Question 1: Structure of a Bacterial Cell with Labeled Diagram and Functions (10 Marks)

Introduction

Labeled Diagram of a Bacterial Cell

Structures and Their Functions

1. Capsule (Glycocalyx / Slime Layer)

• Structure: A loose polysaccharide or polypeptide layer surrounding the cell wall. When compact and well-defined, it is called a capsule; when loose and diffuse, it is a slime layer.
• Functions:
  • Protects the bacterium from phagocytosis by host immune cells (major virulence factor)
  • Helps bacteria adhere to surfaces and host tissues
  • Prevents desiccation (drying out)
  • Confers resistance to some antimicrobial agents
• Examples: Streptococcus pneumoniae, Klebsiella pneumoniae

2. Cell Wall

• Structure: A rigid layer made primarily of peptidoglycan (murein) - a polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by peptide bridges.
  • Gram-positive: thick peptidoglycan (up to 40 layers, comprising ~50% of the cell wall)
  • Gram-negative: thin peptidoglycan (1-2 layers) + outer membrane
• Functions:
  • Maintains the shape of the bacterium
  • Provides rigid structural support and prevents osmotic lysis
  • Determines Gram staining reaction
  • Contains surface antigens involved in immune recognition
  • Is the target of beta-lactam antibiotics (penicillins, cephalosporins) and vancomycin

3. Cytoplasmic (Plasma) Membrane

• Structure: A phospholipid bilayer with embedded proteins. Unlike eukaryotes, it contains no sterols (except Mycoplasma).
• Functions:
  • Selective permeability - controls entry and exit of nutrients and waste
  • Site of electron transport chain and ATP (energy) production (performs functions of mitochondria)
  • Contains transport proteins (permeases) for active transport of metabolites
  • Contains enzymes for biosynthesis (e.g., lipid synthesis)
  • Site of DNA attachment during replication and cell division
  • Houses ion pumps that maintain membrane potential

4. Nucleoid (Bacterial Chromosome)

• Structure: A single, circular, double-stranded DNA molecule (haploid genome) located in the nucleoid region of the cytoplasm. There is no nuclear membrane enclosing it.
• Functions:
  • Contains all genetic information for bacterial growth, reproduction, and metabolism
  • Since transcription and translation are coupled (no nuclear membrane), ribosomes bind mRNA while it is still being synthesized

5. Plasmids

• Structure: Small, circular, extrachromosomal double-stranded DNA molecules that replicate independently.
• Functions:
  • Carry genes for antibiotic resistance, toxin production, virulence factors, and metabolic enzymes
  • Can be transferred between bacteria by conjugation, contributing to horizontal gene transfer
  • Important in the spread of antibiotic resistance

6. Ribosomes

• Structure: 70S ribosomes made of a 30S small subunit and a 50S large subunit (unlike eukaryotic 80S ribosomes).
• Functions:
  • Site of protein synthesis
  • The structural differences from eukaryotic ribosomes make them targets for many antibiotics (aminoglycosides, macrolides, tetracyclines, chloramphenicol)

7. Flagella

• Structure: Long, whip-like filamentous appendages made of the protein flagellin. May be arranged as:
  • Monotrichous (single polar flagellum)
  • Lophotrichous (tuft at one end)
  • Amphitrichous (flagella at both ends)
  • Peritrichous (flagella all around)
• Functions:
  • Primary organ of motility - enables bacteria to swim toward nutrients (chemotaxis) and away from harmful substances
  • Act as virulence factors (help bacteria reach and colonize host tissues)

8. Pili (Fimbriae)

• Structure: Short, hair-like, protein filaments (made of pilin). Two main types:
  • Common pili (fimbriae): numerous, short, involved in adhesion
  • Sex pili (F-pili): longer, fewer in number, involved in conjugation
• Functions:
  • Mediate adherence of bacteria to host cell surfaces and mucosal membranes (critical first step in infection)
  • Sex pili form conjugation bridges for transfer of plasmids and DNA between bacteria (conjugation)

9. Inclusion Bodies (Granules)

• Structure: Storage granules within the cytoplasm; may include glycogen granules, polyphosphate granules (volutin), lipid granules, sulfur granules.
• Functions:
  • Storage of nutrient reserves (carbon, energy, phosphate) used during periods of starvation or stress

10. Endospores (in select bacteria only)

• Structure: Highly resistant dormant structures formed within the cell by bacteria such as Bacillus and Clostridium. Contain dipicolinic acid and have multiple protective layers.
• Functions:
  • Allow the bacterium to survive extreme conditions - heat, desiccation, UV radiation, disinfectants, and lack of nutrients
  • Germinate back into vegetative cells when conditions improve

11. Outer Membrane (Gram-negative bacteria only)

• Structure: An additional lipid bilayer outside the thin peptidoglycan layer, containing lipopolysaccharide (LPS) on its outer leaflet and porin proteins that form channels.
• Functions:
  • Acts as a permeability barrier
  • LPS (endotoxin) triggers a strong immune response and septic shock in the host
  • Porin proteins control passage of small molecules across the outer membrane
  • Confers resistance to many antibiotics and lysozyme

12. Periplasmic Space (Gram-negative bacteria)

• Structure: The space between the inner cytoplasmic membrane and the outer membrane, containing the thin peptidoglycan layer and periplasmic proteins.
• Functions:
  • Contains enzymes for nutrient processing, detoxification (beta-lactamases), and transport

Question 2: What Are Bacteria? Classification Based on Shape (10 Marks)

Definition of Bacteria

• Size: approximately 0.5-3.0 µm
• Single, circular DNA (haploid genome) in a nucleoid region
• 70S ribosomes (30S + 50S subunits)
• Cell wall containing peptidoglycan (murein)
• Reproduce by binary fission
• Can be free-living, commensal, or pathogenic
• Found in virtually every environment on Earth - soil, water, air, and within the bodies of all living organisms

Classification of Bacteria Based on Shape (Morphology)

1. Cocci (Spherical Bacteria)

• Shape: Round or spherical, approximately 1 µm in diameter
• Arrangements:
  • Diplococci: pairs of cocci - e.g., Streptococcus pneumoniae, Neisseria gonorrhoeae
  • Streptococci: chains of cocci - e.g., Streptococcus pyogenes (Group A Streptococcus)
  • Staphylococci: grape-like irregular clusters - e.g., Staphylococcus aureus
  • Tetrads: groups of four cocci - e.g., Micrococcus species
  • Sarcinae: cubical packets of eight cocci
• Clinical significance: Cause pneumonia, meningitis, gonorrhoea, skin infections, toxic shock syndrome

2. Bacilli (Rod-shaped Bacteria)

• Shape: Cylindrical/rod-shaped bacteria, varying in length
• Sub-types:
  • Bacillus (true rod): straight rods, e.g., Escherichia coli, Bacillus anthracis
  • Coccobacillus: short, oval rods intermediate between cocci and bacilli - e.g., Haemophilus influenzae, Bordetella pertussis
  • Fusiform bacillus: spindle-shaped, tapered at both ends - e.g., Fusobacterium species
  • Palisade/Chinese letter arrangement: seen in Corynebacterium diphtheriae
  • Pairs (diplobacilli): two rods end-to-end
  • Chains (streptobacilli): e.g., Streptobacillus moniliformis
• Clinical significance: Cause tuberculosis, diphtheria, typhoid, E. coli infections, anthrax, gas gangrene

3. Spiral Bacteria

a) Vibrio (Comma-shaped)

• Shape: Short, curved rods resembling a comma; incomplete spiral (less than one full turn)
• Example: Vibrio cholerae (cholera), Campylobacter jejuni
• Note: Highly motile with a single polar flagellum

b) Spirillum (Rigid Spiral)

• Shape: Rigid, helical (corkscrew-like) rods with multiple complete turns; have a stiff cell wall
• Example: Spirillum minus (rat-bite fever)
• Motility: By flagella

c) Spirochete (Flexible Spiral)

• Shape: Thin, flexible, tightly coiled spiral bacteria; move by means of internal axial filaments (endoflagella) wound around the cell body
• Examples:
  • Treponema pallidum (syphilis)
  • Borrelia burgdorferi (Lyme disease)
  • Leptospira interrogans (leptospirosis)
• Distinguishing feature: Unique internal locomotion by axial filaments; do not stain well with Gram stain

Summary Table: Classification by Shape

Question 3: Study of Bacteria, and Difference Between Gram-Positive and Gram-Negative Bacteria (10 Marks)

What is the Study of Bacteria Called?

• Bacteriology is a branch of Microbiology - the study of all microorganisms
• The term comes from the Greek: bakterion (small rod/staff) + logos (study)
• Hans Christian Gram (1884) developed the Gram staining technique, a cornerstone of bacteriology
• Key related sub-disciplines:
  • Clinical Bacteriology: study of bacteria causing human diseases
  • Medical Microbiology: encompasses bacteriology in the context of human health and disease
  • Environmental Bacteriology: study of bacteria in nature
  • Industrial Microbiology: uses bacteria for biotechnology, fermentation, etc.

Gram Staining - The Key Differentiating Test

Steps of the Gram Stain:

Differences Between Gram-Positive and Gram-Negative Bacteria

Structural Diagram:

Detailed Comparison Table:

Key Differences Explained:

• Wall teichoic acids (covalently attached to peptidoglycan) and lipoteichoic acids (anchored in cytoplasmic membrane) are unique to Gram-positive bacteria
• Function in cell wall integrity, regulation of cell division, and act as adhesins
• Are important antigens for immune recognition

• Lipopolysaccharide (LPS) is composed of lipid A (toxic component), core polysaccharide, and O-antigen
• Lipid A = endotoxin - released when bacteria lyse; triggers fever, hypotension, disseminated intravascular coagulation (DIC), and septic shock
• Gram-positive bacteria do NOT produce endotoxin

• Forms an additional permeability barrier - makes Gram-negative bacteria intrinsically more resistant to antibiotics, detergents, and lysozyme
• Porin proteins in the outer membrane create size-selective channels for small hydrophilic molecules

Clinical Relevance:

• The Gram stain result guides antibiotic selection BEFORE culture results are available
• Gram-positive infections are often treated with beta-lactams, vancomycin, or clindamycin
• Gram-negative infections often require broader-spectrum antibiotics (aminoglycosides, fluoroquinolones, carbapenems) because of the outer membrane barrier
• Gram-negative sepsis is particularly dangerous due to endotoxin (LPS) release