Bacterial Cell Structure Quiz

Bacterial Cell Structure

• Structure of bacterial cells, from outside to inside:
  • Cell appendages (flagella, fimbriae, axial filaments)
  • Cell envelope (capsule, cell wall, cytoplasmic membrane)
  • Cytoplasmic contents (nucleic acid, plasmids, ribosomes)

Cell Appendages

• Flagella:

  • Origin: cytoplasm
  • Structure: protein
  • Shape: long, thick, and twisted
  • Gram-positive and Gram-negative bacteria
  • Number: one, two, numerous per cell
  • Function: motility (locomotion)
  • Examples: Salmonella, Vibrio cholerae, Escherichia coli

• Axial Filaments (Endoflagella):

  • Location: periplasmic space
  • In spirochetes (like Treponema)
  • Function: rotation motility

• Pili & Fimbriae:

  • Origin: cytoplasmic membrane
  • Structure: protein
  • Common pili:
    • Shape: short, thin
    • Presence: all gram-negative bacteria and some gram-positive
    • Number: numerous (up to 1000 per cell)
    • Function: adherence to host cells and other bacteria
  • Sex pili:
    • Shape: long, thick
    • Presence: only in gram-negative bacteria
    • Number: 1-10 per cell
    • Function: gene transfer by conjugation

Cell Envelope

• Capsule:

  • Location: outermost layer
  • Formed mainly in vivo conditions
  • Presence: most pathogenic bacteria
  • Composition: polysaccharides (e.g., Streptococcus pneumoniae, Neisseria meningitides); exception: Bacillus anthracis (polypeptides)
  • Function: important virulence factor (enhances infection by preventing the immune cells from engulfing the bacteria); also important for identification/diagnosis, and vaccine preparation.

• Cell Wall:

  • Location: outside the cytoplasmic membrane, determines the shape of the cell
  • Structure: Peptidoglycan backbone (chains of two alternating sugars, NAM, and NAG, with peptide chains attached, forming cross-links.)
  • Gram-positive cell wall: thick peptidoglycan layer, contains lipoteichoic acid for attachment
  • Gram-negative cell wall: thin peptidoglycan layer in periplasmic space; additional outer membrane contains lipopolysaccharides (LPS) and Lipid A

Cytoplasmic membrane

• Structure: phospholipid bilayers and proteins
• Function:
  • Selective permeability and transport of solutes
  • Energy production (electron transport chain)
  • Excretion of hydrolytic exoenzymes and toxic waste
  • Biosynthesis of cell wall
• Mesosomes: invaginations of the cytoplasmic membrane into the cytoplasm.
  • Increase surface area and function of the cytoplasmic membrane.
  • Role in cell division

Cytoplasm

• Chromosome:
  • One DNA, double-stranded (helix)
• Plasmid:
  • Circular extra DNA segment
  • In some bacteria
  • Capable of autonomous replication
  • Function: carry genes of antibiotic resistance
• Ribosomes (70S):
  • Composed of rRNA and protein
  • Function: protein synthesis

Bacterial Spores

• Spores: resistant structures formed under adverse conditions (chemicals, radiations, freezing, heat).
  • Can remain viable for many years in dormant state
  • Fomred by certain bacterial types (like Bacillus and Clostridium).
• Sporulation: process of producing inactive bacterial form from active vegetative cells.
  • Spores contain DNA, encased in a multi-layered rigid envelope, strengthened by keratin + calcium dipicolinate
• Spore Germination: In suitable environment(like in "vivo", the spore coat is disintegrated, the bacteria form vegetative cells by binary fission.

Antimicrobial Chemotherapy

• Antimicrobials: Chemicals that kill or inhibit microbes (bacteria, fungi, viruses, parasites)
• Classification:
  • Source: natural, synthetic, semisynthetic
  • Action: bactericidal (lethal), bacteriostatic (inhibit bacterial growth).
    • Broad spectrum: active against many bacteria types
    • Narrow spectrum: active against limited types of bacteria
• Classification according to the targets of antibiotics: -Cell wall inhibitors (Beta-lactams, Penicillins, Cephalosporins, Glycopeptides, e.g,. Vancomycin)
  • Cell membrane inhibitors (Polymyxins)
  • Protein synthesis inhibitors (Macrolides, Aminoglycosides, Tetracyclines)
  • Nucleic acid inhibitors (Sulfonamides, Quinolones, Rifampicin)