PHB is produced by microorganisms (such as Ralstonia eutrophus, Methylobacterium rhodesianum or Bacillus megaterium) apparently in response to conditions of physiological stress;[2] mainly conditions in which nutrients are limited. The polymer is primarily a product of carbon assimilation (from glucose or starch) and is employed by microorganisms as a form of energy storage molecule to be metabolized when other common energy sources are not available.