Growth Cycle

Bacteria reproduce by binary fission:  one cell divides to form 2 progeny cells.  This results in exponential (logarithmic) growth.


  • Number of cells     1     2     4     8     16
  • Exponential           20     21     22     23     24
  • One bacterial cell produces 16 cells after just 4 generations….yikes!


  1. Some bacteria, like E. coli, which can cause diarrhea, have a generation time of 20 minutes….a short generation time + exponential growth = big trouble.
  2. Some bacteria, like Mycobacterium tuberculosis, have a longer generation time; this can also cause big trouble, but it takes longer.
  3. Bacteria can grow in a variety of forms in the laboratory:
    1. as colonies on a solid nutrient medium.
    2. in a broth culture in suspension.
    3. in biofilms in which growth is spread over an inert surface and nutrients are obtained from a fluid that bathes the surface.
  1. The growth cycle of bacteria in broth culture has 4 major phases:
    • lag phase:  period of adaptation to environment and preparation for cell division with high metabolic activity, no cell division.
    • log phase:  period of rapid cell division and production of cell constituents, including peptidoglycan.  β–lactam drugs are most effective during this phase.
    • stationary phase:  growth slows due to nutrient depletion and accumulation of toxic products; cell number in a steady state due to a balance of cell division and cell death.
    • death phase: number of viable bacteria declines.
    • Gene expression can vary greatly during these different phases, e.g. cells in stationary phase are often more resistant to stresses than cells in log phase.
  1. How are bacteria counted?
    • Direct cell count: does not determine if cells can divide
    • Turbidity of culture: proportional to the cell mass
    • Viable cell counts: dilutions of the specimen or culture are tested for growth on nutrient agar plates.  Used to determine the number of colony-forming units (CFU) per milliliter, i.e. the number of cells capable of dividing under provided conditions.  Ex.  Lab test for diagnosis of UTIs.

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