colonised mainly by aerobic marine
bacteria, such as Alteromonadales and Hahellaceae in gills and Flavobacteriaceae
in hemolymph. This study (Musella et al. 2020) supports the idea of there being an
active role of hemolymph and gills in the selection of microbial symbionts composing the microbiota of internal tissues. Interestingly, vibrios were present at low
relative abundances (3% to 5%) being a subdominant genera and they were more
represented in the surrounding seawater (10.6%) of the mussel farm reflecting those
hosts having less affinity to Vibrio species when compared with other marine
organisms, such as oysters.
As a result of their nature, a persistent contact between the bivalve host and
potential Vibrio pathogens over several generations can lead into an arm race like
host parasite interaction with fast coevolutionary changes on both sides. On the one
hand, bivalves possess a powerful innate immune system that relies mainly on
circulating hemocytes, which can infiltrate tissues as well, plus soluble factors
such as reactive oxygen species (ROS) and antimicrobial peptides (AMPs), and
tissue-mediated immune responses, creating a hostile environment for vibrios
(Destoumieux-Garzón et al. 2014; Canesi and Pruzzo 2016; Gerdol et al. 2018).
These defence mechanisms seem to be shared by the hemocytes of different molluscs, although comparative studies are not available. An essential question arises on
how the host discriminates commensal vibrios from pathogenic strains when they
face an encounter with Vibrio populations. The study carried out by Rubio et al.
(2019) demonstrated that vibrios from the Splendidus clade were recognised by the
oyster immune system, but only nonvirulent strains were controlled and virulent
Vibrio species use different mechanisms to evade host cellular defences. In general,
hemocytes have been found to play a key role in Vibrio-bivalve interactions by
controlling infections through phagocytosis and by producing antimicrobial compounds. However, differences in susceptibility to Vibrio infections between oysters
and mussels, the latter of which are less affected, appear to be related to the
expression of AMPs and soluble plasma factors.
On the other hand, and similarly to other microbes adapted to metazoan hosts,
vibrios either infecting or simply colonising bivalves have developed
24 Coevolution of Molluscs and Their Microbes
519
populations. A recent study based on 16S rRNA gene-based analysis (Vezzulli et al.
2018) investigated the different susceptibility of oysters and mussels to Vibrio
colonisation. The authors showed that in hemolymph and the digestive gland of
Crassostreae gigas, vibrios accounted the larger fraction of the microbiota compared
to Mytilus galloprovincialis, suggesting that oysters may provide better conditions
for their survival.
Indeed, a study characterising the structural variation of the microbiota of
M. galloprovincialis at the tissue scale (Musella et al. 2020) showed specific
alterations in the different tissues (gill, digestive gland, foot, stomach and hemolymph) analysed. The dominant microbial families were different in each tissue
reflecting an adaption to the respective tissue. For instance, the digestive gland was
characterised by the presence of bacteria that ferment complex polysaccharides of
dietary origin (Ruminococcacea and Lachnospiraceae), stomach and foot were
dominated by anaerobic microorganisms (Spirochaetaceae and Mycoplasmataceae,
respectively), while gills and hemolymph were colonised mainly by aerobic marine
bacteria, such as Alteromonadales and Hahellaceae in gills and Flavobacteriaceae
in hemolymph. This study (Musella et al. 2020) supports the idea of there being an
active role of hemolymph and gills in the selection of microbial symbionts composing the microbiota of internal tissues. Interestingly, vibrios were present at low
relative abundances (3% to 5%) being a subdominant genera and they were more
represented in the surrounding seawater (10.6%) of the mussel farm reflecting those
hosts having less affinity to Vibrio species when compared with other marine
organisms, such as oysters.
As a result of their nature, a persistent contact between the bivalve host and
potential Vibrio pathogens over several generations can lead into an arm race like
host parasite interaction with fast coevolutionary changes on both sides. On the one
hand, bivalves possess a powerful innate immune system that relies mainly on
circulating hemocytes, which can infiltrate tissues as well, plus soluble factors
such as reactive oxygen species (ROS) and antimicrobial peptides (AMPs), and
tissue-mediated immune responses, creating a hostile environment for vibrios
(Destoumieux-Garzón et al. 2014; Canesi and Pruzzo 2016; Gerdol et al. 2018).
These defence mechanisms seem to be shared by the hemocytes of different molluscs, although comparative studies are not available. An essential question arises on
how the host discriminates commensal vibrios from pathogenic strains when they
face an encounter with Vibrio populations. The study carried out by Rubio et al.
(2019) demonstrated that vibrios from the Splendidus clade were recognised by the
oyster immune system, but only nonvirulent strains were controlled and virulent
Vibrio species use different mechanisms to evade host cellular defences. In general,
hemocytes have been found to play a key role in Vibrio-bivalve interactions by
controlling infections through phagocytosis and by producing antimicrobial compounds. However, differences in susceptibility to Vibrio infections between oysters
and mussels, the latter of which are less affected, appear to be related to the
expression of AMPs and soluble plasma factors.
On the other hand, and similarly to other microbes adapted to metazoan hosts,
vibrios either infecting or simply colonising bivalves have developed
24 Coevolution of Molluscs and Their Microbes
519