Title: Winter is coming–Temperature affects immune defenses and susceptibility to Batrachochytrium salamandrivorans
Environmental temperature is a key factor driving various biological processes, including immune defenses and host-pathogen interactions. Here, we evaluated the effects of environmental temperature on the pathogenicity of the emerging fungal pathogen, Batrachochytrium salamandrivorans ( Bsal ), using controlled laboratory experiments, and measured components of host immune defense to identify regulating mechanisms. We found that adult and juvenile Notophthalmus viridescens died faster due to Bsal chytridiomycosis at 14°C than at 6 and 22°C. Pathogen replication rates, total available proteins on the skin, and microbiome composition likely drove these relationships. Temperature-dependent skin microbiome composition in our laboratory experiments matched seasonal trends in wild N . viridescens , adding validity to these results. We also found that hydrophobic peptide production after two months post-exposure to Bsal was reduced in infected animals compared to controls, perhaps due to peptide release earlier in infection or impaired granular gland function in diseased animals. Using our temperature-dependent susceptibility results, we performed a geographic analysis that revealed N . viridescens populations in the northeastern United States and southeastern Canada are at greatest risk for Bsal invasion, which shifted risk north compared to previous assessments. Our results indicate that environmental temperature will play a key role in the epidemiology of Bsal and provide evidence that temperature manipulations may be a viable disease management strategy. more »« less
Wilber, Mark Q.; Carter, Edward Davis; Gray, Matthew J.; Briggs, Cheryl J.; Pedersen, ed., Amy(
, Functional Ecology)
Abstract
Resistance and tolerance are unique host defence strategies that can limit the impacts of a pathogen on a host. However, for most wildlife–pathogen systems, there are still fundamental uncertainties regarding (a) how changes in resistance and tolerance can affect disease outcomes and (b) the mechanisms underlying resistance and tolerance in host populations.
Here, we first compared observed patterns of resistance and tolerance and their effects on disease outcomes among salamander species that are susceptible to infection and mortality from the emerging fungal pathogenBatrachochytrium salamandrivorans(Bsal). We then tested whether two putative mechanisms that contribute to host resistance and tolerance, skin sloughing and skin lesion reduction, predicted reducedBsalgrowth rate or increased host survival during infection, respectively.
We performed multi‐doseBsalchallenge experiments on four species of Salamandridae found throughout North America. We combined the laboratory experiments with dynamic models and sensitivity analysis to examine how changes in load‐dependent resistance and tolerance functions affectedBsal‐induced mortality risk. Finally, we used our disease model to test whether skin sloughing and lesion reduction predicted variability in infection outcomes not described byBsalinfection intensity.
We found that resistance and tolerance differed significantly among salamander species, with the most susceptible species being both less resistance and less tolerant ofBsalinfection. Our dynamic model showed that the relative influence of resistance versus tolerance on host survival was species‐dependent—increasing resistance was only more influential than increasing tolerance for the least tolerant species where changes in pathogen load had a threshold‐like effect on host survival. Testing two candidate mechanisms of resistance and tolerance, skin sloughing and lesion reduction, respectively, we found limited support that either of these processes were strong mechanisms of host defence.
Our study contributes to a broader understanding of resistance and tolerance in host–pathogen systems by showing that differences in host tolerance can significantly affect whether changes in resistance or tolerance have larger effects on disease outcomes, highlighting the need for species and even population‐specific management approaches that target host defence strategies.
A freePlain Language Summarycan be found within the Supporting Information of this article.
Le Sage, Emily H.; LaBumbard, Brandon C.; Reinert, Laura K.; Miller, Brian T.; Richards‐Zawacki, Corinne L.; Woodhams, Doug C.; Rollins‐Smith, Louise A.; Bielby, ed., Jon(
, Journal of Animal Ecology)
Abstract
Accurately predicting the impacts of climate change on wildlife health requires a deeper understanding of seasonal rhythms in host–pathogen interactions. The amphibian pathogen,Batrachochytrium dendrobatidis(Bd), exhibits seasonality in incidence; however, the role that biological rhythms in host defences play in defining this pattern remains largely unknown.
The aim of this study was to examine whether host immune and microbiome defences againstBdcorrespond with infection risk and seasonal fluctuations in temperature and humidity.
Over the course of a year, five populations of Southern leopard frogs (Rana[Lithobates]sphenocephala) in Tennessee, United States, were surveyed for host immunity, microbiome and pathogen dynamics. Frogs were swabbed for pathogen load and skin bacterial diversity and stimulated to release stored antimicrobial peptides (AMPs). Secretions were analysed to estimate total hydrophobic peptide concentrations, presence of known AMPs and effectiveness ofBdgrowth inhibition in vitro. The diversity and proportion of bacterial reads with a 99% match to sequences of isolates known to inhibitBdgrowth in vitro were used as an estimate of predicted anti‐Bdfunction of the skin microbiome.
Batrachochytrium dendrobatidisdynamics followed the expected seasonal fluctuations—peaks in cooler months—which coincided with when host mucosal defences were most potent againstBd. Specifically, the concentration and expression of stored AMPs cycled synchronously withBddynamics. Although microbiome changes followed more linear trends over time, the proportion of bacteria that can function to inhibitBdgrowth was greatest when risk ofBdinfection was highest.
We interpret the increase in peptide storage in the fall and the shift to a more anti‐Bdmicrobiome over winter as a preparatory response for subsequent infection risk during the colder periods when AMP synthesis and bacterial growth is slow and pathogen pressure from this cool‐adapted fungus is high. Given that a decrease in stored AMP concentrations as temperatures warm in spring likely means greater secretion rates, the subsequent decrease in prevalence suggests seasonality ofBdin this host may be in part regulated by annual immune rhythms, and dominated by the effects of temperature.
ABSTRACT Mucosal defenses are crucial in animals for protection against pathogens and predators. Host defense peptides (antimicrobial peptides, AMPs) as well as skin-associated microbes are key components of mucosal immunity, particularly in amphibians. We integrate microbiology, molecular biology, network-thinking, and proteomics to understand how host and microbially derived products on amphibian skin (referred to as the mucosome) serve as pathogen defenses. We studied defense mechanisms against chytrid pathogens, Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal), in four salamander species with different Batrachochytrium susceptibilities. Bd infection was quantified using qPCR, mucosome function (i.e., ability to kill Bd or Bsal zoospores in vitro ), skin bacterial communities using 16S rRNA gene amplicon sequencing, and the role of Bd-inhibitory bacteria in microbial networks across all species. We explored the presence of candidate-AMPs in eastern newts and red-backed salamanders. Eastern newts had the highest Bd prevalence and mucosome function, while red-back salamanders had the lowest Bd prevalence and mucosome function, and two-lined salamanders and seal salamanders were intermediates. Salamanders with highest Bd infection intensity showed greater mucosome function. Bd infection prevalence significantly decreased as putative Bd-inhibitory bacterial richness and relative abundance increased on hosts. In co-occurrence networks, some putative Bd-inhibitory bacteria were found as hub-taxa, with red-backs having the highest proportion of protective hubs and positive associations related to putative Bd-inhibitory hub bacteria. We found more AMP candidates on salamanders with lower Bd susceptibility. These findings suggest that salamanders possess distinct innate mechanisms that affect chytrid fungi. IMPORTANCE How host mucosal defenses interact, and influence disease outcome is critical in understanding host defenses against pathogens. A more detailed understanding is needed of the interactions between the host and the functioning of its mucosal defenses in pathogen defense. This study investigates the variability of chytrid susceptibility in salamanders and the innate defenses each species possesses to mediate pathogens, thus advancing the knowledge toward a deeper understanding of the microbial ecology of skin-associated bacteria and contributing to the development of bioaugmentation strategies to mediate pathogen infection and disease. This study improves the understanding of complex immune defense mechanisms in salamanders and highlights the potential role of the mucosome to reduce the probability of Bd disease development and that putative protective bacteria may reduce likelihood of Bd infecting skin.
Jani, Andrea J.; Bushell, Jessie; Arisdakessian, Cédric G.; Belcaid, Mahdi; Boiano, Daniel M.; Brown, Cathy; Knapp, Roland A.(
, The ISME Journal)
Abstract
Infectious pathogens can disrupt the microbiome in addition to directly affecting the host. Impacts of disease may be dependent on the ability of the microbiome to recover from such disturbance, yet remarkably little is known about microbiome recovery after disease, particularly in nonhuman animals. We assessed the resilience of the amphibian skin microbial community after disturbance by the pathogen, Batrachochytrium dendrobatidis (Bd). Skin microbial communities of laboratory-reared mountain yellow-legged frogs were tracked through three experimental phases: prior to Bd infection, after Bd infection (disturbance), and after clearing Bd infection (recovery period). Bd infection disturbed microbiome composition and altered the relative abundances of several dominant bacterial taxa. After Bd infection, frogs were treated with an antifungal drug that cleared Bd infection, but this did not lead to recovery of microbiome composition (measured as Unifrac distance) or relative abundances of dominant bacterial groups. These results indicate that Bd infection can lead to an alternate stable state in the microbiome of sensitive amphibians, or that microbiome recovery is extremely slow—in either case resilience is low. Furthermore, antifungal treatment and clearance of Bd infection had the additional effect of reducing microbial community variability, which we hypothesize results from similarity across frogs in the taxa that colonize community vacancies resulting from the removal of Bd. Our results indicate that the skin microbiota of mountain yellow-legged frogs has low resilience following Bd-induced disturbance and is further altered by the process of clearing Bd infection, which may have implications for the conservation of this endangered amphibian.
Host-associated microbial communities can influence physiological processes of macroorganisms, including contributing to infectious disease resistance. For instance, some bacteria that live on amphibian skin produce antifungal compounds that inhibit two lethal fungal pathogens, Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). Therefore, differences in microbiome composition among host species or populations within a species can contribute to variation in susceptibility to Bd/Bsal. This study applies 16S rRNA sequencing to characterize the skin bacterial microbiomes of three widespread terrestrial salamander genera native to the western United States. Using a metacommunity structure analysis, we identified dispersal barriers for these influential bacteria between salamander families and localities. We also analysed the effects of habitat characteristics such as percent natural cover and temperature seasonality on the microbiome. We found that certain environmental variables may influence the skin microbial communities of some salamander genera more strongly than others. Each salamander family had a somewhat distinct community of putative anti-Bd skin bacteria, suggesting that salamanders may select for a functional assembly of cutaneous symbionts that could differ in its ability to protect these amphibians from disease. Our observations raise the need to consider host identity and environmental heterogeneity during the selection of probiotics to treat wildlife diseases.
Carter, Edward Davis, Bletz, Molly C., Le Sage, Mitchell, LaBumbard, Brandon, Rollins-Smith, Louise A., Woodhams, Douglas C., Miller, Debra L., and Gray, Matthew J. Winter is coming–Temperature affects immune defenses and susceptibility to Batrachochytrium salamandrivorans. Retrieved from https://par.nsf.gov/biblio/10213895. PLOS Pathogens 17.2 Web. doi:10.1371/journal.ppat.1009234.
Carter, Edward Davis, Bletz, Molly C., Le Sage, Mitchell, LaBumbard, Brandon, Rollins-Smith, Louise A., Woodhams, Douglas C., Miller, Debra L., & Gray, Matthew J. Winter is coming–Temperature affects immune defenses and susceptibility to Batrachochytrium salamandrivorans. PLOS Pathogens, 17 (2). Retrieved from https://par.nsf.gov/biblio/10213895. https://doi.org/10.1371/journal.ppat.1009234
Carter, Edward Davis, Bletz, Molly C., Le Sage, Mitchell, LaBumbard, Brandon, Rollins-Smith, Louise A., Woodhams, Douglas C., Miller, Debra L., and Gray, Matthew J.
"Winter is coming–Temperature affects immune defenses and susceptibility to Batrachochytrium salamandrivorans". PLOS Pathogens 17 (2). Country unknown/Code not available. https://doi.org/10.1371/journal.ppat.1009234.https://par.nsf.gov/biblio/10213895.
@article{osti_10213895,
place = {Country unknown/Code not available},
title = {Winter is coming–Temperature affects immune defenses and susceptibility to Batrachochytrium salamandrivorans},
url = {https://par.nsf.gov/biblio/10213895},
DOI = {10.1371/journal.ppat.1009234},
abstractNote = {Environmental temperature is a key factor driving various biological processes, including immune defenses and host-pathogen interactions. Here, we evaluated the effects of environmental temperature on the pathogenicity of the emerging fungal pathogen, Batrachochytrium salamandrivorans ( Bsal ), using controlled laboratory experiments, and measured components of host immune defense to identify regulating mechanisms. We found that adult and juvenile Notophthalmus viridescens died faster due to Bsal chytridiomycosis at 14°C than at 6 and 22°C. Pathogen replication rates, total available proteins on the skin, and microbiome composition likely drove these relationships. Temperature-dependent skin microbiome composition in our laboratory experiments matched seasonal trends in wild N . viridescens , adding validity to these results. We also found that hydrophobic peptide production after two months post-exposure to Bsal was reduced in infected animals compared to controls, perhaps due to peptide release earlier in infection or impaired granular gland function in diseased animals. Using our temperature-dependent susceptibility results, we performed a geographic analysis that revealed N . viridescens populations in the northeastern United States and southeastern Canada are at greatest risk for Bsal invasion, which shifted risk north compared to previous assessments. Our results indicate that environmental temperature will play a key role in the epidemiology of Bsal and provide evidence that temperature manipulations may be a viable disease management strategy.},
journal = {PLOS Pathogens},
volume = {17},
number = {2},
author = {Carter, Edward Davis and Bletz, Molly C. and Le Sage, Mitchell and LaBumbard, Brandon and Rollins-Smith, Louise A. and Woodhams, Douglas C. and Miller, Debra L. and Gray, Matthew J.},
editor = {Wang, Chengshu}
}
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