Growth of Microbes
Living organisms grow and reproduce. The growth indicates that an organism is in active metabolism. In plants and animals one see the increase in height or size. In a butterfly, a small larva hatching from egg grows in size, moults, pupates and become an adult butterfly through metamorphosis. Growth in a common use refers to increase in size but with microorganisms particularly with bacteria, this term refers to changes in total population rather than increase in size or mass of an individual organism. With fungi linear growth of hyphae and radial growth of colony is observed for growth on solid media but a biomass or mycelial dry weight on liquid media. In unicellular fungi like yeast that reproduce by fission or budding the population change is considered as growth.
The change in population in bacteria chiefly involves trans verse binary fission in most of the bacteria while budding is observed in Hyphomicrobium. In actinomycetes, fragmentation of hyphae and sporulation results in population change. In yeasts, budding and fission are observed that this depends upon the species. In fungi, growth fragmentation asexual and sexual spores serve as propagates for population increase.
The transverse binary fission, an asexual reproductive process is the most common in the growth cycle of bacterial population. A single cell divides after developing a transverse septum (cross wall) and continues to grow by continuous dividing with out cell death till it is subjected to stress.
A cell dividing by binary fission is immortal unless subjected to stress by nutrient depletion or environmental stress. Therefore a single bacterium continuously divides. One cell divides providing two cells and two cells divide giving four and so on. Therefore the population increases by geometric progression.
1 2 4 8 16 32———2n
1 2 22 23 24 25———2n
When a single bacterium is incubated into the liquid medium in flask and incubated, bacterium divides by fission and attains a period of rapid growth in which cells multiply at an exponential rate. If the logarithmic number of bacterium versus time is plotted a growth curve is obtained with different phases of growth.
Soon after transfer of an inoculum to a new medium, cells do not immediately multiply and the population remains unchanged. The cells however increase in size synthesizing newer protoplasm and enzyme necessary to the newer environment. The organisms are metabolizing but require more for adjustments to the physical environment around each cell and hence there is a lag for cell division (lag phase).
At the end of lag phase cells divide and there is a gradual increase in the population. When all the cells complete their lag, there is division at regular intervals. The cells divide steadily at a constant rate in the logarithmic or exponential phase and when log number of cells are plotted against time there is a straight line. The population in this phase is almost uniform in chemical composition, metabolic activity and physiological characteristics. Generation time is the time required for the population to double and this can be determined by the number of generation that occurs at a particular time interval. Not all bacteria have the same generation time. It varies from 15 – 20 minutes for Escherichia coli to many hours in others and is also dependent upon the nutrients and physical conditions of the environment. With the growth of the bacterium, there will be a depletion of nutrients. At high concentration of nutrients a small change may not cause significant effect but at low concentration the growth rate decreases significantly.
At the end of the exponential phase growth rate decreases due to exhaustion of some nutrients or due to production of toxic products during growth. The population remains constant due to complete cessation of division or reproduction rate equals to death rate. The stationary phase is followed by Decline or Death phase as bacteria divide faster than the new cells produced. The depletion of nutrients, accumulation of solubilising products like acids.
The number of viable cells decreases exponentially. G-ve Cocci divide faster but others divide slowly but viable cells may persist for minutes or even years.