7.4.7 Growth Rate of Microorganisms

Phases of Microbial Growth

1. Lag Phase:

• No immediate increase in population as microorganisms adjust to their new environment.
• Cells are metabolically active, synthesising enzymes and other molecules needed for growth.

2. Log (Exponential) Phase:

• Microorganisms grow and divide at their maximum rate under optimum conditions.
• Population size doubles at regular intervals.
• Nutrients are plentiful, and waste products are minimal.

3. Stationary Phase:

• Growth rate slows as nutrients become limited and waste products accumulate.
• Birth rate equals death rate, so the population size remains constant.

4. Death Phase:

• Nutrients are depleted, and waste products reach toxic levels.
• Death rate exceeds birth rate, causing a decline in population size.

Factors Affecting Microbial Growth Rate

1. Temperature:

• Each species has an optimum temperature for enzyme activity.
• Growth slows outside the optimum range, and extreme temperatures can denature enzymes.

2. pH:

• Microorganisms thrive within a specific pH range, which varies by species.
• Extreme pH levels can disrupt enzyme activity and cell membrane stability.

3. Nutrient Availability:

• Sufficient nutrients are required for energy production and biosynthesis.
• Limiting nutrients (e.g., glucose or nitrogen) restrict growth.

4. Oxygen Levels:

• Aerobic microorganisms need oxygen for respiration, while anaerobic microorganisms may be inhibited by oxygen.

5. Water Availability:

• Essential for metabolic processes.
• Microorganisms in low-water environments may become dormant or die.

Calculating Microbial Growth Rate

• Growth rate can be expressed as the time taken for the population to double, known as the generation time.
• Formula:

Where:

$$N = N_0 \times 2^n$$

• $N$= final population size
• $N_0$= initial population size
• $n$= number of generations