In conclusion, the presence of bacteria significantly affects Hydra longevity. Bacteria-free Hydra oligactis can survive significantly longer compared to those with an intact microbiome when undergoing starvation at low temperature, indicating that the native microbiome has a net cost to longevity in this context. Keeping the hydras at 12 °C made them die sooner, suggesting that warmer conditions speed up this process due to a higher metabolic cost. At the same time, reintroducing the microbiome after antibiotic treatment seems to have a harmful effect, which highlights the importance of a finely balanced native microbiota. Rather than protecting the host from dying, the microbiome might take part in age-associated decline – possibly by opportunistic overgrowth or insufficient support of host tissue maintenance when the host is subjected to stressful conditions. Our results provide experimental evidence supporting the idea that Hydra’s immortality (or mortality) is not a fully intrinsic trait but is a result of symbiotic relations with its microbial partners.