Antifungal skin bacteria, embryonic survival, and communal nesting in four-toed salamanders, Hemidactylium scutatum

Abstract

We examined a novel hypothesis for the maintenance of communal nesting in the salamander, Hemidactylium scutatum, namely that communal nests are more likely than solitary nests to be associated with cutaneous antifungal bacteria, which can inhibit fungal infections of embryos. A communal nest contains eggs of two or more females of the same species. The nesting behavior of H. scutatum females and survival of embryos were determined by frequent nest surveys at three ponds. For communal nests, embryonic survival tended to be higher and catastrophic nest failure was lower. Pure bacterial cultures of resident species were obtained from the salamanders' skins by swabbing and tested against a fungal pathogen of embryos (Mariannaea sp.) in laboratory assays. We found that 27% of females had skin bacteria inhibitory to Mariannaea sp. Communal nests were more likely to have at least one female with antifungal bacteria than were solitary nests. Using a culture-independent assay (denaturing gradient gel electrophoresis of 16S rRNA gene fragments), we found that bacterial species on females and embryos were more similar to each other than they were to bacterial species found in soil within the nest, suggesting that females transmitted skin bacteria to embryos. The presence of anti-Mariannaea skin bacteria identified from the laboratory assays did not prevent fungal presence in field nests. However, once a nest was visibly infected with fungi, presence of anti-Mariannaea bacteria was positively correlated with survival of embryos. Microbe transmission is usually thought to be a cost of group living, but communal nesting in H. scutatum may facilitate the transmission of antifungal bacteria to embryos.

Composition of symbiotic bacteria predicts survival in Panamanian golden frogs infected with a lethal fungus

“…Additionally, the fact that the bacterial community structure on frogs that eventually cleared infection and those that died of chytridiomycosis was significantly different prior to Bd exposure suggests the community was not responding to Bd and therefore is responsible for the clearance of Bd. This conclusion is consistent with results from other studies demonstrating the importance of cutaneous symbiotic bacteria and their antifungal metabolites in protecting amphibians from fungal pathogens, including Bd [10][11][12]. Predicting disease susceptibility with microbial community data has also been done for human intestinal pathogens [52,53].…”

In an early branching metazoan, bacterial colonization of the embryo is controlled by maternal antimicrobial peptides

“…For example, embryos of the crustacean species Palaemon macrodactylus are colonized by symbiotic bacteria producing a secondary metabolite that is active against a pathogenic fungus (31). Similarly, in the salamander Hemidactylium scutatum, a protective bacterial community producing antifungal molecules resides in the organism´s skin (32) and can be passed directly from the mother to offspring in each generation (33). These bacteria may protect eggs and embryos from fungal infection.…”

Embryo protection in contemporary immunology

“…34 In addition to adult salamanders, these bacteria may protect also eggs and embryos from fungal infection since they can be passed directly from the mother to offspring in each generation and since communal nesting increases the likelihood of the transmission of the protective bacteria to the eggs. 35 Preliminary observations in Hydra also point to a role associated bacteria in microbial defense since bacteria-free polyps and embryos are …”