PHB is produced by microorganisms (such as Ralstonia eutrophus, Methylobacterium rhodesianum or Bacillus megaterium) apparently in response to conditions of physiological stress;[3] 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.
Microbial biosynthesis of PHB starts with the condensation of two molecules of acetyl-CoA to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB.[4] PHAs granules are then recovered by disrupting the cells