Control of Growth and Viability

Solution Preview

Physical and chemical control of growth and viability

I am needing help with these three topics. I have done research but I really dont understand it. If I could get at least two references in APA format it would be greatly appreciated.

1. Discuss the four phases of the bacterial growth curve.
2. Explain two methods used to measure microbial growth.
3. Discuss two methods used to control microbial growth.

1. Four Phases of the Bacterial Growth Curve
When bacteria are inoculated into fresh medium, they typically undergo four different distinct phases of growth: A) Lag Phase, B) Exponential Phase, C) Stationary Phase, and D) Death Phase.

Figure 1. The Bacterial Growth Curve.
Courtesy of http://www.mansfield.ohio-state.edu/~sabedon/black06.htm

A. Lag Phase
Lag Phase is the delay phase before the start of exponential growth. It happened whenever bacteria are transfer from one medium to another, which result in the chemical difference between the two media. Lag phase also happened when a bacteria is freshly supplemented with new medium, which result in their adjustment to the new environment. The lag phase is a physiological event in which the bacteria are slow to divide because of physiological stress and adaptation to the new environment. The delay in cell division in the lag phase suggested that bacteria need time to adjust to their new environment. During lag phase, the bacteria cells may increase in size or mass, but the cells do not undergo "binary fission". (1) During the lag phase, the population of bacteria cells remain unchanged.
The period of lag phase allow the bacteria cells to adapt to their new growth environment consisting of new chemical composition and temperature. The adaptations include the synthesis of cellular and bacterial components necessary for bacteria growth, and the repair of macromolecular damage that accumulated during stationary phase. (2) The length and duration of the lag phase depend on the nature of the cells used for inculation into the media. For example, if the cells are obtained from an agar plate, they are small and increase in size before the first cell divisions. Cells in the lag phase cannot be measured because its optical density (OD) and cell concentration are too low. If cells are transferred from a rich medium to a poor medium, the lag phase can be longer because there are new biosynthetic pathways that need to be regenerated before growth can resume. (3)

B. Exponential Phase

After the lag phase, the bacteria cells then enter cell division or binary fission, which typically occurs during exponential phase. During exponential growth, the doubling time of bacteria is only about 20 min, and requires a number of nutritional factors presence in the growth medium, such as carbon, nitrogen, phosphate, and iron. In exponential bacterial growth, multiple round of DNA replication and synthesis occurred and coupled with transcription and translation for the synthesis of necessary macromolecules. (2) The type of growth in exponential growth is logarithmic or exponential because "the rate of increase in cell number is a multiplicative function of cell number." (1) In a graph of cell number versus time, the rate of increase is a function of absolute cell number. The more cells present, the faster the population of cells that increase in size.
In an exponential growth, when we are dealing with a batch culture in a closed system, the cell growth will eventually come to a halt if the nutrient become depleted. The exponential phase can be maintained in a batch culture by periodical dilution of the culture. In a chemostat, an open system in which the medium is refreshed continuosly, the growth rate is limited and depends on the dilution rate ( u = D,dilution rate). In a batch culture, growth rate is maximal and depends on the nature of the carbon source ( u = u max). Biomass increases exponentially in a batch culture, whereas it remains constant in a chemostat. In a batch culture, the growh yield depends on the concentration of the carbon source. In a chemostat, the growth yield is depending on the concentration of the limiting compound. (3)
Exponential growth is considered a balanced growth system meaning that all cellular constituents are manufactured at constant rates ...