<P> Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class that are of interest as bio-derived and biodegradable plastics . The poly - 3 - hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but other polymers of this class are produced by a variety of organisms: these include poly - 4 - hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers . </P> <P> PHB is produced by microorganisms (such as Ralstonia eutrophus, Methylobacterium rhodesianum or Bacillus megaterium) apparently in response to conditions of physiological stress; 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 . </P>

Pha and phb are microbial products which can be used for