UNIT 2: Microbial Growth and Genetic Exchange in Bacteria

Exam Focus: Draw and label the four phases of the microbial **Growth Curve** and know what limits growth in each phase. Genetic exchange mechanisms (**Conjugation, Transformation, Transduction**) are critical, particularly understanding the differences between them.

Table of Contents

1. Microbial Growth
   • Growth Curve
   • Generation Time
   • Factors Affecting Growth of Bacteria
   • Nutritional Categories of Micro-organisms
2. Genetic Exchange in Bacteria
   • Conjugation
   • Transformation
   • Transduction

1. Microbial Growth

Microbial growth refers to the increase in the **number of cells** in a population, rather than an increase in the size of individual cells.

Growth Curve

When bacteria are inoculated into a new liquid medium (batch culture) and counted periodically, the population typically exhibits four distinct phases, visualized on a semilog plot (log of cell number vs. time).

1. **Lag Phase:** Cells adapt to the new environment, synthesizing necessary enzymes and macromolecules. No or very little increase in cell number.
2. **Log (Exponential) Phase:** Cells divide at a constant, maximum rate (exponential growth). The population is most uniform metabolically.
3. **Stationary Phase:** The growth rate slows down as the death rate equals the division rate. This is usually due to nutrient depletion or accumulation of toxic waste products.
4. **Death (Decline) Phase:** The number of viable cells decreases exponentially as the death rate exceeds the division rate.

Generation Time

Generation time (g) is the time required for a population of cells to **double** in number. It is the reciprocal of the growth rate constant (k).

It is shortest during the exponential (log) phase. For E. coli under optimal conditions, g can be as short as 20 minutes.

Factors Affecting Growth of Bacteria

Bacterial growth is heavily influenced by physical and chemical conditions in the environment.

• **Temperature:** Microbes are classified based on their optimal growth temperature (e.g., psychrophiles, mesophiles, thermophiles). Most human pathogens are **mesophiles** (optimal 20^°C to 45^°C).
• **pH:** Classified as acidophiles, neutrophiles (most common), and alkaliphiles.
• **Oxygen:** Classified as obligate aerobes, obligate anaerobes, facultative anaerobes (can grow with or without O₂), and microaerophiles.
• **Water Activity and Osmotic Pressure:** Affects water movement across the cell membrane. Halophiles (salt-lovers) require high salt concentrations.
• **Nutrients:** Availability of C (carbon), N (nitrogen), P (phosphorous), S (sulfur), and trace elements.

Nutritional Categories of Micro-organisms

Microorganisms are categorized based on their sources of **energy** and **carbon**.