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Bacterial Growth Curve

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1. Lag Phase

• Bacteria are added to a new medium and require time to adapt before dividing.
• No significant increase in cell number occurs.
• Cells are metabolically active - synthesizing enzymes, ribosomes, and other machinery needed for growth.
• Duration depends on the age of the inoculum and how different the new environment is from the original.

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2. Exponential (Log) Phase

• Bacteria divide at a constant, characteristic doubling time determined by strain and environmental conditions.
• Cell number increases as 2^n, where n = number of generations (doublings).
• This is the phase of most rapid, predictable growth.
• Cells are most uniform in size, metabolic activity, and biochemical composition.
• Bacteria are most sensitive to antibiotics (especially cell-wall active agents like beta-lactams) during this phase, since they are actively dividing.

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3. Stationary Phase

• The culture runs out of nutrients, or toxic metabolic byproducts accumulate in the medium.
• The rate of cell division equals the rate of cell death - net cell count plateaus.
• Cells begin to show stress responses: DNA may be cannibalized for deoxyribonucleotide precursors, peptidoglycan and proteins are degraded for metabolites, and cells shrink.
• In some species, signals here initiate sporulation.
• Some organisms promote uptake of foreign DNA (transformation) as a survival strategy.

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4. Decline (Death) Phase

• Cell death exceeds cell division; viable count falls.
• Some bacteria stop dividing but remain viable (persisters) - these are often insensitive to antibiotics, which is clinically important for chronic or relapsing infections.
• Cell lysis, loss of structural integrity, and final population collapse occur.

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Key Clinical Relevance

• Generation time (doubling time) varies widely: E. coli ~20 min, Mycobacterium tuberculosis ~18-24 hours - which is why TB treatment takes months.
• The curve is generated from batch (closed) culture - in continuous culture (chemostat), bacteria can be maintained indefinitely in log phase by constantly adding nutrients and removing waste.

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🦠 General Microbiology - BPT Question Bank

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SECTION 1: Bacterial Structure & Classification

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• Medical Microbiology 9e, "Bacterial Structure"

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• Maintains cell shape and prevents osmotic lysis
• Is the target of beta-lactam antibiotics (penicillin, cephalosporins) which inhibit its cross-linking
• Is thicker in Gram-positive bacteria
• Is absent in human cells - making it an ideal antibiotic target

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• Cell wall (peptidoglycan)
• 70S ribosomes (human cells have 80S)
• Pili (fimbriae)
• Flagella with unique basal body structure
• Plasmids
• Endospores (in some species)
• Capsule (in some species)

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• Protects bacteria from phagocytosis (main virulence factor)
• Helps in adherence to host tissues and surfaces
• Protects against desiccation
• Detected by the Quellung reaction (capsular swelling with specific antibody)
• Examples of encapsulated bacteria: Streptococcus pneumoniae, Klebsiella, Haemophilus influenzae, Neisseria meningitidis

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• One cell forms one spore (sporulation), and the spore is liberated when the mother cell undergoes autolysis
• Can survive for centuries, resistant to heat, drying, radiation, and many chemicals
• All medically important spore-formers are Gram-positive rods
• Examples: Clostridium tetani (tetanus), C. botulinum (botulism), C. difficile (pseudomembranous colitis), Bacillus anthracis (anthrax)
• Autoclave at 121°C for 15 min is required to destroy spores

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SECTION 2: Staining Techniques

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• Primary stain: Carbol fuchsin (red) - requires heat to penetrate the waxy mycolic acid wall
• Decolorizer: Acid-alcohol
• Counterstain: Methylene blue
• Result: Acid-fast organisms stain red/pink (AFB+), others stain blue

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SECTION 3: Sterilization & Disinfection

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• Ethylene oxide - heat-sensitive instruments
• Formaldehyde, glutaraldehyde - endoscopes
• Chlorine, hypochlorite - water, surfaces

Key point for BPT: Autoclave is the gold standard for sterilizing surgical instruments and physiotherapy equipment.

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SECTION 4: Bacterial Growth & Reproduction

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1. DNA replicates
2. Cell wall and membrane extend
3. Septum forms in the middle
4. Cell divides into two identical daughter cells

• E. coli doubles every ~20 minutes; M. tuberculosis every ~18-24 hours (explains why TB treatment takes months).

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SECTION 5: Infection & Immunity

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1. Infectious agent (pathogen)
2. Reservoir (human, animal, environment)
3. Portal of exit (respiratory tract, blood, feces)
4. Mode of transmission (droplet, contact, airborne, vector, fecal-oral)
5. Portal of entry (skin, respiratory, GI, urogenital)
6. Susceptible host

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SECTION 6: Viruses, Fungi & Parasites (Brief)

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• Resistant to all beta-lactam antibiotics due to altered penicillin-binding protein (PBP2a)
• Spreads via direct contact and contaminated surfaces/equipment
• BPT relevance: Physiotherapy equipment (mats, ultrasound probes, hydrotherapy pools), close patient contact, and hospital wards are common transmission sites
• Prevention: Hand hygiene, gloves, surface disinfection, isolating infected patients

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SECTION 7: Hospital-Acquired Infections (HAI) - High Yield for BPT

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• Common sites: UTI (most common), pneumonia, surgical site, bloodstream
• Common pathogens: E. coli, Klebsiella, Pseudomonas, Staph aureus, Candida
• BPT students must know: Physiotherapists are at risk of transmitting HAIs through hands, equipment, and close contact. Hand washing (WHO 5 moments) is the single most effective prevention.

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• Up to 1000x more resistant to antibiotics than planktonic (free-floating) bacteria
• Form on medical devices: catheters, prosthetic joints, endoscopes
• Highly relevant in chronic wound infections - a major physiotherapy concern
• Examples: Pseudomonas aeruginosa in chronic lung infections, dental plaque (Streptococcus mutans)

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QUICK REVISION TABLE - One-Liners

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Tip for BPT exams: Focus especially on infection control (sterilization, HAIs, chain of infection) and common pathogens in wound/respiratory infections, as these are most clinically relevant for physiotherapy practice. Would you like detailed questions on any specific topic?