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Unformatted text preview: i plates, you should not lift the lid off completely; rather, tilt it at an angle and introduce a 100μl sample of the bacteria culture to the center of the plate using your pipettor. There are small red biohazard bags on your bench for disposing used tips. If you accidentally touch a non‐ sterile area with a pipette tip, eject the tip and get a new one. B. Serial Dilution Method for Determining Concentration of Bacteria in a Liquid Sample Bacteria are small, reproduce rapidly, and can grow to very high concentrations under optimal growth conditions. There are many reasons why one would want to know the concentration of bacteria in a given sample, and there are several ways to count bacteria. One common method is to perform serial dilutions on a given sample, plate the different dilutions onto a medium that supports growth, and then physically count the number of colonies that grow following incubation. The advantage of this method is that it is relatively easy and gives a good approximation of the number of viable (living) bacteria in the sample. (Simply counting bacteria gives an estimate that includes dead and living cells.) Many samples contain billions of bacteria per milliliter (ml) and plating such a high concentration would result in a “lawn” of bacteria on your petri dish that is uncountable (Fig 1A). The main strategy behind the serial dilution method is to dilute the bacterial sample to the point where individual colonies appear and can be easily counted (Fig. 1B). 15 A B Figure 1. Bacterial Growth at Two Concentrations (A) A sample containing a high concentration of bacteria was plated without dilution. It grows into a “lawn.” (B) The same sample was diluted to 10‐4 and plated. There are a countable number of colonies. If a sample contained 108 bacteria/ml and you wanted to reduce this population to contain a single bacterium/ml, you would need to make a 10‐8 dilution. Doing this in a single step is impractical because of the large volume of di...
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This note was uploaded on 02/06/2014 for the course BIO 110 taught by Professor Hass during the Fall '11 term at Pennsylvania State University, University Park.

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