Spore-forming bacteria of concern in foods are mainly from two genera of Gram-positive bacteria: Clostridium and Bacillus. Members of the Clostridium genus are strict (i.e., will only grow anaerobic) or facultative (prefer to grow in anaerobic) anaerobes. Members of the Bacillus genera are aerobic to facultatively anaerobic. A more detailed discussion is in the background information of Laboratory 14. Spore-forming bacteria can also be divided by optimal growth temperature. Within both genera, there are spore-forming organisms that grow as mesophiles or thermophiles. In addition, the heat resistance of spores can vary between species and will also be dependent upon food product composition.