Lab3answers - LABORATORY EXERCISE #3: Bacteria, Born to...

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LABORATORY EXERCISE #3: Bacteria, Born to Split Objectives: By the end of lab, students will be able to… Perform serial dilutions using a bacterial culture Use the spread plate technique to grow bacterial cultures on solid media Calculate the number of viable bacterial cells present in the plated dilutions Describe the exponential growth of bacteria Explain how bacterial colonies can be used to identify bacteria Use terminology ( bold words ) associated with microbiology correctly Introduction: Carefully read the introduction on this page and circle any words that you do not understand. Define these words for yourself before lab. Studies involving the analysis of materials such as food, water, milk, and in some cases, air, require quantitative enumeration of microorganisms in these substances. In other words, it is important to count the number of bacteria living in an environment with which humans are closely acquainted. Knowing the number of bacteria may prevent illnesses that can be acquired from certain types of bacteria. Counting the number of bacteria in a particular sample is also necessary when conducting laboratory experiments in microbiology. In this lab, you will begin with a sample of bacteria. These bacteria will be living in a nutrient rich liquid broth medium and the culture will contain an unknown number of bacterial cells . By the end of lab, you will be able to count the number of cells present in this culture. The number of bacterial cells living in a liquid culture is measured in colony forming units (cfu). A colony is a cluster of bacteria growing on the surface of a solid medium. All of the bacterial cells within a colony are the same species because they all originated from the same single cell. When a single bacterial cell is present in a fixed location, it will reproduce via binary fission . One cell will become 2, 2 cells will become 4, 4 to 8, 8 to 16, etc. Bacteria divide about every 20 min., depending on the species. In 24 hours, there will be 72 divisions and one cell will become 1.8 x 10 31 cells. Colony forming units refer to viable (living) bacterial cells. Only living cells can form a colony and a researcher is not often interested in the dead cells. From the liquid bacterial culture, you will be performing a series of serial dilutions . In completing the pre-lab, you began to become acquainted with serial dilutions. Your dilutions will be used to gradually decrease the number of bacterial cells (cfu’s) in your liquid media, thus making them easier to count. It is easier to count 100 colonies than it is to count 1000 colonies. Following the serial dilutions, you will use a common method of transferring bacteria from a liquid medium to a solid medium. Using the spread plate method, you will be applying a sample of the diluted bacterial solutions to a Petri dish containing a solid nutrient medium called agar . These bacteria will be allowed to grow for 24 hours and you will see them during the next lab. You will then be able to count
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Lab3answers - LABORATORY EXERCISE #3: Bacteria, Born to...

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