Resident Brain Bacteria and Behavioral Fever in Rainbow Trout: Salinas Team Talk on Four Brain Regions

The Salinas team's examination of four brain regions in laboratory rainbow trout showed resident bacteria in the brains of healthy individuals: after intracardiac perfusion removed >99.5% of blood, culture, 16S amplicon sequencing, and confocal imaging with universal bacterial probes all confirmed the presence of bacteria, while cerebrospinal fluid and multiple environmental controls yielded no cultured bacteria (the entire work is still under submission, not formally published, pending verification).; Brain bacterial load and immune markers showed a rostrocaudal gradient: the olfactory bulb had the lowest bacterial load, and the cerebellum had the lowest IL-1β and TNF-α expression, while the telencephalon and optic tectum constitutively expressed high levels of IL-6 and IL-10.; SourceTracker2 analysis showed that water contributed only about 2% of the brain microbiome's sources, blood plus gut together about 30–50%, and the remaining 50–70% unknown, which the team calls the "core brain microbiome" (pending verification).; Plesiomonas isolated from the brain were all of the long-genome type, carrying complete T6SS and T3SS secretion system modules and a unique spermine synthesis pathway (arginase, speF); only long-genome strains produced spermine in vitro—suggesting these features may help them colonize brain tissue (pending verification).

• The Salinas team's examination of four brain regions in laboratory rainbow trout showed resident bacteria in the brains of healthy individuals: after intracardiac perfusion removed >99.5% of blood, culture, 16S amplicon sequencing, and confocal imaging with universal bacterial probes all confirmed the presence of bacteria, while cerebrospinal fluid and multiple environmental controls yielded no cultured bacteria (the entire work is still under submission, not formally published, pending verification).
• Brain bacterial load and immune markers showed a rostrocaudal gradient: the olfactory bulb had the lowest bacterial load, and the cerebellum had the lowest IL-1β and TNF-α expression, while the telencephalon and optic tectum constitutively expressed high levels of IL-6 and IL-10.
• SourceTracker2 analysis showed that water contributed only about 2% of the brain microbiome's sources, blood plus gut together about 30–50%, and the remaining 50–70% unknown, which the team calls the "core brain microbiome" (pending verification).
• Plesiomonas isolated from the brain were all of the long-genome type, carrying complete T6SS and T3SS secretion system modules and a unique spermine synthesis pathway (arginase, speF); only long-genome strains produced spermine in vitro—suggesting these features may help them colonize brain tissue (pending verification).
• Intravenous injection of commensal bacteria or LPS derived from them induced "behavioral fever" in rainbow trout (preference for warmer water with reduced activity in an 11–19°C annular tank); blocking bacterial polyamine synthesis with DFMA/DFMO abolished the fever and lethargy, and the team calls polyamines a key mediator of the fever (first report by the team, pending verification).
• The telencephalon is particularly "leaky" to systemic inflammation: Evans Blue tracing showed the most pronounced blood-brain barrier leakage in this region, and only the telencephalon (not the hypothalamus) showed bacterial invasion and polyamine pathway responses; during the "zombie salmon" stage of spawning migration in chinook salmon, bacterial loads in the telencephalon and cerebellum and circulating LPS were significantly elevated.