When inoculated in a growth media under optimal conditions, bacteria grow in a distinct pattern of four phases. The growth of the microbes is quantified in numbers and not the size. The phases are lag, log or exponential, stationary and death phases, which are characteristic of bacteria growth in a batch or closed system such as a flask or test tube.
The lag phase is the initial stage of growth after cells are inoculated in fresh medium, but there is no cell division in this phase. However, the microbes may grow in mass or volume, are metabolically active synthesizing DNA, RNA, proteins and enzymes in readiness for the next phase. The period of the log phase is dependent on various factors such as inoculum size, length of time for the synthesis of new metabolic enzymes and essential coenzymes, and recovery time from physical shock during transfer.
The exponential phase is quite rapid with intense replication of bacteria by binary fission. The number of bacterial colonies increases significantly during this phase with a uniform pattern of division, and the growth rate is by geometric progression. However, the constant growth rate is dependent on the constituents of the culture medium and inoculation conditions. The cells are also susceptible to adverse environmental conditions such as antibiotics and radiation, but positive growth is recorded when the number of generated cells exceeds that of the dead cells (Rosenberger, 2008).
During the stationary phase, the growth rate begins to stabilize with little metabolic activity. There is no increase in population size, and the rate of cell division is slow. Bacteria growth in this phase is limited by various intrinsic factors such as exploitation of resources, waste accumulation, insufficient oxygen, acidic pH medium, and limited space. However, this phase is favorable to bacteria producing secondary metabolites such as antibiotics because it is the peak of production.
In the decline or death phase, the population size reduces, and the cells cannot divide anymore due to harsh inoculation conditions. The rate of cell decrease is geometric at a logarithmic rate and the number of dead cells exceeds that of cells generated. There is no more growth beyond this phase and the cells cycle back to the lag phase and the growth pattern continues (Rosenberger, 2008).
Viable microbial cells can be quantified through various methods such as direct and indirect microscopic count and electronic counting chamber. The indirect cell count, otherwise known as plate counts, involves spreading culture sample on nutrient agar and diluting with water before plating. Viable cells grow on the medium and form a colony referred to as colony forming unit (cfu), and each cfu represents the viable number of bacteria cells in culture. The direct microscopic method is executed using counting chambers, which are special slides. The method distinguishes dead cells from the live ones, and only dense cell suspensions of more than 107 cells per ml are quantified. The sensitivity of the cells can be enhanced by increasing the sample suspension through centrifugation. Electronic counting chambers mainly quantify the number and size of the cells such as eukaryotic cells (Kumar, 2012).
Control of microbial growth involves inhibition or prevention of microbes by use of chemical and physical methods. The physical methods include radiation, heat, filtration, high pressure, and desiccation while the chemical methods include disinfection and antisepsis. Complete destruction of microbes is called sterilization, which is achieved through various means. Heat is an effective sterilization method, and both dry and moist heat is used (Todar, 2009).
Dry heating methods include incineration, which is a method used to destroy microorganisms by burning them in an incinerator at high temperatures. Used needles, syringes, glassware, and inoculating loops are sterilized using incineration. Moist heating methods include autoclaving and boiling at high temperatures for about 15-30 minutes. Autoclaving is the preferred method of sterilization in hospitals and laboratories. Pasteurization uses mild heat to reduce microbial load in foods and food products such as milk. The use of low heat through refrigeration and deep freezing is also an effective sterilization method. Desiccation methods include osmotic pressure and lyophilization, which are mainly used for food and drug preservation.
Sterilization using irradiation aims at killing the nucleic acid of microorganisms, completing destroying them. Ultraviolet light is the most common irradiation method used, but others such as x-rays and gamma rays are used to disinfect surfaces. Filtration method is used to sterilize heat-sensitive gases and liquids such as vitamins, injectable medicine, antibiotics, and amino acids. The solutions are filtered by use of filters with ultramicroscopic pores to remove the smallest microbial cells (Todar, 2009).
Chemical agents such as alcohol, phenols, iodine, chlorine, aldehydes, quaternary ammonium compounds, and heavy metals such as mercury and silver are examples of disinfectants used to kill microbes on inanimate objects. Antiseptics such as iodine, detergents, hydrogen peroxide and dilute alcohol are mild agents used on living tissues. Cidal agents totally kill microbes on surfaces while those that inhibit their growth are static agents. For example, bactericides kill bacteria while bacteriostatic agents inhibit the growth of bacteria (Todar, 2009).
Kumar, S. (2012). What are the methods of measuring microbial growth? Preserve Articles. Retrieved on 18 March 2014 from com/2012042631158/what-are-the-methods-of-measuring-microbial-growth.html”>http://www.preservearticles.com/2012042631158/what-are-the-methods-of-measuring-microbial-growth.html
Rosenberger, E. (2008). What are the four phases of bacterial growth. Sciences 360. Retrieved on 18 March 2014 from http://www.sciences360.com/index.php/what-are-the-four-phases-of-bacterial-growth-18035/
Todar, K. (2009). The control of microbial growth. The Microbial World. Retrieved on 18 March 2014 from http://textbookofbacteriology.net/themicrobialworld/control.html
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