Chapter 6_ Microbial Growth

• Microbial growth refers to the increase in the number of cells, not the size.

Requirements for Microbial Growth

Physical Requirements

• Temperature

  • Minimum, optimum, and maximum growth temperatures vary by species.
  • Categories:
    • Hyperthermophiles: ~93-94°C
    • Thermophiles: 60-61°C
    • Mesophiles: ~37°C (most lab bacteria and pathogens)
    • Psychrotrophs: ~24°C (refrigerator temps, can cause food spoilage)
    • Psychrophiles: Very low temps
  • Danger Zone: 60°F - 130°F (15°C - 52°C)
  • Food preservation: maximize surface area to cool food quickly.

• pH

  • Most bacteria: 6.5 - 7.5 (neutral)
  • Molds and yeasts: ~5-6
  • Acidophiles: Very low pH (e.g., Helicobacter pylori in the stomach)

• Osmotic Pressure

  • Hypertonic environments (high salt/sugar) can cause plasmolysis (cell shrinkage and death).
  • Obligate halophiles: Require high osmotic pressure.
  • Facultative halophiles: Can tolerate high osmotic pressures.

Chemical Requirements

• Carbon
  • Backbone of organic molecules: proteins, lipids, carbohydrates, nucleic acids.
  • Chemoheterotrophs: Use carbon and obtain energy from other sources.
  • Autotrophs: Use CO2 for carbon (e.g., photosynthetic bacteria).
• Nitrogen
  • Found in amino acids and proteins.
  • Can be used as ammonia (NH4+), nitrate (NO3-), or fix nitrogen gas (N2).
• Sulfur
  • Found in amino acids, can decompose proteins and use sulfates (SO4 2-) or H2S.
• Phosphorus
  • Found in DNA, RNA, ATP, and membranes as phosphate (PO4 3-).
• Trace Elements
  • Needed in small amounts, often enzyme cofactors.
• Oxygen
  • Different bacteria have different oxygen requirements:
    • Obligate aerobes: Need oxygen.
    • Facultative anaerobes: Can grow with or without oxygen (better with).
    • Obligate anaerobes: Cannot tolerate oxygen.
    • Aerotolerant anaerobes: Tolerate but do not use oxygen.
    • Microaerophiles: Require small amounts of oxygen.

Enzymes for Oxygen Detoxification

• Superoxide dismutase: Converts superoxide radicals to hydrogen peroxide.
• Catalase: Breaks down hydrogen peroxide to water and oxygen.
• Peroxidase: Breaks down hydrogen peroxide to water.

Biofilms

• Biofilms are bacterial communities.
• Bacteria communicate via quorum sensing.
• Biofilms share nutrients and provide shelter from harmful factors.

Culture Media

• Agar: Complex polysaccharide, solidifies at 40°C, not digested by bacteria.
• Chemically defined media: Exact chemical composition known.
• Complex media: Undefined chemical composition extracts (e.g., yeast, beef).

Culture Techniques

• Anaerobic culture methods
  • Reducing media, gas jars, anaerobic chambers.
• Capnophiles: Prefer high CO2 concentrations.

BSL (Biosafety Levels)

• BSL-1: Basic lab safety (e.g., handwashing).
• BSL-2: Includes use of gloves, eye protection.
• BSL-3: Biosafety cabinets to prevent airborne transmission.
• BSL-4: Highest safety (e.g., CDC). Complete isolation, specialized training.

Media Types

• Selective media: Suppress some microbes, encourage others (e.g., MSA for Staphylococci).
• Differential media: Distinguish between bacteria (e.g., blood agar).
• Pure culture: Isolated colony obtained using streak plate method.

Preservation Techniques

• Deep freezing or freeze-drying.

Bacterial Reproduction

• Mainly by binary fission (1 cell becomes 2).
• Doubling time: Time required for one cell to become two.

Generational Calculations

• Use the log of the number of cells to make calculations simpler.
• Example problem: Generation time calculation based on initial and final cell numbers over time.

Growth Phases

• Lag phase: Preparation for growth.
• Log phase: Exponential growth.
• Stationary phase: Plateau (growth rate = death rate).
• Death phase: Decline in population.

Measuring Microbial Growth

• Plate counts: Counting CFUs.
• Filtration: Counting colonies after filtering.
• Direct microscopy: Counting cells directly.
• Turbidity: Measuring cloudiness of culture (e.g., spectrophotometer).

Key Activities and Study Tips

• Engage in hands-on lab exercises for deeper understanding.
• Be well-versed with complex media definitions and practical applications.
• Prepare for problem-solving activities and calculations in exams.