Unit 7: Microbial growth-curve determination - Practice Quiz

1 What does a microbial growth curve show?

2 Which microorganism is used in this experiment?

3 What does turbidity of a culture mainly indicate?

4 Which instrument is commonly used to measure culture turbidity?

5 What measurement is commonly used to express turbidity?

6 What is plotted on the horizontal axis of a basic growth curve?

7 What is usually plotted on the vertical axis of a turbidimetric growth curve?

8 Which phase occurs immediately after inoculation when cells adjust to the medium?

9 During which phase do E. coli cells multiply most rapidly?

10 What happens to the cell population during the stationary phase?

11 What generally occurs during the death phase?

12 Why is a sterile uninoculated medium used as a blank?

13 Why are readings taken at different incubation times?

14 What should be done before measuring the optical density of a sample?

15 What is the main purpose of incubating the inoculated culture?

16 What does an increase in optical density usually indicate?

17 Which phase is represented by a nearly horizontal part of the growth curve after rapid growth?

18 What is the purpose of plotting the recorded readings?

19 Which condition can cause a culture to enter the stationary phase?

20 What is the relationship between turbidity and suspended cell mass in a culture?

21 Why is a sterile uninoculated broth used as a blank before measuring the turbidity of an E. coli culture?

22 Which wavelength is commonly selected for measuring bacterial turbidity because it provides a useful estimate of light scattering by cells?

23 A culture has OD600 values of 0.10, 0.20, and 0.40 at successive equal time intervals. Which growth phase is most consistent with these observations?

24 Why are growth-curve data commonly plotted as log cell density or log OD against time?

25 An E. coli culture increases from an OD600 of 0.15 to 0.60 during exponential growth. How many doublings occurred?

26 The OD600 of an E. coli culture increases from 0.20 to 0.80 in 60 minutes during exponential growth. What is the approximate generation time?

27 Which observation most strongly indicates that an E. coli culture has entered the stationary phase?

28 A culture shows little change in OD immediately after inoculation, followed by a rapid increase. What is the most likely explanation for the initial period?

29 Why should a turbidimetric sample be mixed gently before transferring it to the cuvette?

30 An OD600 reading is above the linear range of the spectrophotometer. What should be done before using the value for growth analysis?

31 A 1:10 dilution of a culture gives an OD600 of 0.35. What is the estimated OD600 of the undiluted culture, assuming linearity?

32 Which set of measurements is most suitable for determining the shape of an E. coli growth curve?

33 If time is plotted on the horizontal axis and OD600 on the vertical axis using a linear scale, what feature is usually expected during exponential growth?

34 Which change would most likely shorten the lag phase when preparing an E. coli growth curve?

35 Why should the same cuvette orientation be maintained when measuring a series of turbidity samples?

36 Two replicate OD600 readings at the same time are 0.42 and 0.58. What is the best immediate response?

37 What is a major limitation of using OD600 alone to estimate viable E. coli cell numbers?

38 During the late part of a growth experiment, OD600 decreases gradually. Which interpretation is most appropriate?

39 The slope of a plot of versus time during exponential growth is . What does this slope represent?

40 Why should bubbles be removed from a cuvette before recording OD600?

41 An E. coli culture gives blank-corrected OD readings of 0.08, 0.16, and 0.32 at 20-minute intervals during exponential growth. What is the estimated specific growth rate?

42 A culture has an observed OD of 0.62, while the uninoculated medium has an OD of 0.08. The culture is diluted fourfold before measurement. What is the estimated original culture OD?

43 The corrected OD of an E. coli culture increases from 0.10 to 0.80 during a 90-minute interval in exponential phase. What is the approximate generation time?

44 A plot of versus time gives a slope of during exponential growth. What is the corresponding generation time?

45 An OD-versus-time curve reaches a stable plateau. Which conclusion is scientifically justified from turbidimetric data alone?

46 The observed OD of a culture is 0.42, and the medium blank is 0.12. What OD should be plotted for the culture?

47 An E. coli sample gives an OD of 1.40, exceeding the validated linear range of the spectrophotometer. What is the best procedure?

48 Which plotting strategy most reliably identifies the exponential phase from turbidimetric measurements?

49 Two cultures have identical initial OD values, but one was prepared from a late-stationary-phase inoculum and shows a prolonged delay before exponential increase. What is the most likely explanation?

50 During apparent exponential growth, one OD reading is unexpectedly lower than the preceding reading, although the culture is otherwise healthy. Which procedural issue is most plausible?

51 A calibration relationship states that an OD of 1.00 corresponds to cells/mL. What cell concentration corresponds to a corrected OD of 0.45?

52 The corrected OD increases from 0.20 to 0.50 in 40 minutes during exponential growth. What is the approximate generation time?

53 A sample is mixed with an equal volume of sterile diluent. Its measured OD is 0.20, and the blank-corrected value is negligible. What is the estimated original OD?

54 Why can a straight line on a raw OD-versus-time plot be misleading when identifying exponential growth?

55 Triplicate OD readings for one time point are 0.31, 0.32, and 0.47, while adjacent time points are consistent. What is the most appropriate response?

56 Why should the medium blank ideally be incubated and measured under the same conditions as the inoculated culture?

57 An E. coli culture has reached a turbidity plateau. Which additional measurement is most useful for determining whether viable cell numbers are declining?

58 Repeatedly removing aliquots from a single culture flask can distort a growth curve primarily because it changes the culture volume and may introduce contamination. What is the best design for frequent sampling?

59 The corrected OD rises from 0.125 to 1.000 in 75 minutes during exponential growth. What is the generation time?

60 An exponential-phase fit gives and an initial corrected OD of 0.20. What is the predicted OD after 60 minutes, and what experimental concern follows?