Cell Division and the Bacterial Growth Cycle

Binary Fission and Cell Cycle

Bacteria primarily reproduce by binary fission, a process in which one parent cell divides into two genetically identical daughter cells. The cell cycle involves several coordinated steps to ensure accurate DNA replication and cell division.

• Cell Elongation: The cell grows in size, elongating its structure.

• Septum Formation: A septum (division wall) forms at the mid-cell, guided by the FtsZ ring and associated proteins (divisome complex).

• Completion of Division: The septum is completed, and the cell separates into two daughter cells.

• Variation: Some bacteria, such as Hyphomicrobium, divide asymmetrically by budding rather than binary fission.

Key Proteins: The FtsZ ring is essential for septum formation, recruiting other proteins to form the divisome complex. The process is regulated to ensure proper chromosome segregation and cell wall synthesis.

Difference in Shape: Rod-shaped and coccoid bacteria differ in the spatial organization of their division machinery, but both rely on similar molecular components.

Generation Time and Exponential Growth

The generation time is the time required for a bacterial population to double in number. Under optimal conditions, some bacteria can have a generation time as short as 20 minutes.

• Exponential Growth: The rate of increase in cell number or biomass is proportional to the current population size, resulting in exponential (logarithmic) growth.

• Mathematical Expression:

• Where is the final cell number, is the initial cell number, and is the number of generations.

• Some cyanobacteria divide by multiple fission, producing more than two daughter cells per division.

Cell Cycle Timing: DNA replication and cell division are tightly coordinated. Overlapping rounds of replication can allow generation times shorter than the time required for a single round of DNA replication.

Bacterial Growth Curve

Bacterial populations in batch culture exhibit a characteristic growth curve with four phases:

• Lag Phase: Cells adapt to new conditions and prepare for growth.

• Log (Exponential) Phase: Cells divide at a constant, maximum rate.

• Stationary Phase: Growth rate slows as nutrients are depleted and waste accumulates; cell division balances cell death.

• Death Phase: Cells die at an exponential rate due to unfavorable conditions.

Continuous Culture and Biofilms

Continuous Culture (Chemostat)

In a continuous culture system, such as a chemostat, fresh medium is continuously supplied, and waste is removed, maintaining the culture in a steady state.

• Key Variables: Dilution rate, cell mass, and generation time are interrelated.

• Applications: Used to study microbial physiology under constant conditions and in industrial fermentation.

Biofilms

Biofilms are structured communities of microorganisms attached to surfaces and embedded in a self-produced extracellular matrix.

• Development Stages:

  1. Attachment of planktonic cells to a surface

  2. Formation of microcolonies

  3. Production of exopolysaccharide (EPS) matrix

  4. Maturation into complex, three-dimensional structures

  5. Dissolution and dispersal of cells

• Examples: Dental plaque, biofilms in streams, and industrial pipelines.

Cell-Cell Communication in Biofilms

Bacteria within biofilms communicate using chemical signals in a process called quorum sensing. This allows coordinated behavior, such as the regulation of gene expression and differentiation within the biofilm.

• Significance: Cell-cell communication is essential for biofilm development, maintenance, and dispersal.

Environmental Influences on Microbial Growth

Normal and Extreme Growth Conditions

Microbes are adapted to a wide range of environmental conditions. "Normal" growth conditions are considered to be:

• Sea level pressure

• Temperature: 20°C–40°C

• Neutral pH

• 0.9% salt concentration

• Ample nutrients

Microbes that thrive outside these conditions are called extremophiles.

Classification by Environmental Niche

Temperature Adaptations

• Psychrophiles: 0–20°C

• Mesophiles: 15–45°C (includes most human pathogens)

• Thermophiles: 40–80°C

• Hyperthermophiles: 65–121°C

Each group has membranes and proteins optimized for their preferred temperature range.

Pressure Adaptations

• Barophiles/Piezophiles: Grow optimally at very high pressures (up to 1,000 atm).

• Barotolerant: Grow over a range of pressures but not optimally at the highest pressures.

• Many barophiles are also psychrophiles, adapted to cold, high-pressure environments such as the deep ocean.

Water Activity, Salt, and Osmotic Stress

• Water Activity (): A measure of water availability in the environment; lower means less available water.

• Osmolarity: The concentration of solute molecules; high osmolarity can cause osmotic stress.

• Aquaporins: Membrane proteins that facilitate rapid water movement across the cell membrane, protecting cells from osmotic stress.

• Halophiles: Require high salt concentrations for growth.

pH Adaptations

• Neutralophiles: Grow at pH 5–8 (most pathogens).

• Acidophiles: Grow at pH 0–5 (often chemoautotrophs).

• Alkaliphiles: Grow at pH 9–11 (found in soda lakes).

• Bacteria regulate internal pH and can be killed by weak acids that disrupt pH homeostasis (used in food preservation).

Oxygen Requirements

• Strict aerobes: Require oxygen for growth.

• Microaerophiles: Grow only at low oxygen levels.

• Strict anaerobes: Cannot tolerate oxygen.

• Aerotolerant anaerobes: Tolerate oxygen but do not use it for metabolism.

• Facultative anaerobes: Can grow with or without oxygen, using fermentation or respiration as needed.

Control of Microbial Growth

Physical, Chemical, and Biological Control

• Sterilization: Complete destruction of all living cells, spores, and viruses.

• Disinfection: Removal or killing of pathogens from inanimate objects (not necessarily all microbes).

• Antisepsis: Removal of pathogens from living tissues.

• Sanitation: Reduction of microbial population to safe levels.

Physical Methods

• Autoclaving: 121°C at 15 psi for 20 minutes (sterilizes by moist heat).

• Pasteurization: Three main protocols:

  • LTLT: 63°C for 30 min

  • HTST: 72°C for 15 sec

  • UHT: 134°C for 1–2 sec (produces nearly sterile milk)

• Filtration: Removes microbes from liquids using micropore filters (not effective for viruses).

• Irradiation: UV light (surface sterilization), gamma rays, electron beams, and X-rays (penetrate deeper, used for food and heat-sensitive items).

Chemical Methods

• Factors Affecting Efficacy: Presence of organic matter, type of organism, corrosiveness, stability, odor, and surface tension.

• Common Disinfectants: Ethanol, iodine, chlorine, ethylene oxide (gas sterilant).

• These agents damage proteins, lipids, and/or DNA to reduce or eliminate microbial content.

Microbial Death Kinetics

• Microbes die at a logarithmic rate when exposed to lethal agents.

• Decimal Reduction Time (D-value): The time required to kill 90% of the microbial population under specific conditions.

Summary Table: Environmental Classification of Microorganisms

Example Applications

• Food Preservation: Use of weak acids and pasteurization to control microbial growth.

• Medical Sterilization: Autoclaving surgical instruments, using chemical disinfectants for surfaces.

• Industrial Fermentation: Continuous culture systems for large-scale microbial production.

Additional info: The notes also include questions for review, such as the basis of cell division, methods to observe bacterial growth, and definitions of extremophiles, which are useful for exam preparation.