FUNDAMENTALS OF MICROBIOLOGY - V. M. Samygin - 2015

CHAPTER 6. TYPES AND MECHANISMS OF PROKARYOTIC NUTRITION

The chemical composition of a bacterial Cell can vary depending on the composition and quality of the nutrient medium, the age of the culture, and the cultivation conditions of the microbe. A microorganism can function normally only if its surrounding medium contains the essential factors required to meet its energy and biosynthetic needs. From the nutrient substrate, the microbe extracts all necessary elements and returns its Metabolic waste products back into it.

Microbial Nutrition has distinct characteristics. Microorganisms lack specialized digestive Organs, and nutrients enter through the entire cell envelope. Consequently, these substances must be dissolved in the medium surrounding the microorganism. The surface-to-volume ratio in microorganisms is very high, which facilitates the rapid uptake of nutrients from the environment. Other important factors include the size of the nutrient molecules, their charge, the pH of the medium, and the Chemical Nature of the substance. Certain substances (Proteins, fats, Polysaccharides) are pre-hydrolyzed by exoenzymes into simpler compounds that can diffuse into The Cell. Thus, the digestive process in Bacteria occurs, to a certain extent, outside the cell. Macro- and microelements are absorbed as anions and cations. Typically, Water moves toward the higher concentration, while salts move toward the lower concentration.

The type of nutrition is determined based on the assimilation of two primary organogens — carbon and nitrogen, whereas water serves as the source of hydrogen and oxygen.

Based on the source of carbon nutrition, microorganisms are classified into:

- autotrophs (from Latin *autos* — self, *trophe* — nourishment, i.e., self-nourishing organisms) that derive carbon from Inorganic Compounds (carbonates) and atmospheric carbon dioxide. Examples of such microorganisms include nitrifying bacteria, iron bacteria, sulfur bacteria, etc.

- heterotrophs (from Latin *heteros* — other, i.e., nourished by others) that utilize Organic compounds (Amino Acids, CARBOHYDRATES, polyhydric alcohols, etc.) as a carbon source. The majority of heterotrophic microorganisms survive by utilizing organic substrates of animal and plant plant origin, playing a vital role in the decomposition of dead organic residues. Such bacteria are called Saprophytes (from Greek *sapros* — rotten, *phyton* — plant). They are harmless to humans.

Some heterotrophs live On the surface or inside another Organism and derive their nutrition at the host's expense. They are referred to as parasites (from Greek *parasitos* — a person who eats at the table of another). Parasites include pathogens that cause diseases in humans, animals, and plants. The highest degree of heterotrophy is characteristic of obligate (strict) intracellular parasites, which include rickettsiae, chlamydiae, and Viruses.

Most parasitic microbes grow on complex artificial nutrient media and are classified as facultative (optional) parasites. However, there is no sharp boundary between facultative parasites and saprophytes. Parasites account for a relatively small proportion of microbial species (0.1%) that have adapted to this lifestyle through evolution. Certain species of human-pathogenic bacteria can exist in the external environment as saprophytes and, conversely, some saprophytes under unfavorable conditions can cause diseases in humans and animals.

Depending on the source of nitrogen, microorganisms are subdivided into aminoautotrophs, which are capable of synthesizing amino acids from inorganic compounds, and aminoheterotrophs, which require preformed amino acids from an external source. The first group includes nitrogen-fixing soil and nodule bacteria capable of assimilating free atmospheric nitrogen. Aminoheterotrophs include all parasitic species and the majority of saprophytic species. They obtain nitrogen by breaking down proteins, amino acids, urea, Purines, Pyrimidines, and Other Compounds, releasing ammonia, which then becomes available for biosynthetic processes.

Some microorganisms require specific organic nutrients known as growth factors. These include Certain amino acids, nitrogenous bases (purines and pyrimidines), and Vitamins.

Microorganisms that have lost The ability to independently synthesize essential organic compounds and thus require growth factors are called auxotrophs. They frequently arise As a result of Mutations. Prototrophs satisfy their nutritional requirements using the primary nutrient source and do not require growth factors. They are considered the "wild" type of microorganisms.

Based on quantitative nutritional requirements, a distinction is made between oligotrophs, which grow at low nutrient concentrations (ranging from fractions of a milligram to 100 mg/L), and copiotrophs, which include parasitic microorganisms that grow at standard concentrations of essential nutrients ranging from 1 to 100 g/L.

Nutrients are utilized by microorganisms not only as structural building blocks but also as a source of energy. Accordingly, chemotrophs obtain energy through oxidation-reduction reactions involving nutrient substrates. Phototrophs (e.g., cyanobacteria, purple bacteria, green bacteria) use radiant energy (light) as their energy source.

During the biochemical oxidation of a substrate, organotrophic bacteria use organic substances (e.g., carbohydrates) as electron Donors, whereas lithotrophic bacteria (from Greek *lithos* — stone) use inorganic substances (H, C, S, sulfites, H_2S, inorganic nitrogen compounds, etc.).

The uptake of substances into the bacterial cell can occur via several pathways:

- passive diffusion, which is driven by the substance's concentration gradient and continues until equilibrium is reached between the concentration outside and inside the cell. This process occurs at a low rate and requires no Energy Expenditure. All uncharged molecules (water, gases) enter the cell via standard (passive) diffusion;

- Facilitated Diffusion, which is mediated by specialized Membrane Proteins called permeases that bind to the substrate, "drag" it across the membrane in an unchanged form, and release it on the inner surface. This process requires no energy expenditure because it does not operate against a concentration gradient. However, the transport rate is significantly increased. Glycerol uptake in *Escherichia coli* serves as an example of this pathway;

- Active Transport, in which substances are transported into the cell against a concentration gradient. Such mechanisms require metabolic energy expenditure;

- group translocation, which differs from the previous mechanisms in that certain compounds undergo chemical modification during transport—meaning the transport process is accompanied by substrate modification. Sugars (e.g., phosphorylated glucose), purines, and pyrimidines are transported in this manner.

The diverse Nutritional Requirements of microorganisms must be taken into account when cultivating them on artificial nutrient media. A single universal nutrient medium cannot be formulated for all species. A vast array of diverse nutrient media has been developed in bacteriological practice to meet the species-specific requirements of bacteria for necessary organic and inorganic compounds.