Microbial Growth Lecture Notes

Physical and Chemical Requirements for Microbial Growth

Physical Requirements

• Temperature: Organisms have specific temperature ranges for growth:
  • Psychrophiles: Cold-loving, grow at freezing or below.
  • Mesophiles: Moderate temperature, around human body temperature.
  • Thermophiles: Heat-loving, optimum growth at 50-60°C.
  • Hyperthermophiles: Extremely high temperatures (near 200°C).
• pH: Most bacteria prefer pH 6.5-7.5. Molds/Yeasts prefer pH 5-6.
  • Acidophiles: Grow in very acidic environments.
• Osmotic Pressure: Influences cell water balance.
  • Hypertonic: Higher solute concentration outside, causes plasmolysis.
  • Hypotonic: Lower solute outside, causes cells to swell.
  • Isotonic: Equal solute concentration, stable.
  • Halophiles: Thrive in high salt concentrations.

Chemical Requirements

• Carbon: Backbone of all organic molecules.
• Nitrogen: Important for DNA, proteins, ATP.
• Sulfur: Found in amino acids and vitamins.
• Phosphorus: In DNA, RNA, ATP; part of cell membrane.
• Trace Elements: Required in small amounts, e.g., iron, zinc.
• Oxygen: Different microbial oxygen tolerances:
  • Obligate Aerobes: Require oxygen.
  • Facultative Anaerobes: Prefer oxygen but can grow without it.
  • Obligate Anaerobes: Cannot tolerate oxygen.
  • Aerotolerant Anaerobes: Only anaerobic growth, tolerate oxygen.
  • Microaerophiles: Require low oxygen concentrations.

Microbial Growth Concepts

• Increase in Cell Numbers: Microbial growth refers to an increase in the number of cells.
• Colony: A group of cells large enough to be seen without a microscope.
• Binary Fission: Process by which bacteria replicate, resulting in exponential growth.

Culture Methods

• Culture Media: Nutrients prepared for microbial growth.
  • Sterile Media: Free of living microbes at the start.
  • Chemically Defined Media: Known exact chemical composition.
  • Complex Media: Includes digests of yeast, meat, plants.
• Anaerobic Culture Methods: Use reducing media to remove oxygen.
• Selective Media: Suppresses unwanted microbes, encourages desired ones.
• Differential Media: Distinguishes colonies of different microbes.

Biofilms

• Microbial Communities: Form slime layers, adhere to surfaces.
• Quorum Sensing: Bacteria communicate with chemical signals.
• Resistance: Biofilms are more resistant to antibiotics.

Growth Phases of Bacterial Populations

• Lag Phase: Adjustment period, no population increase.
• Log Phase: Exponential growth, increasing cell numbers rapidly.
• Stationary Phase: Equilibrium, deaths balance new cell production.
• Death Phase: Population decline, death outpaces growth.

Fun Facts and Exam Tips

• Early Earth Atmosphere: No free oxygen; photosynthesis introduced it.
• Exponential Growth Riddle: Pond with lily pads doubles each day; full on day 30 is half full on day 29.