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Science · Drill 41

Science practice 41

14 questions ~9 min recommended
⏱ 00:00
Score
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Time (min) Trial 1(3°C) Trial 2(20°C) 0 0.011 0.011 48 0.011 0.030 62 0.011 0.067 68 0.011 0.072 82 0.011 0.085 88 0.011 0.090 97 0.011 0.098 108 0.011 0.108 118 0.011 0.117 128 0.011 0.126 138 0.011 0.135 148 0.011 0.144 157 0.011 0.152 168 0.011 0.162 180 0.011 0.173 188 0.011 0.180 198 0.011 0.189 600 0.011 0.551 1,470 0.011 0.553 Trial 3(37°C) Trial 4(45°C) 0.011 0.012 0.022 0.045 0.035 0.081 0.047 0.056 0.070 0.075 0.081 0.078 0.099 0.078 0.109 0.079 0.137 0.088 0.149 0.089 0.167 0.098 0.174 0.106 0.190 0.109 0.207 0.114 0.222 0.117 0.297 0.118 0.288 0.126 0.800 0.125 0.800 0.130 Trial 5 (60°C) 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013

Time (min) Trial 1 (pH 3) Trial 2 (pH 5) Trial 3 (pH 6) Trial 4 (pH 7) Trial 5 (pH 9) 0 48 62 68 82 88 97 108 118 128 138 148 157 168 180 188 198 600 1,470 0.012 0.012 0.011 0.012 0.026 0.022 0.012 0.031 0.035 0.012 0.032 0.047 0.012 0.037 0.070 0.012 0.038 0.081 0.012 0.041 0.099 0.012 0.044 0.109 0.012 0.047 0.137 0.012 0.050 0.149 0.012 0.053 0.167 0.012 0.056 0.174 0.012 0.059 0.190 0.012 0.062 0.207 0.012 0.066 0.222 0.012 0.068 0.297 0.012 0.071 0.288 0.012 0.192 0.800 0.012 0.195 0.851 0.011 0.045 0.054 0.059 0.068 0.073 0.079 0.087 0.094 0.101 0.108 0.115 0.121 0.129 0.137 0.143 0.150 0.431 0.440 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013

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Staphylococcus aureus (S. aureus) is a bacterium found on the skin of 25% of healthy individuals with no adverse effects. However, when transferred to food products, the toxins it can produce have been known to cause food-borne illness, particularly in cooked and cured meats. It is important to determine conditions that promote the growth of pathogenic species of bacteria and apply that knowledge in food preparation and handling. Two groups of students set out to determine the ideal growth conditions for S. aureus. In each group, bacteria were inoculated in a nutrient broth and allowed to grow over a period of time.

When analyzing the growth of bacteria in a liquid medium, an increase in turbidity corresponds to an increase in the bacteria growing in suspension. Because the bacterial cells scatter light, spectrophotometry can be used to determine bacterial growth. Changes in the logarithmic absorbance scale on the spectrophotometer correspond to changes in the number of cells, and a growth curve can be plotted by graphing the absorbance readings from the spectrophotometer versus time. The rate of growth can be determined by the slope of the lines, and the type of growth occurring at a given time can be determined by the shape of the curve.

Group 1

The students in Group 1 set out to determine whether temperature affected the growth rate of S. aureus in a nutrient-rich medium. They tested S. aureus by inoculating a flask of nutrient-rich broth with a pH of 6 at the following temperatures: 3°C, 20°C, 37°C, 45°C, and 60°C. The spectrophotometer absorbance readings are shown in Table 11.4.

TABLE 11.4

Group 2

Students in Group 2 planned to determine the effect of pH on the growth of S. aureus. They wanted to test growth at a range of values from highly acidic (low pH) to highly basic/alkaline (high pH). The cultures were grown at a temperature of 37°C, and the pH levels of each culture were as follows: 3, 5, 6, 7, and 9. Table 11.5 shows their data.

TABLE 11.5

1. (A) Which of the following is a valid assessment of the data from Group 1?

2. The stationary phase of growth is entered as the nutrients in the medium begin to run out and the growth of bacteria changes the conditions in the flask. Cell division slows, and the turbidity ceases to increase because the overall population remains unchanged. When did the bacteria in Group 1/Trial 3 likely begin the stationary phase of growth?

3. Which of the following statements is most accurate?

4. Generation time is the amount of time it takes for the bacterial population to double. Some bacteria, such as E. coli, have a doubling time of 20 minutes under ideal conditions, while other bacteria may take days to double their population size. Which of these is most likely the approximate generation time for the bacteria in Group 1/Trial 3 during the first 100 minutes of the experiment?

5. Which of the following does NOT represent a controlled variable for both groups?

6. Which of the following is a valid assessment of the data in Table 11.5?

7. The data for Group 2/Trial 5 indicate that:

8. Which of the following trials were run under the same conditions for Groups 1 and 2?

9. If scientists wanted to do further testing on the growth of S. aureus to inform food processing and handling decisions, such as the effect of salinity, what conditions would be best as the controls for their test?

10. Which of the following graphs most closely resembles the shape of a graph that could be drawn for Group 1/Trial 3?

11. Chemical inhibitors such as sodium benzoate are often used as food preservatives because of their ability to retard, although not completely inhibit, bacterial growth. If a study were done on the effects of sodium benzoate on the growth of S. aureus at 37°C, one would expect to see results similar to those of:

12. What is the independent variable for both Group 1 and Group 2?

13. Meat in a delicatessen tested positive for the presence of bacteria, and this bacteria was to be identified using spectrophotometry. Which of the following test results would most clearly indicate that the bacteria found was S. aureus?

14. Which of the following statements is the best explanation for what occurred in Group 2/Trial 2 between 10 and 24 hours?

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