Heterotrophs are organisms that cannot produce their own food, but instead take nutrition from other sources of organic carbon — typically plants and animals. Hence the name of the term, which comes from the Greek words hetero (other) and trophe (food). All animals are heterotrophs, as are most bacteria and fungi. In this article, we shall focus our attention on the role and significance heterotrophic bacteria in the ecosystem.
Heterotrophic bacteria are found in soil, water and in other environments where they help with the recycling of organic compounds. They also play a role in decomposition of dead and decaying plant and animal material, and they are responsible for the breakdown of organic matter such as chitin, cellulose, lignin and keratin. Heterotrophs can be either anaerobic or aerobic.
There are several different subtypes of heterotrophic bacteria. Some are photoheterotrophs that obtain energy from sunlight and require organic sugars from their environment to survive. Other bacteria are chemoheterotrophs, which get their energy by chemical reactions with inorganic substances. Finally, some bacteria are lithoheterotrophs, which get energy by oxidation of inorganic minerals or metals.
Some heterotrophic bacteria are pathogenic, which means they cause disease in humans or animals. These include the bacteria that cause Typhoid fever – Salmonella typhi, Cholera – Vibrio cholerae, Tetanus – Clostridium tetani, Diphtheria – Corynebacterium diphtheriae and Gonorrhoea – Neisseria gonorrhoeae.
In examining the significance of heterotrophic bacteria, we must note that there are other heterotrophic bacteria that are not pathogenic, and are also part of the microbial flora in human skin, mucous membranes, digestive tract and other organs, where they coexist without harming the host organism. In addition, many heterotrophic bacteria are used in the production of antibiotics, curd and other chemicals. There are even a few examples of bacteria that act as symbiont in plants or other organisms, for example Rhizobia in the roots of legumes.
Despite the fact that some heterotrophic bacteria can be opportunistic pathogens, many studies have shown that drinking water with high concentration of heterotrophic bacteria does not pose any health risk to consumers. In addition, the national primary drinking water regulations established by the U.S. EPA require that the total heterotrophic count in drinking water should be less than 500 CFU/mL, measured using Standard Methods for the Examination of Water and/or Heterotrophic Plate Count methodology. The bacteriological method for the analysis of water quality has been developed by International Organization for Standardization (ISO). Moreover, numerous epidemiological studies have also indicated that ingestion of drinking water with high concentration of heterotrophic bacterial counts does not result in an increased incidence of gastroenteritis in human participants. In a 242 sample set of tap water, the mean heterotrophic bacteria count was lower than the recommended limit. In most of the samples, the counts were below the standard coliform and E. coli recovery limits. The concentration of heterotrophic bacteria in these tap water samples may be attributed to a number of factors, including sampling techniques, environmental conditions, and water treatment processes. The lower values of heterotrophic bacteria in the tap water also reflect a better maintenance of the treatment and distribution system. However, the lower values of heterotrophic bacteria in the water may interfere with some total coliform and E. coli recovery methods (ISO, 2006).
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 significance of 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 the classification of heterotrophic bacteria and in accessing the significance of heterotrophic bacteria 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 be built from smaller units. Hence they are restricted to those nutritional substrates which can supply their individual needs.