Microbial Growth and Viruses Lecture

Bacterial Growth and Division

  • Binary Fission: Bacterial cell division method involving elongation, DNA replication, cross wall formation, and separation into daughter cells.

  • Generation Time: Duration between cell divisions. Example times:

    • E. coli: 20-30 min

    • Clostridium perfringens: 10 min (fastest)

    • Staphylococcus aureus: 20-35 min in lab, 2 hours in host.

    • Mycobacterium tuberculosis: 12-16 hours.

Bacterial Growth Curve Phases

  • Lag Phase: Initial adjustment period.

  • Log Phase: Exponential growth.

  • Stationary Phase: Nutrient depletion slows growth; cell numbers stabilize.

  • Death Phase: Nutrients exhausted; decline in cell numbers.

  • Quorum sensing occurs in the stationary phase.

Metabolites

  • Primary Metabolites: Essential for growth; produced during log phase.

  • Secondary Metabolites: Produced during stationary phase; important for survival and competition.

Environmental Factors Influencing Growth

  • Factors include temperature, oxygen, pH, nutrients, moisture, and pressure.

  • Optimal Temperature Classification:

    • Psychrophiles: 0-20°C

    • Mesophiles: 20-45°C

    • Thermophiles: 45-80°C

    • Hyperthermophiles: >80°C

Bacterial Oxygen Requirements

  • Obligate Aerobes: Require oxygen.

  • Facultative Anaerobes: Prefer oxygen but can grow without it.

  • Obligate Anaerobes: Cannot grow in oxygen.

  • Microaerophiles: Require low oxygen levels.

  • Aerotolerant Anaerobes: Indifferent to oxygen presence.

Mechanisms of Protection Against Oxygen Toxicity

  • Superoxide Dismutase, Catalase, and Peroxidases detoxify harmful reactive oxygen species.

Antibiotics Overview

  • Antibiotics: Substances used to treat bacterial infections; their spectrum can be broad or narrow.

  • Originating from bacteria (e.g., Streptomyces) or fungi (e.g., Penicillium).

  • Selective Toxicity: Ideally, drugs should kill bacteria without harming host cells.

  • Mechanisms of resistance include mutation, enzyme production (like penicillinase), and reduced permeability.

Horizontal Gene Transfer in Bacteria

  • Transformation: Uptake of naked DNA from the environment.

  • Transduction: Involves bacteriophages (viruses that infect bacteria).

  • Conjugation: Direct transfer between bacteria via a sex pilus; involves plasmids.

  • Hfr Cells: High frequency recombination cells resulting from F plasmid integration into the bacterial chromosome.

Key Definitions

  • Auxotroph: Bacteria that require additional growth factors due to inability to synthesize certain compounds.

  • Prototroph: Bacteria that can synthesize all essential compounds from minimal medium.

  • MIC (Minimal Inhibitory Concentration): Lowest concentration of an antimicrobial that inhibits visible growth of bacterium.

Viral Characteristics

  • Viruses: Acellular infectious particles that replicate only inside host cells.

  • Structure includes nucleic acids and protein coats; may or may not have lipid envelopes.

  • Virus classification based on genome type (DNA/RNA) and replication strategy.

  • Virus-host interaction includes attachment, penetration, replication, assembly, and release of progeny.