Introduction to bacteria: Classification and structure; virulence and pathogenicity

Bacterial classification (Phenotypic)

• Gram Stain: Differentiates based on cell wall structure.

• Morphology: Cocci, bacilli, curved rods, clusters, chains.

• Growth Requirements: Oxygen tolerance (aerobes vs anaerobes).

• Biochemical Reactions: Catalase, coagulase, oxidase, lactose fermentation.

• Serologic Systems: Group A vs B Strep (based on surface antigens).

Gram stain mechanism

• Crystal Violet → binds peptidoglycan.

• Iodine → forms CV-I complex.

• Alcohol → decolorizes Gram-negatives.

• Safranin → counterstains Gram-negatives pink.

• Gram-Positive: Thick peptidoglycan, teichoic acids, no outer membrane → purple.

• Gram-Negative: Thin peptidoglycan, outer membrane with LPS → pink.

Bacterial cell structure

• All bacteria/Prokaryotes: No nucleus, no membrane-bound organelles, single circular chromosome, 70S ribosomes, replication via binary fission.

• All bacteria have a cell wall with peptidoglycan (Mycoplasma is an exception). Structural integrity is provided by peptide cross-linking by transpeptidase (PBP): Target of β-lactams.

• Additional structural features (some bacteria).

  • Capsule: Polysaccharide; anti-phagocytic (e.g., S. pneumoniae, H. influenzae).

  • Appendages: Pili (adhesion, conjugation), Flagella (motility).

  • Spores: Dormant, resistant forms (e.g., Clostridium, Bacillus).

Gram-positive cell wall

• Thick peptidoglycan layer confers the purple color on gram stain.

• Teichoic and lipoteichoic acids are G+ specific structural features that are recognized by immune system (pathogen associated molecular pattern, PAMP).

Gram-negative cell wall

• Thin peptidoglycan layer leads to the pink color on gram stain.

• Outer membrane with LPS (endotoxin): Lipid A (toxic), O-antigen (serotyping).

• Porins: Selective permeability.

• Protein secretion systems: Types I–VI (Type III = injectosome).

Atypical cell walls

• Mycobacteria: Mycolic acids, acid-fast stain (Ziehl-Neelsen), slow-growing.

• Mycoplasma/Chlamydia: No peptidoglycan → not visible on Gram stain.

Spores

• Metabolically inactive, highly resistant.

• Spore coat: Keratin-like.

• Examples: Clostridium, Bacillus.

• Not killed by antibiotics (non-replicating).

Metabolism and growth

• Obligate aerobes: Only aerobic respiration, no fermentation. (e.g. TB, Pseudomonas.

• Obligate anaerobes: No aerobic respiration, no catalase or SOD = oxygen is poison (e.g., Clostridium, Bacteroides.)

• Facultative anaerobes: Can metabolize energy aerobically or anaerobically and small amounts of catalase and SOD protect from ROS (e.g., E. coli, S. aureus).

• Microaerophilic: Need oxygen for aerobic respiration, but small amounts of catalase and SOD mean they are poisoned by high O2 (e.g., Helicobacter pylori).

Biochemical tests used in differentiation of organisms

• Catalase: H₂O₂ → H₂O + O₂ (Staph +, Strep –).

• Coagulase: Fibrinogen → fibrin (S. aureus +).

• Lactose fermentation: MacConkey agar (E. coli +).

• Oxidase: Cytochrome c oxidase (Pseudomonas +).

Board tips and mnemonics

• No Cell Wall = No Gram Stain → Mycoplasma, Chlamydia.

• Spores = Survival → Think Clostridium difficile in hospitals.

• Positive = Purple → Gram-positive retains crystal violet.

• Lipid A = Lethal A → Endotoxin effects: fever, shock, DIC.

Infection process overview

• Steps in Infection:

  Attach → Persist → Invade → Adapt → Replicate → Exit/Spread

• Virulence: Ability to cause disease; not all colonizing bacteria cause infection.

• Host Factors: Skin integrity, immune status, social determinants, environment.

Attachment and persistence

• Adhesins (bacterial) bind receptors (host).

• Pili often enhance adhesion.

• Site-specific colonization: e.g., Strep pyogenes → pharyngitis.

• Biofilms:

  • Aggregates of bacteria in a matrix.

  • Resist immune clearance and antibiotics.

  • Common in prosthetics, CF lungs, ischemic wounds.

• Mechanisms used.

• Nutrient acquisition:

  • Siderophores: High-affinity iron chelators.

  • Nutritional Immunity: Host sequesters nutrients (e.g., iron via transferrin/lactoferrin).

  • Bacteria evolve mechanisms to extract metals, amino acids, sugars, vitamins.

• Intracellular survival or transition

  • Example: Listeria monocytogenes uses actin polymerization to move between cells.

Virulence factors: Toxins

• Endotoxins (LPS)

  • Found in Gram-negative outer membrane.

  • Lipid A activates TLR4 → cytokine storm (TNF, IL-1).

  • Causes fever, shock, complement activation.

• Exotoxins (Secreted Proteins)
  • Mechanisms:

1. 1. 1. Membrane damage (pore-forming):

         • S. aureus α-toxin.

         • Strep pyogenes: Streptolysin O.

         • C. perfringens- phospholipase.

      2. Immune Activation:

         • Superantigens (e.g., TSS toxins) → massive cytokine release.

      3. Neurotoxins:

         • Prevention of release of neurotransmitters at neuromuscular junction = contraction or paralysis depending on which cell is targeted.

      4. Cellular Signaling Interference:

         • ADP-ribosylation (e.g., Cholera, Pertussis, C. difficile).

      5. Protein Synthesis Inhibition:

         • Inhibition of Elongation Factor 2 (EF2).

         • Diphtheria toxin, Shiga toxin.

      6. Extracellular Matrix Damage:

         • Collagenase, hyaluronidase = tissue damage.

         • IgA protease = immune evasion(e.g., Neisseri, Strep pneumo).

Genetic adaptability and resistance

• Horizontal Gene Transfer

  • Transformation: Uptake of naked DNA.

  • Conjugation: Plasmid transfer via sex pilus.

  • Transduction: Phage-mediated DNA transfer.

  • Transposition: Mobile genetic elements (“jumping genes”).

• Specialized Transduction
  • Example: Corynebacterium diphtheriae acquires toxin gene via β-phage.

Board tips and mnemonics

• Biofilms = Bacterial bunkers → immune evasion + antibiotic resistance.

• Superantigens = Cytokine storm → shock.

• ADP-ribosylation = Signal hijack. 

• Horizontal gene transfer = Resistance spread.