Population Growth

Population Growth

1. The concept of Exponential Growth

   • During cell division one cell becomes two.
   • Many Bacteria have minimum generation times of 0.5-6 h
   • Exponential Growth
     • It beginning with a single cell having a doubling time of 30 min.
     • Where the number of cells doubles during a constant time interval
     • Semilogarithic graphs are also convenient to use to estimate the generation time of a microbial culture.
       • The consequences of exponential growth
         • The increase in cell number is initially rather slow.
           • Increases at an ever faster rate.
         • For a nonsterile and nutrient-rich food product
           • For a nonsterile and nutrient-rich food product to stand at room temperature for a few hours during the early stages of exponential growth, is not detrimental
             • These harmless organisms grow only very slowly at refrigeration temperatures (-4°C)

2. The mathematics of Exponential Growth

   • The increase in cell number in an exponentially growing bacterial culture approaches a geometric progression of the number 2.
     • A fixed relationship exists between the initial number of cells in the culture and the number present after a period of exponential growth:
       • • N is the final cell number, N0 is the initial cell number, and n is the number of generations during the period of exponential growth
         • The generation time of the exponentially growing population is t/n ( t is the duration of exponential growth).
           • Other Growth Expressions
             • g can be calculated from the slope of the straight-line function obtained in a semilogarithmic plot of exponential growth
               • The slope is equal to 0.301*n/t

3. The Microbial Growth Cycle

   • An organism growing in an enclosed vessel, cannot grow exponentially indefinitely.
   • Instead, a typical growth curve for the population is obtained.
   • Lag phase
     • When a microbial culture is inoculated into a fresh medium, growth usually begins only after a period of time.
     • If the inoculum is taken from an old culture and transferred into the same medium, there is usually a lag .
       • A lag also ensues when the inoculum consists of cell that have been damaged by significant temperature shifts, radiation or toxic chemicals.
     • Exponential Phases
       • Cells in exponential growth
         • Are typically in their healthiest state and most desirable for studies of their enzymes or other cell components.
       • Rates of exponential growth vary greatly.
         • It is influenced by environmental conditions, as well as by genetic characteristics of the organism itself.
       • Stationary Phase
         • In a batch culture exponential growth is limited.
         • Typically, either one or both of two situations limit growth
           • An essential nutrient of the culture medium is used up
           • A waste product of the organism accumulates in the medium and inhibits growth.
         • Death Phase
           • If incubation continues after a population reaches the stationary phase, the cells may remain alive and continue to metabolize
             • They eventually die.

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