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Unit 4: Total count and viable count determination - Practice Quiz

BTY331 — Microbiology Laboratory 60 Questions
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1 What is the main purpose of the spread plate technique?

To determine total count of bacteria by spread plate technique Easy
A. To count viable bacterial cells
B. To stain bacterial cells
C. To measure bacterial cell size
D. To identify bacterial shapes

2 What is spread over the surface of an agar plate in the spread plate method?

To determine total count of bacteria by spread plate technique Easy
A. A fixed bacterial smear
B. A melted agar mixture
C. A concentrated stain solution
D. A measured bacterial sample

3 Which instrument is commonly used to spread the sample on the agar surface?

To determine total count of bacteria by spread plate technique Easy
A. Sterile inoculating needle
B. Sterile measuring cylinder
C. Sterile microscope slide
D. Sterile glass spreader

4 Why are serial dilutions made before plating a bacterial sample?

To determine total count of bacteria by spread plate technique Easy
A. To change bacteria into spores
B. To obtain countable colonies
C. To remove all living bacteria
D. To increase the sample volume

5 What does one visible colony usually represent in a viable count?

To determine total count of bacteria by spread plate technique Easy
A. One bacterial species
B. One microscope field
C. One type of culture medium
D. One colony-forming unit

6 What is the usual unit for reporting bacterial counts from a liquid sample?

To determine total count of bacteria by spread plate technique Easy
A. CFU/mL
B. CFU/g
C. Cells/mm
D. Colonies/cm

7 Which type of agar is commonly used for counting bacteria in a general sample?

To determine total count of bacteria by spread plate technique Easy
A. Nutrient agar
B. Fixation agar
C. Indicator paper
D. Distilled water agar

8 What is the purpose of labeling the agar plates before inoculation?

To determine total count of bacteria by spread plate technique Easy
A. To increase bacterial growth
B. To prevent dilution of samples
C. To identify sample details
D. To sterilize the agar surface

9 Which condition helps prevent contamination during plating?

To determine total count of bacteria by spread plate technique Easy
A. Using aseptic technique
B. Mixing different samples
C. Using unsterilized spreaders
D. Leaving plates uncovered

10 Why should a diluted sample be mixed before transferring it for plating?

To determine total count of bacteria by spread plate technique Easy
A. To increase agar thickness
B. To distribute cells evenly
C. To change the dilution factor
D. To destroy sensitive cells

11 What is a suitable incubation position for most agar plates?

To determine total count of bacteria by spread plate technique Easy
A. Upright with agar downward
B. Inverted with agar upward
C. Sideways with agar outward
D. Open with agar upward

12 What does a dilution of mean?

To determine total count of bacteria by spread plate technique Easy
A. One thousand parts in one
B. One part in three hundred
C. Three parts in one hundred
D. One part in one thousand

13 Which plates are generally selected for calculating a reliable bacterial count?

To determine total count of bacteria by spread plate technique Easy
A. Plates with no visible colonies
B. Plates with completely merged colonies
C. Plates with heavy surface contamination
D. Plates with countable colonies

14 A plate contains 50 colonies after plating mL of a dilution. What is the bacterial concentration?

To determine total count of bacteria by spread plate technique Easy
A. CFU/mL
B. CFU/mL
C. CFU/mL
D. CFU/mL

15 A plate contains 50 colonies after plating mL of a dilution. What is the bacterial concentration?

To determine total count of bacteria by spread plate technique Easy
A. CFU/mL
B. CFU/mL
C. CFU/mL
D. CFU/mL

16 If 100 colonies grow from mL of a dilution, what is the original concentration?

To determine total count of bacteria by spread plate technique Easy
A. CFU/mL
B. CFU/mL
C. CFU/mL
D. CFU/mL

17 What may happen if too many bacterial cells are plated on an agar plate?

To determine total count of bacteria by spread plate technique Easy
A. Colonies may become sterile
B. The agar may become liquid
C. The dilution may disappear
D. Colonies may merge together

18 What may happen if a plate receives very few bacterial cells?

To determine total count of bacteria by spread plate technique Easy
A. The sample becomes more concentrated
B. The agar must become contaminated
C. The count may be less reliable
D. The colonies must merge together

19 Why should the spreader be sterilized before use?

To determine total count of bacteria by spread plate technique Easy
A. To increase colony size
B. To change the sample color
C. To prevent contamination
D. To reduce agar nutrients

20 Which observation is recorded after incubation in a spread plate count?

To determine total count of bacteria by spread plate technique Easy
A. Length of the spreader
B. Number of visible colonies
C. Weight of the agar plate
D. Color of the tube label

21 A sample from a dilution produces 86 colonies on an agar plate. What is the bacterial concentration in the original sample?

To determine total count of bacteria by spread plate technique Medium
A.
B.
C.
D.

22 A plate prepared from a dilution has 245 colonies, while a plate from a dilution has 27 colonies. If was spread on each plate, which result is most suitable for calculating the bacterial concentration?

To determine total count of bacteria by spread plate technique Medium
A. The average of both plates without adjustment
B. The plate because it has more colonies
C. Neither plate because both contain fewer than 300 colonies
D. The plate because it is less crowded

23 Why is a diluted sample used in the spread plate technique?

To determine total count of bacteria by spread plate technique Medium
A. To increase the original bacterial concentration
B. To obtain separated and countable colonies
C. To prevent bacteria from growing on agar
D. To eliminate the need for sterile technique

24 A portion of a dilution gives 64 colonies. What is the concentration of bacteria in the original sample?

To determine total count of bacteria by spread plate technique Medium
A.
B.
C.
D.

25 Two replicate plates from the same dilution contain 92 and 108 colonies. What concentration should be reported if of a dilution was plated?

To determine total count of bacteria by spread plate technique Medium
A.
B.
C.
D.

26 A student plates of a dilution and observes a confluent lawn. What is the best next step?

To determine total count of bacteria by spread plate technique Medium
A. Add more agar to the same plate
B. Repeat using a higher dilution
C. Count the lawn as one colony
D. Incubate the plate for a shorter time

27 A plate from a dilution contains 312 colonies, and a plate from a dilution contains 34 colonies. Which plate should be selected for the most reliable calculation?

To determine total count of bacteria by spread plate technique Medium
A. The plate because its count is countable
B. Neither plate because counts must exceed 100
C. Both plates because their counts differ tenfold
D. The plate because dilution is lower

28 Why should the diluted sample be mixed thoroughly immediately before transferring an aliquot for plating?

To determine total count of bacteria by spread plate technique Medium
A. To sterilize the diluted suspension
B. To distribute cells evenly throughout the suspension
C. To increase the volume delivered by the pipette
D. To make all bacterial cells form larger colonies

29 A dilution gives 150 colonies when is plated. If of the same dilution had been plated, approximately how many colonies would be expected?

To determine total count of bacteria by spread plate technique Medium
A. 750 colonies
B. 30 colonies
C. 300 colonies
D. 150 colonies

30 A sample is diluted by transferring into of sterile diluent. What is the dilution factor of this tube?

To determine total count of bacteria by spread plate technique Medium
A.
B.
C.
D.

31 A dilution is followed by a dilution in a serial dilution scheme. What is the overall dilution relative to the original sample?

To determine total count of bacteria by spread plate technique Medium
A.
B.
C.
D.

32 A plate prepared from a dilution has 72 colonies after plating . What result would be expected from plating the same volume of a dilution?

To determine total count of bacteria by spread plate technique Medium
A. Approximately 7 colonies
B. Approximately 72 colonies
C. Approximately 7,200 colonies
D. Approximately 720 colonies

33 A spread plate contains several merged colonies that cannot be distinguished individually. How should this plate be treated during calculation?

To determine total count of bacteria by spread plate technique Medium
A. Use it as an exact count without adjustment
B. Divide the merged area by the dilution factor
C. Record it as unsuitable for accurate counting
D. Count each merged area as one colony

34 A sample produces 48 colonies from of a dilution. If the same sample is plated at of the same dilution, what count is expected?

To determine total count of bacteria by spread plate technique Medium
A. 96 colonies
B. 480 colonies
C. 24 colonies
D. 48 colonies

35 What is the primary purpose of spreading the inoculum evenly over the agar surface with a sterile spreader?

To determine total count of bacteria by spread plate technique Medium
A. To prevent nutrient absorption by the agar
B. To concentrate cells at the plate center
C. To reduce the incubation temperature of the agar
D. To distribute cells for separate colony formation

36 A plate from a dilution has 125 colonies, and was plated. What is the original bacterial concentration?

To determine total count of bacteria by spread plate technique Medium
A.
B.
C.
D.

37 A negative control plate receives sterile diluent instead of the bacterial sample and develops colonies. What is the most appropriate interpretation?

To determine total count of bacteria by spread plate technique Medium
A. The agar naturally produces bacterial colonies
B. The sample concentration is unusually high
C. The dilution factor should be increased
D. Contamination likely occurred during the procedure

38 A plate has 260 colonies, but 20 are clearly different in color and morphology from the others. If only the target organism is being counted, what number should be used?

To determine total count of bacteria by spread plate technique Medium
A. 280 colonies
B. 240 colonies
C. 20 colonies
D. 260 colonies

39 Three replicate plates from the same dilution have counts of 82, 85, and 143 colonies. What is the best action before reporting the result?

To determine total count of bacteria by spread plate technique Medium
A. Use only the highest count
B. Average all counts without review
C. Investigate the variable replicate
D. Use only the lowest count

40 A dilution plate has 40 colonies from . A dilution plate has 38 colonies from . What is the most likely explanation?

To determine total count of bacteria by spread plate technique Medium
A. The higher dilution contains more viable cells
B. The plates prove the sample has no bacteria
C. The lower dilution must always have fewer colonies
D. The dilution series was prepared accurately

41 A aliquot of a dilution produces 146 colonies. What is the bacterial concentration in the original sample?

To determine total count of bacteria by spread plate technique Hard
A.
B.
C.
D.

42 Two replicate plates from a dilution contain 32 and 38 colonies. If was spread from each plate, what concentration should be reported?

To determine total count of bacteria by spread plate technique Hard
A.
B.
C.
D.

43 A serial dilution is prepared by transferring into of diluent four consecutive times. A sample from the final tube produces 82 colonies. What is the original concentration?

To determine total count of bacteria by spread plate technique Hard
A.
B.
C.
D.

44 A sample is diluted 1:100 and then diluted 1:10. A aliquot of the second dilution gives 63 colonies. What is the original bacterial concentration?

To determine total count of bacteria by spread plate technique Hard
A.
B.
C.
D.

45 Counts from successive tenfold dilutions are 248 colonies at and 29 colonies at , with plated. Which result is most defensible?

To determine total count of bacteria by spread plate technique Hard
A. based on the dilution average
B. based on both plates
C. based on the lower dilution
D. based on the countable plate

46 A sterile-diluent control plate receives the same volume and incubation conditions as the sample plates but develops 6 colonies. How should the sample counts be handled?

To determine total count of bacteria by spread plate technique Hard
A. Report the sample counts without any adjustment
B. Average the control colonies with sample colonies
C. Subtract six colonies from every sample plate
D. Treat the sample run as contaminated and repeat it

47 A plate shows a nearly continuous lawn with several isolated colonies, although the calculated count would exceed 300 colonies. What is the principal analytical problem?

To determine total count of bacteria by spread plate technique Hard
A. The organisms were all killed during incubation
B. The spreader converted colonies into nonviable cells
C. The dilution factor becomes irrelevant at high counts
D. The colony count is likely underestimated by coalescence

48 A bacterial suspension contains many cell aggregates. Compared with a well-dispersed suspension, the spread-plate result will most likely show which pattern?

To determine total count of bacteria by spread plate technique Hard
A. A zero CFU value because aggregates cannot grow
B. An underestimated CFU value because aggregates form single colonies
C. An unchanged CFU value because each cell forms a colony
D. An inflated CFU value because aggregates multiply faster

49 Why is spreading rather than on a standard agar surface often unsuitable without modification?

To determine total count of bacteria by spread plate technique Hard
A. The volume changes the dilution inside the sample tube
B. The agar surface may not absorb the volume rapidly
C. The larger volume prevents bacterial metabolism
D. The larger volume always sterilizes the agar surface

50 A aliquot from a dilution yields 25 colonies. Assuming the plate is acceptable for the method, what is the calculated concentration?

To determine total count of bacteria by spread plate technique Hard
A.
B.
C.
D.

51 Three replicate plates from one dilution contain 64, 68, and 72 colonies. What conclusion is best supported by these results?

To determine total count of bacteria by spread plate technique Hard
A. The mean count should be replaced by the highest count
B. The plates must contain three different species
C. The replicate variation is reasonably low
D. The dilution series is certainly inaccurate

52 A sample is plated on a medium containing an antibiotic that suppresses some bacteria. The resulting count should be interpreted as what?

To determine total count of bacteria by spread plate technique Hard
A. The count of bacteria able to grow under the selection
B. The total count of all bacteria in the sample
C. The direct microscopic count of living bacteria
D. The count of bacterial genomes in the sample

53 Which statement most accurately distinguishes a spread-plate viable count from a direct microscopic total count?

To determine total count of bacteria by spread plate technique Hard
A. A spread plate counts DNA molecules, whereas microscopy counts enzymes
B. A spread plate counts all cells, whereas microscopy counts colonies
C. A spread plate counts dead cells, whereas microscopy counts viable cells
D. A spread plate counts culturable units, whereas microscopy counts cells

54 Counts are 280 at , 4 at , and too numerous to count at ; each plate received . Which value should normally be reported?

To determine total count of bacteria by spread plate technique Hard
A. from the countable plate
B. from all dilution levels
C. from both numeric plates
D. from the most dilute plate

55 A student transfers into of diluent and labels the tube as a tenfold dilution. What is the actual dilution and the likely effect on the calculated original count?

To determine total count of bacteria by spread plate technique Hard
A. It is and causes a tenfold underestimate
B. It is and causes a tenfold overestimate
C. It is and causes a hundredfold underestimate
D. It is and causes a tenfold overestimate

56 One milliliter of sample is added to 99 mL of diluent, followed by transferring into 9.9 mL of diluent. A aliquot from the final tube gives 75 colonies. What is the original concentration?

To determine total count of bacteria by spread plate technique Hard
A.
B.
C.
D.

57 Two plates from the same dilution contain 118 and 182 colonies, although the plating volumes and dilution labels are identical. Which response is most appropriate before reporting a result?

To determine total count of bacteria by spread plate technique Hard
A. Report the lower plate because it avoids crowding
B. Report the higher plate because it is more sensitive
C. Investigate mixing and plating variability before averaging
D. Multiply both counts by their difference and report the product

58 A sample is spread immediately after adding inoculum, but the agar surface is visibly wet and colonies later appear as streaks or merged patches. Which factor most directly compromises enumeration?

To determine total count of bacteria by spread plate technique Hard
A. Excess surface moisture causing nonuniform cell distribution
B. Insufficient dilution causing every cell to remain dormant
C. High agar concentration preventing all colony formation
D. Low incubation temperature causing dilution reversal

59 A plate from a dilution contains 7 colonies after spreading . What is the most important limitation of reporting the resulting value without qualification?

To determine total count of bacteria by spread plate technique Hard
A. The dilution factor cannot be used with spread plates
B. The calculated value is mathematically impossible
C. The low count has high relative sampling uncertainty
D. The colonies necessarily represent dead bacterial cells

60 A dilution is prepared correctly, but the analyst accidentally records it as . A plate with 156 colonies is then used for calculation. How will the reported concentration compare with the true value?

To determine total count of bacteria by spread plate technique Hard
A. It will be ten times the true concentration
B. It will be equal to the true concentration
C. It will be one hundred times the true concentration
D. It will be one-tenth of the true concentration
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