Microbiology

Chapter 6: Microbial Nutrition and Growth

Metabolism

  • Metabolism controls chemical reactions within cells, essential for all living organisms.

  • The ultimate outcome is reproduction.

Growth Requirements

  • Organisms use nutrients to meet energy needs and build organic molecules.

  • Common nutrients: Carbon, Nitrogen, Oxygen, Hydrogen.

  • Nutrients must enter cells via active or passive transport.

Chemical Energy Requirements

  • Essential for metabolism:

    • Carbon: Inorganic (Autotrophs) vs. Organic (Heterotrophs).

    • Energy: Source from redox reactions (Aerobic Respiration, Anaerobic Respiration, Fermentation).

    • Electrons or Hydrogen Atoms: Obtained from either organic or inorganic sources.

Categories of Organisms Based on Carbon and Energy Sources

  1. Photoautotrophs: Use light and CO2 (e.g., plants, algae).

  2. Chemoautotrophs: Use CO2 for carbon, catabolizing organic molecules for energy.

  3. Photoheterotrophs: Use light energy, gaining nutrients by catabolizing organic compounds.

  4. Chemoheterotrophs: Use organic compounds for energy and carbon (e.g., animals, fungi).

Oxygen Requirements

  • Obligate Aerobes: Require O2 as the final electron acceptor.

  • Obligate Anaerobes: Cannot tolerate O2; it's poisonous to them.

  • Toxic forms of O2 include:

    • Singlet Oxygen: Highly reactive.

    • Superoxide Radicals: Reactive species formed during O2 reduction, detoxified by enzymes (e.g., superoxide dismutase).

    • Peroxide Anion (H2O2): Formed during reactions and can be detoxified by catalase.

    • Hydroxyl Radical (OH): Most reactive; does not accumulate in aerobic cells.

Organism Types Based on Oxygen Tolerance

  • Facultative Anaerobes: Can live with or without O2 (e.g., E. Coli).

  • Aerotolerant Anaerobes: Do not utilize aerobic metabolism but can tolerate O2.

  • Microaerophiles: Require reduced O2 levels.

Nitrogen Requirements

  • Nitrogen makes up ~14% of the dry weight of microbial cells, crucial for proteins and nucleotides.

  • Limiting nutrient causes a halt in anabolism.

  • Nitrogen fixation by organisms like cyanobacteria converts nitrogen gas to ammonia, essential for life on Earth.

Other Chemical Requirements

  • Phosphorus: Vital for nucleic acids, ATP, phospholipids.

  • Sulfur: Important for amino acids and vitamins.

  • Trace Elements: Essential in small amounts for various cellular functions.

Physical Requirements

  • Microorganisms require specific conditions:

    • Temperature: Affects protein structure and membrane fluidity.

    • pH: Influences hydrogen bonding; microorganisms have pH preferences.

    • Water Availability: Essential for metabolism; osmotic pressure is critical.

Growth Phases

  • Psychrophiles: Thrive below 15°C.

  • Mesophiles: Optimal growth at 20-40°C (most human pathogens).

  • Thermophiles: Prefer above 45°C.

Water and Osmotic Pressure

  • Microorganisms need a moist environment; osmotic pressure influences cell viability.

  • Halophiles: Thrive under high salt conditions.

Relationships among Microorganisms

  • Antagonistic Relationships: One microbe harms another.

  • Synergistic Relationships: Members cooperate for mutual benefits.

  • Biofilms: Structures formed by microbial interactions; significant in disease.

Culturing Microorganisms

  • Laboratory growth of pathogens involves inoculating a medium.

  • Pure Cultures: Isolated organisms from a single progenitor.

  • Techniques: Streak plates, pour plates for isolation and growth assessment.

Growth Measurement Methods

  • Direct Methods: Viable plate counts, membrane filtration.

  • Indirect Methods: Measuring turbidity with a spectrophotometer, metabolic activity assessments.

  • Molecular methods: PCR for identifying unculturable prokaryotes.