Identification and Genome Analysis of Vibrio coralliilyticus Causing Mortality of Pacific Oyster (Crassostrea gigas) Larvae

Vibrio coralliilyticus is known as a coral pathogen that also infects marine bivalve larvae worldwide. It is considered to be one of the major constraints in artificial marine bivalve seed production as it causes mortality. In this study, we first isolated and characterized a high virulent of V. coralliilyticus designated as SNUTY-1 that was the cause of Pacific oyster larvae mortality in Korea. In the pathogenicity test, exposure to 2.14 × 105 CFU/mL for 24 h caused mortality to 88.65 ± 2.4% of the tested healthy Pacific oyster larvae. SNUTY-1 showed anti-microbial resistance to β-lactams, such as penicillins, cephalosporins, and carbapenems. We sequenced and assembled the complete genome of SNUTY-1 (5,842,676 bp), consisting of two chromosomes (Chr I and Chr II) and two plasmids (pSNUTY1 and pSNUTY2). The COG functional analysis confirmed that Chr I had more genes associated with basic cellular functions in comparison to Chr II. The results of the phylogenetic trees based on OrthoANI values indicated that the SNUTY-1 was closely related to V. coralliilyticus strains. SNUTY-1 had a unique plasmid (pSNUTY2), which could mean that the Korean isolate is different from other sequenced V. coralliilyticus strains from different geographical origins. Toxic proteins such as cytolysin/hemolysin and extracellular metalloprotease genes were encoded on Chr I and Chr II of SNUTY-1. These data facilitate the control of V. coralliilyticus infections in aquaculture by providing valuable insights into the biodiversity of this organism and valuable information for the study of virulence factors.

Microscale tracking of coral disease reveals timeline of infection and heterogeneity of polyp fate

Coral disease is often studied at scales ranging from single colonies to the entire reef. This is particularly true for studies following disease progression through time. To gain a mechanistic understanding of key steps underlying infection dynamics, it is necessary to study disease progression, and host-pathogen interactions, at relevant microbial scales. Here we provide a dynamic view of the interaction between the model coral pathogen Vibrio coralliilyticus and its coral host Pocillopora damicornis at unprecedented spatial and temporal scales. This view is achieved using a novel microfluidics-based system specifically designed to allow microscopic study of coral infection in-vivo under controlled environmental conditions. Analysis of exudates continuously collected at the system’s outflow, allows a detailed biochemical and microbial analyses coupled to the microscopic observations of the disease progression. The resulting multilayered dataset provides the most detailed description of a coral infection to-date, revealing distinct pathogenic processes as well as the defensive behavior of the coral host. We provide evidence that infection in this system occurs following ingestion of the pathogen, and may then progress through the gastrovascular system. We further show infection may spread when pathogens colonize lesions in the host tissue. Copious spewing of pathogen-laden mucus from the polyp mouths results in effective expulsion of the pathogen from the gastrovascular system, possibly serving as a first line of defense. A secondary defense mechanism entails the severing of calicoblastic connective tissues resulting in the controlled isolation of diseased polyps, or the survival of individual polyps within infected colonies. Further investigations of coral-pathogen interactions at these scales will help to elucidate the complex interactions underlying coral disease, as we as the versatile adaptive response of the coral ecosystems to fluctuating environments.

Alien vs. Predator: Pathogens open niche space for opportunists, unless controlled by predators

Coral microbiomes are known to play important roles in organismal health, response to environmental stress, and resistance to disease. Pathogens invading the coral microbiome encounter diverse assemblages of resident bacteria, ranging from defensive and metabolic symbionts to opportunistic bacteria that may turn harmful in compromised hosts. However, little is known about how these bacterial interactions influence the overall structure, stability, and function of the microbiome during the course of pathogen challenge. We sought to test how coral microbiome dynamics were affected by interactions between two of its members: Vibrio coralliilyticus, a known temperature-dependent coral pathogen, and Halobacteriovorax, a unique bacterial predator of Vibrio and other gram-negative bacteria. We challenged specimens of the important reef-building coral Montastraea cavernosa with Vibrio coralliilyticus pathogens in the presence or absence of Halobacteriovorax predators, and monitored microbial community dynamics with 16S rRNA gene time-series. In addition to its direct effects on corals, pathogen challenge reshaped coral microbiomes in ways that allowed for secondary blooms of opportunistic bacteria. As expected, Vibrio coralliilyticus addition increased the infiltration of Vibrio into coral tissues. This increase of Vibrios in coral tissue was accompanied by increased richness, and reduced stability (increased beta-diversity) of the rest of the microbiome, suggesting strong secondary effects of pathogen invasion on commensal and mutualistic coral bacteria. Moreover, after an initial increase in Vibrios, two opportunistic lineages (Rhodobacterales and Cytophagales) increased in coral tissues, suggesting that this pathogen opens niche space for opportunists. Based on the keystone role of predators in many ecosystems, we hypothesized that Halobacteriovorax predators might help protect corals by consuming gram-negative pathogens. In keeping with a protective role, Halobacteriovorax addition alone had only minor effects on the microbiome, and no infiltration of Halobacteriovorax into coral tissues was detected in amplicon libraries. Simultaneous challenge with both pathogen and predator eliminated detectable V. corallyticus infiltration into coral tissue samples, ameliorated changes to the rest of the coral microbiome, and prevented secondary blooms of opportunistic Rhodobacterales and Cytophagales. Thus, we show that primary infection by a coral pathogen is sufficient to cause increases in opportunists, as seen in correlational studies. These data further provide a proof-of-principle demonstration that, under certain circumstances, host-associated bacterial predators can mitigate the ability of pathogens to infiltrate host tissue, and stabilize the microbiome against complex secondary changes that favor growth of opportunistic lineages.