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Bacterial Growth and Growth Curve

    In this bacterial growth and growth curve post we have briefly explained about bacterial growth curve phases lag phase, logarithmic phase, stationary phase and decline phase.

    If a suitable liquid medium is inoculated with bacterium and incubated, its growth follows a definitive course. Small samples are taken at regular intervals after inoculation and plotted in relation to time.

    A plotting of the data will yield a characteristic growth curve. The changes of slope on such a graph indicate the transition from one phase of development to another. Bacterial growth curve phases can be divided into four major phases:

    Lag Phase

    Logarithmic Phase

    Stationary Phase

    Decline Phase.

    Bacterial Growth Curve Phases

    Lag phase

    When microorganisms are introduced into fresh culture medium, usually no immediate increase in cell number occurs, and therefore this period is called the lag phase of bacterial growth curve phases. After inoculation, there is an increase in cell size at a time when little or no cell division is occurring.

    During this time, however, the cells are not dormant. This initial period is the time required for adaptation to the new environment, during which the necessary enzymes and metabolic intermediates are built up in adequate quantities for multiplication to proceed.

    The lag phase lag phase of bacterial growth curve phases varies considerably in length with the species, nature of the medium, size of inoculum and environmental factors such as temperature and nutrients present in the new medium.

    Bacterial Growth Curve

    The viable count shows lag, log, stationary and decline phases. In the total count, the phase of decline is not evident.

    Logarithmic Phase

    Following the lag phase of bacterial growth curve phases, the cells start dividing and their numbers increase exponentially or by geometric progression with time.

    If the logarithm of the viable count is plotted against time, a straight line will be obtained. The population is most uniform in terms of chemical and physiological properties during this phase.

    Therefore, exponential phase of bacterial growth curve phases cultures are usually used in biochemical and physiological studies. Exponential phase of bacterial growth curve phases is of limited duration because of Exhaustion of nutrients, Accumulation of toxic metabolic end products, Rise in cell density, Change in pH, Decrease in oxygen tension (in case of aerobic organisms).

    The log phase of bacterial growth curve phases is the time when cells are most active metabolically and is preferred for industrial purposes. However, during their log phase of growth, microorganisms are particularly sensitive to adverse conditions.

    Radiation and many antimicrobial drugs, e.g. the antibiotic penicillin exert their effect by interfering with some important step in the growth process and are, therefore, most harmful to cells during this phase.

    Stationary phase

    After a varying period of exponential growth, cell division stops due to depletion of nutrients and accumulation of toxic products.

    Eventually growth slows down, and the total bacterial cell number reaches a maximum and stabilizes.

    The number of progeny cells formed is just enough to replace the number of cells that die. The growth curve becomes horizontal.

    The viable count remains stationary as equilibrium exists between the dying cells and the newly formed cells.

    Death phase

    The death phase of bacterial growth curve phases is the period when the population decreases due to cell death. Eventually the rate of death exceeds the rate of reproduction, and the number of viable cells declines.

    Like bacterial growth, death is exponential cell death may also be caused by autolysis besides nutrient deprivation and build-up of toxic wastes.

    Finally, after a variable period, all the cells die and culture becomes sterile. When the total count is plotted, it parallels the viable count up to the stationary phase, but it continues steadily without any phase of decline. Even the total count shows a phase of decline with autolytic enzymes.

    Cell Changes

    The various stages of the growth curve are associated with morphological and physiological alterations of the cells. It has been possible to define the effect of growth rate on the size and specialized growth techniques.

    Lag phase: Bacteria have the maximum cell size towards the end of the lag phase.

    Log phase: Cells are smaller and stain uniformly in the log phase.

    Stationary phase: In the stationary phase, cells frequently are Gram variable and show irregular staining due to the presence of intracellular storage granules. Sporulation occurs at this stage and also many bacteria produce secondary metabolic products such as exotoxins and antibiotics.

    Decline phase: In the phase of decline, involution forms are common.

    Further Readings