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Unformatted text preview: lutant necessary (e.g., 10μl into 1 L). A more practical method is to make a series of dilutions that reduces the numbers of bacteria per ml to the appropriate level in a stepwise fashion. Dilutions are multiplicative, therefore, if you dilute a sample 1:10 (add 10μl to 90μl of liquid), mix and then dilute again by 1:10 (mix and add 10μl of the first dilution to a second vial of 90μl of dilutant), you will have made a 1:100 dilution (10‐2). Similarly, if you made four 1:100 dilutions, you will have diluted your sample by a factor of 10‐8 (i.e., 10‐2x4). This can be accomplished by taking four tubes containing 990μl of dilutant and performing four serial dilutions of 10μl each (Fig 2). Note, in the end you will have a 10‐8 dilution, but instead of using 1L, you will have used less than 4mls of dilutant. Figure 2. The Serial Dilution Method. This figure illustrates four serial dilutions and that only a small amount of total dilutant (less than 4 mls) is needed to make a relatively large final dilution. 16 C. Bacterial Plasmids and Natural Selection In a world where the presence of antibiotics is common, there is natural selection for the genes that confer resistance to antibiotics. The source of these genes is often naturally‐occurring or can result from a new mutation in a bacterium. Regardless of the source, in the presence of antibiotics these genes are beneficial and will increase in frequency in the bacterial population as sensitive bacteria die and resistant bacteria survive and reproduce. The genes that confer resistance to these bacteria will be passed to the next generation. These genes that confer resistance are typically carried on bacterial plasmids that are distinct from the main bacterial chromosome. These plasmids can not only be passed to new bacteria via binary fission, they can also be passed via conjugation, transformation, and transduction in wild populations (see Tutorial 4 – Prokaryotes I to review these processes). These processes allow for the rapid spread of plasmids that contain antibioti...
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- Fall '11