Metabolism of Bacteria


The physiology of bacteria studies the vital activity of microbial cells – processes of their nutrition, respiration, growth, and reproduction.

Metabolism is a complex of biochemical pathways providing energy accumulation and cell structure synthesis. It is composed of two closely related sets of reactions: catabolism and anabolism.

The catabolism (energy metabolism) is a process of degradation of large molecules into more simple ones, resulting in the energy accumulation in the form of proton-motive force, ATP, or GTP.

Anabolism (the synthetic metabolism) ensures the synthesis of macromolecules the cell is created from. It uses the energy, accumulated from catabolism. Metabolism of bacteria is of high speed and provides fast microbial adaptation to varying environmental conditions.

Nutrients present in the environment or growth media must contain all the elements necessary for the microbial biosynthesis.

Autotrophs (Gk. autos – self, trophe – nutrition) are photosynthetic and chemosynthetic microorganisms capable of producing organic molecular substances from inorganic precursors. They don’t need carbon of organic origin, and they build the structures of their cells by utilization of carbon dioxide, water, and primary nitrogen-containing inorganic compounds (ammonia and its salts, nitrates, etc.). For instance, environmental nitrifying bacteria and sulphur-containing bacteria pertain to the autotrophic microorganisms.

Heterotrophs uptake the carbon for their growth and development from any external organic source (from carbohydrates, proteins, lipids and fatty acids, etc.). It is worthy to note that organic carbon of these substances should be easily available for next assimilation.

Heterotrophic microorganisms fall into 2 basic groups depending on their source of organic matter – saprophytes and parasites.

  1. Saprophytes (Gk. sapros – decaying, phyton – plant) assimilate organic compounds acquired from non-living matter of the environment. The majority of bacteria belongs to saprophytes. Their activity is absolutely essential for global turnover of basic chemical elements as well as any complex substances on the Earth.
  2. Parasites inhabit another living body (host organism) and exploit the host for their nutrition and/or energy donation. This group comprises relatively small amount of species of microbes that in the process of evolution have adapted themselves to the parasitic mode of life.

Parasites can be divided into obligate and facultative.

The obligate parasites are able to survive only intracellularly without possibility to change their parasitic mode of behavior (e.g. rickettsiae and chlamydiae).

The facultative parasites in proper situations can change their source of nutrition being capable of propagating not only within the live host, but also on artificial nutrient media.

According to possible energy source, chemotrophic bacteria gain energy from transformations of various chemical substances. Phototrophic bacteria obtain energy from light.

Lithotrophs (Gk. lithos – stone, trophe – nutrition) utilize some inorganic substrates as electron donors (e.g., hydrogen, sulfur, or ammonia serve as reductants), whereas organotrophs use the wide number of organic substances.

The majority of pathogenic microorganisms pertain to chemoorganoheterotrophs.

Many bacteria need special growth factors for their optimal growth and development. They use vitamins; essential amino acids and fatty acids; peptides, purine and pirimidine bases, etc.

Bacteria that require one or several growth factors for their propagation are termed auxotrophs.