Classification of heterotrophic bacteria: The common characteristic of these bacteria is that they can utilize for their own purposes, only the energy which has been incorporated into organic compounds by other living things.We can group or classify heterotrophic bacteria according to their requirements of energy-supplying and body-building substances as follows:
- Those which must have organic carbon such as carbohydrates, but are able to make use of gaseous nitrogen as a nitrogen source. With water and certain mineral ions, they can then synthesize all their requirements including accessory growth factors. Such organisms are the nitro-fixing bacteria such as the free-living Azotobacter species, chroococcum, agile, beijerincki and indicum, the free-living species Clostridium pastorianum and the symbiotic Rhizobium leguminosarium.
- Those which must have organic carbon such as carbohydrate but are unable to use gaseous nitrogen. They use nitrate or ammonia as a nitrogen source instead. Many saprophytic bacteria growing on a wide variety of substrates are in this category.
- Those which must have organic carbon and are unable to utilize any form of inorganic nitrogen. Certain amino-acids are therefore essential. From a limited number of these, together with the carbohydrate, water and necessary mineral ions, all protoplasmic constituents can be synthesized including the accessory growth substances. Saprophytic bacteria occurring on a limited range of substrates are in this group.
- Those which must have organic carbon, certain amino-acids and accessory growth substances. In this group are all the parasitic bacteria and the saprophytic organisms which will grow in only one particular kind of medium, e.g. milk.
From this brief summary of the heterotrophic bacteria it must not be concluded that even the most thorough-going saprophyte or parasite cannot utilize inorganic materials. It must be remembered that we have used the term heterotrophe to mean an organism which cannot tap for itself sources of energy in the physical environment external to the living world. As long as some other source is available to provide the energy for synthetic reactions, inorganic materials can be incorporated into the system quite readily. Indeed, this seems to be the universal condition, since the presence of water and mineral ions is essential to all protoplasmic processes in both plants and animals.
In accessing the position of heterotrophe in the scheme of living things, two factors must be taken into account. First, it must rely on the energy-fixing ability of other organisms for its supply of energy with which to accomplish its own syntheses. A carbohydrate respired in its own cells suffices for most heterotrophic organisms in this respect. Secondly, different heterotrophes possesses different enzyme systems and therefore all cannot use the same starting substances from which to build their protoplasm. When supplied with the substances with which their enzymes can cope, they can then build protoplasm, using the energy released in their respiratory activity. The autotrophe, on the other hand, possesses the complete range of enzymes necessary for the synthesis of all protoplasmic substances from carbon(IV) oxide, an inorganic nitrogen source, water and some mineral ions. In the form of proteins, some synthesized compounds perform the enzymic function of catalyzing the formation of the others and once the system is in full operation, it becomes self-increasing, as long as raw materials and energy source last out. The heterotrophes, in varying degree, lack the ability to synthesize all these compounds and at fewer or more places, gaps must be filled by direct access to the substances which cannot bebuilt from smaller units. Hence they are restricted to those nutritional substrates which can supply their individual needs.