A brain microbiome in salmonids at homeostasis.

Ectotherms have peculiar relationships with microorganisms. For instance, bacteria are recovered from the blood and internal organs of healthy teleosts. However, the presence of microbial communities in the healthy teleost brain has not been proposed. Here, we report a living bacterial community in the brain of healthy salmonids with bacterial loads comparable to those of the spleen and 1000-fold lower than in the gut. Brain bacterial communities share >50% of their diversity with gut and blood bacterial communities. Using culturomics, we obtained 54 bacterial isolates from the brains of healthy trout. Comparative genomics suggests that brain bacteria may have adaptations for niche colonization and polyamine biosynthesis. In a natural system, Chinook salmon brain microbiomes shift from juveniles to reproductively mature adults. Our study redefines the physiological relationships between the brain and bacteria in teleosts. This symbiosis may endow salmonids with a direct mechanism to sense and respond to environmental microbes.

The telencephalon is a neuronal substrate for systemic inflammatory responses in teleosts via polyamine metabolism.

Systemic inflammation elicits sickness behaviors and fever by engaging a complex neuronal circuitry that begins in the preoptic area of the hypothalamus. Ectotherms such as teleost fish display sickness behaviors in response to infection or inflammation, seeking warmer temperatures to enhance survival via behavioral fever responses. To date, the hypothalamus is the only brain region implicated in sickness behaviors and behavioral fever in teleosts. Yet, the complexity of neurobehavioral manifestations underlying sickness responses in teleosts suggests engagement of higher processing areas of the brain. Using in vivo models of systemic inflammation in rainbow trout, we find canonical pyrogenic cytokine responses in the hypothalamus whereas in the telencephalon and the optic tectum il-1b and tnfa expression is decoupled from il-6 expression. Polyamine metabolism changes, characterized by accumulation of putrescine and decreases in spermine and spermidine, are recorded in the telencephalon but not hypothalamus upon systemic injection of bacteria. While systemic inflammation causes canonical behavioral fever in trout, blockade of bacterial polyamine metabolism prior to injection abrogates behavioral fever, polyamine responses, and telencephalic but not hypothalamic cytokine responses. Combined, our work identifies the telencephalon as a neuronal substrate for brain responses to systemic inflammation in teleosts and uncovers the role of polyamines as critical chemical mediators in sickness behaviors.

Sex-dependent effects of mechanical delousing on the skin microbiome of broodstock Atlantic salmon (Salmo salar L.).

Delousing strategies, including mechanical delousing, are typically used to treat Atlantic salmon (Salmo salar) sea lice infestations. In this study, we evaluate the impact of mechanical delousing (Hydrolicer) on the skin bacterial microbiome of broodstock female and male Atlantic salmon. 16S rDNA sequencing of salmon skin microbial communities was performed immediately before delousing, right after delousing and 2 and 13 days post-delousing (dpd). The skin bacterial community of female salmon was more diverse than that of males at the start of the experiment. Overall, hydrolycer caused losses in alpha diversity in females and increases in alpha diversity in males. Hydrolicer also caused rapid shifts in the skin microbial community composition immediately after delicing in a sex-specific manner. There was a decrease in abundance of Proteobacteria and Bacteriodetes in both female and male salmon, whereas Firmicutes and Tenericutes abundances increased. Interestingly, the female community recovered faster, while the male community remained dysbiotic 13 dpd due to expansions in Bacteroidetes (Pseudomonadaceae) and Firmicutes. Our data suggest that female broodstock are more resilient to Hydrolicer treatment due to their more diverse skin microbiota community, and that sex influences the skin microbial community and therefore host health outcomes during common farming manipulations.

Secretory IgM (sIgM) is an ancient master regulator of microbiota homeostasis and metabolism.

The co-evolution between secretory immunoglobulins (sIgs) and microbiota began with the emergence of IgM over half a billion years ago. Yet, IgM function in vertebrates is mostly associated with systemic immunity against pathogens. sIgA and sIgT are the only sIgs known to be required in the control of microbiota homeostasis in warm- and cold-blooded vertebrates respectively. Recent studies have shown that sIgM coats a large proportion of the gut microbiota of humans and teleost fish, thus suggesting an ancient and conserved relationship between sIgM and microbiota early in vertebrate evolution. To test this hypothesis, we temporarily and selectively depleted IgM from rainbow trout, an old bony fish species. IgM depletion resulted in a drastic reduction in microbiota IgM coating levels and losses in gutassociated bacteria. These were accompanied by bacterial translocation, severe gut tissue damage, inflammation and dysbiosis predictive of metabolic shifts. Furthermore, depletion of IgM resulted in body weight loss and lethality in an experimental colitis model. Recovery of sIgM to physiological levels restores tissue barrier integrity, while microbiome homeostasis and their predictive metabolic capabilities are not fully restituted. Our findings uncover a previously unrecognized role of sIgM as an ancient master regulator of microbiota homeostasis and metabolism and challenge the current paradigm that sIgA and sIgT are the key vertebrate sIgs regulating microbiome homeostasis. One-Sentence Summary: IgM, the most ancient and conserved immunoglobulin in jawed vertebrates, is required for successful symbiosis with the gut microbiota.

The knowns and many unknowns of CNS immunity in teleost fish.

Many disease agents infect the central nervous system (CNS) of teleost fish causing severe losses for the fish farming sector. Yet, neurotropic fish pathogens remain poorly documented and immune responses in the teleost CNS essentially unknown. Previously thought to be devoid of an immune system, the mammalian CNS is now recognized to be protected from infection by diverse immune cells that mostly reside in the meningeal lymphatic system. Here we review the current body of work pertaining immune responses in the teleost CNS to infection. We identify important knowledge gaps with regards to CNS immunity in fish and make recommendations for rigorous experimentation and reporting in manuscripts so that fish immunologists can advance this burgeoning field.

Effects of ploidy and salmonid alphavirus infection on the skin and gill microbiome of Atlantic salmon (Salmo salar).

The microbial communities that live in symbiosis with the mucosal surfaces of animals provide the host with defense strategies against pathogens. These microbial communities are largely shaped by the environment and the host genetics. Triploid Atlantic salmon (Salmo salar) are being considered for aquaculture as they are reproductively sterile and thus cannot contaminate the natural gene pool. It has not been previously investigated how the microbiome of triploid salmon compares to that of their diploid counterparts. In this study, we compare the steady-state skin and gill microbiome of both diploid and triploid salmon, and determine the effects of salmonid alphavirus 3 experimental infection on their microbial composition. Our results show limited differences in the skin-associated microbiome between triploid and diploid salmon, irrespective of infection. In the gills, we observed a high incidence of the bacterial pathogen Candidatus Branchiomonas, with higher abundance in diploid compared to triploid control fish. Diploid salmon infected with SAV3 showed greater histopathological signs of epitheliocystis compared to controls, a phenomenon not observed in triploid fish. Our results indicate that ploidy can affect the alpha diversity of the gills but not the skin-associated microbial community. Importantly, during a natural outbreak of Branchiomonas sp. the gill microbiome of diploid Atlantic salmon became significantly more dominated by this pathogen than in triploid animals. Thus, our results suggest that ploidy may play a role on Atlantic salmon gill health and provide insights into co-infection with SAV3 and C. Branchiomonas in Atlantic salmon.

Radiological signs of periorbital trauma - the Singapore experience.

PURPOSE: To describe the radiological signs and demographic characteristics of patients who suffered facial trauma in South-East Asia. MATERIALS AND METHODS: This is a retrospective cross-sectional study of a 399 patients who presented with facial trauma over a 2-year period in a tertiary referral centre in South-East Asia. Patients with available CT scan films were included. Demographics, bony and soft tissue radiology characteristics were analyzed. RESULTS: Male to female ratio was 320 (80%):79 (20%). Most of the facial trauma was due to Road Traffic Accident. Of 399 patients, 273 (68%) showed radiological signs of bone or soft tissue trauma. Of these 273 patients, left to right side involvement was 114 (41.7%) / 82 (30.03%), and 77 (28.2%) had bilateral involvement. Floor was the most involved wall (229[83.8%]) and the most involved rim was the inferior rim (164[60.07%]). There were 39 (14.2%) patients with blowout fracture. Bony nasolacrimal duct was broken in 56 (20.5%) patients. Optic canal fracture was found in 7 (2.5%) patients and it was associated with roof and superior rim fracture (P-values were 0.016, 0.046, respectively). Thirty-three (12.08%) patients had radiological signs of extraocular muscle involvement. Traumatic intracranial signs were associated with roof and superior rim fractures (P < 0.001 for both). CONCLUSION: Floor and inferior rim were the most affected orbital structures in facial trauma. We described radiological risk factors association with optic canal, NLD fracture and intracranial involvement. We described extraocular muscle morphological signs, which could alert to the possibility of orbital floor fracture in equivocal cases.