3. You work for the U.S. Department of Agriculture\'s Agricultural Marketing Ser
ID: 279343 • Letter: 3
Question
3. You work for the U.S. Department of Agriculture's Agricultural Marketing Services, which is responsible e microbiological testing of foods that are purchased for federal nutritional programs such as the Nati onal School Lunch Program. Specifically, your job involves monitoring the total bacterial counts as well as the E. coli counts in ground beef using standard plate count techniques. The figure below shows the procedure you used and the results you obtained after testing a sample of ground beef. 1 ml 1 ml 1 ml 1 ml 1 gram ground beef 9 mi sterile saline in each tube 10-1 10-3 10-4 105 0.4 ml /I 0.4 ml mi 402 0.4 ml 0.4 ml 0.4 ml Total bacterial colony counts 821 162 16 0.8 ml plated 14 0.8 ml plated 0.8 ml plated 0.8 ml plated 0.8 ml plated 108 E. coli colony 367 zero counts Based on the figure and the information provided answer the following questions
Explanation / Answer
Answer:
(i)
Fourth tube with 10^-4 gave countable number of colonies of 162
Original CFU/ml = ?
Dilution factor plate D= 10^-4
Number of colonies= 162
Volume of sample =0.4 mL to the plate
cfu/ml = (no. of colonies)/( dilution factor x volume of culture plate)
cfu/ml = (162)/ (10^-4 x 0.4)
cfu/ml = 4.05 x 10^6 cfu/mL
Original bacterial load in beef sample = 4.05 x 10^6 cfu/g
(ii)
Fourth tube with 10^-2 gave countable number of colonies of 108
Original CFU/ml = ?
Dilution factor plate D= 10^-2
Number of colonies= 108
Volume of sample =0.8 mL to the plate
cfu/ml = (no. of colonies)/( dilution factor x volume of culture plate)
cfu/ml = (108)/ (10^-2 x 0.8)
cfu/ml = 1.35 x 10^4 cfu/mL
Original E Coli count= 1.35 x 10^4 cfu/gram
(iii)
USDA regulations
Bacterial count= 2 x 10^5 CFU/gram
E Coli count = 1 x 10^4 CFU/gram
Since we found that the
Original bacterial load in beef sample = 4.05 x 10^6 cfu/gram, it does not meet the tested specifications
But the E coli load is complying with the given specifications.
(iv)
Using the same tip would carry the residual sample to the next tube, also a wet tip holds a different volume compared to dry tip. This could be the reason for the varying results
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