MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 5. MICROBIAL METABOLISM

Nutritional Requirements of Microorganisms. Culture Media

Like All living organisms, microorganisms require specific nutrients for their GROWTH AND DEVELOPMENT, which must be present in the culture medium and continuously transported into The Cell in an assimilable form.

Microbial development depends primarily on the availability of Water. Fungi, for instance, can grow on substrates containing about 12% water, whereas Bacteria require over 20% for survival. Water is the main constituent of the microbial cell, accounting for 80-90% of its total mass.

Microbial Cells also consist of the following elements (expressed as a percentage of dry weight): carbon - 50; oxygen - 20; nitrogen - 14; hydrogen - 8; phosphorus - 3; sulfur - 1; potassium - 1; sodium - 1; calcium - 0.5; magnesium - 0.5; chlorine - 0.5; iron - 0.2; others - 0.3. C, O, N, and H form The basis of organic molecules, which is why they are found in cells in substantial quantities, making them macroelements.

Microorganisms also require specific Trace Elements in the medium, which include: manganese, molybdenum, zinc, copper, cobalt, nickel, vanadium, boron, and silicon. These elements are integral components of Enzymes and are needed in very small quantities.

Particularly fastidious microorganisms and mutants with certain genetic defects additionally require specific substances that they cannot synthesize on their own. Such essential supplementary compounds are called growth factors. They may include Amino Acids, Vitamins, Purines, and Pyrimidines, among others. Microorganisms that require growth factors are termed auxotrophic. If auxotrophy arises As a result of a mutation, the "wild-type" (parental) strain that does not require growth factors is referred to as prototrophic.

Culture media are used to grow microorganisms under laboratory or industrial conditions. A culture medium is a formulation of chemical compounds that Supports the life processes of microorganisms. Media must meet the following requirements:

- contain necessary nutrients;

- contain an adequate amount of water;

- have optimal pH and Eh values;

- be isotonic, meaning they must possess the same osmotic pressure as the internal environment of the microbial cell;

- be sterile.

Culture media are classified into groups According to the following criteria: origin, consistency, and purpose (Table 5.1).

Class="center">Table 5.1. Classification of culture media

Based on their origin, culture media are categorized into natural (empirical), synthetic, and semisynthetic. Natural media are of plant or animal origin and have an undefined composition. These include grain, fruit, and vegetable decoctions; milk, Blood, serum, urine, animal cells, avian eggs and embryos; Yeast autolysate; meat broths, manure and soil extracts; pond and sea water. Examples include meat-peptone broth (MPB), meat-peptone Agar (MPA), meat-peptone gelatin (MPG), peptone water (PW), and wort agar (WA). Synthetic media are prepared from chemically pure substances at precisely specified concentrations. They have a defined composition and are easily reproducible, such as Czapek's, Ashby's, Winogradsky's, and Giltay's media. Semisynthetic media have a partially undefined composition. They contain both chemically pure compounds (CARBOHYDRATES, phosphates, nitrates, ammonium) and components of strictly undetermined composition (yeast extract, brewers' wort, peptone).

According to their consistency, culture media are divided into liquid, semisolid, and solid. Liquid media contain nutrients dissolved in water (MPB, PW, Hiss's media, etc.). Solid and semisolid media are prepared from liquid bases by adding a solidifying agent. Typically, agar-agar—a complex polysaccharide derived from red Algae and composed of agarose and agaropectin—is used to solidify culture media. In water, agar-agar forms a gel that melts at -100 ⁰С and solidifies at -40 ⁰. Solid media contain 1.5-2.0% agar-agar, whereas semisolid media contain 1%. A valuable property of agar-agar is that it is not assimilated by the vast majority of microorganisms. Examples of solid media include MPA, WA, starch-ammonia agar (SAA); semisolid media include Hiss's medium, semisolid nitrate medium, etc.

Depending on their purpose, culture media are classified as general-purpose or special-purpose. General-purpose media are used for the isolation and cultivation of most known microorganisms. Examples of such media include MPB and MPA. Special-purpose media are formulated for specific objectives and particular Applications. Among these, selective and differential-diagnostic media are distinguished. Selective media were introduced into microbiological practice by S. Winogradsky. They provide optimal conditions for the growth of only specific microorganisms while inhibiting the growth of others. They are used primarily for isolating microorganisms from natural sources and obtaining enrichment cultures. An example of a selective medium is Ashby's medium for nitrogen-fixing bacteria. Differential-diagnostic media are used to study the PHYSIOLOGICAL AND BIOCHEMICAL properties of microorganisms and for their identification. For instance, Hiss's medium is used to study the ability of microorganisms to assimilate carbohydrates, whereas Endo, Levine, and Ploskirev media are used to identify enteric bacteria.