Chapter 9 – Microbial Nutrition, Ecology & Growth

Essential, Macro- and Micronutrients

• Essential elements (96–97 % of cell mass): C, H, O, N, P, S (mnemonic: CHONPS)
• Macronutrients
  • Needed in large quantities for structure & metabolism
  • Examples: proteins, lipids, carbohydrates, nucleic acids
• Micronutrients (trace elements)
  • Needed in small amounts, usually as enzymatic cofactors
  • Examples: Mn, Zn, Ni, Co, Cu, Mo, etc.

Nutritional & Energy Classifications

• Heterotroph – obtains organic carbon from other organisms (must eat)
• Autotroph – fixes inorganic CO₂ (self-feeds)
  • Photoautotroph: light energy → photosynthesis
  • Chemoautotroph: inorganic chemicals (e.g., NH3, H2S)
• Phototroph – energy from sunlight
• Chemotroph – energy from chemical compounds
  • Chemoorganoheterotroph: organic e⁻ donors (most pathogens)
  • Methanogens (a type of chemoautotroph): anaerobic, produce CH₄
  • Typical reaction: CO2 + 4H2 \rightarrow CH4 + 2H2O
  • Found in swamps, sediments, ruminant guts; NOT in human intestines (our farts lack CH₄)
• Saprobe – feeds on dead/decaying matter
• Parasite – lives in/on host, harms host, derives nutrients

Transport Across the Cell Membrane

• Passive (no ATP)
  • Simple diffusion: high → low concentration
  • Facilitated diffusion: high → low via specific carrier/channel; exhibits saturation & competition
  • Osmosis: water moves through semipermeable membrane from lower solute → higher solute concentration until equilibrium (key in hypo-/iso-/hyper-tonic solutions)
• Active (ATP required)
  • Moves substances against gradient via pumps/carriers
  • Examples: Na⁺/K⁺ pump, proton pumps

Environmental Factors Affecting Microbes

Temperature (Cardinal Temperatures)

• Psychrophiles: optimum < 15^\circ\text{C}; grow at 0^\circ\text{C}; die > 20^\circ\text{C} (polar seas)
• Mesophiles: 10–50^\circ\text{C} range; optimum 20–40^\circ\text{C} (human body ≈ 37^\circ\text{C}) → most pathogens
  • Fever (>38^\circ\text{C}) slows mesophile growth
• Thermophiles: optimum > 45^\circ\text{C} (hot springs, compost)
• Extreme thermophiles / hyperthermophiles: 80–121^\circ\text{C} (geysers, deep-sea vents)

Gases (O₂, CO₂)

• Oxygen is reactive; forms toxic derivatives (superoxide O2^-, peroxide H2O_2)
• Protective enzymes
  • Superoxide dismutase (SOD): O2^- \rightarrow H2O_2
  • Catalase: 2H2O2 \rightarrow 2H2O + O2 (basis of bubbling in H₂O₂ wound test—indicates Staphylococcus presence)
• Oxygen relationships
  • Obligate aerobe: requires O₂; possesses SOD & catalase
  • Facultative anaerobe (better term: facultative aerobe): grows with or without O₂; prefers O₂; can switch to fermentation (e.g., E. coli)
  • Obligate anaerobe: lacks SOD/catalase; O₂ lethal; live in deep soil, gut, tissue necrosis; killed by hyperbaric O₂ therapy
  • Aerotolerant anaerobe: doesn’t use O₂ but tolerates it (has SOD only)
  • Microaerophile (not emphasized in lecture but foundational): needs 1–10 % O₂
• Lab culture of anaerobes: anaerobic jars with O₂-absorbing chemical packs

pH

• Acidophiles: thrive at low pH (< 5); e.g., Lactobacillus in pickled foods (food preservation)
• Neutrophiles: pH 6–8 (most pathogens)
• Alkalinophiles: pH > 8

Osmolarity & Salinity

• Halophiles: require high NaCl (≥ 9 %)
• Non-halophiles face plasmolysis in salty environments (basis of food curing)

Radiation

• UV, X-ray, γ-ray damage DNA; used for sterilization
• Some phototrophs possess pigments/enzymes that repair UV damage

Pressure & Moisture

• Barophiles: thrive under extreme hydrostatic pressure (deep-sea trenches)
• Water availability critical; desiccation inhibits most microbes; some spores, cysts survive dry environments

Ecological Relationships

• Symbiosis: close partnership of different species
  • Mutualism: both benefit (e.g., E. coli in gut synthesizing vitamin K)
  • Commensalism: one benefits, other unaffected (skin Staphylococcus)
  • Parasitism: one benefits, host harmed (pathogens)
• Synergism: cooperative but non-essential; combined effect > individuals (mixed biofilm infections)
• Antagonism / Competition: one organism inhibits another (antibiotic production)
• Biofilms
  • Structured communities encased in extracellular matrix (plaque, catheters)
  • Quorum sensing: chemical communication that monitors population density & coordinates gene expression (e.g., virulence, EPS production)

Microbial Growth: Generation Time & Growth Curve

Generation (Doubling) Time Formula

• Nt = N0 \times 2^{n}
  • N_0 = initial number of cells
  • n = \dfrac{t}{g} where g = generation time
• Example: start with 100 cells; generation time g = 15\;\text{min}; find population after 2 h.
  • n = \dfrac{120}{15} = 8
  • N_t = 100 \times 2^{8} = 25{,}600 cells

Typical Bacterial Growth Curve (Closed System)

1. Lag Phase
   • No immediate increase; cells adjust, synthesize enzymes
2. Exponential (Log) Phase
   • Maximal, constant division; cells most vulnerable to antibiotics & disinfectants
3. Stationary Phase
   • Nutrients deplete, wastes accumulate; growth rate = death rate; survival mode
4. Death (Decline) Phase
   • Exponential cell death; some form spores, persisters

Implications: long-stored sealed food may contain mostly dead cells; timing of antimicrobial therapy targets log phase.

Practical / Clinical Connections

• Fever therapy: elevates body temp to slow mesophilic pathogens & denature microbial enzymes
• Hydrogen peroxide wound care: bubbling confirms catalase-positive staphylococci; addition of clavulanic acid (β-lactamase inhibitor) enhances amoxicillin efficacy
• Pickling & fermentation: low pH + salt inhibit spoilage microbes while promoting probiotic acidophiles
• Pasteurization kills mesophiles but thermoduric microbes may survive (spore-formers)
• UV lamps in hospitals & sun-drying clothes: leverage radiation & desiccation to reduce microbial load
• Hyperbaric oxygen chambers treat anaerobic infections (e.g., Clostridium perfringens gas gangrene)
• Chemical O₂-absorber packets enable lab culture of strict anaerobes without expensive glove boxes