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.

Photo by Internet Archive Book Images

Rate this post