Nutrients in growth media must contain all the elements necessary for the biosynthesis of pathogenic microorganisms. This rule is especially true of media that are intended to have a selective (select for a specific group of bacteria) or differential (differentiate between a pathogenic and a normal or usual strain of a group) purpose. Bacteria and plants use photosynthetic energy to reduce carbon dioxide expending water. The organisms that can use CO2 as a sole source of carbon are termed autotrophs. Chemolithotrophs are organisms that use an inorganic substrate, such as hydrogen or thiosulfate, as a reductant and CO2 a carbon source. Heterotrophs require organic carbon for growth. Many organisms possess the ability to assimilate nitrate (NO.3) and nitrite (NO2) reductively by conveing these forms to ammonia (NH3). Dissimilation pathways are taken by microorganisms that use these ions as terminal electron acceptors in respiration. This process is known as denitrification, with the final product being nitrogen gas (N2). Fermentation is characterized by substrate phosphorylation, in which products such as glucose are involved for energy production. Autotrophs should not be confused with auxotrophs. Auxotrophs are strains that require a nutrient that is not required by the parental or prototype strain. They are usually derived from mutants with defective synthetic capabilities. Ref: Ananthanarayana textbook of microbiology 9th edition pgno: 45