The influence of dietary carotenoid supplementation on the cutaneous bacterial communities of the critically endangered Southern Corroboree Frog (Pseudophryne corroboree)

Amphibians support a diverse range of cutaneous bacteria that may contribute to innate immune defence through mutualistic associations. These associations, which have come about through the coevolution of bacteria and amphibians, are potentially capable of supressing infection from deadly skin pathogens such as Batrachochytrium dendrobatidis. Conditions experienced within ex-situ conservation programs may affect the composition of the cutaneous bacterial community. In particular, nutritional conditions such as the provision of dietary carotenoids may influence cutaneous bacterial communities and have immune response implications for amphibians in captivity and post-release. Carotenoids exhibit efficient biological antioxidant activity and are known to influence vertebrate immune function through enhancing proliferation and functioning of immune response components. Vertebrates are unable to biosynthesize carotenoids de novo and must acquire these compounds via dietary means. This study aims to: 1) characterise the cutaneous bacterial community of a captive colony of the critically endangered Southern Corroboree Frog (Pseudophryne corroboree), and 2) test the effect of dietary carotenoid supplementation on these cutaneous bacterial populations. Dietary carotenoid availability was manipulated throughout juvenile (tadpole) and post-metamorphic (frog) life stages. Bacterial culturing methods were applied to frog skin swabs and bacteria were isolated and identified to determine the effect of dietary carotenoid supplementation on cutaneous bacterial communities. As expected, the provision of dietary carotenoids significantly increased bacterial abundance and species richness, and also affected overall community composition of bacteria. These findings provide support for the hypothesis that dietary carotenoid supplementation can enhance the cutaneous bacteria community of amphibians. It is expected that carotenoid supplementation enhances the cutaneous bacterial community by: 1) bacteria utilising carotenoids directly for physiological functions and/or; 2) the provision of a more suitable microhabitat for bacteria to reside through host utilisation of dietary carotenoids. Outcomes of this study contribute to a body of empirical evidence demonstrating the benefits of developing standardised ex-situ breeding conditions to maximise the mutualistic properties of cutaneous bacterial communities. This knowledge has the potential to improve the immune capabilities of amphibians both within captivity and upon release, and potentially aid in the suppression of amphibian pathogens