Microbial Growth and Nutrition

Nutritional Requirements - Fueling the Bugs

• Macronutrients: C, H, O, N, P, S (📌 CHONPS); K, Mg, Ca, Fe.
• Micronutrients (Trace Elements): Mn, Zn, Co, Mo, Ni, Cu.
• Carbon Source:
  • Autotrophs: Use inorganic $CO_2$.
  • Heterotrophs: Use organic compounds.
• Energy Source:
  • Phototrophs: Light.
  • Chemotrophs: Chemical compounds (Lithotrophs - inorganic; Organotrophs - organic).
• Growth Factors: Essential organic compounds if not synthesized (e.g., vitamins, amino acids).
• Iron: Crucial for growth; often acquired via siderophores.

⭐ Siderophores are low molecular weight compounds produced by bacteria to chelate ferric iron from the environment, crucial for virulence.

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Culture Media - Bug Food Galore

• Supply nutrients for microbial growth.
• Consistency Types:
  • Solid (Agar: 1.5-2%), Semi-solid (Agar: 0.2-0.5%), Liquid (Broth).
• Functional Types:
  • Basal: Basic support, e.g., Nutrient Agar.
  • Enriched: Added nutrients (blood, serum), e.g., Blood Agar, Chocolate Agar.
  • Selective: Inhibit some, grow others, e.g., LJ medium, TCBS.
  • Differential: Distinguish microbes, e.g., CLED agar.
  • Transport: Maintain viability, e.g., Stuart's, Amies.
  • Anaerobic: For anaerobes, e.g., RCMB, Thioglycollate.

⭐ MacConkey agar is both a selective (bile salts & crystal violet inhibit Gram-positives) and differential (lactose fermentation indicated by neutral red pH indicator) medium.

Environmental Factors - Happy Place Hunt

• Temperature: Optimal range varies.
  • Psychrophiles: Cold-loving (<15°C)
  • Mesophiles: Moderate (20-45°C); most human pathogens.
  • Thermophiles: Heat-loving (50-80°C)
  • Hyperthermophiles: Extreme heat (>80°C)
• pH: Measure of acidity/alkalinity.
  • Acidophiles: Grow at low pH (<5.5)
  • Neutrophiles: Grow at neutral pH (5.5-8.5); most pathogens.
  • Alkaliphiles: Grow at high pH (>8.5)
• Oxygen (O₂):
  • Obligate Aerobes: Require O₂ (e.g., Mycobacterium tuberculosis).
  • Facultative Anaerobes: Prefer O₂, but can grow without (e.g., E. coli).
  • Obligate Anaerobes: O₂ is toxic (e.g., Clostridium spp.).
  • Aerotolerant Anaerobes: Tolerate O₂, but don't use it.
  • Microaerophiles: Require low O₂ concentrations (e.g., Campylobacter jejuni).
• Osmotic Pressure: Halophiles require high salt concentrations.
• Radiation: UV light (DNA damage), ionizing radiation (breaks DNA).

⭐ Most human pathogenic bacteria are mesophiles, growing optimally at 37°C, and neutrophiles, preferring a pH around 7.0-7.4.

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Bacterial Growth Curve - Life & Times

• Lag Phase: Adaptation, no ↑ in cell number; ↑ metabolic activity.
• Log (Exponential) Phase: Rapid cell division; constant generation time. Population doubles each generation.
• Stationary Phase: Growth rate = death rate; nutrient depletion, waste accumulation.
• Decline (Death) Phase: Death rate > growth rate; viable cell count ↓.

⭐ Antibiotics like Penicillin, which target cell wall synthesis, are most effective during the log (exponential) phase of bacterial growth due to active cell division.

Measuring Microbial Growth - Counting Critters

• Direct Methods: Count cells directly.
  • Microscopic Count (e.g., Petroff-Hausser): Total cells (live + dead). Rapid.
  • Viable Plate Count:

  ⭐ Viable plate count (e.g., spread plate or pour plate method) quantifies only live, culturable bacteria, reported as Colony Forming Units (CFU)/mL.

  • Membrane Filtration: Concentrates microbes from dilute samples.
• Indirect Methods: Assess growth via other indicators.
  • Turbidity (OD): Estimates biomass (live + dead).
  • Metabolic Activity: Product (acid/gas) or substrate use.
  • Dry Weight: Filamentous microbes.
  • Genetic (qPCR): Quantifies DNA/RNA.

High‑Yield Points - ⚡ Biggest Takeaways

• Bacterial growth curve proceeds through Lag, Log (Exponential), Stationary, and Death phases.
• Generation time (doubling time) is shortest during the Log phase.
• Obligate aerobes (e.g., Mycobacterium tuberculosis) require O2; obligate anaerobes (e.g., Clostridium perfringens) are harmed by O2.
• Facultative anaerobes (e.g., E. coli) can grow with or without O2, while microaerophiles (e.g., Campylobacter jejuni) need low O2.
• Capnophiles (e.g., Neisseria gonorrhoeae, Haemophilus influenzae) require increased CO2 concentrations.
• Siderophores are crucial for bacterial iron acquisition.
• Active transport moves nutrients against concentration gradients, requiring energy (ATP).