Bacterial nutrition

Classification by carbon source

Carbon serves as a key building block for any microbial cell. Without a constant supply of its atoms from the environment, a bacterium cannot synthesize vital organic polymers: proteins, carbohydrates, lipids, and nucleic acids. Depending on the form in which microorganisms can assimilate this macronutrient, they are generally divided into two fundamental groups:

1. Autotrophs (from the Greek word autos — self). These are bacteria that use inorganic compounds as their sole or main carbon source. In the vast majority of cases, common carbon dioxide ($CO_2$) serves as this source. Autotrophs are capable of independently synthesizing complex organic molecules from simple inorganic precursors.

1. Heterotrophs (from the Greek word heteros — other). This group of microorganisms requires organic carbon sources. For optimal growth and reproduction, they must consume preformed, easily digestible organic compounds. Among the most preferred sources for heterotrophs are various hexoses, polyhydric alcohols, and amino acids.

Classification by type of energy metabolism

All vital activities of a microbial cell are based on energy, which is reliably stored in the form of ATP. However, the pathways for obtaining this energy, as well as the sources of electron donors for metabolic reactions, differ significantly among various species.

According to their main energy source, bacteria are divided into:

• Phototrophs — microorganisms capable of capturing and using light quanta energy to meet their physiological needs.
• Chemotrophs — microorganisms that extract energy exclusively through oxidation-reduction chemical reactions.

Among chemotrophs, there is a further subdivision based on the chemical nature of the substances acting as electron donors:

• Lithotrophs (from the Greek lithos — stone). They use inorganic substrates as electron donors. Gaseous hydrogen, ammonia, hydrogen sulfide, iron ions, and a number of other inorganic molecules can act in this capacity.
• Organotrophs. Unlike lithotrophs, these bacteria extract electrons by oxidizing various organic compounds.

Ecological classification of heterotrophs

Heterotrophic bacteria demonstrate varying degrees of dependence on the macroorganism (host) and differ in their ability to assimilate specific nutrient substrates. Based on these ecological criteria, they are classified into saprophytes and parasites.

Saprophytes (from the Greek sapros — rotten) are microorganisms that specialize in feeding on dead organic substrates. The most important feature of saprophytes is that their life cycle does not depend on other living macroorganisms at all; they live freely in the environment.

Parasites (from the Greek parasitos — parasite), on the contrary, are directly dependent on a living macroorganism. It is from the host that they obtain all the nutrients necessary for life. The group of parasites is heterogeneous and is 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 of such bacteria are representatives of the genera Rickettsia, Coxiella, Ehrlichia, and Chlamydia.
• Facultative parasites. They possess high metabolic plasticity. They are capable of not only living and reproducing in host tissues, but also existing outside the host organism, feeding like saprophytes. Due to this feature, facultative parasites can be successfully cultured in vitro using artificial nutrient media.

Class and exam questions

What else to study on this topic

• Mechanisms of synthesis of organic polymers (proteins, carbohydrates, lipids) from carbon dioxide in autotrophs.
• Biochemical features of oxidation-reduction reactions in chemotrophic bacteria.
• Specifics of artificial nutrient media for culturing facultative parasites in vitro.
• Life cycle and mechanisms of intracellular reproduction of obligate intracellular parasites.

Other topics in the "Microbial Physiology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.