Bacterial Cell Structure and Function PDF

Chapter 2 STRUCTURE AND FUNCTION OF BACTERIAL CELL 17  The structure and function of bacterial cell ❖ Bacteria are minute unicellular organisms, have many sizes and several shapes. ❖ Most bacteria range from 0.2 to 2.0 µm in diameter and from 2 to 8 µm in length. They hav...

Chapter 2 STRUCTURE AND FUNCTION OF BACTERIAL CELL 17  The structure and function of bacterial cell ❖ Bacteria are minute unicellular organisms, have many sizes and several shapes. ❖ Most bacteria range from 0.2 to 2.0 µm in diameter and from 2 to 8 µm in length. They have a few basic shapes: spherical coccus, rod- shaped bacillus, and spiral. Note: 1 Micrometer (µm) = 10-6 meter The Bacterial Structure The bacteria are formed of essential components and non-essential components. 1. Essential structures, present in all bacteria: The cytoplasm containing ribosomes; The cytoplasmic membrane (or plasma membrane); The rigid cell wall (exception, mycoplasmas). The nucleoid (circular chromosome) 2. Structures present in some species (primary taxonomic characters):  Flagella;  Spores;  Inclusion granules. 18 3. Structures present in some strains of some species: Fimbriae; Sex pili; Glycocalyx Capsule 1- Glycocalyx - Glycocalyx is a sticky, sugary envelope composed of polysaccharides and/or polypeptides that surround the cell. - Glycocalyx can be firmly ‫ بقوة‬attached to the cell's surface, called capsule, or loosely attached, called slime layer. - A slime layer is water-soluble and is used by the procaryotic cell to adhere to surfaces external to the cell. - It covers surfaces and is an important component of biofilms. - glycocalyx-producing strains of Pseudomonas aeruginosa, which cause respiratory tract infections in cystic fibrosis patients 19 2- Capsule: - Gelatinous layer composed of polysaccharide. - Plays a role in pathogenicity (adherence to human tissues). - Capsule will increase the pathogenicity of the bacteria because capsulated bacteria are not as easily phagocytosed and destroyed by host cell. 3- Cell wall: - The cell wall is the outermost component common to all bacteria (except Mycoplasma species, which are bounded by a cell membrane, not a cell wall). - Rigid structure composed of peptidoglycan in inner site, the outer depends on bacterial type. It protects the cell and gives the shape to the cell. - Peptidoglycan made by N-acetylglucosamine and N-acetylmuramic acid. tetrapeptide (L-alanine- isoglutamine-lysine-alanine) is attached. The entire cell wall structure is cross linked by covalent bonds. - The peptidoglycan provides structural support and maintains the characteristic shape of the cell. 20 Cell Walls Of Gram-positive And Gram-negative Bacteria The peptidoglycan layer is much thicker in gram-positive than in gram- negative bacteria. Some gram-positive bacteria also have fibers of teichoic acid, which protrude outside the peptidoglycan, whereas gram-negative bacteria do not. 21 Cell walls of acid-fast bacteria Mycobacteria, e.g., Mycobacterium tuberculosis, have an unusual cell wall, resulting in their inability to be Gram-stained. These bacteria are said to be acid-fast because they resist decolorization with acid–alcohol after being stained with carbol fuchsin. This property is related to the high concentration of lipids, called mycolic acids, in the cell wall of mycobacteria. 4- Cytoplasmic membrane ▪ Surrounds the cytoplasm and its organelles. ▪ Its main function is a s a selective permeability barrier that regulates the passage of substances into and out of the cell. ▪ Secretion of enzymes and toxins. 22  5- Flagella ▪ Flagella are organs of motility. ▪ They are long, whip like appendages that move bacteria toward nutrients and other attractants, composed of protein. ▪ The flagellar filament is rotated by a motor apparatus in the plasma membrane allowing the cell to swim in fluid environments. ▪ The location of flagella varies in various bacteria. The bacteria which lack flagella are referred as a trichous. ▪ The number and position of the attachments of the flagella on the bacterial wall vary according to the species. Therefore bacteria can be divided into following types: (i) Monotrichous: Single flagellum at one end of the cell. (ii) Lophotrichous: Two or more flagella at one or both ends of the cell. (iii) Amphitrichous: One or many flagella at the end of the cell. (iv) Peritrichous: Many flagella attached all-round the cell 6- Fimbria and Pili: Nonmotile extensions Fimbriae – Short, hair-like filaments extend from the cell surface. – Used by bacteria to adhere to one another, to hosts, and to substances in environment. – May be hundreds per cell and are shorter than flagella – Serve an important function in biofilms 23  Pili – Longer than fimbriae but shorter than flagella – Bacteria typically only have one or two per cell – Called sex pili because it can join two bacterial cells and mediate the transfer of DNA from one cell to another (conjugation) 7- Ribosomes - Fine granular structure in the cytoplasm, each cell has very large numbers of ribosomes. - They are the sites of protein synthesis as in eukaryotic cells, but they differ in size and chemical composition, bacterial ribosomes are 70S in size with 50S and 30S subunits, while eukaryotic ribosomes are 80S in size with 60S and 40S subunits. - The differences in both the ribosomal RNAs and proteins constitute the basis of the selective action of several antibiotics that inhibit bacterial, but not human, protein synthesis. 24 8- Mesosomes - Projections of Cytoplasmic membrane. - Play part in cell division. - 9- The Cytoplasm: The cytoplasm is everything that is present inside the bacterium. One major component of the cytoplasm is the gel-like cytosol, a water- based solution that contains ions, small molecules, and macromolecules. Contains the mechanisms necessary for maintaining the cell and allowing reproduction. 10- Granules: - The cytoplasm contains several different types of granules that serve as storage areas for nutrients. 11- Nucleoid (Bacterial chromosome) - Circular, Single molecule Function: Control center of the cell (controls reproduction and metabolism), Contains genetic code 25  Nucleic Acids (DNA&RNA) DNA (Deoxyribonucleic acid) DNA Structure A. Deoxyribose: 5 carbon sugar B. Phosphate group (PO4) C. Nitrogen bases: 1. Pyrimidines: Thymine, Cytosine, (Uracil in RNA) 2. Purines: Adenine, Guanine 3. Pairing of Nitrogen bases is specific a. Adenine to thymine - 2 “weak” hydrogen (H) bonds b. Guanine to cytosine - 3 “weak” H bonds 4. double helix 2. RNA (Ribonucleic acid) A. Consists of RNA nucleotides a. Ribose (instead of deoxyribose) b. Uracil (instead of thymine) B. Single helix (instead of double helix) 26 Major RNAs: There are three major classes of RNA in the cell: 1- mRNA Messenger RNA carries the genetic information that will be expressed ultimately as proteins. 2- tRNA Transfer RNA is bonded to the appropriate amino acid. 3- rRNA Ribosomal RNA is found in the ribosomes. MUTATIONS Permanent change in the sequence of nucleotides in DNA, a mutation is any chemical change in a cell's genotype (genes) that may or may not lead to changes in a cell's phenotype (specific characteristics displayed by the organism). Mutations often make an organism unable to synthesize one or more proteins. Spontaneous mutations – occur by chance, usually during DNA replication. Induced mutations are caused by chemical, physical, or biological agents called mutagens, some chemicals UV light, X-rays, gamma radiation, & decay of radioactive elements 27  Types of mutation: 1. Substitution mutations, (A > G) 2. Insertions add one or more extra nucleotides into the DNA. 3. Deletions remove one or more nucleotides from the DNA. Which can significantly alter the gene product. Bacterial Genetic Recombination The transfer of DNA from one organism to another with subsequent integration into the DNA of the recipient. Three mechanisms Transformation Transduction Conjugation 1- Transformation: Genetic recombination in which a DNA fragment from a dead, degraded bacterium enters a competent recipient bacterium and it is exchanged for a piece of the recipient's DNA. 28 2- Transduction Genetic recombination in which a DNA fragment is transferred from one bacterium to another by a bacteriophage 3- Conjugation: Bacterial Conjugation is genetic recombination in which there is a transfer of DNA from a living donor bacterium to a recipient bacterium. 12- Plasmids 29 - Plasmids are extra-chromosomal, double-stranded, circular DNA molecules that are capable of replicating independently of the bacterial chromosome. - They can be integrated into the bacterial chromosome. - Plasmids occur in both gram-positive and gram-negative bacteria - Plasmids contain antibiotic resistance genes. - Plasmids contain resistance genes to heavy metals such as mercury (the active component of some antiseptics). - Plasmids contain resistance genes to ultraviolet light, which is mediated by DNA repair enzymes. - Plasmids contain genes that responsible for Pili (fimbriae) formation, which mediate the adherence of bacteria to epithelial cells. - Plasmids contain genes fro exotoxins, including several enterotoxins. - - 30 13- Spores in the bacteria  Endospores are formed by a few genera of bacteria, such as Bacillus.  By forming spores, bacteria can survive in hostile conditions.  Spores are resistant to heat, dessication, chemicals, and radiation.  Bacteria can form endospores in approximately 6 to 8 hours after being exposed to adverse conditions.  The normally-growing cell that forms the endospore is called a vegetative cell.  Spores are metabolically inactive and dehydrated.  They can remain viable for thousands of years.  When spores are exposed to favorable conditions, they can germinate into a vegetative cell within 90 minutes.  Endospores can form within different areas of the vegetative cell. They can be central, subterminal, or terminal. 31