Microbial Growth & Division

Introduction

Microbial growth in microbiology refers to the increase in population size through reproduction, rather than an increase in individual cell size. Understanding microbial growth is essential for studying population dynamics, disease progression, and biotechnological applications.

Prokaryotic Reproductive Strategies

Overview of Strategies

• Binary Fission: The most common method, where a single cell divides into two identical daughter cells.

• Multiple Fission (Cyanobacteria): Division of cytoplasm to form basal and apical cells, with multiple rounds of division.

• Budding: New cells form as outgrowths (buds) from the parent cell, seen in some bacteria like Hyphomicrobium.

• Sporulation: Formation of spores, which are resistant structures for survival under adverse conditions.

Binary Fission Process

• Step 1: Young cell at early phase of cycle.

• Step 2: Cell prepares for division by enlarging its cell wall, membrane, and overall volume.

• Step 3: Septum begins to grow inward as chromosomes move toward opposite ends.

• Step 4: Septum is synthesized completely, dividing the cell center and forming two separate chambers.

• Step 5: Daughter cells separate completely or remain attached, forming chains, clusters, or other arrangements.

Most studies are done in rod-shaped bacteria such as E. coli and Bacillus subtilis.

Eukaryotic vs. Prokaryotic Cell Cycles

Eukaryotic Cell Cycle

• Consists of four stages: G1, S, G2, and M phase.

• Includes checkpoints (G1/S, G2/M, spindle-assembly) to ensure proper division.

• Mitosis and cytokinesis are key events for nuclear and cell division.

Prokaryotic Cell Cycle

• Two main stages: Chromosome replication/partition and cell division (septation).

• Generation time varies with nutrient availability: ~60 min in minimal media, ~20 min in rich media for E. coli.

DNA Replication and Chromosome Segregation in E. coli

Mechanisms

• MreB (actin homolog): Responsible for partitioning chromosomes into daughter cells.

• Initiation of replication occurs at a specific mass threshold.

• Chromosomes are separated and septum formation is initiated.

Mutant Phenotypes

• mreB mutant: Defective chromosome partitioning.

• minCD mutant: Abnormal septation site selection, leading to irregular cell division.

Bacterial Growth Curve

Phases of Growth

• Lag Phase: Adaptation period; cells prepare for division.

• Exponential (Log) Phase: Rapid cell division; population doubles at a constant rate.

• Stationary Phase: Nutrient depletion slows growth; cell division equals cell death.

• Death Phase: Cell death exceeds division; population declines.

Growth is slower in minimal media and faster in rich media.

Generation Time

Definition and Measurement

• Generation time: Time required for a population to double.

• Measured during the exponential (log) phase.

Examples of Generation Times

Calculating Generation Time

• Use the formula: where G = generation time, t = time elapsed, n = number of generations.

• Number of generations: where Nt = number of bacteria at end point, N0 = number at start point.

Sample Calculation

For a population increasing from 10,000 cells to 10,000,000 cells in 4 hours:

• minutes (if t = 240 min)

Alternative Bacterial Lifestyles

Biofilms

• Multicellular communities with implications for pathogenesis and environmental quality.

• Examples: Cyanobacterial biofilm in Yellowstone, dental plaque on teeth.

Biofilm Development

• Stages: Substratum preconditioning, cell deposition, adsorption, cell-cell signaling, growth, secretion of matrix, detachment.

• Quorum sensing: Communication via chemical signals to coordinate group behavior.

Bacterial Endospores

Super Resistant Structures

• Formed by some bacteria (e.g., Bacillus and Clostridium).

• Extreme resistance due to:

  • Spore coat

  • Dehydrated core

  • Small acid soluble spore proteins (SASPs)

Endospore Formation

• Occurs in response to adverse conditions.

• Stages: Vegetative growth, sporulation, axial filament formation, forespore formation, cortex and coat synthesis, exosporium formation, germination.

Importance: Endospores are highly resistant to heat, chemicals, and radiation, making them significant in medical and environmental contexts.

Additional info: Expanded explanations and context were added for clarity and completeness, including definitions, examples, and formulas.