The resting stage of certain bacteria, formed in response to adverse conditions. The bacterial cell changes into a partially dehydrated core, enclosed in a multilayered protein coat. On return to favourable conditions the spore germinates and reverts to the normal vegetative form of the organism. Endospores can remain viable for long periods, perhaps millions of years. There are reports of successful germination of endospores obtained from the guts of prehistoric bees preserved in amber for 25–40 million years and from brine isolated in 250-million-year-old salt deposits. Endospores of low G-C content Gram-positive bacteria, such as some members of the Firmicutes (e.g. Bacillus subtilis), are particularly resistant to the usual arsenal of antimicrobial treatments.