1. why are the results expressed as CFU/ ml instead of number of bacteria/ml 2.
ID: 257864 • Letter: 1
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1. why are the results expressed as CFU/ ml instead of number of bacteria/ml 2. Why are samples often diluted prior to plating 3. What might be done if a sample had so few organisms that even an undiluted sample too few to count. 1. Why are the results expressed as CFU/ml instead of number of bacteria/ml? 2. Why are samples often diluted prior to plating? What might be done if a sample had so few organisms that even an undiluted sample contained too few to count? 3. 4. A food-borne outbreak has resulted in a number of cases of illness. 1 gram of food was collected from the potential food source and mixed into 99 ml of sterile water, and then two additional 1/100 serial dilutions were made. After the serial dilutions were completed you plated out 0.1-ml aliquots from the last dilution blank. The 0.1 ml plates showed 72, 78, and 85 colonies. What was the CFU/gExplanation / Answer
1. Cfu/ml is the colony forming units and is a measure of the viable count of bacteria or fungi. When the cells are plated, a viable count is obtained based on the number of colonies that grow. It does not indicate the total number of non viable (dead) bacteria. A colony is group of bacteria that have originated from single mother bacteria. A sample contains both viable and non-viable organisms. Bacteria/ml will be indicative of both viable and non viable bacteria in the sample. However, when the bacteria are serially diluted and plated on culture plates, each bacteria will form that are composed of a number of bacteria originating from the single bacteria originally present. Hence, cfu/ml is a better indication of the count than bacteria/ml.
2. A sample will have a number of bacteria present and is concentrated. If the sample is directly applied to the plate, then the bacteria will grow and form colonies that will merge due to lack of space. Further, bacteria tend to clump. Hence, individual colonies cannot be distinguished, making the counting difficult. Further, too many bacteria will exhaust the nutrient faster, hindering the growth of other bacteria. When the sample is diluted, the number of bacteria will progressively decrease with the dilution. Hence, individual colonies will be obtained that can be easily counted.
3. Viable counts may not be accurate if the undiluted sample has less than 25 bacteria. If the number of bacteria are low, then no dilution can be performed for the sample. The undiluted sample should be concentrated before performing the plate count. Bacteria can be concentrated by ultrafiltration, centrifugation and evaporation.
4. 1 g of food sample was dissolved in 99 ml of distilled water is a 1:100 or 10-2 dilution. First addition 1/100 dilution means it is 10-4 dilution (or 1:10000). A second 1/100 dilution is 10-6 (or 1: 1000000).
Viable count or Plate count= Number of colonies X Dilution factor/volume of sample
= (72+78+85)/3 * 106/0.1= 78.33*10*106= 7.833*108 cfu/g
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