MICROBIAL GROWTH: DOUBLING TIME AND BINARY FISSION

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Growthis simply defined as an irreversible increase in the size of an organism. It is one of the characteristics of living organisms. In microbiological terms, growth is basically the increase in the cell size and cell mass of a microbial (bacterial) cell, and it occur during the development or reproduction process of the organism. Bacterial growth involves the asexual reproduction of the organism, in which the bacterium undergoes binary fission to divide into two daughter cells. Growth in microorganisms unlike other living organisms is usually measured in terms of increase in the number of cells or population of the microbial cells rather than increase in the size of an individual organism. Several methods exist for enumeration of the growth of bacteria and other unicellular organisms in the microbiology laboratory. Some of these techniques used for microbial enumeration and which are usually based on measuring changes in the cell mass and cell numbers of the growing organism are as expanded in the later part of this chapter. Microorganisms are ubiquitous, and can grow in virtually all environments that support life. Some microorganisms can even grow successfully in extreme conditions that scarcely support other forms of life. For example, some microbes grow in extreme hot conditions as that found in boiling springs (for example, Thermus species and some Archaea) and extreme cold conditions as that found in frozen Antarctic sea ice (for example, Polaromonas bacteria, Archaea and some green algae or diatoms).

Microbial growth in the environment can be viewed in two perspectives viz: beneficial growth and harmful growth depending on the conditions and/or circumstances the growth is occurring. The multiplication of beneficial bacteria in the environment is significant to the sustenance of life on earth because their activities impact on various important life processes such as nutrient recycling, nitrogen fixation (in agriculture), biodegradation and a multitude of other vital actions that maintain equilibrium in terms of nutrient distribution in the environment. However, some other groups of microorganisms are not beneficial, and are pathogenic in nature because they cause a variety of diseases and infections to man, animals and plants in the environment. Such pathogenic microorganisms will continue to replicate and multiply in the body cells of the host that they invaded; and if antimicrobial agents targeted at the invaded organism fails to work or the host’s immune is compromised and cannot stop the further proliferation of the organism, a disease condition is likely to set in. When the growth of pathogenic microorganisms is not controlled or inhibited, they cause several havoc to man, animal or plant, and this may result in colossal economic loss. Such pathogenic microorganisms growing on either biotic or abiotic surfaces in hospitals and non-hospital environments are responsible for plethora of nosocomial- and community-acquired infections now plaguing humanity as well as the plant and animal populations.

BINARY FISSION

The growth of microbial cells can be affected by the physical and chemical composition of their environment; and thus the growth of pathogenic organisms in vitro or in vivo could be effectively contained through proper antimicrobial actions. The techniques used to control the growth of harmful (pathogenic) microbial cells on abiotic and biotic surfaces include disinfection, sterilization, sanitation and other forms of decontamination. The growth of bacteria occurs when bacterial cells divide into two daughter cells by an asexual reproductive mechanism known as binary fission. Such cells are said to be viable because they are able to reproduce and increase the size or cell number of their population. On the other hand, a microbial cell can be said to be “dead” when it can no longer reproduce (i.e., grow and divide) when introduced into a growth medium or environment that support its development or replication. Such a cell is said to be non-viable (i.e., they are not alive and cannot divide or multiply).  

Binary fission is defined as the cell division process in which bacterial cells divide to form two new identical cells from the parent organism. It is a duplication process in which a microbial cell divides to form two identical cells known as the daughter cells (Figure 1). Binary fission is one of the most common forms of reproduction in bacteria (prokaryotes); however, it can also be seen in some unicellular eukaryotes. Through binary fission, bacteria can add more bacterial cells to their immediate environment; and this ultimately lead to increase in the population size or number of the organism in that environment. After binary fission must have taken place, the resulting daughter cells produced are usually genetically identical to each other and to their parent cell. However, the daughter cells might as well not be identical to their parent cell, and this happens when mutation occurs during the process of cell division. The increase in a microbial population when the bacterial cells are growing exponentially through binary fission normally follows a geometric progression pattern. For example, one parent bacterial cell yields two daughter cells (this is the first generation). In the second generation, four cells are produced; in the third generation, 8 cells are produced; in the fourth generation, 16 cells are produced, and this process continues on and on until the microbial growth is inhibited by an agent that prevents its growth (for example, antibiotics).

Figure 1. Schema of binary fission. In binary fission, the parent cell splits into two identical parts; and each of the daughter cells produced are also identical to their progenitor or parent cell. Photo courtesy: https://www.microbiologyclass.com

Figure 2. Exponential cell division of a bacterial cell into different daughter cells. Photo courtesy:

The bacterial cells proliferate and increase their numbers or population size in a process known as local doubling. Bacterial cells increases their population size in a process known as doubling (generation) time. Doubling time is simply defined as the time it takes a bacterial population to double and increase its population size in twofold. It is the time required for a microbial population to double. Bacteria grow in an exponential fashion; and in every exponential cell division of a bacterium, each cell division of the bacterial cell results in a doubling of the initial cell number in the growth medium (Figure 2). The increase in the number of bacterial cell is usually very low at low cell number, but after a few generations, the cell numbers increase exponentially or explosively (as shown in Figure 2); and after “n” divisions for example, we have 2n cells. The letter ‘n’ here, refers to the number of dividing cells or number of bacterial generations.

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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