Gastroenteritis or “food poisoning” is characterized by sudden nausea, vomiting,
ID: 87028 • Letter: G
Question
Gastroenteritis or “food poisoning” is characterized by sudden nausea, vomiting, abdominal cramps, diarrhea, headache chills and fever up to 39 ºC. The symptoms can be mild to severe and may last between 5-7 days. Salmonella enterica causes an estimated 1.3 billion cases of gastroenteritis and 3 million deaths annually (1.4 million cases and 600 deaths in the US alone). The infectious dose for S. enterica is 103 bacilli. Infectious dose is the number of organisms you need to enter the body in order to cause disease. This means if the food you eat is contaminated with 1000 cells or greater you risk contracting food poisoning. You have chicken salad you made for lunch and decide to determine the amount of Salmonella it contains. To do this you will blend 1 gram of chicken salad with 9 ml of sterile saline, remove 1 ml of the blended sample and mix with another 9 ml. You then plate 1 ml of sample from tube 1 and 1 ml of sample from tube 2 onto XLD media. You count 45 cells from tube 1 and 7 cells from tube 2. 1. Which plate (1 or 2) should you use to calculate your CFU/ml? Why? 2. Calculate the number of organisms in the original chicken salad sample. Will you get sick from eating the chicken salad? Temperature Generation time 40 No growth 45 12 hours 72 65 minutes 98.6 20 minutes 108 24 minutes 3. You bring the chicken salad to a picnic on a nice June day where it is in the low 70s outside. The chicken salad sits out on the picnic table for 3 hours before you eat lunch. Use the information in the table to the right to calculate how many bacteria will be in the chicken salad after the 3 hours. Are you at risk of food poisoning now? 3. Look at the table in question 2, what are the minimum, maximum, and optimum growth temperatures for Salmonella based on the given information. What temperature growth classification would Salmonella fall into? please answer all questions
Explanation / Answer
Ans. 1. Tube 1 gives 45 cells (colonies). It means there are at least 45 bacteria cells in the inoculum (1.0 mL). Tube 1 is 1: 10 dilutions (i.e. 1 mL sample is diluted upto 10 mL volume by adding 9 mL sterile diluent).
So, total bacterial count in sample = CFU count on plate 1 / dilution factor
= 45 cells / (1 : 10)
= 450 cells
Therefore, using tube 1 calculation, there are 450 bacterial cells in 1.0 g sample.
Tube 2 (the inoculum volume is exactly the same as for tube 1) gives 7 cells. Tube 2 is a 1: 100 dilution of original sample (two successive 1: 10 dilutions for tube 1 and 2, respectively).
So, total bacterial count in sample = CFU count on plate 2 / dilution factor
= 7 cells / (1 : 100)
= 700 cells
Therefore, using tube 2 calculation, there are 700 bacterial cells in 1.0 g sample.
Conclusion: Accept the value of tube 2 inoculum for CFU count/ mL; generally, the FCU count that gives maximum viable count for the unit mass or volume of the sample.
All lower CFU count may be due to the reason- “One bacterial cell in the inoculum gives rise to one colony on the culture plate after incubation period. The number of colony forming unit (CFU) of exactly the same inoculum on two plates may differ due difference in homogenous mixing during inoculation, spreading, clumping of cells in inoculum, etc.”
#2. The number of organisms in original chicken salad sample = 700
[see #1 for calculation].
Yes, eating the salad would make sick because its bacterial load would increase the threshold of 1000 Salmonella cells after eating 1.5 gram (1 gram salad has 700 cells) salad- of course consumption of salad is always greater than 1.5 gram.
Note: Eating 1000 cells or more (NOT 1000 cells/ mL) will make you sick.
#3. Bacterial growth rate is given by-
ln (N2/ N1) = k (t2-t1) - equation 1
where, t2 = final time (4 hrs), t1= initial time (0 hrs),
N2= population at t2 (final population),
N1= population at t1 (initial),
k = specific growth rate = ln 2/ g
g = generation time
Given, T = 700F outside. So, generation time, g = 65 min
N1 = 700 ; [Assuming 1 g salad because mass of salad is not mentioned]
Putting the values in equation 1-
ln (N2/ 700) = (ln2 /g) (180 min)
or, ln N2 – ln 700 = (0.693 / 65 min) (180 min)
or, ln N2 – 6.551 = 1.919
or, 2.303 log N2 = 1.919 + 6.551 = 8.47
or, log N2 = 8.47 / 2.303 = 3.678
or, N2 = antilog 3.678 = 4764
Therefore, number of bacterial after 3 hours (= 180 min) = 4764 cell/ gram salad
#4. Minimum temperature = 450F with generation time 12 hours. There is NO growth at 400F.
Optimum growth temperature = 98.60F with shortest generation time (20 min).
#5. Salmonella is mesophile because it grows optimally at normal temperature (98.60F = around 370C)
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