Vences, M., V. Schulz, L. Heldt, F. Kamprad, K. Ruthsatz, K. Preissler, M. Müsken & S. Steinfartz

Comparative abundance of cutaneous bacteria in Central European amphibians. pp. 275-288 plus Supplementary document.

Abstract. The composition of cutaneous (skin-associated) bacterial communities of amphibians has been intensively studied in light of the potential of some of these commensal bacterial taxa to mitigate infection with the chytrid fungi Batracho­chytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal). However, surprisingly, the absolute densities in which these bacteria occur on the skin are only poorly known. We here combine quantification of bacterial 16SrDNA copies from skin swabs by quantitative PCR with counts from scanning electron microscopy (SEM) images to assess and compare bacterial abundances on the skin of various central European amphibians. We focus on the fire salamander (Salamandra salamandra) which is severely threatened by the spread of Bsal. Based on counts from SEM pictures of selected skin parts, local densities of ~43,000 bacteria per mm² were ascertained in frogs, although the data are insufficient to understand whether these values apply to the whole body surface. Bacterial densities are doubtless much lower in fire salamanders in which we observed almost no bacteria in SEM examination. From qPCR data, we find: (i) statistically relevant differences in bacterial abundances among species, with the lowest abundances in terrestrial salamanders and the highest abundances in toads; (ii) higher bacterial loads in captive compared to wild fire salamanders, and much higher loads in Bsal-infected captive specimens, in agreement with the hypothesis that Bsal-infection leads to blooming of opportunistic bacteria that may cause secondary infection; (iii) an only weak decrease of bacterial load after repeated swabbing, in agreement with the hypothesis that skin swabs capture only a part of the bacteria of the swabbed skin surface. We discuss the multiple sources of uncertainty in absolute estimates of abundances of cutaneous bacteria and suggest further research to clarify and reduce these uncertainties.

Key words. Amphibia, Anura, Caudata, Batrachochytrium salamandrivorans, qPCR, scanning electron microscopy, skin microbiome, skin microbiota, bacterial abundance.