Unit 2: Microbiological Quality Assessment of Organic Manures - Practice Quiz

1 What does total viable count measure in an organic manure sample?

2 Which unit is commonly used to report the total viable count of solid organic manure?

3 What does the abbreviation CFU stand for?

4 Why is serial dilution performed before plating a manure sample?

5 Which laboratory item is commonly used to prepare a tenfold serial dilution?

6 Which medium is commonly used for a general viable bacterial count?

7 What is the main purpose of incubating an inoculated agar plate?

8 Why should sterile equipment be used during total viable count testing?

9 Which action helps distribute microorganisms evenly before transferring a diluted sample?

10 What is a visible spot of microbial growth on solid agar called?

11 Which plate is most suitable for estimating a total viable count?

12 What does confluent growth on an agar plate mean?

13 What is the purpose of an uninoculated control plate?

14 Which instrument is commonly used to transfer a measured liquid volume during dilution?

15 A sample is diluted by mixing 1 mL with 9 mL of sterile diluent. What is the resulting dilution?

16 Which factor is needed when calculating CFU/g from a diluted manure sample?

17 If 50 colonies grow from 1 mL of a dilution, what is the estimated count in the original sample?

18 Why are replicate plates used in total viable count testing?

19 Why may total viable count underestimate the total number of microorganisms in manure?

20 What does a higher CFU/g value generally indicate in an organic manure sample?

21 A technician plates mL of a dilution of an organic manure suspension and counts 84 colonies. What is the total viable count of the manure?

22 Duplicate plates inoculated with mL of a dilution contain 142 and 158 colonies. What is the estimated mean viable count?

23 Plates from three consecutive dilutions contain 326, 74, and 8 colonies. Which plate should normally be selected for calculating bacterial TVC?

24 A manure sample gives a lower TVC by the pour-plate method than by the spread-plate method under otherwise similar conditions. Which explanation is most reasonable?

25 An aerobic plate count of composted manure is reported as CFU/g. Which conclusion is scientifically appropriate?

26 During serial dilution, a technician transfers mL into mL blanks but fails to mix each tube before the next transfer. What is the most likely effect?

27 A manure sample has a TVC of CFU/g on a wet-weight basis and contains 60% moisture. What is its TVC on a dry-weight basis?

28 During composting, the TVC decreases from CFU/g to CFU/g. What is the logarithmic reduction?

29 A dilution plate is too numerous to count, while a dilution plate inoculated with mL contains 46 colonies. What TVC should be reported?

30 Colonies appear on an uninoculated sterility-control plate during manure TVC analysis. What is the most appropriate response?

31 Direct microscopic examination shows many more bacterial cells than the TVC obtained by plating. What best explains this difference?

32 Two laboratories obtain different TVCs for the same manure sample. One incubates plates at C and the other at C. Why can their results differ?

33 A manure heap is visibly heterogeneous. Which sampling approach is most suitable for estimating its overall microbial status?

34 Which medium is most appropriate when the objective is to estimate the general aerobic bacterial TVC of organic manure?

35 A spread plate prepared with mL of a dilution contains 120 colonies. Why must the calculation include a factor of 10 for plated volume?

36 Duplicate plates from the same manure dilution contain 52 and 241 colonies. Both are within the nominal countable range. What should the analyst do?

37 No colonies grow after plating mL of a manure dilution. Based on this plating scheme, what is the practical detection limit?

38 A fresh manure sample has CFU/g, while a treated sample has CFU/g. By what factor did the viable count decrease?

39 A manure suspension is allowed to stand for 20 minutes before aliquots are removed for dilution. How might this affect the TVC result?

40 Why is a total viable count generally reported as CFU/g rather than as the exact number of viable cells per gram?

41 A manure suspension is prepared so that the plated tube represents an overall sample dilution of . Duplicate plates inoculated with give 72 and 78 colonies. What is the estimated total viable count?

42 Using the weighted plate-count formula , two pour plates at contain 145 and 168 colonies, while two at contain 21 and 18 colonies. What is ?

43 A manure containing 60% moisture has a total viable count of on a wet-mass basis. What is its count on a dry-mass basis?

44 An organic manure plate has 280 colonies at , while the plate is uncountable and the plate has 24 colonies. If was spread and the accepted countable range is 30–300 colonies, which result should be reported?

45 Adding a validated, non-toxic dispersant during homogenization increases the measured count from to without permitting microbial growth. What is the most defensible interpretation?

46 Three plates inoculated from an overall sample dilution show zero colonies. Using one expected CFU in the total plated sample equivalent as the detection-limit criterion, what is the detection limit?

47 Four replicate plates collectively contain 100 colonies. If colony occurrence follows an independent Poisson process and dilution uncertainty is negligible, what is the approximate relative standard error of the pooled count?

48 Composting reduces the aerobic total viable count from to . Which description is correct?

49 Microscopy indicates intact bacterial cells per gram of manure, but aerobic plating yields . Which conclusion is scientifically justified?

50 An acidic manure gives unexpectedly low plate counts, and the pH of the first dilution remains 4.0. Which modification best tests whether acid carryover is suppressing recovery?

51 After correcting for dilution and plated volume, a manure produces estimates of , , and from successive tenfold dilutions. What pattern is most strongly suggested?

52 Duplicate plates from the same dilution contain 12 and 88 colonies, although plating and incubation conditions were nominally identical. What is the most appropriate response?

53 An aerobic plate count is used to assess manure rich in obligately anaerobic bacteria. Which statement best characterizes the result?

54 Two laboratories analyze identical manure. One incubates plates at for 48 hours, while the other uses for 24 hours. Their results differ by one log. What is the strongest conclusion?

55 A non-heat-treated manure suspension yields , while an otherwise identical heat-shocked suspension yields . Assuming heat shock eliminates vegetative cells but allows equivalent recovery of resistant spores, what fraction of the original count is heat resistant?

56 A spread plate contains 45 well-separated visible colonies, but microscopic inspection shows that many inoculum particles contained chains of viable cells. What does represent most accurately?

57 Equal wet masses of manure A and manure B are mixed. A contains 60% moisture and ; B contains 20% moisture and . Assuming no loss of viability, what is the count of the mixture on a dry-mass basis?

58 A manure plate is completely covered by confluent growth. Why is assigning a count based on the visible surface sectors generally invalid?

59 A control manure and a treated manure are plated on different days using different batches of growth medium. The treated manure gives a lower count. Which design change best supports a causal treatment comparison?

60 A sanitizer-treated manure may contain sublethally injured bacteria. Which protocol is most likely to avoid underestimating its total viable count?