BACKGROUNDS: A method based on constraints is called Flux Balance Analysis (FBA), it is used to examine metabolic networks, analyse the flow of metabolites in a network and predict the biomass. Polyhydroxy butyrate (PHB) is a type of polyhydroxy alkanoates, a bio-derived plastic with a wide range of applications in packaging films and as a wrapping material. Bacillus spp., a Gram-positive bacterium, is capable of PHB biosynthesis.