6.6.1 Phases of Growth

Introduction

The growth curve of bacteria is known as a logarithmic or exponential curve. Such growth curves can be converted to straight lines by plotting the logarithms of growth against time. The growth cycle of bacteria has four major phases.

Definition

Phases of Growth: The growth cycle of bacteria has four major phases: lag phase, log phase, stationary phase, and death phase.

In simple words: The growth of bacteria passes through four distinct phases.

Core Concept

The growth cycle of bacteria has four major phases:

• Lag Phase – No growth
• Log Phase – Rapid growth period
• Stationary Phase – Bacterial rate of death and reproduction is equal
• Death Phase – Bacteria start dying

Detailed Explanation

1. Lag Phase – No Growth

• During the lag phase, the bacteria are adapting to their new environment.
• Growth has not yet achieved its maximum rate.
• The bacteria, for example, may be synthesizing new enzymes to digest the particular spectrum of nutrients available in the new medium.

Key Point: The lag phase is a period of adaptation, not growth.

2. Log Phase – Rapid Growth Period

• The log phase is the phase when growth is proceeding at its maximum rate.
• Growth is closely approaching a logarithmic increase in numbers.
• When the growth curve would be a straight line.

Key Point: The log phase is the period of maximum growth rate.

3. Stationary Phase – Bacterial Rate of Death and Reproduction is Equal

• Eventually, growth of the colony begins to slow down.
• It starts to enter the stationary phase where growth rate is zero.
• There is much greater competition for resources.
• Rate of production of new cells is slower and may cease altogether.
• Any increase in the number of cells is offset by the death of other cells.
• So that the number of living cells remains constant.
• This phase is a result of several factors, including:
• Exhaustion of essential nutrients
• Accumulation of toxic waste products of metabolism
• Depletion of oxygen (if the bacteria are aerobic)

Key Point: The stationary phase is a period of equilibrium between cell division and cell death.

4. Death Phase – Bacteria Start Dying

• During the final phase, the death phase or phase of decline, the death rate increases.
• Cells stop multiplying.

Key Point: The death phase is the period when the death rate exceeds the rate of reproduction.

Summary Table of Phases of Growth

Intermediate Level

Phases of Growth in Detail

Factors Affecting Stationary Phase

Growth Curve

• The growth curve of bacteria is known as a logarithmic or exponential curve.
• Such growth curves can be converted to straight lines by plotting the logarithms of growth against time.

Summary Table of Phases of Growth

Important Points

• 1. Bacteria reproduce by binary fission.
• 2. Because one cell gives rise to two progeny cells, bacteria are set to undergo exponential growth (Logarithmic growth).
• 3. Number of cells: 1 → 2 → 4 → 8
• 4. Exponential: 2⁰ → 2¹ → 2² → 2³
• 5. Thus, one bacterium will produce 16 bacteria after four generations.
• 6. Phases of Growth:
• - The curve in graph is known as a logarithmic or exponential curve.
• - Such growth curves can be converted to straight lines by plotting the logarithms of growth against time.
• - The growth cycle of bacteria has four major phases:
• 1. Lag Phase – No Growth
• 2. Log Phase – Rapid Growth Period
• 3. Stationary Phase – Bacterial Rate of Death and Reproduction is Equal
• 4. Death Phase – Bacteria Start Dying
• 7. Lag Phase – No Growth:
• - During the lag phase the bacteria are adapting to their new environment and growth has not yet achieved its maximum rate.
• - The bacteria for example may be synthesizing new enzymes to digest the particular spectrum of nutrients available in the new medium.
• 8. Log Phase – Rapid Growth Period:
• - The log phase is the phase when growth is proceeding at its maximum rate, closely approaching a logarithmic increase in numbers when the growth curve would be a straight line.
• 9. Stationary Phase – Bacterial Rate of Death and Reproduction is Equal:
• - Eventually growth of the colony begins to slow down, and it starts to enter the stationary phase where growth rate is zero, and there is much greater competition for resources.
• - Rate of production of new cells is slower and may cease altogether.
• - Any increase in the number of cells is offset by the death of other cells, so that the number of living cells remains constant.
• - This phase is a result of several factors, including exhaustion of essential nutrients, accumulation of toxic waste products of metabolism and possibly, if the bacteria are aerobic, depletion of oxygen.
• 10. Death Phase – Bacteria Start Dying:
• - During the final phase, the death phase or phase of decline, the death rate increases and cells stop multiplying.

Remember

• - Binary fission = reproduction in bacteria.
• - Exponential growth = logarithmic growth.
• - Four phases of growth: Lag, Log, Stationary, Death.
• - Lag phase = no growth; adaptation.
• - Log phase = rapid growth.
• - Stationary phase = growth rate equals death rate.
• - Death phase = death rate increases.
• - Growth curve = logarithmic or exponential curve.
• - Logarithms of growth vs. time = straight line.

Common Mistakes

• 1. Confusing the phases of growth – Lag = no growth; Log = rapid growth; Stationary = equilibrium; Death = decline.
• 2. Thinking bacteria grow immediately after inoculation – Bacteria first go through the lag phase.
• 3. Thinking the log phase lasts forever – The log phase is followed by the stationary phase.
• 4. Confusing stationary phase and death phase – Stationary = growth rate equals death rate; Death = death rate exceeds growth rate.
• 5. Thinking the growth curve is always a straight line – The growth curve is a logarithmic or exponential curve.
• 6. Forgetting the factors affecting stationary phase – Exhaustion of nutrients, accumulation of toxic waste, depletion of oxygen.
• 7. Thinking bacteria die immediately after the log phase – Bacteria enter the stationary phase before the death phase.
• 8. Confusing binary fission with other types of reproduction – Binary fission is the primary method of asexual reproduction in bacteria.
• 9. Thinking all bacteria have the same growth rate – Growth rate varies among species and conditions.
• 10. Forgetting the generation time – Generation time varies among species (e.g., E. coli = 20 minutes; M. tuberculosis = 24 hours).
• 11. Thinking the death phase is the end of all bacteria – Some bacteria may survive and resume growth under favorable conditions.
• 12. Confusing the phases of growth with the phases of the cell cycle – Phases of growth are for bacterial populations; phases of the cell cycle are for individual cells.

Quick Revision

• Bacteria reproduce by binary fission.
• Because one cell gives rise to two progeny cells, bacteria are set to undergo exponential growth (Logarithmic growth).
• Number of cells: 1 → 2 → 4 → 8
• Exponential: 2⁰ → 2¹ → 2² → 2³
• Thus, one bacterium will produce 16 bacteria after four generations.
• Phases of Growth:
• The curve in graph is known as a logarithmic or exponential curve.
• Such growth curves can be converted to straight lines by plotting the logarithms of growth against time.
• The growth cycle of bacteria has four major phases:
• Lag Phase – No Growth:
• During the lag phase the bacteria are adapting to their new environment and growth has not yet achieved its maximum rate.
• The bacteria for example may be synthesizing new enzymes to digest the particular spectrum of nutrients available in the new medium.
• Log Phase – Rapid Growth Period:
• The log phase is the phase when growth is proceeding at its maximum rate, closely approaching a logarithmic increase in numbers when the growth curve would be a straight line.
• Stationary Phase – Bacterial Rate of Death and Reproduction is Equal:
• Eventually growth of the colony begins to slow down, and it starts to enter the stationary phase where growth rate is zero, and there is much greater competition for resources.
• Rate of production of new cells is slower and may cease altogether.
• Any increase in the number of cells is offset by the death of other cells, so that the number of living cells remains constant.
• This phase is a result of several factors, including exhaustion of essential nutrients, accumulation of toxic waste products of metabolism and possibly, if the bacteria are aerobic, depletion of oxygen.
• Death Phase – Bacteria Start Dying:
• During the final phase, the death phase or phase of decline, the death rate increases and cells stop multiplying.

60-Second Revision

• Binary fission = reproduction in bacteria.
• Exponential growth = logarithmic growth.
• Four phases of growth: Lag, Log, Stationary, Death.
• Lag phase = no growth; adaptation.
• Log phase = rapid growth.
• Stationary phase = growth rate equals death rate.
• Death phase = death rate increases.
• Growth curve = logarithmic or exponential curve.
• Logarithms of growth vs. time = straight line.
• Warning: Bacteria first go through the lag phase before growing. The growth curve is logarithmic, not linear. Stationary phase = growth rate equals death rate. Death phase = death rate exceeds growth rate. Factors affecting stationary phase: nutrient exhaustion, toxic waste accumulation, oxygen depletion. Examples: E. coli generation time = 20 minutes; M. tuberculosis = 24 hours.