Effects of exogenous elevation of corticosterone on immunity and the skin microbiome of eastern newts ( Notophthalmus viridescens )

Abstract

The amphibian chytrid fungus, Batrachochytrium salamandrivorans ( Bsal ) threatens salamander biodiversity. The factors underlying Bsal susceptibility may include glucocorticoid hormones (GCs). The effects of GCs on immunity and disease susceptibility are well studied in mammals, but less is known in other groups, including salamanders. We used Notophthalmus viridescens (eastern newts) to test the hypothesis that GCs modulate salamander immunity. We first determined the dose required to elevate corticosterone (CORT; primary GC in amphibians) to physiologically relevant levels. We then measured immunity (neutrophil lymphocyte ratios, plasma bacterial killing ability (BKA), skin microbiome, splenocytes, melanomacrophage centres (MMCs)) and overall health in newts following treatment with CORT or an oil vehicle control. Treatments were repeated for a short (two treatments over 5 days) or long (18 treatments over 26 days) time period. Contrary to our predictions, most immune and health parameters were similar for CORT and oil-treated newts. Surprisingly, differences in BKA, skin microbiome and MMCs were observed between newts subjected to short- and long-term treatments, regardless of treatment type (CORT, oil vehicle). Taken together, CORT does not appear to be a major factor contributing to immunity in eastern newts, although more studies examining additional immune factors are necessary. This article is part of the theme issue ‘Amphibian immunity: stress, disease and ecoimmunology’.

Transport from the wild rapidly alters the diversity and composition of skin microbial communities and antifungal taxa in spring peeper frogs

“…This may suggest that a host response like glucocorticoid release may play a role. In eastern newts (Notophthalmus viridescens), corticosterone was not considered to be a major factor in immunity (Pereira et al, 2023), but in salamanders (Plethodon shermani) differences were seen between corticosterone and chytridiomycosis resistance (Fonner et al, 2017), suggesting that this relationship may need to be further evaluation, specifically in frogs. Additionally, longitudinal sampling of the same individuals throughout the transportation process would allow for better temporal resolution, identifying shifts in composition and taxa, and improved statistical power.…”

Introduction to the special issue Amphibian immunity: stress, disease and ecoimmunology

“…The authors found decreased plasma bacterial killing ability and melanomacrophage centre and lower skin microbiome after long-term treatment (26 days), compared with short-term treatment (5 days), but these changes were not associated with corticosterone treatment. These results demonstrate plasma corticosterone increases may not result in immune changes in the eastern newt, at least in the investigated immune aspects [ 53 ].…”

“…It is worth mentioning that in amphibians, the resident skin microbiota plays an antimicrobial role, helping to eliminate pathogens such as the Bd fungus and the ranavirus [47][48][49][50][51]. This theme issue adds to evidence that the microbiome composition is an essential component of the immune system in tadpoles and adult anurans (X. laevis and Pseudacris regilla) and salamanders (Notophthalmus viridescens) [52][53][54]. Specifically, [52] demonstrates that experimental reductions of the microbiome, by using antimicrobial treatments, during embryonic and larval stages reduced microbial richness and diversity and altered community composition in tadpoles (X. laevis) prior to metamorphosis, which could lead to increased disease susceptibility in amphibian tadpoles and adults.…”

“…The authors also show that frogs implanted with corticosterone (1 cm silastic tube filled with powdered corticosterone for 48 to 61 days) tended to heal more slowly than controls (with empty silastic tube), suggesting that the chronic increase in corticosterone can show immunosuppressive effects in these animals [ 73 ]. In addition, [ 53 ] tested the hypothesis that short- and long-term corticosterone treatment modulates the immune system in a salamander model ( N. viridescens ). The authors found decreased plasma bacterial killing ability and melanomacrophage centre and lower skin microbiome after long-term treatment (26 days), compared with short-term treatment (5 days), but these changes were not associated with corticosterone treatment.…”

The gut microbiota-immune-brain axis in a wild vertebrate: dynamic interactions and health impacts

“…However, several parts of the feedback cascade appeared to be functional (see below). Despite the well-established influence of the gut microbiota on immune system modulation and its impact on the CNS, these tripartite connections remain elusive in wild populations (Hird, 2017;Davidson et al, 2020;Madden et al, 2022;Pereira et al, 2023), particularly during the early stages of development, in contrast to the studies in humans and mice (Martin et al, 2018;Francella et al, 2022;Lynch et al, 2024).…”