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Unit 5: Purification of bacterial culture - Practice Quiz

BTY331 — Microbiology Laboratory 60 Questions
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1 What is the main purpose of the streak plate method?

To purify the bacterial culture by streak plate method. Easy
A. To identify bacterial shape
B. To obtain isolated bacterial colonies
C. To increase the culture temperature
D. To measure bacterial movement

2 Which instrument is commonly used to streak bacteria on an agar plate?

To purify the bacterial culture by streak plate method. Easy
A. Inoculating loop
B. Graduated cylinder
C. Glass thermometer
D. Microscope slide

3 What type of medium is commonly used for the streak plate method?

To purify the bacterial culture by streak plate method. Easy
A. Staining solution
B. Distilled water
C. Liquid broth medium
D. Solid agar medium

4 Why is the inoculating loop sterilized before use?

To purify the bacterial culture by streak plate method. Easy
A. To make bacteria grow faster
B. To change the colony color
C. To increase the agar thickness
D. To remove unwanted microorganisms

5 What should be done to the loop before it touches the bacterial culture?

To purify the bacterial culture by streak plate method. Easy
A. Allow it to cool
B. Dip it in oil
C. Place it in water
D. Cover it with agar

6 What is the basic action performed during streaking?

To purify the bacterial culture by streak plate method. Easy
A. Dragging the loop across agar
B. Shaking the plate strongly
C. Scraping the agar away
D. Pouring broth over agar

7 Why are different sections of the agar plate streaked in sequence?

To purify the bacterial culture by streak plate method. Easy
A. To change the medium color
B. To dilute the bacterial cells
C. To increase the plate volume
D. To soften the agar surface

8 What does a well-isolated colony usually develop from?

To purify the bacterial culture by streak plate method. Easy
A. A single bacterial cell
B. A drop of stain
C. A piece of glass
D. A section of empty agar

9 What is the appearance of a pure culture on a streak plate generally expected to be?

To purify the bacterial culture by streak plate method. Easy
A. Colonies with similar features
B. Colonies of different organisms
C. No visible colonies
D. Colonies with many colors

10 How should the lid of an agar plate be handled during streaking?

To purify the bacterial culture by streak plate method. Easy
A. Open it slightly when necessary
B. Seal it before streaking
C. Remove it for the entire procedure
D. Place it under the culture

11 Why should the agar surface not be gouged during streaking?

To purify the bacterial culture by streak plate method. Easy
A. It can sterilize the culture
B. It can remove all bacteria
C. It can increase the incubation time
D. It can damage the agar surface

12 What should be used to label a culture plate?

To purify the bacterial culture by streak plate method. Easy
A. A thermometer and a ruler
B. A pipette and a burner
C. A stain and a coverslip
D. A marker and sample details

13 Where is the label usually written on a culture plate?

To purify the bacterial culture by streak plate method. Easy
A. On the bottom of the plate
B. On the inside of the lid
C. On the edge of the loop
D. On the agar surface

14 Why are inoculated agar plates commonly incubated upside down?

To purify the bacterial culture by streak plate method. Easy
A. To increase agar melting
B. To prevent condensation drops
C. To expose the culture to dust
D. To stop bacterial growth

15 What is a colony on an agar plate?

To purify the bacterial culture by streak plate method. Easy
A. A mark made by the loop
B. A visible mass of bacterial cells
C. A drop of culture medium
D. A piece of unused agar

16 Which area of a streak plate is most likely to contain isolated colonies?

To purify the bacterial culture by streak plate method. Easy
A. The unused lid area
B. The plate label area
C. The final streaked area
D. The initial inoculation area

17 What does contamination mean in a bacterial culture?

To purify the bacterial culture by streak plate method. Easy
A. Growth of the intended bacterium
B. Loss of the plate label
C. Growth of unwanted microorganisms
D. Formation of solid agar

18 Why should aseptic technique be used during streaking?

To purify the bacterial culture by streak plate method. Easy
A. To prevent contamination
B. To increase agar volume
C. To remove the culture label
D. To change bacterial shape

19 After isolated colonies appear, what can be used to start a purified culture?

To purify the bacterial culture by streak plate method. Easy
A. A single isolated colony
B. The empty plate lid
C. The agar plate label
D. A condensation drop

20 What is the main reason for flaming or sterilizing the loop between streaking steps?

To purify the bacterial culture by streak plate method. Easy
A. To reduce cells carried forward
B. To increase the number of contaminants
C. To melt the agar completely
D. To dry the culture plate

21 What is the main purpose of flaming the inoculating loop between streaking sections?

To purify the bacterial culture by streak plate method Medium
A. To increase the number of bacterial cells transferred
B. To make the inoculum spread more rapidly
C. To prevent the agar surface from becoming too dry
D. To sterilize the loop before reducing the bacterial load

22 Why is a small amount of inoculum preferred when beginning a streak plate?

To purify the bacterial culture by streak plate method Medium
A. It allows bacteria to grow only in the first section
B. It prevents the agar from absorbing any culture fluid
C. It prevents excessive growth and supports colony separation
D. It ensures that every colony receives identical nutrients

23 A student obtains growth covering the entire agar surface after streaking. Which procedural error most likely caused this result?

To purify the bacterial culture by streak plate method Medium
A. The plate was incubated with the lid uppermost
B. The agar medium contained an appropriate nutrient level
C. The plate was labeled before the culture was inoculated
D. The loop was not sterilized between streaking sections

24 Why should the loop be cooled after flaming before it touches the culture?

To purify the bacterial culture by streak plate method Medium
A. A hot loop may prevent the culture from mixing with air
B. A hot loop may cause colonies to develop too slowly
C. A hot loop may kill the cells being transferred
D. A hot loop may increase the agar's nutrient concentration

25 Which streaking pattern is most effective for progressively diluting bacteria across an agar plate?

To purify the bacterial culture by streak plate method Medium
A. A single continuous line across the entire plate
B. A series of sections separated by loop sterilization
C. Parallel strokes made without changing the inoculum
D. Repeated circular motions in the center of the plate

26 During a four-quadrant streak, where should the first streaks be made?

To purify the bacterial culture by streak plate method Medium
A. Along the edge using only the unused medium
B. At the plate center using a sterilized loop
C. In the first quadrant using the original inoculum
D. In the final quadrant using the original inoculum

27 When starting a new streaking section, why should the loop pass through only a small part of the previous section?

To purify the bacterial culture by streak plate method Medium
A. To keep bacterial concentration equal in every section
B. To transfer enough cells while maintaining progressive dilution
C. To avoid exposing the agar to any bacterial cells
D. To collect the maximum number of cells from the plate

28 A plate has isolated colonies only in the fourth section. What does this result indicate?

To purify the bacterial culture by streak plate method Medium
A. The dilution across sections was sufficient for isolation
B. The original culture contained no viable bacterial cells
C. The agar surface was completely unsuitable for bacterial growth
D. The loop should have been sterilized after every single stroke

29 Why is the Petri plate commonly incubated in an inverted position after streaking?

To purify the bacterial culture by streak plate method Medium
A. To reduce condensation dripping onto the streaked agar
B. To prevent the bacterial cells from contacting nutrients
C. To make the inoculating loop remain sterile during growth
D. To increase oxygen entry through the agar surface

30 A student presses the loop deeply into the agar while streaking. What is the most likely consequence?

To purify the bacterial culture by streak plate method Medium
A. The agar may be gouged, interfering with uniform streaking
B. The plate may contain fewer nutrients after incubation
C. The bacteria may become genetically identical immediately
D. The loop may produce isolated colonies without dilution

31 Which colony should be selected to prepare a purified subculture?

To purify the bacterial culture by streak plate method Medium
A. A colony touching several neighboring colonies
B. The heaviest growth near the original inoculation point
C. A mixed-looking area with several colony appearances
D. A well-isolated colony with typical growth characteristics

32 If a mixed culture produces colonies with two distinct morphologies, what is the best next step for purification?

To purify the bacterial culture by streak plate method Medium
A. Continue incubating the same plate until one type disappears
B. Combine both colony types and incubate them in broth
C. Pick one isolated colony of each type and restreak separately
D. Select the largest colony and discard the remaining plate

33 What is the primary reason for keeping the Petri dish lid open only briefly during streaking?

To purify the bacterial culture by streak plate method Medium
A. To limit contamination from airborne microorganisms
B. To stop all bacterial growth during inoculation
C. To prevent the agar from receiving sufficient oxygen
D. To keep the inoculating loop from cooling

34 A student streaks all four quadrants with the same unsterilized loop. Which change would most improve colony isolation?

To purify the bacterial culture by streak plate method Medium
A. Flame and cool the loop between successive quadrants
B. Press the loop more firmly into each quadrant
C. Use a larger inoculum in the first quadrant
D. Incubate the plate for a shorter period only

35 Why should the culture tube be opened near a flame or within an appropriate aseptic workspace?

To purify the bacterial culture by streak plate method Medium
A. To make the medium solidify before inoculation
B. To reduce the chance of introducing environmental contaminants
C. To ensure that all cells remain attached to the loop
D. To increase the bacterial concentration in the tube

36 What is the purpose of labeling the bottom of the agar plate before inoculation?

To purify the bacterial culture by streak plate method Medium
A. To identify the sample while the lid can be exchanged safely
B. To increase the thickness of the agar during incubation
C. To sterilize the surface before the loop is introduced
D. To prevent bacterial cells from spreading between quadrants

37 A streak plate has no growth in any section, although the source culture was viable. Which error is most likely?

To purify the bacterial culture by streak plate method Medium
A. The plate was streaked in several separate sections
B. The loop was lightly drawn across the agar surface
C. The lid was opened for a limited time during inoculation
D. The loop was used while still hot after flaming

38 Why is it preferable to streak near the agar surface rather than dig into the medium?

To purify the bacterial culture by streak plate method Medium
A. Surface streaking eliminates the need for loop sterilization
B. Surface streaking prevents all contact between cells and oxygen
C. Surface streaking causes every transferred cell to form a colony
D. Surface streaking distributes cells without damaging the agar

39 Which observation best supports that a streak plate culture is pure?

To purify the bacterial culture by streak plate method Medium
A. The plate has colonies with different colors and textures
B. Isolated colonies show consistent morphology and staining
C. The culture grows rapidly only near the first streak
D. The plate contains dense growth with several colony types

40 If the final streaking section has no colonies but earlier sections have heavy growth, what is the most likely explanation?

To purify the bacterial culture by streak plate method Medium
A. Too few cells were transferred from the preceding section
B. The first section contained no viable bacterial cells
C. The final section received the largest inoculum on the plate
D. The agar became more nutrient-rich in the earlier sections

41 A mixed culture contains two colony types, but only a small number of cells are available. Which streaking strategy best maximizes the probability of obtaining isolated colonies while preserving viable cells?

To purify the bacterial culture by streak plate method. Hard
A. Spread the entire inoculum densely over one quadrant to avoid losing cells.
B. Apply the inoculum to every quadrant before beginning the dilution streak.
C. Use repeated parallel streaks without sterilizing the loop between sectors.
D. Use a four-quadrant streak with progressive dilution and sterilize the loop between quadrants.

42 After incubation, the first quadrant has confluent growth, the second has heavy growth, and the third and fourth have no colonies. Which procedural error most directly explains this pattern?

To purify the bacterial culture by streak plate method. Hard
A. The plate was incubated in an inverted position.
B. The loop was sterilized after the second quadrant and not cooled before reuse.
C. The plate was incubated at a temperature slightly below the optimum.
D. The agar surface was too dry for bacterial growth.

43 A student obtains isolated colonies, but colonies from both morphologies are still present on the final streak area. What is the most appropriate next action?

To purify the bacterial culture by streak plate method. Hard
A. Repeat the same plate incubation without changing the inoculum.
B. Use the entire final streak area as the purified inoculum.
C. Select one well-isolated colony of the desired morphology and restreak it.
D. Combine several colonies of the desired morphology before making a new streak.

44 Which observation most strongly indicates that a streak plate has produced isolated colonies rather than merely small aggregates of the original inoculum?

To purify the bacterial culture by streak plate method. Hard
A. Colonies are all located near the initial inoculation site.
B. Colonies have identical pigmentation and approximate diameter.
C. Colonies are separated by visible areas of uninoculated agar.
D. Colonies appear only along the heaviest streak lines.

45 A plate shows two colony types, but one type occurs only where the streak lines overlap. Which conclusion is most scientifically justified?

To purify the bacterial culture by streak plate method. Hard
A. The second type is definitely a contaminant introduced during incubation.
B. The second type may be a mixed-colony artifact and requires restreaking for confirmation.
C. The second type can be ignored because overlapping streaks do not affect purity.
D. The second type must be a sector-specific variant of the first organism.

46 Why should the loop be sterilized and cooled before entering each new streak sector?

To purify the bacterial culture by streak plate method. Hard
A. Sterilization removes oxygen, while cooling prevents condensation on the lid.
B. Sterilization prevents carryover, while cooling preserves viability during transfer.
C. Sterilization increases nutrient concentration, while cooling prevents agar dehydration.
D. Sterilization improves colony color, while cooling increases bacterial motility.

47 A student streaks deeply into soft agar and produces grooves that collect inoculum. What is the most likely effect on purification?

To purify the bacterial culture by streak plate method. Hard
A. The deeper inoculation increases oxygen diffusion and improves isolation.
B. Cells become more genetically uniform because grooves restrict colony expansion.
C. Grooves selectively remove contaminants while retaining the desired organism.
D. Cells accumulate in grooves, reducing even dilution and impairing isolation.

48 A student touches the original inoculum repeatedly while streaking every quadrant. The result is confluent growth throughout the plate. Which modification is most effective?

To purify the bacterial culture by streak plate method. Hard
A. Streak each quadrant from the center toward the original inoculation point.
B. Flame or sterilize the loop between sectors and begin each sector from the preceding edge.
C. Incubate the plate longer so confluent growth separates into colonies.
D. Increase the inoculum volume so that isolated colonies form more rapidly.

49 A plate contains isolated colonies of one morphology and several tiny colonies near the edge of the agar. Which interpretation is most appropriate before selecting a colony?

To purify the bacterial culture by streak plate method. Hard
A. The isolated colonies are necessarily contaminated because all edge colonies are abnormal.
B. The tiny colonies should always be selected because slower growth indicates greater purity.
C. The edge colonies are necessarily the same organism because they grew on the same medium.
D. The edge colonies may represent contaminants or edge effects and require comparison with controls.

50 Why is selecting a colony from the region of greatest isolation preferable to selecting one from the initial streak area?

To purify the bacterial culture by streak plate method. Hard
A. It ensures that the colony contains more cells than an early colony.
B. It prevents all future mutations during subculture.
C. It guarantees that the selected cells have the fastest growth rate.
D. It lowers the chance that cells from neighboring organisms are transferred together.

51 A pure culture is streaked correctly, but no colonies appear. The uninoculated control is also sterile. Which explanation is most likely if the inoculum came from an old culture?

To purify the bacterial culture by streak plate method. Hard
A. The absence of colonies confirms that the culture was genetically pure.
B. The plate must contain too many nutrients for the organism to grow.
C. The sterile control proves that the streaking loop was contaminated.
D. The source culture may have lost viability or entered a nonrecoverable state.

52 Which plate design provides the strongest evidence that apparent colony purity is not caused by contamination during handling?

To purify the bacterial culture by streak plate method. Hard
A. A test streak incubated without labeling the inoculum source.
B. A test streak accompanied by an uninoculated sterility-control plate.
C. A test streak incubated only after the lid has been left open.
D. A test streak made with the loop repeatedly returned to the broth.

53 Two colonies look identical on a nonselective agar plate, but one may be a contaminant with similar morphology. Which approach best addresses this uncertainty?

To purify the bacterial culture by streak plate method. Hard
A. Discard the plate because similar morphology makes purification impossible.
B. Assume both colonies are identical because morphology is a sufficient identity test.
C. Pool both colonies to increase the chance of recovering the intended organism.
D. Restreak separate colonies and verify purity using additional appropriate tests.

54 A student uses a very small inoculum but makes only one short streak line. Why may this fail to purify the culture?

To purify the bacterial culture by streak plate method. Hard
A. The cells become nonviable whenever the streak line is straight.
B. Short streaks sterilize the agar surface and prevent all bacterial growth.
C. A small inoculum always produces genetically unstable colonies.
D. The cells are not distributed across enough surface area to separate mixed populations.

55 A student obtains isolated colonies only after incubating a plate with a wet surface. Which concern is most important when interpreting the result?

To purify the bacterial culture by streak plate method. Hard
A. Moisture guarantees that every isolated colony originated from one cell.
B. Wet agar selectively prevents contaminants from forming colonies.
C. Surface moisture may allow cells to spread, making apparent isolation unreliable.
D. Condensation permanently sterilizes regions where droplets accumulate.

56 Which sequence best represents an effective four-sector streak for purifying a mixed bacterial culture?

To purify the bacterial culture by streak plate method. Hard
A. Inoculate sector one, sterilize and cool the loop, drag from sector one into sector two, then repeat progressively.
B. Sterilize the loop after every stroke, avoiding contact with the previous sector entirely.
C. Begin in sector four, return to sector one, and increase inoculum with each sector.
D. Inoculate all sectors heavily, sterilize the loop once, and incubate immediately.

57 A streak plate yields colonies with a gradual decrease in density, but none are fully isolated. Which change is most likely to improve the next attempt?

To purify the bacterial culture by streak plate method. Hard
A. Incubate at a higher temperature to force neighboring colonies apart.
B. Use more inoculum and retrace the first sector during every subsequent pass.
C. Avoid sterilizing the loop so that cells reach the final sector more efficiently.
D. Use fewer initial loopfuls and extend the later sectors farther into unused agar.

58 Several colonies from a supposedly purified streak plate are transferred to broth, and the broth later shows two colony morphologies on subculture. What is the best interpretation?

To purify the bacterial culture by streak plate method. Hard
A. The broth caused a mutation that reliably created a second species.
B. The original selection likely included more than one organism or the culture was not adequately verified.
C. The streak plate method cannot produce pure cultures under any conditions.
D. The two morphologies prove that the agar medium was completely sterile.

59 Why should the plate label identify the organism or sample, date, and relevant conditions before streaking?

To purify the bacterial culture by streak plate method. Hard
A. It sterilizes the area beneath the label and improves bacterial attachment.
B. It increases the number of isolated colonies by reducing agar evaporation.
C. It preserves traceability and prevents comparison of colonies from different cultures.
D. It ensures that all organisms on the plate express identical colony morphology.

60 A student opens the plate widely during streaking, and colonies later appear in the sterility control. Which process failure is most likely?

To purify the bacterial culture by streak plate method. Hard
A. The inoculum was too dilute, causing sterile-control colonies to become visible.
B. Excessive exposure allowed airborne or environmental cells to enter the control.
C. The loop was cooled too long, causing the control medium to become selective.
D. The streak sectors were too large, forcing contaminants to grow in the control.
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