Bacteria propagate when times are good. When times are bad, certain bacteria have the capability to shrink down and encrust critical cellular elements in a tough casing. This is called an endospore.***The process works like this: The, bacterium senses environmental stress, which triggers activation of genes which create a tough shell within the stressed cell, containing all the basics needed for life when conditions become more favorable. Eventually, the former bacterial cell degrades away, leaving just the shell behind. The photograph below shows what the hardy shell looks like when stained. The endospores are a purified Clostridum difficile spore suspension.
Unlike fungal spores, where one fungus can make many spores, bacterial endospores are a "one cell makes one endospore" affair.
Endospores exhibit no signs of life, however when the environment returns to a favorable state for bacterial growth the bacterial endospore will germinate and return to a normal state.
Understanding endospores is important to the study of disinfection and antimicrobial chemials, because the same features that make bacterial endospores resistant to environmental stresses render them resistant to many disinfectants.
Two great examples of bacteria that are difficult to disinfect because they persist in the environment as endospores are Bacillus anthracis (the cause of anthrax) and Clostridium difficile (the cause of pseudomembraneous colitis - a common enteric infection acquired in hospitals).
As a general rule, only a few disinfectants are strong enough to kill bacterial endospores within a short period of time. Usually they are strong oxidants like 1:10 diluted bleach. Among antimicrobial devices, strong ultraviolet light-based systems are the best bet for endospore killing.