Bioinformatics analysis revealed 910,604 reads, which were used to classify bacterial OTUs and their relative abundance for each sample (Table 5). Rarefaction curves, based on maximum OTU richness were generated and all the samples reached asymptotic levels, indicating sufficient sequencing depth to detect the highest number of bacterial OTUs via 16S amplicon sequencing (Fig 1). The healthy HP sample (CKH) showed a shift toward higher OTU richness, with 3552 OTUs and a Shannon alpha index of 9.7. EHP-infected HP samples (IAH, IEH and ISH) had 1943, 980 and 499 OTUs with Shannon alpha indexes of 4.7, 4.8 and 4.0, respectively.
Our analysis showed substantial changes in the shrimp microbiome abundance during EHP infection. Surprisingly, ANOSIM analysis revealed no significant difference (P=0.667) in the core microbiome between healthy and EHP-infected hepatopancreas samples. This suggests that EHP might alter the abundance of specific pathogenic bacteria within the core microbiome of EHP-infected samples without major changes in the baseline microbiome composition. These findings align with studies such as
Wang et al., (2019), who also observed no significant differences in the microbiome composition between healthy and white spot syndrome virus (WSSV)-infected shrimp samples. Similarly,
Holt et al., (2021) found no significant microbiome differences between healthy and diseased shrimp larvae or those affected by blue shell syndrome.
EHP and its relationship with WFS and AHPND
Kumar et al., (2022) documented maximum EHP load when the shrimps were showing symptoms of WFS, with no conclusive evidence that high EHP spore concentration is the sole reason for such white fecal manifestation. Traditionally, WFS and AHPND have been associated with over-representation of
Vibrio parahaemolyticus in infected shrimps
(Boopathi et al., 2023). In recent studies, the genera
Vibrio and
Propionigenium, alongside species such as
V. harveyi and
V. alginolyticus have been identified as potential causative agents of white feces in shrimp in the presence of EHP spores
(Subash et al., 2023; Munkongwongsiri et al., 2022; Aranguren et al., 2021). In contrast,
Tangprasittipap et al., (2013), implied that EHP is not the causative agent for WFS and that another parasite, sharing 99% sequence similarity with EHP, might be the contributing factor to WFS. These conflicting outcomes create uncertainty regarding EHP’s role in WFS. Our analysis unveils new insights into these intricate interactions.
EHP-infected HP samples, IAH, IEH and ISH were dominated by the Vibrionales (order), with abundances of 61.80%, 65.0% and 77.78%, respectively, against a lower abundance of 0.23% in CKH (Table 6). This is noteworthy, since dominant Vibrionales abundance is a pattern observed during AHPND infection
(Kumar et al., 2020; Restrepo et al., 2021). Similarly,
Photobacterium damselae was one of the most abundant bacteria in IAH (7.9%), IEH (21.8%) and ISH (27.9%). whereas CKH displayed only a 0.1% abundance of
P. damselae. Additionally, the genus
Vibrio showed high abundance in IAH, IEH and ISH (0.4%, 6.6% and 15.9%), while CKH exhibited a notably low 0.005% level (Fig 3). These high abundances of Vibrionales,
Vibrio and
P. damselae in EHP-infected HP samples, suggests a potential pathobiome-like associations along with EHP, as evidenced by its close clustering in the hierarchical clustering analysis, with one branch predominantly comprised of the genus
Photobacterium and another branch displaying subdivisions that include the
Vibrio genus in IAH, IEH and ISH, respectively (Fig 4). This aligns with the study conducted by
Somboon et al., (2012), where seven different
Vibrio spp. along with
P. damselae were isolated from WFS-affected shrimp samples. Notably, the higher abundance level of Vibrionales was not only limited to hepatopancreas, but it was also reported in the EHP-infected shrimp gut
(Babu et al., 2023). Additionally, the microbiome plasticity we observed may possibly stem from the higher abundance of the genus
Vibrio and this is consistent with multiple studies that have highlighted the role of
Vibrio in microbiome disruption in shrimp gut samples
(Alvarez et al., 2022; Hou et al., 2018; Boopathi et al., 2023).
It was found that an unclassified OTU from the
Pseudoalteromonadaceae family was most common in all of the EHP-infected samples (52.6%, 35.4% and 32.1%). This is a noteworthy finding, since certain bacteria from the
Pseudoalteromonadaceae family are highly prevalent during white fecal disease in shrimps
(Alfiansah et al., 2020). This further supports the hypothesis that EHP plays a major role in favoring the growth of bacteria that may contribute to the development of white feces syndrome as a co-infection in shrimp
(Prachumwat et al., 2021; Munkongwongsiri et al., 2022).
One concerning aspect is that EHP may serve as a risk factor for the occurrence of AHPND infection
(Aranguren et al., 2017), or
vice versa. This possibility stems from the fact that multiple
Vibrio spp., such as
V. campbellii,
V. owensii,
V. harveyi and
V. alginolyticus, also cause AHPND infection in addition to
V. parahaemolyticus by plasmid transfer of pirAB genes or through pathobiome-like associations
(Dong et al., 2017; Liu et al., 2018; Subash et al., 2023; Zhang et al., 2020; Muthukrishnan et al., 2019). Interestingly
, V. shilonii, an emerging AHPND associated bacteria, that also carries the pirA and pirB toxin genes
(Quang et al., 2020), showed a moderate abundance of 3.2% and 0.3% in IEH and ISH.
Vibrio harveyi also showed a considerable abundance of 0.1%, 0.4% and 0.6% in IAH, IEH and ISH. In contrast, CKH exhibited no abundance for
V. shilonii and
V. harveyi, respectively.
The exact mechanism by which EHP favours a higher abundance of pathogenic bacteria remains unclear. It is speculated that EHP negatively influences the non-specific immune responses, leading to decreased activity of superoxide anion (SOA), catalase (CAT), alkaline phosphatase (ALP) and prophenoloxidase (PPO)
(Kumar et al., 2022), which may favour the growth of opportunistic pathogenic bacteria. This possibly elucidates the dominance of pathogenic bacteria in the EHP-infected HP samples.
Microbiome abundance alterations in shrimp hepatopancreas during EHP infection
Principal Coordinate Analysis (PCoA) showed clear clustering patterns, that indicates a clear difference in the microbiome composition of healthy and EHP-infected
P. vannamei HP samples. The first principal component (PC1) explained 48.94% of the total variation, with the second principal component (PC2) indicating 30.41%, collectively accounting for 79.35% of the variance (Fig 5). It is noteworthy that there is a significant difference in microbiome abundance between healthy and EHP-infected HP samples.
The CKH sample distinctly diverged and formed a separate cluster from IAH, IEH and ISH, while all the EHP-infected samples formed closely clustered groups. Particularly, IAH and IEH exhibited a closer sub-cluster within the EHP-infected samples (Fig 5). These clustering patterns suggest a shared, altered microbiome population shift attributable to EHP infection. These findings are consistent with the concept of specific pathogenicity exhibited by EHP in the shrimp hepatopancreas organ and contemporary studies have already emphasised the significance of maintaining a balanced microbiome for optimal host functioning
(Khac et al., 2018; Biedermann and Rogler, 2015;
Sommer et al., 2017).
Promising biomarkers for the health assessment of shrimp hepatopancreas
CKH exhibited a higher diversity of OTUs, occupying the top 20 positions when compared to the EHP-infected HP samples. At the family level, the highest abundance was exhibited by
Sphingomonadaceae at 7.7% in CKH (Fig 2). In contrast, its abundance was extremely low, with 0.16% in IAH, 0.52% in IEH and 0.2% in ISH. Interestingly, one of the important traits of bacteria, is their ability to form biofilm for their survival, as seen in
Sphingomonadaceae bacteria
(Fazeli-Nasab et al., 2022). The decreased abundance of such bacteria explains EHP’s extreme plasticity-inducing nature.
Chitinophagaceae, predominantly found in soil and marine habitats, have exhibited decreased abundance, possibly due to the influence of EHP (
Glaeser and Kämpfer, 2014). It ranked as the second most abundant OTU in CKH at 6.0%, against a low abundance level of 0.06%, 0.1% and 0.03% in IAH, IEH and ISH (Table 6). Similarly, the bacterial genus
Kaistobacter, a beneficial plant bacterium, also showed reduced abundance in EHP-infected HP samples, yet it occupied the second most abundant OTU at 5.3% for CKH. Eventhough the role of these bacteria in relation to shrimp is not clear, the impact on the sustenance of bacteria such as
Chitinophagaceae and
Kaistobacter, which are involved in the processes of organic matter removal and hydrocarbon degradation
(Hou et al., 2021; Borowik et al., 2021), elucidates the broader impact of EHP against other bacteria that may potentially carry a beneficial role in the ecosystem. It is also important to note that, certain bacterium such as
Cytophagaceae,
Verrucomicrobiaceae and
Xanthomonadaceae, specifically exhibited higher abundance in CKH at 3.3%, 4.6% and 3.6%, respectively.
The bacterial genus,
Pseudomonas, which is a pathogenic bacterium for both humans and aquatic animals (
Golemi-Kotra, 2008;
Duman et al., 2021), showed the highest abundance at 5.5% in CKH. This is interesting to note, as recent studies have shown
Pseudomonas as a potential biocontrol agent against
Vibrio sp. and that it is present in higher abundance in healthy shrimp
(Anyanwu et al., 2017; Piamsomboon et al., 2022). Its optimal abundance may have a protective role for the homeostatic microbiome.
Therefore, the distinctly higher abundance of
Sphingomonadaceae, Chitinophagaceae, Cytophagaceae, Verrucomicrobiaceae, Xanthomonadaceae, Pseudomonas and
Kaistobacter (Table 6) in CKH, can be potentially used as a biomarker to assess the overall health of the microbiome of shrimp hepatopancreas.