MICROBIOLOGY Study Guide - 2012

CHAPTER 3. BACTERIAL MORPHOLOGY

3.2. CHEMICAL COMPOSITION OF THE BACTERIAL CELL

In its chemical composition, the bacterial Cell is generally similar to the Cells of other living organisms. It contains the same biocomponents: Water, mineral compounds, and organic substances such as Proteins, Nucleic Acids, CARBOHYDRATES, and Lipids (Table 2).

Class="center">Table 2. Chemical composition of the bacterial cell

Water constitutes the major part of a vegetative bacterial cell—70–85% of its total mass. The Role of water in cellular life is immense. It serves as a solvent for organic and mineral substances, a dispersion medium for colloids, an environment for metabolic reactions, and a source of hydrogen and hydroxyl ions. In addition, water performs a mechanical role in maintaining turgor pressure (tension). In a hypertonic solution, The Cell loses water, causing the protoplast to pull away from the Introduction/37.html">Bacterial Cell wall. This phenomenon is known as plasmolysis.

The loss of a large amount of water disrupts METABOLISM and leads to cell death. Water exists within the cell both in a free state and bound to cellular structural elements. In bacterial spores, the water content drops to 18–20%, and practically all of it is in a bound state.

Proteins make up a significant portion of the dry cell mass, and their content depends on The Nature and cultivation conditions of the Bacteria. Proteins are found in various Cellular Structures and perform diverse Functions. Many proteins act as Enzymes—catalysts for biochemical reactions; some serve as Structure/83.html">Structural elements of the cell, making up The cell wall, membrane, flagella, capsule, and others. Permease Proteins perform a transport function, ensuring the uptake of nutrients into the cell. Specific proteins (toxins) determine bacterial pathogenicity.

Bacterial proteins contain a unique amino acid not found in the cells of higher organisms: diaminopimelic acid:

Diaminopimelic acid was first discovered in the Cells of the diphtheria bacillus. Later, it turned out that this acid is present in most species of rod-shaped bacteria and actinomycetes. An exception is found in spherical bacteria (cocci), in which it is replaced by Lysine, an amino acid structurally similar to diaminopimelic acid.

Alongside simple proteins, complex proteins are found in bacterial cells: nucleoproteins, Glycoproteins, and Lipoproteins.

Nucleic acids in bacteria perform functions analogous to those in any living cell. DNA, in the form of the bacterial chromosome, is responsible for storing and transmitting hereditary information. In some bacteria, small DNA molecules are found in the Cytoplasm and are called Plasmids. The total DNA content accounts for 3–4% of the dry cell mass. Ribonucleic Acids (messenger, transfer, and ribosomal) participate in Protein Biosynthesis. On average, they account for up to 10% of the dry cell mass.

Carbohydrates are Components of the cell wall and capsule, and they also serve as nutrient reserves for the bacterial cell. Carbohydrates found in bacterial cells include mono-, di-, and Polysaccharides. Polysaccharides—such as starch, Glycogen, and granulose—make up the bulk of carbohydrates; they function as reserve substances and are utilized by The Cell as a carbon source as needed. The cell wall contains aminosugars (N-acetylglucosamine, N-acetylgalactosamine) as well as muramic acid, a derivative of D-glucosamine. In addition to intracellular carbohydrates, many microorganisms synthesize extracellular polysaccharides that form part of the capsule and slime layers. Notably, Leuconostoc mesenteroides synthesizes the polysaccharide dextran in a sucrose-containing medium. Some spore-forming bacilli synthesize levan, which is a polymer of fructose. Acetic acid bacteria of the species Acetobacter xylinum produce Cellulose.

Lipids are primarily constituents of membranes and the bacterial cell wall. Lipids located in the cytoplasm of bacteria act as nutrient reserves. Bacterial lipids are represented by Phospholipids, Fatty acids, and glycerides. The total lipid content in the cells of various bacteria varies widely: from 5% (in the diphtheria bacillus) to 30–40% (in the tuberculosis bacterium).

The mineral substances of bacteria are detected after cellular incineration. Among them, phosphorus is dominant, being a component of nucleic acids, phospholipids, and A number of Coenzymes. Potassium, sodium, sulfur, iron, calcium, and magnesium are also detected in significant quantities. Other minerals (copper, cobalt, barium, manganese, etc.) are found as Trace Elements.