Learning Objective

By the end of this section, you should be able to describe the phases of the bacterial growth curve and explain how each phase influences bacterial metabolism and susceptibility to antibiotics.

Bacterial Growth and Death

Lag Phase

• This is the initial adjustment phase of bacterial growth (there is only one lag phase).
• Bacteria are metabolically active but not yet dividing.
• Cells are:
  • Detoxifying the new environment
  • Synthesizing and activating enzymes needed to utilize available nutrients
• Key exam point:
  The number of bacterial cells at the beginning of the lag phase is equal to the number at the end of this phase.

Log (Exponential) Phase

• Period of rapid, exponential cell division.
• Bacteria divide at a constant and maximal rate by binary fission.
• Generation time (the time required for one bacterium to divide into two) is determined during this phase.

Stationary Phase

• Occurs when:
  • Nutrients become depleted, and
  • Toxic metabolic byproducts (e.g., acids or alkaline substances) accumulate.
• The rate of new cell formation equals the rate of cell death, resulting in no net increase in population size.

Clinical Correlation

• Antibiotics are most effective during the logarithmic (log) phase, when bacteria are actively growing and dividing.

Activity