Experiment 9: Growth curve of Serratia marcescens
Bacteria grows by binary fission. The aim of this experiment is to follow the growth of Serratia marcescens in nutrient broth at 37oCby recording the changes in turbidity (cloudiness) by measuring the absorbance of visible light (600 nm) and also to prove that there is an increase in the cell number and not just in mass during the growth. In the experiment we measure the full growth curve of Serratia marcescens by measuring the absorbance at 600nm at every 10 mins. I also determined the viable count at the start and the end of the exponential phase of growth. Using the growth curve I calculated the growth curve and it was 1.2. Using this I found the doubling time which was 34s. Introduction
Bacteria grows by binary fission. One bacteria becomes two, two bacteria becomes four bacteria and it so goes on. A lot of changes occur in the bacterial cell during growth and due to this reason it is difficult to measure growth quantitatively. Growth is therefore usually measure by changing one or two easily measure parameters which will ignore the complicated cellular change. Therefore different type of analysis may yield different results and the measure of cell growth is only a crude method. We can measure the bacteria growth by either monitoring the changes in the absorbance reading ( the culture becomes more cloudy as the numbers increase and the cells grow in size ) and to prove that there is an increase in cell number and not just the increase in mass during growth we determine the viable count at the start and end of the period where there is most active growth or we can measure the chemical or biochemical property of the cell (e.g.: protein concentration) In this experiment we measure the absorbance reading and plot a growth curve.
Fig 1: Bacterial growth curve showing phases
During batch culture, a typical bacterial growth curve shows five distinct phases of growth: lag phase, the delay before the start of exponential growth; exponential phase, where cell division proceeds at a constant rate; stationary phase, when conditions become unfavourable for growth and bacteria stop replicating; death phase, when cells lose viability; and, finally, long-term stationary phase, which can extend for years.
1. Lag phase
When a microorganism is introduced into the fresh medium, it takes some time to adjust with the new environment. This phase is termed as Lag phase, in which cellular metabolism is accelerated, cells are increasing in size, but the bacteria are not able to replicate and therefore no increase in cell mass or cell number.
2. Exponential (log) Phase- The exponential phase of growth is a pattern of balanced growth wherein all the cells are dividing regularly by binary fission. The cells divide at a constant rate depending upon the composition of the growth medium and the conditions of incubation. The rate of exponential growth of a bacterial culture is expressed as generation time, also the doubling time of the bacterial population. Generation time (G) is defined as the time (t) per generation (n = number of generations). Hence, G=t/n is the equation from which calculations of generation time (below) derive.
3. Stationary phase- As the bacterial population continues to grow, all the nutrients in the growth medium are used up by the microorganism for their rapid multiplication. This result in the accumulation of waste materials, toxic metabolites and inhibitory compounds such as antibiotics in the medium. This shifts the conditions of the medium such as pH and temperature, thereby creating an unfavourable environment for the bacterial growth. The reproduction rate will slow down, the cells undergoing division is equal to the number of cell death, and finally bacterium stops its division completely. The cell number is not increased and...
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