BISC209/F13: Assignment 209 GPCA

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Wellesley College- BISC209 Microbiology- Fall 2013


The Great Plate Count Anomaly

Due by the beginning of Lab 4.
Advances in molecular tools for gene sequencing and in other types of microorganism identification have dramatically expanded our knowledge of the contribution of microbes in their (and our) environment. It is estimated that 99.9% of microbes are "unculturable" - that is, currently not able to be cultured by traditional methods! The large number of these uncultured bacteria are responsible for the, so-called, "Great Plate Count Anomaly": the recognition that many more bacteria are present than appear as colonies on agar plates. Culture-independent estimates of the number of bacteria in a gram of soil are 109 - this is several hundred to 9,000 orders of magnitude greater than the number derived from culture-dependent methods. It has been speculated that there might be 10 billion species of bacteria on Earth!

Please use published information on "The Great Plate Count Anomaly" to write a discussion section on your enumeration results. You must formally cite references in the text and include a reference page with full citation information for all sources. Please use the journal Cell's references-citation format. Model it exactly. Attention to detail matters. The Lab WIKI will provide you with a pdf file of a published research report in Cell for modeling formatting. You may use this one or find your own as a model of how, where, and when to cite. As always, see your instructor for guidance if you have questions or problems.

Two important references are supplied for you here: : | THE UNCULTURED MICROBIAL MAJORITY. Rappe & Giovannoni. 2003. Annual Review of Microbiology. Vol. 57: 369-394. First published online as a Review available through the Wellesley College Library and | Uncultivated Microorganisms by Slava Epstein in Microbiology Monographs Vol. 10, 2009 DOI: 10.1007/978-3-540-85465-4 available as an e-book through Springerlink at the Wellesley College Library or as a pdf file in the Resources section of the lab Sakai site.

DIscussion Format in a Primary Research Report:
Think of this discussion as a continuation of the results analysis that you turned in last week. Start with a BRIEF summary of your conclusions and the data from your results analysis that allows those conclusions. (Note that whenever you mention your experimental findings or specific data, you must include in parenthesis a reference to the figure/table number where those data are found. ) After the single, brief paragraph summary of results and conclusions you can put your findings together with the consensus of information available in the literature. This does not mean you need to mention and cite every paper on the topic! Use a recent review article or the introduction section of a recent study as well as the references supplied above to find out the history of investigation on this topic and what is currently thought to be the number of microbes or bacteria in a gram of soil. Put the results from other studies together with your findings to help your reader understand why your answers came out wildly different when you used two different tools to measure the same thing and which answer, if either, might be closer to the truth.

BE positive. DO NOT trash your experiments, your data, or yourself as an investigator. THere is no "sources of error" section in the discussion section of a primary research report. If you can not determine exactly the number of microbes in a gram of the soil you are investigating, that is not failure. It's a type of conclusion that needs to be explained, not in terms of what was wrong with your study but in terms of the limitations of our tools and the sheer, mindboggling abundance of the microbial world.

Include the significance of your findings in a broader context. Where might this study of soil microbial abundance lead? What are other tools that you might consider adding to enumerate the microbial population?

As in all good, clear writing, information should move from old to new and broad to narrow. Think about paragraphs, sentences, and the whole discussion as different types of "units of discourse". In a unit of discourse, put the context of the information at the beginning and the new information at the end. Remember that the "story is about what shows up first".

Good luck.