Bacterial Cell Structure Quiz

Bacterial Cell Structure

• Bacterial cells have three main components: appendages, envelopes, and cytoplasmic contents.
• Appendages include flagella, fimbriae/pili, and axial filaments (endoflagella).
• The cell envelope consists of the capsule, cell wall, and cytoplasmic membrane, layers from outside to inside.
• Cytoplasmic contents include nucleic acid, plasmids, and ribosomes.

Cell Appendages (Flagella, Fimbriae, Axial Filaments)

• Flagella:
  • Originate in cytoplasm
  • Long, thick, and twisted protein structures
  • Present in Gram-positive and Gram-negative bacteria.
  • Function in motility
  • Different arrangements include monotrichous (one flagellum), lophotrichous (multiple flagella at one end) , amphitrichous (flagella at both ends) and peritrichous (flagella diffused over the entire surface).
  • Examples of bacteria with flagella are Salmonella, Vibrio cholerae, and E. coli.
• Fimbriae/Pili:
  • Originate in cytoplasmic membrane
  • Short, thin protein structures.
  • Present in all Gram-negative bacteria and some Gram-positive bacteria.
  • Numerous per bacterial cell.
  • Function in adhesion to surface.
  • Adhere to host cells and other bacteria
  • Sex pili involve in gene transfer by conjugation.
• Axial Filaments/Endoflagella:
  • Located in periplasmic space
  • Present in spirochetes (such as Treponema)
  • Run along the axis of the bacteria, enabling rotation and motility.

Cell Envelope (Capsule, Cell Wall, Cytoplasmic Membrane)

• Capsule:

  • Outermost layer, sometimes formed in vivo.
  • Present in most pathogenic bacteria.
  • Composition can be polysaccharides (e.g., Streptococcus pneumoniae) or polypeptides (e.g., Bacillus anthracis).
  • Demonstrated via capsule stain or India ink wet mount techniques.
  • Importance in virulence as it aids in preventing phagocytosis and serving as a protective barrier from the host’s immune cells.

• Cell Wall:

  • Lies outside the cytoplasmic membrane.
  • Shape determinant.
  • Structure: Peptidoglycan backbone composed of alternating sugars (N-acetylglucosamine and N-acetylmuramic acid) and peptides.
  • Gram positive bacteria have a thick peptidoglycan layer.
  • Gram negative bacteria have a thin peptidoglycan layer covered by an outer membrane.
  • Gram negative outer membrane: Lipopolysaccharide (LPS) and Lipid A.

• Cytoplasmic Membrane:

  • Consists of phospholipid bilayers and proteins.
  • Functions include selective permeability, transport of solutes, energy production(electron transport chain), excretion of hydrolytic exoenzymes and toxic wastes, and cell wall biosynthesis.
  • Mesosomes are invaginations found in some bacteria that increase membrane surface area and play a role in cell division.

Cytoplasmic Contents (Chromosome, Plasmid, Ribosomes)

• Chromosome:

  • Single, double-stranded DNA (helix).

• Plasmid:

  • Extra circular double stranded DNA segment.
  • Found in some bacteria.
  • Capable of autonomous replication.
  • Function: carrying genes for antibiotic resistance.

• Ribosomes:

  • Composed of rRNA and proteins.(70s: 50s and 30s subunits)
  • Function: protein synthesis.

Bacterial Spores

• Spores:
  • Resistant structures formed under adverse environmental conditions (chemicals, radiation, freezing, and heat).
  • Remain viable for extended periods.
  • Formed by some bacteria, including Bacillus and Clostridium.
  • Sporulation: Process forming inactive spore from active vegetative cells.
  • Spores contain DNA enclosed in a multi-layered, rigid envelope.
  • Envelope is strengthened by keratin and calcium dipicolinate.
• Spore Germination:
  • Spores germinate in suitable conditions.
  • Spore coat destruction creates active vegetative bacterial cells.
  • Germinated spore divides by binary fission.

Antimicrobial Chemotherapy

• Antimicrobials:

  • Chemicals that kill or inhibit the growth of microorganisms (bacteria, fungi, viruses, and parasites).

• Classification:

  • Source (natural, synthetic, or semisynthetic).
  • Action (bactericidal, bacteriostatic).
  • Spectrum (broad or narrow).
  • Bactericidal antibiotics kill the bacteria.
  • Bacteriostatic antibiotics inhibit bacterial growth.

• Target Sites for Antibiotics:

  • Cell wall synthesis
  • Cell membrane
  • Protein synthesis
  • Nucleic acid synthesis
  • Examples of antibiotics targeting specific sites are Penicillins, Cephalosporins, Glycopeptides(Vancomycin), Polymyxins, Macrolides (erythromycin, azithromycin), Aminoglycosides (gentamicin), Tetracyclines, Fluoroquinolones, Sulfonamides, and Rifampicin.