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Structure OF Bacterial CELL
KLE University
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Structure of bacteria.
Definition “Bacteria are unicellular organisms belonging to the prokaryotic group where the organisms lack a few organelles and a true nucleus”. Classification of Bacteria Bacteria can be classified into various categories based on their features and characteristics. The classification of bacteria is mainly based on the following: - Shape - Composition of the cell wall - Mode of respiration - Mode of nutrition
Classification of bacteria based on Shape
Type of Classification Examples Bacillus (Rod-shaped) Escherichia coli (E. coli) Spirilla or spirochete (Spiral) Spirillum volutans Coccus (Sphere) Streptococcus pneumoniae Vibrio (Comma-shaped) Vibrio cholerae
Classification of bacteria based on the
Composition of the Cell Wall
Type of Classification Examples Peptidoglycan cell wall Gram-positive bacteria Lipopolysaccharide cell wall Gram-negative bacteria
Classification of bacteria based on the Mode
of Nutrition
Type of Classification Examples Autotrophic Bacteria Cyanobacteria Heterotrophic Bacteria All disease-causing bacteria
- Cell Wall: Provides structural support and protection.
- Capsule (Slime Layer): Outer layer aiding attachment, protection, and immune evasion.
- Flagella: Whip-like structures for motility.
- Pili (Fimbriae): Short, hair-like appendages for adhesion and DNA transfer.
- Outer Membrane (Gram-negative): Additional lipid bilayer. Cell Membrane:
- Plasma Membrane: Semi-permeable membrane regulating substance transport.
- Lipopolysaccharides (LPS) (Gram-negative): Outer membrane components.
- Periplasmic Space: Region between inner and outer membranes. Cytoplasmic Components:
- Cytoplasm: Jelly-like substance containing metabolic pathways.
- Nucleoid: Region containing bacterial DNA.
- Ribosomes: Site of protein synthesis.
- Inclusions: Specialized structures for storage or specific functions.
- Cytoskeletal Elements: Maintaining cellular shape and structure. Genetic Material:
- Chromosomal DNA: Bacterial genome.
- Plasmids: Small, self-replicating circular DNA.
- Transposons: Mobile genetic elements. Internal Structures:
- Mesosomes: Infoldings of the cell membrane.
- Vesicles: Membrane-bound compartments.
- Granules: Storage structures for nutrients or waste.
Other Components:
- Spores: Highly resistant, dormant structures.
- S-layers: Crystalline protein layers.
- Pilus-like structures: Additional adhesion and signaling structures. Variations:
- Gram-positive: Thick peptidoglycan cell wall.
- Gram-negative: Thin peptidoglycan layer with outer membrane.
- Acid-fast: Unique mycolic acid-based cell wall. A simplified diagram of a bacterial cell: Outer Structures:
- Cell Wall (Peptidoglycan)
- Outer Membrane (Lipopolysaccharides)
- Capsule (Slime Layer)
- Flagella
- Pili (Fimbriae) Cell Membrane:
- Plasma Membrane (Phospholipid Bilayer)
- Periplasmic Space Cytoplasmic Components:
- Cytoplasm
- Nucleoid (DNA)
- Ribosomes
- Inclusions (Storage Granules)
- Mesosomes Internal Structures:
Nucleoid:
- Contains genetic material (DNA)
- Regulates gene expression
- Replicates DNA during cell division Ribosomes:
- Site of protein synthesis (translation)
- Reads mRNA and builds polypeptide chains Flagella:
- Provides motility and movement
- Helps bacteria colonize new environments Pili:
- Facilitates DNA transfer (conjugation)
- Mediates bacterial adhesion and attachment Inclusions:
- Storage of nutrients (glycogen, lipids)
- Regulation of metabolic pathways
- Protection against environmental stress *Functions by Type: Phototrophic bacteria:
- Photosynthesis
- Energy production
Chemolithotrophic bacteria:
- Oxidation of inorganic compounds
- Energy production Heterotrophic bacteria:
- Decomposition of organic matter
- Nutrient uptake and utilization Symbiotic bacteria:
- Mutualistic relationships with hosts
- Nutrient exchange and sharing Pathogenic bacteria:
- Invasion and colonization of hosts
- Toxin production and disease causation Detailed explanation of bacterial cell structure: 1. Cell Wall:
Provides structural support, maintains cell shape, and prevents cell lysis.
Composed of peptidoglycan (also known as murein) in most bacteria.
Gram-positive bacteria have a thicker cell wall (20-80 nm) than Gram- negative bacteria (7-8 nm).
Functions:
DNA is organized into a compact, supercoiled structure.
Functions:
- Stores genetic information.
- Regulates gene expression.
- Replicates DNA during cell division. 5. Ribosomes:
Site of protein synthesis.
Composed of rRNA and proteins.
Functions:
- Translates mRNA into proteins.
- Synthesizes proteins essential for cellular functions. 6. Pili (Fimbriae):
Short, hair-like appendages for adhesion and attachment.
Composed of proteins (e., pilin).
Functions:
- Facilitates bacterial attachment to surfaces.
- Mediates DNA transfer (conjugation). 7. Flagella:
Long, whip-like structures for motility.
Composed of flagellin protein.
Functions:
- Propels bacteria through environments.
- Enhances chemotaxis (movement towards attractants).
Additional Structures:
- Capsule (Slime Layer): Protective outer layer, composed of polysaccharides or polypeptides.
- Mesosomes: Infoldings of the cell membrane, involved in cellular processes (e., DNA replication).
- Inclusions: Specialized structures for storage or specific functions (e., gas vesicles).
- Plasmids: Small, self-replicating circular DNA, often carrying antibiotic resistance genes. Variations in Bacterial Cell Structure:
- Gram-positive bacteria (e., Staphylococcus aureus):
- Thick peptidoglycan cell wall.
- No outer membrane.
- Gram-negative bacteria (e., Escherichia coli):
- Thin peptidoglycan cell wall.
- Outer membrane containing lipopolysaccharides.
- Acid-fast bacteria (e., Mycobacterium tuberculosis):
- Unique cell wall composition (mycolic acid). BACTERIAL STRUCTURE FLAGELLA are long, whip-like structures that enable bacteria to move through their environment. Bacterial flagella are long (3 to 12 μm), filamentous surface appendages about 12 to 30 nm in diameter. Structure:
Gene regulation (e., FlhDC master regulator). Importance:
Pathogenesis: Flagella aid bacterial colonization and infection.
Biofilm formation: Flagella facilitate initial attachment.
Environmental adaptation: Flagella enable bacteria to explore and exploit new environments. Interesting facts:
Flagella rotate at 100-1000 Hz.
Bacteria can move up to 50 μm/s.
Flagella are 10-20 nm in diameter. THE CAPSULE, also known as the slime layer, is a protective outer layer surrounding some bacterial cells. It plays a crucial role in bacterial survival, virulence, and interaction with the environment. Composition: The capsule is composed of:
Polysaccharides (e., cellulose, xylan, or alginate)
Polypeptides (e., poly-D-glutamic acid)
Glycoproteins
Teichoic acids (in Gram-positive bacteria) Functions:
Protection: Shields bacteria from environmental stress, desiccation, and phagocytosis.
Adhesion: Facilitates attachment to surfaces, host cells, and other bacteria.
Immune evasion: Interferes with complement activation, phagocytosis, and antibody recognition.
Biofilm formation: Contributes to the development of complex bacterial communities.
Virulence: Enhances bacterial pathogenicity by facilitating invasion and colonization. *Types of Capsules:
Well-defined capsules: Thick, structured layers (e., Klebsiella pneumoniae).
Microcapsules: Thin, poorly defined layers (e., Escherichia coli).
Slime layers: Loose, amorphous layers (e., Pseudomonas aeruginosa). Importance in Human Health:
Infectious diseases: Capsule plays a key role in bacterial pathogenesis (e., pneumonia, meningitis).
Antibiotic resistance: Capsule can reduce antibiotic effectiveness.
Vaccine development: Capsular polysaccharides are used in vaccines (e., pneumococcal conjugate vaccine). Interesting Facts:
The capsule can account for up to 20% of the bacterial cell’s dry weight.
Functions:
Survival: withstand extreme conditions
Dispersal: facilitate spread to new environments
Infection: initiate disease in hosts Examples:
Bacillus anthracis (anthrax)
Clostridium botulinum (botulism)
Clostridium tetani (tetanus)
Bacillus subtilis (soil bacterium) Importance:
Pathogenesis: spores can initiate disease
Food safety: spores can contaminate food
Environmental persistence: spores can survive in harsh environments
Biotechnology: spores used in industrial applications Germination:
Triggered by environmental cues (e., nutrients, moisture)
Activation: spore becomes metabolically active
Outgrowth: spore germinates into vegetative cell Inactivation:
Heat: autoclaving, pasteurization
Radiation: UV, gamma radiation
Chemicals: disinfectants, sterilants
Pressure: high-pressure processing THE CELL WALL The cell wall of bacteria is a critical component that provides structural support, maintains cellular shape, and regulates interactions with the environment. Composition:
Peptidoglycan (also known as murein): main component of bacterial cell walls
Teichoic acids: found in Gram-positive bacteria
Lipoteichoic acids: found in Gram-positive bacteria
Lipopolysaccharides (LPS): found in Gram-negative bacteria
Proteins: structural, enzymatic, and receptor functions Structure:
Peptidoglycan layer: 20-80 nm thick
Periplasmic space: between inner and outer membranes (Gram- negative)
Outer membrane (Gram-negative): contains LPS and porins Functions:
Maintains cellular shape and integrity
Regulates osmotic pressure
Provides structural support
Facilitates cell division
Interacts with environment (adhesion, signaling)
RIBOSOMES
Ribosomes in bacteria are complex molecular machines responsible for protein synthesis. Structure:
- 70S ribosome (small subunit: 30S, large subunit: 50S)
- Composed of rRNA (ribosomal RNA) and proteins
- 30S subunit: 16S rRNA, 21 proteins
- 50S subunit: 23S rRNA, 5S rRNA, 34 proteins Function:
- Translate mRNA into protein
- Synthesize polypeptide chains
- Catalyze peptide bond formation Types of Ribosomes:
- 70S ribosomes (prokaryotic)
- 80S ribosomes (eukaryotic) Components:
- rRNA (ribosomal RNA)
- Proteins (ribosomal proteins)
- mRNA (messenger RNA)
- tRNA (transfer RNA)
- Amino acids
Process of Protein Synthesis:
- Initiation: ribosome binds mRNA, tRNA
- Elongation: amino acids added to polypeptide chain
- Translocation: ribosome moves along mRNA
- Termination: polypeptide chain released Importance:
- Essential for bacterial growth and survival
- Target for antibiotics (e., tetracycline, erythromycin)
- Regulates gene expression Regulation:
- Transcriptional regulation
- Translational regulation
- Ribosome biogenesis regulation Examples:
- E. coli ribosomes
- Bacillus subtilis ribosomes
- Staphylococcus aureus ribosomes Differences from Eukaryotic Ribosomes:
- Size (70S vs 80S)
- Structure (subunit composition)
- Function (initiation, elongation, termination)
Structure OF Bacterial CELL
Course: Patient Examination And Nursing
University: KLE University
