Chapter 3: Growth Cycle

Growth Cycle

• Bacteria reproduce by binary fission, a process by which one parent cell divides to form two progeny cells, undergoing exponential growth (logarithmic growth).
• The growth cycle of bacteria has four major phases: lag phase, log phase, stationary phase, and death phase.

Lag Phase

• The lag phase is the first phase of bacterial growth, during which vigorous metabolic activity occurs, but cells do not divide.
• This phase can last from a few minutes to many hours.

Log Phase

• The log phase is when rapid cell division occurs, and antibiotics such as penicillin are most effective during this phase.
• β-Lactam drugs, such as penicillin, act during this phase.

Stationary Phase

• The stationary phase occurs when nutrient depletion or toxic products cause growth to slow until the number of new cells produced balances the number of cells that die, resulting in a steady state.

Death Phase

• The death phase is marked by a decline in the number of viable bacteria.

Aerobic & Anaerobic Growth

• An adequate supply of oxygen enhances metabolism and growth in bacteria.
• Oxygen acts as the hydrogen acceptor in the final steps of energy production catalyzed by flavoproteins and cytochromes.
• Facultative bacteria grow faster in the presence of oxygen, while obligate anaerobes vary in their response to oxygen exposure.

Iron Metabolism

• Bacteria produce iron-binding compounds called siderophores to obtain iron for their growth.
• Siderophores, such as enterobactin produced by E. coli, capture iron by chelating it, then attach to specific receptors on the bacterial surface, and are actively transported into the cell where the iron becomes available for use.

Fermentation of Sugars

• Fermentation of sugars, such as glucose, results in the production of ATP and pyruvic acid or lactic acid.
• These acids lower the pH, which can be detected by the change in color of indicator dyes.
• Fermentation is used to distinguish between certain bacteria, such as Neisseria gonorrhoeae and Neisseria meningitidis, based on the fermentation of glucose or maltose.

Key Points

• Bacteria have a complex and specific mechanism for obtaining iron, testifying to its importance in their growth and metabolism.
• The bacterial growth cycle consists of four phases: lag, log, stationary, and death.