Bacterial Growth, Division, and Environmental Influences: Microbiology Study Notes

Bacterial Population and Division

Population and Colonies

Bacterial populations refer to groups of cells that grow and divide, often forming visible colonies on solid media such as agar plates.

• Population: A group of bacteria growing together.

• Colony: A visible mass of bacterial cells arising from a single cell on an agar plate.

Binary Fission

Most bacteria reproduce by binary fission, a process where one cell divides into two identical daughter cells.

• Binary fission: "Binary" means two; "fission" means division.

• Steps in binary fission:

  1. One cell divides in the center, giving rise to two cells.

  2. Cell elongation, septum formation, completion of septum, formation of walls, and cell separation.

  3. Chromosome is attached to the cell membrane so each daughter cell gets a copy.

  4. Chromosome replicates, chromosomes pull apart, ring of FtsZ protein forms, FtsZ ring constricts, membrane and cell wall infold, division is complete.

• Most bacteria divide by this process.

Exceptions: Budding Process

Some bacteria, such as Caulobacter, reproduce by budding rather than binary fission.

• Commonly found in water.

• Stock cell with one chromosome begins to form hypha projection, which grows in length while chromosome duplicates and makes two copies.

• One copy moves towards the end of the hypha, which creates a bud. The bud pinches off to form another cell.

• Polar flagella eventually pinches off from mother cell and swims out (called swarmer cell).

• This is an example of unequal cell division.

Streptomyces

Streptomyces are soil bacteria that produce many antibiotics. They have a filamentous life cycle similar to fungi.

• Start with a spore that germinates and grows mycelium (like fungi).

• Aerial mycelia grow and produce antibiotics, which kill surrounding bacteria and feed on the lysed bacteria.

• Eventually, aerial mycelia produce spores.

Bacterial Growth in the Laboratory

Culture Methods

Bacteria can be grown in nutrient-rich media in the lab, either in broth or batch cultures.

• Broth: Nutrient-rich medium in a flask, usually closed to prevent contamination.

• Batch culture: Closed-system microbial culture of fixed volume.

Growth Phases in Batch Culture

Bacterial growth in closed systems follows a characteristic pattern of phases:

• Lag phase: Cells are adjusting to the environment; no increase in cell number.

• Log/Exponential phase: Cells divide actively and exponentially; growth rate depends on nutrients and temperature.

• Stationary phase: No net increase or decrease in cell number; cells are still dividing but nutrients are exhausted and waste accumulates.

• Death phase: Cells die off due to lack of nutrients or accumulation of toxic products.

Types of Cells in Death Phase

• Viable but not culturable cells: Can survive death phase but do not divide.

• Persister cells: Can survive death phase and regrow if nutrients are provided.

• Altruistic cells: Sacrifice themselves for other cells by releasing their cytoplasm.

Continuous Culture

Continuous culture allows for the maintenance of bacterial populations in a constant growth phase.

• Fresh nutrients are added and waste products removed.

• Concentration of limiting nutrient controls growth rate.

• Cell number remains constant.

• Limiting nutrient is essential for cell growth.

• System uses a pump to regulate input and output.

Bacterial Growth Rate and Generation Time

Exponential Growth

Bacterial populations double each division during exponential growth.

• In binary fission, one cell gives rise to two, then four, etc.

• Generation time: Time it takes for a population to double.

Equations for Binary Fission

• For cells undergoing binary fission: $N_t = N_0 \times 2^n$ where $N_t$ is the final cell number, $N_0$ is the original cell number, and $n$ is the number of generations.

• Number of generations: $n = \frac{\log_{10} N_t - \log_{10} N_0}{0.301}$

• Generation time (inverse of growth rate): $g = \frac{t}{n}$

Logarithmic vs Arithmetic Scale

• Log scale creates a linear relationship between number of generations and log of number of cells.

• Arithmetic scale is not linear; not useful for growth calculations.

• Inverse of generation time is growth rate.

Environmental Factors Influencing Bacterial Growth

Temperature

Temperature affects bacterial growth by influencing nutrient transport and metabolism.

• If nutrients cannot be transported, cells will not grow.

• Enzymatic reactions generate energy and macromolecules for growth; enzyme activity depends on temperature.

• Three cardinal temperatures for every organism:

  • Minimum temperature: Lowest rate of growth.

  • Optimum temperature: Highest rate of growth.

  • Maximum temperature: Growth rate falls; proteins denature.

• If temperature is below minimum, cells stop growing but do not die.

Effects of Temperature on Enzymes and Membranes

• Between minimum and optimal, enzymatic reactions occur at increasing rates.

• At maximum temperature, protein denaturation and membrane collapse occur.

• At minimum temperature, membrane gelling occurs; transport processes slow down.

• At optimum temperature, enzymatic reactions occur at maximal rate.

• Enzymes need thermal energy to work; as temperature increases, catalysis improves.

• Liquid-ordered state (crystalline state) at low temperature; lipids tightly packed, transport slow.

• At high temperature, liquid-disordered state; transport increased.

Protein Denaturation

• Denaturation is the unfolding of proteins by breaking bonds.

• Agents: pH, temperature, ionic strength, solubility.

• Hydrophobic interactions break first, then hydrogen bonds, then ionic bonds.

Types of Bacteria by Temperature Preference

• Psychrophiles: Grow at low temperatures; optimal growth at 15°C or lower; found in polar regions, high altitudes, deep oceans. Example: Polaromonas vacuolate (isolated from Antarctic sea ice).

Summary Table: Bacterial Growth Phases

Phase	Description	Key Features
Lag	Adjustment to environment	No increase in cell number
Log/Exponential	Active cell division	Population doubles each division
Stationary	Growth rate slows, nutrients exhausted	No net increase/decrease in cell number
Death	Cells die off	Viable, persister, and altruistic cells present

Key Equations

• Final cell number: $N_t = N_0 \times 2^n$

• Number of generations: $n = \frac{\log_{10} N_t - \log_{10} N_0}{0.301}$

• Generation time: $g = \frac{t}{n}$

Environmental Factors Affecting Growth Rate

• Temperature

• Pressure

• pH

• Concentration of nutrients

• Concentration of CO2 or O2

Additional info:

• Some context and examples were expanded for clarity and completeness.

• Equations and table were formatted for study purposes.