Experiment2-Bacteria

Experiment2-Bacteria - Experiment 2(Lab Periods 2 and 3 Determining the Number of Bacteria in a Colony Bacteria are found throughout the biosphere

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Experiment 2 (Lab Periods 2 and 3) Determining the Number of Bacteria in a Colony Bacteria are found throughout the biosphere, inhabiting places that eukaryotes find uninhabitable, sharing all the places where eukaryotes live, and living in and on the bodies of eukaryotes. Most of the time, however, their presence is not obvious, and detection of live bacteria requires the employment of special techniques that must achieve two ends at once: (l) Allowing bacteria in a given sample to multiply until a number sufficient for them to be observable is attained, while (2) Avoiding allowing bacteria from any other material to grow and multiply. A commonly-used technique for the first purpose is the viable count, in which a sample of bacteria is mixed with a liquid (and diluted further in liquid if necessary) then spread out on the surface of a growth- supporting medium, usually agar in a petri plate. The bacteria are then allowed to grow and reproduce until each bacterium has produced so many offspring that the accumulated mass is macroscopically visible. Such a mass of bacteria is a colony, and the cell or cluster of cells that initiated development of the colony is a colony-forming unit (cfu). (The container of growth medium plus growing bacteria is a culture, whether it is liquid or solidified with agar.) The number of colonies on the plate’s surface tells you how many bacterial cells were in the solution you added. The methods for achieving the second purpose are collectively known as aseptic technique, or asepsis (literally, without infection). The first basic rule for asepsis is that all materials that come into contact your sample must be sterile. With reasonable care, this is possible except for air, which contains bacteria and fungi. Therefore, the second basic rule of asepsis is that all containers must be kept closed except when the experimenter must work within the containers, e.g., to inoculate (introduce the bacteria into) the medium or to remove some of the culture for one purpose or another. In this experiment, you will perform a viable count aseptically in order to determine how many live bacteria (cfu) are in a colony that you scrape off of an agar plate. Petri plates containing colonies different microorganisms (one organism per plate) will be distributed to the class. The colonies formed by different species of bacteria look different--in shape, size (diameter), color, whether they are shiny or matte in appearance, and the smoothness of the edge of the colony. At some time during the laboratory period, examine the colonies of all the organisms and record a description (including maximum diameter, in mm) of a typical colony of each organism; your description should be adequate to help you recognize the colonies of each organism. Each group of four students will be assigned to work with one of the organisms.
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This note was uploaded on 01/30/2012 for the course BIO 2004 taught by Professor Morton during the Spring '11 term at Columbia.

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Experiment2-Bacteria - Experiment 2(Lab Periods 2 and 3 Determining the Number of Bacteria in a Colony Bacteria are found throughout the biosphere

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