Binary Fission

• A method of asexual reproduction in which a single 'mother' cell divides into two identical 'daughter' cells.
• Characterized by symmetric division.

Exponential Growth

• Describes a growth pattern where cells double in number at a constant rate over time.
• Characteristics of all cellular components (DNA, lipids, proteins) doubling at the same rate.
• The time between generations remains constant.

Types of Growth Environments

• Batch Culture:

  • Closed system where no new nutrients enter and waste products do not leave.
  • Good model for contaminated food or certain infections.
  • Common in laboratory settings but poor for environmental modeling.

• Continuous Culture:

  • Open system where fresh nutrients flow in and waste products flow out.
  • Supports sustained growth conditions unlike batch culture.

Batch Growth Curve Stages

1. Lag Phase:

   • Occurs post-inoculation into a fresh medium.
   • Characterized by slow or no growth as cells adjust physiologically.

2. Exponential Phase:

   • Known as 'steady-state' or 'balanced growth'.
   • Uniform metabolism, regular doubling of cell numbers.
   • Only phase where growth equations apply.

3. Stationary Phase:

   • No net increase in population size.
   • Growth rate equals death rate due to nutrient depletion or toxin accumulation.

4. Death Phase:

   • Cells begin to lyse, resulting in a population decline with death rate exceeding growth rate.

Growth Metrics

• Generation Time (g):

  • Time required for the cell population to double.
  • Equation: $g = \text{time required for doubling}$

• Growth Rate (k):

  • Inverse of generation time.
  • Equation: $k = \frac{1}{g}$
  • Measured in generations per hour.

• Example:

  • If generation time is $20$ minutes, then:
  • $g = 0.33 ext{ hours/generation}$
  • $k = \frac{1}{0.33} \approx 3 ext{ generations/hour}$

Stringent Response

• Response occurs during nutrient limitation, especially low amino acids.
• RelA protein detects ribosome pausing and synthesizes alarmone ppGpp to signal starvation-induced physiological changes.

Physiology of Stationary Phase

1. Reduced cell size.
2. Increased cell wall crosslinking.
3. Decreased membrane fluidity.
4. Compacted nucleoid structure.
5. Increased detoxifying enzymes in the periplasm.
6. Recycling of proteins.
7. Enhanced motility.
8. Induction of sporulation in some bacteria.

Nutrient Effects on Growth

• Maximum cell yield is proportional to nutrient concentration.
• Growth rate proportional to nutrient concentration, especially when low.

Continuous Culture - The Chemostat

• Continuous inflow of fresh nutrients enables perpetual exponential growth.
• Bacteria in continuous culture bypass stationary phase.

Environmental Growth Conditions

• Some environments replicate continuous culture (e.g., gastrointestinal tract, flowing water).
• Others mimic batch culture (e.g., pathogenesis, sudden nutrient spikes).
• Exponential growth is rare; many bacterial populations exist in stationary phase.

Factors Limiting Bacterial Growth

1. Temperature
2. Desiccation
3. Oxygen
4. Radiation
5. Acidity
6. Pressure
7. Chemical agents
8. Nutritional availability