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Cards In This Set
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What is s standard plate count?
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Estimating the number of microbes in a sample
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Explain the standard plate method.
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Original sample is diluted with bottles or tubes. A set volume (aliquot) of the dilutions is then put onto plates, then the number of colonies on the plates is counted after incubation of 24 hrs or more
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How is the number calculated?
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Number of bacteria in the original sample can be found by multiplying the number of colonies on the plate by the dilution factor
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How is accuracy achieved when counting?
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By counting 2 or 3 plates and averaging the counts
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What is the valid range in the standard method?
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30-300
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What is written on plates with fewer than 30 colonies?
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TFTC, too few to count
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What is written on plates with more than 300 colonies?
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TNTC, too numerous to count
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Each colony arises from how many cells?
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A single cell
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Because cells my not always arise from a single cell, what are the counts in the surface plate count reported as?
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Colony-forming Units or CFU's
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What is an advantage of the surface plate count method over directly counting microbes under the microscope?
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Both living and dead cells look the same under the microscope but only living cells will grow on a plate
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Why do plate counts provide a better indicator of disease risk?
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Because both living and dead cells look the same under the microscope but only living cells will grow on a plate
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What is the standard plate count used for?
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Environmental samples, drinking water, juices and foods. For the safety of where we swim and what we eat or drink
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How are federal or state symbols as a maximum number of allowable cells?
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CFU per milliliter
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What is dilution?
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Necessary to reduce the number in a sample to a level that can be counted
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Howis the formula for calculating a dilution states?
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Volume of sample/ (volume of sample + volume of diluent)
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