Environmental perturbations (e.g. urbanization) have been shown to act indirectly through host physiology to alter an individual’s microbiome. This is likely important for amphibians whose cutaneous microbiome acts as a line of defense against the global fungal wildlife disease Batrachochytrium dendrobatidis (Bd). By combining research on host physiology and microbial ecology, we can explore the relationship between stress-induced responses and the cutaneous microbiome in mediating Bd infection. Using redback salamanders (Plethodon cinereus), an indicator species in the northeast United States which is highly urbanized, we can investigate the interaction between physiology and degree of urbanization in disease susceptibility.
We sampled the cutaneous microbiome and blood characteristics (e.g. corticosterone and leukocytes) of P. cinereus along a 65-km urban-to-rural gradient originating in New York City. We sampled male and female individuals before, during, and after mating season to determine if behavioral variation in blood characteristics may influence microbiome structure and Bd susceptibility. We used blood smears and a corticosterone ELISA to quantify leukocytes and corticosterone respectively. We extracted all DNA from cutaneous swabs and performed qPCR, assaying for Bd and total bacterial load, to quantify the ratio of infection to known beneficial microbes and then relate this to stress.
Results/Conclusions
Originally, we hypothesized that individuals would exhibit higher corticosterone levels and a greater neutrophil:lymphocyte ratio during mating season. Additionally, we expected to observe lower amounts of beneficial bacteria in urban areas due to a higher prevalence of Bd and increased stress. In support of our hypotheses, our results indicate that individuals have greater corticosterone levels and a higher neutrophil:lymphocyte ratio leading up to and during mating season. Contrary to expectations, urban areas showed lower levels of Bd and higher abundances of beneficial bacteria relative to exurban and rural woodlands (p < 0.05). Greater prevalence of beneficial bacteria may be a consequence of living in a disturbed environment. Lastly, we observed a weak correlation between Bd load and mating season. Moving forward, we aim to characterize the total bacterial community and correlate this to variation in blood characteristics. Understanding how urbanization, natural variation in stress levels, cutaneous bacterial community composition, and disease prevalence act together to mediate amphibian susceptibility to Bd will provide valuable information for conservation efforts.