Microbial Growth Phase

MICROBIAL GROWTH PHASE

B

A

Microbial growth curve was observed when microorganisms are cultivated in batch culture. The culture was incubated in a closed vessel with a single batch of medium. The curve was usually plotted as logarithm of cell number versus time and usually has four distinct phases. Lag Phase At this stage, the cell synthesizing new components such as to replenish spent materials or to adapt to new medium or other conditions. The length of the lag phase is determined in part by characteristics of the bacterial species and in part by conditions in the media - both the medium from which the organisms are taken and the one to which they are transferred. Some species adapt to the new medium in an hour or two; others take several days. Organisms from old cultures, adapted to limited nutrients and large accumulated wastes, take longer to adjust to a new medium than do those transferred from a relatively fresh, nutrient-rich medium.1 (A) Acceleration Phase (once after the cells have adapted to the new environment; cell division occurs at increasing frequency until the maximum growth rate reached) Log/Exponential Phase At this stage, microbes’ population is uniform in terms of chemical and physical properties. • cells are dividing and doubling in number at regular intervals. • each individual cell divides at a slightly different time. • the curve rises smoothly rather than the discrete step.

Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey 1

During log phase, cells exhibit balanced growth, where cellular constituents manufactured at constant rates relative to each other. However, under certain condition (change in nutrient level and environmental condition), unbalanced growth happens (rates of synthesis of cell components vary relative to each other). (B) Deceleration Phase (when level of substrate decreases, it eventually become limiting and no longer sustain maximum growth rate) Stationary Phase During this phase, total number of viable cells remains constant; mainly because of metabolically active cell stop dividing or reproductive rate balanced by death rate. Possible reason for entry into stationary phase • nutrient limitation • limited oxygen availability • toxic waste accumulation • critical population density reached Starvation responses • morphological changes (such as endospore formation) • decrease in size • protoplast shrinkage • nucleoid condensation • production of starvation proteins • long-term survival • increased virulence Death Phase Death phase is a physiological point at which cell deaths exceed cell births. More specifically, viable count declines. During the decline phase, many cells undergo involution - that is, they assume a variety of unusual shapes, which makes them difficult to identify.2 Number of cells in each phase was estimated by using spectrophotometer. In a spectrophotometer, light is transmitted through a dilution of the culture (usually a 1:10 dilution). As microbe numbers increase, light passing through the culture decreases. The output is often absorbance or optical density. Absorbance is a logarithmic expression of the amount of light that gets through the culture. The number of cells per absorbance unit is a known constant for most microorganisms.

Michael J. Waites, N.L. Morgan et all (2001). Industrial Microbiology: An Introduction. Blackwell Science. London, UK. 2