Effects of Pine and Eucalypt ashes on bacterial isolates from the skin microbiome of the fire salamander (Salamandra salamandra)

Environmental contamination influences the diversity of the resident skin microbial community of amphibians, ultimately affecting the individual's immune system. Wildfires are expected to impact the skin microbiome, since post-fire runoff typically transports hazardous substances, that can affect terrestrial and aquatic ecosystems. The present study is the first to assess the effects of Eucalypt and Pine wildfire ash on cultivable bacterial isolates from the skin microbiome of amphibians, in particular the fire salamander (Salamandra salamandra), a common species in fire-prone Mediterranean ecosystems. To achieve this goal, samples of skin bacteria of adult individuals of S. salamandra were collected at a site without influence of wildfires. The bacterial isolates were tested against the pathogenic agent Aeromonas salmonicida for assessing their antimicrobial activity, before exposing them to a series of dilutions of aqueous extracts of Eucalypt and Pine ashes (AAEs) from high severity wildfires. From the 80 bacterial isolates collected, 48 (mostly Pseudomonas spp.) showed antimicrobial activity. Exposure of bacteria with antimicrobial activity to the Eucalypt and Pine AAEs at concentrations of 0, 6.25, 12.5, 25, 50, 75, and 100%, revealed that bacterial growth could be significantly inhibited, stimulated or unaffected by ash. Growth inhibition was found for Pine and Eucalypt AAEs at concentrations as low as 6.25% and 12.5%, respectively, but were more expressive at concentrations equal or above 50%. Eucalypt AAEs had a higher negative impact on bacterial growth than Pine AAEs, likely due to differences in metal concentrations between ash types. These findings raise concern about the future of amphibians in fire-prone regions since the foreseen increase in fire frequency and severity owing to climate changes are likely to alter the skin microbiome of amphibians, weaken the immune system and consequently increasing the incidence of infections or diseases, further contributing to the decline of the populations.