MICRO201L Report1 - Determination of Bacterial Numbers Lab...

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Determination of Bacterial Numbers Lab Report #1 Christy Skoglund Chin Cheun Heather Loges Melika Budimlic Matt Brower The purpose of this lab was to determine the amount of Escherichia coli in a given microtube by performing serial dilutions, standard plate counting, and spectrophotometry. The bacteria Escherichia coli was plated from serial dilutions and the spectrophotometer was used to count how many cells (living and dead) were in the original sample and in all of the dilutions. The reason for diluting the bacteria was to reduce the cell density and get a viable count of the E. coli colonies. A countable plate should contain anywhere from 30 to 300 bacterial colonies. Using both the standard plate count and the spectrophotometer count, we were able to make an educated guess on what the original cell density. This was done by calculating the number of colony forming units on the plate and multiplying that by the plate dilution. Using the spectrophotometer, we measured the percent transmittance and the absorbance of the tube. Our goal was to be able to count the E. coli on the most diluted plate then figure the original cell density. Ideally, all of our techniques for plating were correct and there was no contamination, so we were able to get a viable colony count. There are three techniques used to count bacteria, two of which we performed in lab and are mentioned above. The third technique is a direct count and this is done by counting the number of bacteria in a Petroff-Hausser counting chamber under a microscope. There are several factors to consider when choosing which method to use. I will discuss the standard plate count and the spectrophotometer, since these were the methods used in class. Some advantages of using the standard plate count are it counts only living cells (obviously, you aren’t able to count the dead cells) and it is apparent if more than one organism is growing on the media. Some disadvantages include: it counts only living cells (you can’t take into account the number of cells that are dead), it takes 18 - 24 hours for the colonies to appear and it is prone to contamination. Some advantages of using the spectrophotometer include it counts both dead and living cells, it is not prone to contamination and the results are immediate. Some disadvantages are it counts both dead and living cells (if you are just wanting to know the number of living cells, this method wouldn’t be the one to choose), spectrophotometers are required and they are
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MICRO201L Report1 - Determination of Bacterial Numbers Lab...

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