Spores in bacteria is not the same as spores of fungi. Bacteria do not use spores to reproduce, but to survive adverse conditions (lack of nutrients, extreme pH, high temperature etc). Some bacteria may switch to a resting state by forming spores.
Endospores are formed, as the name suggests, inside the bacterial cell. Endospores are highly resistant and can be formed by members of the phylum Firmicutes. The genera, which are most important in veterinary medicine are Bacillus and the closely related genus Paenibacillus as well as Clostridium and the closely related genera Paeniclostridium and Clostridioides. In each bacterial cell only one spore is formed and it can survive without access to nutrients. Endospores can withstand high and low temperatures, dehydration, chemical disinfectants and UV radiation, and the reason why they are so resistant is that they contain almost no water, and that the cell wall contains dipicolinic acid. The spores also contains much calcium (Ca) and when Ca2+ is pumped into the spore, the water is pumped out. In addition to the envelope (cortex) the spore contains primarily just DNA, ribosomes and polymerases. This is sufficient for the spores to germinate i.e. develop into bacteria again when the conditions become favorable.
Treatment with moist heat (+121°C) for 15 minutes is required to kill endospores, since they are so resistant.
Use in diagnostics
Endospores may be of importance for identification of bacteria. The position of the spore in the mother cell can give information about possible bacteria. B. cereus, B. subtilis and C. tetani for instance, have a central, subterminal and terminal spore, respectively.
Video clip at YouTube that is recommended: "Bacterial Spore Formation".
The orders Nostocales and Stigonematales of the phylum Cyanobacteria form so-called akinetes, which is a type of resting stage for these bacteria. Akinetes is not the same as endospores and they are not as resistant, but in the form of akineter these cyanobacteria survive the cold (winter season) and dehydration.
The genera Actinomyces and Streptomyces of the phylum Actinobacteria can form a type of spores referred to as exospores. Exospores are formed by budding of the mycelium like structures (filaments), which these bacteria exhibit during growth. Exospores are not as resistant as endospores and therefore not of the same clinical significance. They also have a completely different structure.
Is actually something different from spores, see VBNC (Viable but nonculturable) below.