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LectNotes&Reading_Motility - Structures and...

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1 Structures and Functions of Microbial Motility Overview The driving force for the evolution of motility appears to be acquisition of food. Thus, motility has also evolved with a mechanism to sense the environment and control motility in response. Almost all microorganisms have mechanisms for motility, both prokaryotic and eukaryotic. Despite the widespread nature of motility, the mechanisms for motility have independently evolved several times. Some forms of motility are very well understood (e.g. the bacterial flagella), but scientists continue to identify microorganisms that exhibit motility through unknown mechanisms. The study of motility, specifically the bacterial flagella, has made fundamental breakthroughs in how motors, environmental sensing, and protein secretion function. Many microbial pathogens are motile, and this motility plays a major role in their ability to cause disease. Reading. 11.5 Structure and Function of Cell Appendages (pg. 307-314) E.coli and Experiments that Lead to Our Understanding of Bacterial Flagellar Motility Here are presented some of the key experimental methods and observations that lead to our understanding of chemotaxis in E. coli . The point here is not to memorize the particular experiments. These are presented to elucidate the scientific method. Quantification of Chemotaxis Chemotaxis=movement in response to a chemical. This chemical could be either an attractant or repellent. The experiment described in the textbook demonstrated that bacteria exhibited chemotaxis. With an assay to study chemotaxis, scientists could then make perturbations to the system and then quantify what effect those perturbations had on the chemotactic response. Key points: 1. Understand how to measure chemotaxis. 2. Know that the concentration of bacteria in the capillary tube relative to the exterior of the tube will change whether there is an attractant or repellent present in the tube or whether the solution in the tube is the same as the solution in the beaker. 3. Be able to explain why these different results would be obtained. Microscopic Analysis of Chemotaxis Models for how detecting the presence of an attractant or repellent affected the motility of E. coli was developed through the use of time-lapsed photography of microscopic observations. This analysis revealed that E. coli moved in a series of runs and tumbles (see movie of the course web site). This movement did not change as long as E. coli was in a homogeneous solution, whether or not an attractant or repellent was present. But remember, in the tube experiment (above), a gradient of attractant or repellent would have been set up as the
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2 chemical diffused from the tube.
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