Notes, Exercises, Videos, Tests and Things to Remember on Growth cycles of bacteria.
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Growth can be defined as the increase in cellular constituent. It leads to a rise in cell number. When microorganisms reproduce by the process like budding or binary fission. In the latter individual cells enlarge and divide to give two progeny generations of approximately equal size. Growth also results when a cell simply become larger. In ceonocytic microorganisms like some fungi growth result in increase in cell size not in a cell number. In microbial study growth is actually ,easured in terms of change in population number.
Growth curve is the curveobtained by plotting thenumber of population vs time when microorganisms is cultured in open method i.e continous supply of nutrients and output of waste materials the growth curve will only have 3 stages. But when the microorganisms is closed cultured (i.e. limited nutrients is supplied without output of waste product) the growth wil have 4 major stages.During growth of microorganisms initially they adapt and prepare for cell division. Afterwards they undergo rapid competition upto the limit. There will be the competition for the nutrients adn space. Finally they die due to the depletion of nutrients and toxic effect of secondary metabolities. All stages of growth can be seen in growth cure by plotting population vs time . Upward for forwarding curve represents increase in number of population and downward curve represents decrease in number of population of microorganisms.
This is the first phase or period of time taken by microorganisms to adapt and synthesizenew and more cellular constitent before going to cell division. During this phase, microorganisms may undergo respiring, recovering, activationof or synthesis of ATP, enlargement of size. Eventually the cells replicate their DNA synthesize ribisomes and other proteins to increase in mass and finally divide. Based of type of microorganisms and nature of culture mediu, log phase varies. Long lag phase may be due to refrigerated source of innoculum. Reculture/ subculture of the freshly growing microorganisms will short lag period.
It is the second stage of growth curve. When rapid cells division occurs by the results of which microbial population number increases logarithmically exponentially. Based on genetic potential of microorganismsthe natureof the medium and the conditions under whch they are growing, the rate of growth depends. The rate of growthh of particular microorganisms remains constant during the logThe population is mist uniform in terms of chemical and physiological properties. So, log phase culture is usually used in biochemicals and physiological studies. the growth is balanced as cellular constituents manufacturing occurs in same rate in relation to each other. Any change in inutrient level or other envirnoment condition lead to unbalanced growth. The rate of growth ncreases with increases in nutrient concentration in hyperbolicway. At sufficiently higher niutrients level growth rate does not increase above saturation. In closed system of hrowth depletion of nutrients and space tends to decrease the rate of growth after certain time.
This is the third stage of growth curve after log phase. Where formation of new microorganisms coincide with death of old microorganism. Therefore, the gowth curves goes parallel to the horizontall axis. This stage happens due to depletion of nutrients, completiotion for space and toxic effects of secondary metabolites. This stage normally comes after population number reaches to 109 cells. bacteria in batch culture enters this phase in response to starvation. Some microorganisms undergo adaptation like endospore formation and virulance increases in this stage of growth.
After depletition of nutrients and extreme completition for space. The microorganisms either undergo adverse adaptation like endospore formation or go to death. the rate of death of microorganisms is also exponential logarithm as explained in log phase. The death may also occour due to production, secreation of toxic end products like toxins other secondary metabolites etc. After certain time the rate of death of microorganisms may reduce due to seviability few microorganisms by consuming the elements yeild after the death of other microbes. That'swhy the curve becmes more sloopy as shown in the figure and the phase is called survival phase.
The time required by a bacterial cell to double is called generation time. It is not same for all bacteria. It is genetically determined trait. generation time of given bacterial species is also influenced by growth medium. If culture mmedium contains good growth promoting substances generation time is shorter. if the culture is poor with growth promiting substances, generation time is longer. It is also called a time required to microorganism to double it's population so called population doubling time. it is the interval of of two successive cell division of microorganisms. As population increase logarithmically during log phase, population increase by twice fold of the original population.
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The time period o adjustment durin which bacteria remain metaolically active but do not divide is called lag phase.
in log phase bacteria divides continously increasing it's number.
In stationery phase the is no net division of bacteria but they die due to toxic substance through density of bacteria and the numbr of bacteria remains the same.
In death phase number of bacteria continously decreases.
Th etime required by cell to divide is called generation time.