MICROBIAL GROWTH CURVE BSC III SEM UNIT II

Microbial Growth: Refers to an increase in cell number, not in cell size. Bacteria grow and divide by binary fission, a rapid and relatively simple process. Phases of Growth Bacterial Growth Curve : When bacteria are inoculated into a liquid growth medium, we can plot of the number of cells in the population over time. Four phases of Bacterial Growth:

In the laboratory, under favorable conditions, a growing bacterial population doubles at regular intervals. Growth is by geometric progression: 1, 2, 4, 8, etc. or 20, 21, 22, 23.........2n (where n = the number of generations). This is called exponential growth. When a fresh medium is inoculated with a given number of cells, and the population growth is monitored over a period of time, plotting the data will yield a typical bacterial growth curve.

As mentioned above, bacterial growth rates during the phase of exponential growth, under standard nutritional conditions (culture medium, temperature, pH, etc.), define the bacterium's generation time. Generation times for bacteria vary from about 12 minutes to 24 hours or more. The generation time for E. coli in the laboratory is 15-20 minutes, but in the intestinal tract, the coliform's generation time is estimated to be 12-24 hours. For most known bacteria that can be cultured, generation times range from about 15 minutes to 1 hour. Symbionts such as Rhizobium tend to have longer generation times. Many lithotrophs, such as the nitrifying bacteria, also have long generation times. Some bacteria that are pathogens, such as Mycobacterium tuberculosis and Treponema pallidum, have especially long generation times, and this is thought to be an advantage in their virulence.

GROWTH CURVE

LAG PHASE: Period of adjustment to new conditions. Little or no cell division occurs, population size doesn’t increase. Phase of intense metabolic activity, in which individual organisms grow in size. May last from one hour to several days.

LOG PHASE: Cells begin to divide and generation time reaches a constant minimum. Period of most rapid growth. Number of cells produced > Number of cells dying Cells are at highest metabolic activity. Cells are most susceptible to adverse environmental factors at this stage. • Radiation • Antibiotics

Stationary phase: Population size begins to stabilize. Number of cells produced = Number of cells dying Overall cell number does not increase. Cell division begins to slow down. Factors that slow down microbial growth: • Accumulation of toxic waste materials • Acidic pH of media • Limited nutrients • Insufficient oxygen supply

Decline/death phase Population size begins to decrease. Number of cells dying > Number of cells produced Cell number decreases at a logarithmic rate. Cells lose their ability to divide. A few cells may remain alive for a long period of time.

Calculation of Generation Time When growing exponentially by binary fission, the increase in a bacterial population is by geometric progression. If we start with one cell, when it divides, there are 2 cells in the first generation, 4 cells in the second generation, 8 cells in the third generation, and so on. The generation time is the time interval required for the cells (or population) to divide. G (generation time) = (time, in minutes or hours)/n(number of generations) G = t/n t = time interval in hours or minutes B = number of bacteria at the beginning of a time interval b = number of bacteria at the end of the time interval n = number of generations (number of times the cell population doubles during the time interval) b = B x 2n (This equation is an expression of growth by binary fission)

Solve for n: logb = logB + nlog2 n = logb - logB             log2 n = logb - logB             .301 n = 3.3 logb/B G = t/n Solve for G G =        t         3.3 log b/B Example: What is the generation time of a bacterial population that increases from 10,000 cells to 10,000,000 cells in four hours of growth?   G =        t_____        3.3 log b/B G =    240 minutes         3.3 log 107/104 G =   240 minutes             3.3 x 3 G = 24 minutes