Structure of Chromosomes and Microbial Growth Eustace Smith Objectives To give an overview of the structure of bacterial chromosomes To describe the growth of microorganisms and the influence of environmental factors Learning Outcomes
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A topoisomerase is defined as an enzyme that affects the Topology of DNA. Topology is the branch of mathematics that deals with the properties of geometric figures that are unaltered when the figures are twisted or contorted
There is another enzyme that is able to remove the super coiling in DNA. This is called topoisomerase 1.
b. If the inoculum is from an old culture (stationary phase), a lag occurs even if all the cells are viable. This is because these cells are usually depleted of various essential constituents and require time to resynthesize these
d. A lag also occurs when a population is transferred from a rich culture medium to a poorer one. For growth to occur in the poorer culture medium the cells must have a complete complement of enzymes for synthesis of the essential metabolites not present in that medium. The time is thus required for the synthesis of the new enzymes
Exponential growth – This is the growth of a microorganism where the cell number doubles within a fixed time period.
When one plots the cell numbers of an exponentially growing culture against time on arithmetic coordinates one gets a curve with a constantly increasing slope. If one were to plot the log number of cells versus time (on semi log paper). A straight line is the result.
One of the characteristics of exponential growth is that the rate of increase in cell number is slow initially but increases at an ever increasing rate.
A practical implication of this is that when a non sterile product like milk is allowed to stand under ideal conditions of growth, the early stages of growth are not detrimental but doubling or tripling this time results in spoilage.
In the stationary phase no net increase or decrease in cell number occurs. Many cell functions hwvr may still continue such as energy metabolism and some biosynthetic processes. In some microorganisms slow growth may occur accompanied by the death of some cells resulting in no net change in cell number. This is called cryptic growth
If incubation continues after a population reaches the stationary phase, the cells may remain alive & continue to metabolize or they may die. When they die the population is said to be in the death phase. In some cases death may be accompanied by cell lysis, where there is loss of cellular integrity and the release of cytoplasmic contents.
5. Method is not suitable for cell suspensions with less than 106/ml. Here very few bacteria will be seen in the microscope field. Here one would have to first concentrate cells and re-suspend in a small volume.
The number of colonies obtained in a viable count will depend not only on the inoculum size but also on the suitability of the culture medium and the incubation conditions used. The length of incubation will also have an impact
Cells deposited on plates will not all develop into colonies at the same time. If incubation is too short, some viable cell will not develop into colonies
The size of colonies sometimes varies and some tiny colonies may be missed during counting
At high cell concentrations, light scattered away from the detecting unit by one cell can be rescattered by another. This thus appears to the photocell as if the light had not been scattered. The one-one correspondence between cell number and turbidity would thus lose it linearity.