Growth phase of bacteria is defined as the stages or phases that showcase how single or individual bacterial cells mature, divide and proliferate or replicate in their environment. It shows us how bacteria grow, divide, multiply and die, especially in response to the availability or unavailability of important growth nutrients and molecules in the organism’s environment. When the nutrients required for the optimal growth of an organism is sufficiently supplied for the organism to grow, the said organism begins to grow at a steady state or rate after the organism must have acclimatized itself with the new environment it was introduced into.
On the other hand, if nutrient required for growth is meager and starts depleting, the organism’s growth ceases and starts to recede or decline. And this continues until the organism finally dies. Bacteria usually go throw four (4) phases or stages of growth during their life time in their natural environment or artificially-constructed environment such as that provided by a culture media in a Petri dish plate. These four phases of bacteria growth phase include lag phase, log (exponential) phase, stationary phase and decline or death phase. These individual phases of bacterial growth are succinctly summarized in this section.
Lag phase represent the stage in the development of a bacterial cell when no growth occurs. It is the period after the initial seeding, culturing or inoculation of a bacterium in a fresh culture/growth medium before observable growth begins (Figure 1). When bacterial cells are initially introduced into a new medium or environment, they usually require some time to synthesize important biosynthetic intermediates (i.e., primary metabolites) required to keep them alive. Microbial cells initially adjust to new medium until they can conveniently divide frequently by binary fission (as exemplified by the log phase). During lag phase, the bacterial cell monitors their new environment and become acclimatized to it. Lag phase represents a period in the cell division of bacteria in which the respective bacterial cells are maturing or growing but on the other hand, they are not able to divide and increase in their population size. There is enormous synthesis of primary metabolites such as enzymes, nucleic acids and alcohol which are critical to the growth of the bacterial cell.
Log phase is the period in the growth curve of a microorganism in which the growth of the bacteria increases at an exponential time. This stage of microbial growth is also known as exponential growth phase; and it is characterized by doubling of the bacterial cells (Figure 1). In this stage, the number of new bacteria that appears per unit time is equal to the present population of bacteria. The doubling of the bacterial cells continues at a constant rate unless the growth is inhibited by altering some of the growth conditions. Therefore, the number of bacterial cells and the rate of population increase doubles at each consecutive time period. When the natural logarithm of bacterial cell number is plotted against time (hrs) for this type of exponential growth, a straight line graph is obtained. The slope of this graph (line) gives the specific growth rate of the bacteria.
The specific growth rate is a measure of the number of divisions per cell of the bacteria per unit time. Therefore the slope of the line or graph represents the actual rate of the cell growth; and this is usually dependent on the prevailing growth conditions in the medium. This growth conditions may affect the frequency of cell division events in the growth medium, and thus the survivability of the resulting daughter cells. The growth and doubling of the bacterial cells at the log or exponential phase of growth can be inhibited following nutrient depletion and the buildup of toxic wastes. Studies of microbial metabolism and physiology are usually undertaken at the exponential growth phase because the microbial cells are often at their prime or maximum growth in this stage.
Stationary phase is the period in the growth curve of microorganisms (bacteria) in which the growth of the microbial cells stops. The stationary phase is characterized by a slow growth of bacterial cells due to nutrient depletion and toxic waste buildup in the growth medium. Stationary phase sets in as the growing bacterial cells begin to use up or exhaust all the growth nutrients available to them. Once the stationary phase sets in, the line of the growth curve flattens and assumes a horizontal dimension (Figure 1) because the rate of bacterial growth is equal to the rate of bacteria death. Growth rate is stabilized at this phase as the cell number of the organism is no longer increasing. However, the stationary phase is important because it is at this phase that secondary metabolites (e.g. antibiotics) with immense industrial and economic values are synthesized.
DECLINE AND DEATH PHASE
The growth of a bacterial cell begins to decline and recede once growth nutrients become unavailable and drastically depleted in the growth medium. In some circumstances, the microbial cells may continue to grow and metabolize but such growth is not enduring because the microbial population will eventually die (Figure 1). The decline stage ushers in the death phase which is generally characterized by a period of “no growth”. At the death stage, the rate of bacterial death exceeds the rate of bacterial growth. Growth stops entirely because nutrient has been depleted and toxic wastes have started building up. Bacteria completely loses its ability to reproduce (i.e., grow & divide) at this stage of the growth cycle. However, bacterial cells capable of forming endospores and other protective factors that helps it to survive adverse environmental conditions can survive this stage of the growth cycle until it finds a suitable environment to continue its growth process.
Figure 1: Growth curve of bacteria in a closed system. This figure illustrates the typical growth curve of a bacterial cell with time (hrs) and the number of bacteria (log10) in the X-axis and Y-axis respectively. Microbial populations exhibit this pattern of growth when they are grown in a closed culture system (batch culture). A typical bacterial growth curve as obtainable in this illustration is normally obtained when a fresh growth medium is inoculated with a number of cells, and the cell growth is monitored over a certain period of time. Plotting the cell population against the time gives you the bacterial growth curve obtainable in a closed or batch culture system. This curve is termed a sigmoid curve and it is known as a standard growth curve of bacterial cells. MicroDok.
A = Lag phase
B = Log (exponential Phase)
C = Stationary phase
D = Decline & Death Phase
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