Unit 2: Microbial Growth and Genetics

Microbial Growth

In microbiology, growth refers to an increase in the number of cells, not the size of an individual cell. Most bacteria reproduce by binary fission, where one cell divides into two identical daughter cells.

The Bacterial Growth Curve

When bacteria are grown in a batch culture (a closed system with finite nutrients), the population follows a predictable pattern with four distinct phases.

1. Lag Phase:
   • What happens: No increase in cell number.
   • Why: Bacteria are adapting to the new medium, synthesizing enzymes, and preparing for division.
2. Log (Exponential) Phase:
   • What happens: Cells divide at a constant, maximum rate (binary fission). The population increases exponentially.
   • Why: Abundant nutrients, optimal conditions.
   • Note: This is the phase when bacteria are most metabolically active and most sensitive to antibiotics.
3. Stationary Phase:
   • What happens: The growth rate slows, and the number of new cells equals the number of dying cells. Population size stabilizes.
   • Why: Nutrients are becoming depleted, and toxic waste products are accumulating.
4. Death (Decline) Phase:
   • What happens: The number of dying cells exceeds the number of new cells. Population size decreases.
   • Why: Nutrients are exhausted, and the environment is too toxic.

Generation Time

• Definition: The generation time (or doubling time) is the time it takes for a population of bacteria to double in number.
• Example: E. coli can have a generation time of 20 minutes in ideal conditions. One cell can become over 1 billion in 10 hours.
• Formula: G = t / n (Generation time = total time / number of generations).

Factors Affecting Growth

Bacterial growth is heavily influenced by its environment.

Physical Factors

• Temperature:
  • Psychrophiles: Cold-loving (0-20°C).
  • Mesophiles: Moderate-loving (20-45°C). Most human pathogens are mesophiles (body temp is 37°C).
  • Thermophiles: Heat-loving (45-80°C).
• pH:
  • Acidophiles: Grow in acidic (low pH) environments.
  • Neutrophiles: Grow best at neutral pH (~7.0).
  • Alkaliphiles: Grow in alkaline (high pH) environments.
• Oxygen:
  • Obligate Aerobes: Require O2.
  • Obligate Anaerobes: Killed by O2.
  • Facultative Anaerobes: Prefer O2, but can grow without it (e.g., *E. coli*).

Nutritional Factors

• Macronutrients: Needed in large amounts (e.g., Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Sulfur - CHONPS).
• Micronutrients (Trace Elements): Needed in small amounts (e.g., zinc, iron, magnesium).

Microbial Categories based on Nutrition

Microbes can be classified based on how they get energy and carbon.

Genetic Exchange in Bacteria

Bacteria can share genes, which is a major reason for the rapid spread of antibiotic resistance. This is called Horizontal Gene Transfer (HGT).

Conjugation

• Mechanism: Gene transfer through direct cell-to-cell contact.
• Process: A donor cell (F+) extends a sex pilus to a recipient cell (F-). A copy of a plasmid (a small, circular piece of DNA) is transferred from the donor to the recipient.

Transformation

• Mechanism: Uptake of naked DNA from the surrounding environment.
• Process: When a bacterial cell dies, it breaks open (lyses) and releases its DNA. A "competent" recipient cell can pick up fragments of this DNA and incorporate it into its own chromosome.
• Discovery: First demonstrated by Frederick Griffith's experiment with mice and *Streptococcus pneumoniae*.

Transduction

• Mechanism: Gene transfer mediated by a bacteriophage (a virus that infects bacteria).
• Process: When a virus assembles new viral particles inside a host cell, it sometimes "accidentally" packages a piece of the host bacterium's DNA into a new viral capsid. When this virus infects the next cell, it injects the *previous* bacterium's DNA instead of its own.