Growth Cycle in Microbiology

Growth Cycle

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

1.

• 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!

 

2. Some bacteria, like E. coli, which can cause diarrhea, have a generation time of 20 minutes….a short generation time + exponential growth = big trouble.
3. Some bacteria, like Mycobacterium tuberculosis, have a longer generation time; this can also cause big trouble, but it takes longer.
4. 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.

5. 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.

6. 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