ABSTRACT
These characteristics, which are defined in terms of the carbon source, energy source and the electron donor utilized, are indicative of the enzymatic composition of different micro-organisms and can thus be used to distinguish specific nutritional categories, as shown in Table 2.1. Bacteria which obtain carbon from an inorganic source (i.e. CO2) are termed autotrophs, while those which utilize organic carbon for cellular synthesis are called heterotrophs. Similarly, two sources of energy can be identified; phototrophic micro-organisms use a light source to produce energy from the reaction with photosynthetic pigments such as chlorophyll, while chemotrophic micro-organisms oxidize organic or inorganic chemical compounds. The electron donor involved in such reactions may be organic or
inorganic, giving a further division of organotrophism and lithotrophism respectively. The above categorizations are frequently combined to give a full description of a bacterium’s metabolic requirements. Thus a phototrophic organism with an inorganic electron donor such as sulphide would be a photolithotroph, whereas one with an organic electron donor such as acetate would be a photoorganotroph. Likewise, chemotrophic organisms obtaining energy from oxidation of an inorganic electron donor such as hydrogen are chemolithotrophs and those oxidizing reduced organic compounds such as glucose are chemoorganotrophs.
