Bacteria

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Bacteria — Key Facts for NEET PG

• Bacterial Cell Wall: Peptidoglycan (muramic acid + N-acetylmuramic acid + peptide bridges); Gram-positive = thick PG layer + teichoic acid; Gram-negative = thin PG + outer membrane (LPS)
• Gram Stain: Crystal violet + iodine (primary stain) → Alcohol/acetone (decolorizer) → Safranin (counterstain); Purple = Gram-positive; Pink/red = Gram-negative
• Endotoxin: LPS (Lipopolysaccharide) of Gram-negative outer membrane — causes fever, septic shock
• Bacterial Spores: Bacillus, Clostridium — heat-resistant; Dipicolinic acid in core
• ⚡ Exam tip: Antibiotics targeting cell wall (β-lactams, vancomycin) don’t affect mycoplasma (no cell wall)

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Bacteria — NEET PG Study Guide

Bacterial Morphology

Shapes:

• Cocci (round): Staphylococci (clusters), Streptococci (chains), Diplococci (pairs)
• Bacilli (rod-shaped): Single rods (E. coli), Diplobacilli, Streptobacilli
• Spiral: Vibrio (comma), Spirillum (thick spiral), Spirochete (corkscrew)

Bacterial Structures

Appendages:

• Flagella: Motility; Arrangement: Monotrichous, Lophotrichous, Amphitrichous, Peritrichous
• Pili/Fimbriae: Adherence; Sex pili for conjugation

Cell Envelope:

• Gram-positive: Thick peptidoglycan (90%), teichoic acid, no outer membrane
• Gram-negative: Thin peptidoglycan (10%), periplasmic space, outer membrane with LPS

Gram Staining Procedure:

1. Crystal violet (primary stain)
2. Gram’s iodine (mordant)
3. Alcohol/acetone (decolorizer)
4. Safranin (counterstain)

NCE Exam Pattern

Common question types:

1. Bacterial morphology and classification
2. Gram staining and bacterial structures
3. Endotoxins vs. exotoxins
4. Bacterial genetics and plasmids
5. Antimicrobial mechanisms

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Bacteria — Comprehensive NEET PG Notes

Detailed Theory

1. Bacterial Cell Wall — Complete Structure

Peptidoglycan (Murein):

• N-acetylglucosamine (NAG) + N-acetylmuramic acid (NAM) alternating
• Tetrapeptide bridges crosslink NAM units
• Provides rigid structure against osmotic pressure

Gram-Positive Cell Wall:

• Thick peptidoglycan (20-80 nm, 90% of wall)
• Teichoic acids (glycerol/ribitol phosphate polymers)
• Lipoteichoic acids (anchor to membrane)
• No periplasmic space
• Retain crystal violet → purple

Gram-Negative Cell Wall:

• Thin peptidoglycan (2-3 nm, 10% of wall)
• Periplasmic space (between inner and outer membrane)
• Outer membrane (asymmetric lipid bilayer)
• LPS (Lipopolysaccharide) on outer leaflet:
  • Lipid A (endotoxin, toxic component)
  • Core polysaccharide
  • O-antigen (variable, serotype determinant)
• Porins for permeability
• Lose crystal violet → counterstain safranin → pink/red

Acid-Fast Bacteria (Mycobacteria):

• High mycolic acid content (60% of cell wall)
• Not classical Gram staining
• Use Ziehl-Neelsen (carbol fuchsin + heat) or auramine-rhodamine
• Considered Gram-positive but stain poorly
• Examples: M. tuberculosis, M. leprae

2. Cell Membrane and Cytoplasm

Cell (Plasma) Membrane:

• Phospholipid bilayer
• Respiratory enzymes located here
• Site of cell wall synthesis
• Contains penicillin-binding proteins (PBPs)

Cytoplasm:

• Ribosomes (70S = 30S + 50S)
• Nucleoid (circular chromosome, not membrane-bound)
• Plasmids (extrachromosomal DNA)
• Inclusion bodies (metachromatic granules, glycogen, lipid)

Ribosomes:

• 70S (prokaryotic)
• 30S subunit: 16S rRNA + 21 proteins
• 50S subunit: 23S rRNA + 5S rRNA + 34 proteins
• Target for aminoglycosides, tetracyclines, macrolides

3. Surface Structures

Flagella:

• Protein: Flagellin
• Basal body (embedded in membrane) with hook and filament
• Arrangement varies by species:
  • Monotrichous: Single polar flagellum
  • Lophotrichous: Multiple polar flagella
  • Amphitrichous: One at each end
  • Peritrichous: All around

Pili (Fimbriae):

• Common pili: Short, numerous, for adhesion
• Sex pili: Longer, fewer, for conjugation (F pilus)

Capsule:

• Polysaccharide (except Bacillus anthracis which is D-glutamic acid)
• Antiphagocytic
• Virulence factor
• Biofilm formation
• Visualized by India ink (negative staining)

4. Endospores

Sporulation:

• Triggered by nutrient depletion
• Spore forms in 8 hours
• Contains: Dipicolinic acid (calcium salt) — heat resistance
• Core (cortex, spore coat, exosporium)
• NOT reproductive (vegetative cell transforms, not divides)

Germination:

• Triggered by amino acids, sugars
• Single vegetative cell emerges
• Requires 20-30 minutes

Genera: Bacillus (aerobic), Clostridium (anaerobic)

Examples:

• B. anthracis: Cutaneous, inhalation, GI anthrax
• B. cereus: Food poisoning (emetic, diarrheal)
• C. tetani: Tetanus (tetanospasmin)
• C. botulinum: Botulism (botulinum toxin)
• C. difficile: Antibiotic-associated diarrhea
• C. perfringens: Gas gangrene (lecithinase)

5. Bacterial Genetics

Chromosome:

• Single, circular, double-stranded DNA
• ~4,000-5,000 genes
• Attached to cell membrane at origin of replication

Plasmids:

• Extrachromosomal, circular, double-stranded DNA
• Replicate independently
• Conjugative (tra genes), non-conjugative
• R factors (resistance plasmids)
• Examples: F plasmid (conjugation), R100 (multiple drug resistance)

Gene Transfer Methods:

Conjugation:

• Direct cell-to-cell transfer via sex pilus
• F plasmid or Hfr (high frequency recombination)
• Unidirectional (donor → recipient)
• Can transfer chromosomal DNA (Hfr)

Transformation:

• Uptoake of free DNA from environment
• Competent cells (natural transformation)
• Griffith’s experiment with Streptococcus pneumoniae

Transduction:

• Bacteriophage-mediated transfer
• Generalized (any gene)
• Specialized (specific genes, lysogenic conversion)

6. Exotoxins vs. Endotoxins

Property	Exotoxin	Endotoxin
Source	Secreted by living cells	Part of Gram-negative cell wall
Location	Extracellular	Outer membrane (LPS)
Heat	Labile (destroyed >60°C)	Heat stable
Toxicity	High (specific effects)	Low (general effects)
Antigenicity	Strong (forms toxoids)	Weak (cannot form toxoids)
Specificity	Specific targets	General (fever, shock)
Examples	Diphtheria, Tetanus, Botulinum	Salmonella, E. coli O157:H7

Exotoxin Types (by action):

• Cytolytic: Phospholipases, hemolysins
• A-B toxins: Diphtheria (inhibits EF-2), Tetanus (inhibits GABA release), Botulinum (blocks ACh release)
• Superantigens: TSST-1, Staphylococcal enterotoxin (massive cytokine release)
• ADP-ribosylating: Cholera (Gαs, ↑cAMP), Pertussis (Gi, ↑cAMP)

7. Bacterial Growth

Phases:

1. Lag phase: Adaptation, no division
2. Log (Exponential) phase: Active division, most susceptible to antibiotics
3. Stationary phase: Growth = Death
4. Death phase: Decline

Generation Time: Time for population to double (10-20 minutes for most, 12-24 hours for M. tuberculosis)

Anaerobic Growth:

• Obligate anaerobes: Killed by O₂ (lack superoxide dismutase, catalase)
• Facultative anaerobes: Grow with/without O₂
• Aerotolerant anaerobes: Don’t use O₂ but tolerate it
• Microaerophilic: Require low O₂

Culture Media:

• Nutrient broth: General purpose
• Blood agar: Fastidious organisms, hemolysis detection
• MacConkey: Bile salts + lactose + neutral red → Gram-negative enteric differentiation
• Chocolate agar: Lysed blood, for fastidious organisms (H. influenzae, N. gonorrhoeae)

8. Sterilization and Disinfection

Heat Sterilization:

• Autoclave (121°C, 15 psi, 15 min): Most reliable
• Dry heat (160°C, 2 hours)
• Pasteurization (72°C, 15 sec): Kills pathogens, not spores

Filtration: 0.22 μm membrane filters (heat-labile solutions)

Radiation: Gamma, UV (DNA damage)

Disinfectants:

• Alcohol (70%): Denatures proteins
• Chlorine: Oxidizing agent
• Iodine: Oxidizing agent
• Hydrogen peroxide: Oxidizing agent
• Glutaraldehyde: Crosslinks proteins (sterilization)
• Ethylene oxide: Alkylating agent (gas sterilizer)

9. Antibiotics — Mechanisms

Cell Wall Synthesis Inhibitors:

• β-lactams: Penicillins, Cephalosporins, Carbapenems, Monobactams
  • Inhibit transpeptidase (PBPs) → prevent crosslinking
  • bactericidal
• Vancomycin: Inhibits transpeptidase in Gram-positive
  • Binds D-ala-D-ala terminus
  • Bactericidal

Protein Synthesis Inhibitors:

• Aminoglycosides (30S): Streptomycin, Gentamicin — mRNA misreading, bactericidal
• Tetracyclines (30S): Block tRNA binding, bacteriostatic
• Macrolides (50S): Erythromycin, Azithromycin — block translocation, bacteriostatic
• Lincosamides (50S): Clindamycin — block translocation
• Chloramphenicol (50S): Inhibits peptidyl transferase, bacteriostatic

Nucleic Acid Synthesis Inhibitors:

• Quinolones (DNA gyrase/topoisomerase IV): Bactericidal
• Rifampicin (RNA polymerase): Blocks transcription, bactericidal

Antimetabolites:

• Sulfonamides (Folic acid synthesis): Bacteriostatic
• Trimethoprim (Dihydrofolate reductase): Bacteriostatic

10. Bacterial Pathogenesis

Koch’s Postulates:

1. Same pathogen found in all cases
2. Isolated and cultured
3. Causes same disease when inoculated
4. Re-isolated from inoculated host

Modified Koch’s Postulates: For viruses and modern understanding

Virulence Factors:

• Adherence (pili, adhesins)
• Invasion (enzymes, toxins)
• Evasion (capsule, antigenic variation)
• Iron acquisition (siderophores)

Invasiveness: Ability to spread and multiply in tissues

Toxigenicity: Ability to produce toxins

Practice Questions for NEET PG

1. Compare and contrast the cell wall structure of Gram-positive and Gram-negative bacteria.
2. Explain the mechanism of action of β-lactam antibiotics.
3. Differentiate between exotoxins and endotoxins.
4. Describe the process of bacterial conjugation.
5. Discuss the virulence factors of Staphylococcus aureus.

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