Classification by Carbon Source
Carbon serves as the key building block for any microbial cell. Without a continuous supply of carbon atoms from the environment, a bacterium cannot synthesize vital organic polymers: proteins, carbohydrates, lipids, and nucleic acids. Depending on the form in which microorganisms assimilate this macronutrient, they are divided into two fundamental groups:
- Autotrophs (from Greek autos — self). These bacteria use inorganic compounds as their sole or primary carbon source. In the vast majority of cases, this source is carbon dioxide ($CO_2$). Autotrophs are capable of independently synthesizing complex organic molecules from simple inorganic precursors.
- Heterotrophs (from Greek heteros — other). This group of microorganisms requires organic carbon sources. For optimal growth and reproduction, they must consume preformed, readily assimilable organic compounds. Preferred sources for heterotrophs include various hexoses, polyhydric alcohols, and amino acids.
Classification by Energy Metabolism
All microbial activities are powered by energy securely stored as ATP. However, the pathways for obtaining this energy and the sources of electron donors for metabolic reactions vary significantly among species.
Based on their primary energy source, bacteria are divided into:
- Phototrophs — microorganisms capable of capturing and utilizing light quantum energy to meet their physiological needs.
- Chemotrophs — microorganisms that derive energy exclusively through oxidation-reduction chemical reactions.
Among chemotrophs, there is a further subdivision based on the chemical nature of the electron donor substances:
- Lithotrophs (from Greek lithos — stone). They use inorganic substrates as electron donors. These may include hydrogen gas, ammonia, hydrogen sulfide, iron ions, and various other inorganic molecules.
- Organotrophs. Unlike lithotrophs, these bacteria extract electrons by oxidizing various organic compounds.
Ecological Classification of Heterotrophs
Heterotrophic bacteria demonstrate varying degrees of dependence on a host organism and differ in their ability to assimilate specific nutrient substrates. Based on these ecological criteria, they are classified into saprophytes and parasites.
Saprophytes (from Greek sapros — rotten) are microorganisms that specialize in feeding on dead organic matter. A defining feature of saprophytes is that their life cycle is entirely independent of living host organisms; they live freely in the environment.
Parasites (from Greek parasitos — parasite), by contrast, depend directly on a living host organism, obtaining all necessary nutrients from it. The parasite group is heterogeneous and divided into two subgroups based on their degree of adaptation to the host:
- Obligate parasites. These are microorganisms that, in the course of evolution, have completely lost the ability to exist outside a living host cell. Their reproduction occurs exclusively intracellularly. Classic examples include members of the genera Rickettsia, Coxiella, Ehrlichia, and Chlamydia.
- Facultative parasites. These possess high metabolic plasticity. They are capable of both living and multiplying in host tissues and existing outside the host organism by feeding like saprophytes. Due to this feature, facultative parasites can be successfully cultured in vitro using artificial nutrient media.