ACT science practice test 62

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.

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?

A. As temperature increases, the growth rate of S. aureus increases proportionally.
B. S. aureus grows optimally at a temperature of 37°C.
C. As temperature decreases, so does the growth rate of S. aureus.
D. S. aureus exhibits exponential growth at a temperature of 37°C.

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?

A. 97 minutes
B. 148 minutes
C. 600 minutes
D. 1,470 minutes

3. Which of the following statements is most accurate?

A. S. aureus responds well to increases in pH above 7.
B. pH factor does not have an effect on the growth of S. aureus.
C. S. aureus is viable at pH levels below 2.
D. pH is a control for Group 1 and an experimental variable for Group 2.

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?

A. 6 minutes
B. 29 minutes
C. 75 minutes
D. 100 minutes

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

A. The nature of the nutrient media
B. The temperature of incubation
C. The strain of S. aureus used
D. The wavelength of light set on the spectrophotometer

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

A. S. aureus has a faster growth rate at pH 7 than at pH 5.
B. S. aureus achieves a higher overall turbidity at pH 5 than at pH 7.
C. S. aureus achieves maximum growth at pH levels below 4.
D. S. aureus has a faster growth rate at pH 5 than at pH 7.

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

A. at this high level of acidity, S. aureus bacteria cannot grow.
B. at this high level of alkalinity, S. aureus bacteria cannot grow.
C. at this low level of alkalinity, S. aureus bacteria cannot grow.
D. at this high level of alkalinity, S. aureus bacteria thrive.

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

A. Group 1/Trial 1 and Group 2/Trial 1
B. Group 1/Trial 2 and Group 2/Trial 4
C. Group 1/Trial 3 and Group 2/Trial 3
D. Group 1/Trial 5 and Group 2/Trial 5

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?

A. 37°C and pH 6
B. 37°C and pH 5
C. 20°C and pH 6
D. 45°C and pH 7

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


Figure 11.9

Figure 11.10

Figure 11.11

Figure 11.12

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:

A. Group 2/Trial 1.
B. Group 1/Trial 5.
C. Group 2/Trial 2.
D. Group 1/Trial 3.

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

A. Time
B. pH
C. Temperature
D. The type of bacteria being tested

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?

A. The bacteria died at temperatures above 55°C.
B. The bacteria experienced rapid growth at temperatures below 20°C.
C. The bacterial generation time was determined to be approximately 30 minutes at 37°C.
D. The bacteria showed no increase in turbidity when incubated in an ice bath.

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

A. The bacterial population grew exponentially.
B. The birth and death rate of bacterial cells were relatively equal during this time.
C. The bacteria were dying more rapidly than new bacteria could be generated.
D. No new bacteria were generated or died during this time.