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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 obtain isolated bacterial colonies
B. To increase the culture temperature
C. To identify bacterial shape
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. Graduated cylinder
B. Glass thermometer
C. Inoculating loop
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. Liquid broth medium
C. Distilled water
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 increase the agar thickness
B. To change the colony color
C. To remove unwanted microorganisms
D. To make bacteria grow faster

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. Place it in water
B. Allow it to cool
C. Dip it in oil
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. Shaking the plate strongly
B. Dragging the loop across agar
C. Pouring broth over agar
D. Scraping the agar away

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 soften the agar surface
C. To dilute the bacterial cells
D. To increase the plate volume

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 piece of glass
C. A drop of stain
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 of different organisms
B. No visible colonies
C. Colonies with many colors
D. Colonies with similar features

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

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

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

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

12 What should be used to label a culture plate?

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

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

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

14 Why are inoculated agar plates commonly incubated upside down?

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

15 What is a colony on an agar plate?

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

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 plate label area
B. The final streaked area
C. The unused lid 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. Formation of solid agar
B. Growth of unwanted microorganisms
C. Loss of the plate label
D. Growth of the intended bacterium

18 Why should aseptic technique be used during streaking?

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

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 condensation drop
B. The empty plate lid
C. The agar plate label
D. A single isolated colony

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 increase the number of contaminants
B. To reduce cells carried forward
C. To dry the culture plate
D. To melt the agar completely

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 prevent the agar surface from becoming too dry
C. To sterilize the loop before reducing the bacterial load
D. To make the inoculum spread more rapidly

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 ensures that every colony receives identical nutrients
B. It prevents excessive growth and supports colony separation
C. It allows bacteria to grow only in the first section
D. It prevents the agar from absorbing any culture fluid

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 loop was not sterilized between streaking sections
B. The plate was labeled before the culture was inoculated
C. The plate was incubated with the lid uppermost
D. The agar medium contained an appropriate nutrient level

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 kill the cells being transferred
B. A hot loop may prevent the culture from mixing with air
C. A hot loop may cause colonies to develop too slowly
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 series of sections separated by loop sterilization
B. A single continuous line across the entire plate
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. In the first quadrant using the original inoculum
B. Along the edge using only the unused medium
C. In the final quadrant using the original inoculum
D. At the plate center using a sterilized loop

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 avoid exposing the agar to any bacterial cells
C. To collect the maximum number of cells from the plate
D. To transfer enough cells while maintaining progressive dilution

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 loop should have been sterilized after every single stroke
B. The original culture contained no viable bacterial cells
C. The agar surface was completely unsuitable for bacterial growth
D. The dilution across sections was sufficient for isolation

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 make the inoculating loop remain sterile during growth
B. To reduce condensation dripping onto the streaked agar
C. To prevent the bacterial cells from contacting nutrients
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 bacteria may become genetically identical immediately
B. The agar may be gouged, interfering with uniform streaking
C. The plate may contain fewer nutrients after incubation
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. Pick one isolated colony of each type and restreak separately
B. Combine both colony types and incubate them in broth
C. Select the largest colony and discard the remaining plate
D. Continue incubating the same plate until one type disappears

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 stop all bacterial growth during inoculation
B. To prevent the agar from receiving sufficient oxygen
C. To keep the inoculating loop from cooling
D. To limit contamination from airborne microorganisms

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. Press the loop more firmly into each quadrant
B. Incubate the plate for a shorter period only
C. Flame and cool the loop between successive quadrants
D. Use a larger inoculum in the first quadrant

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 ensure that all cells remain attached to the loop
B. To increase the bacterial concentration in the tube
C. To reduce the chance of introducing environmental contaminants
D. To make the medium solidify before inoculation

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

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 lid was opened for a limited time during inoculation
B. The loop was lightly drawn across the agar surface
C. The plate was streaked in several separate sections
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 prevents all contact between cells and oxygen
B. Surface streaking eliminates the need for loop sterilization
C. Surface streaking distributes cells without damaging the agar
D. Surface streaking causes every transferred cell to form a colony

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

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

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

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. Use the entire final streak area as the purified inoculum.
B. Select one well-isolated colony of the desired morphology and restreak it.
C. Repeat the same plate incubation without changing the inoculum.
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 appear only along the heaviest streak lines.
B. Colonies are separated by visible areas of uninoculated agar.
C. Colonies have identical pigmentation and approximate diameter.
D. Colonies are all located near the initial inoculation site.

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

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 improves colony color, while cooling increases bacterial motility.
B. Sterilization increases nutrient concentration, while cooling prevents agar dehydration.
C. Sterilization removes oxygen, while cooling prevents condensation on the lid.
D. Sterilization prevents carryover, while cooling preserves viability during transfer.

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. Cells become more genetically uniform because grooves restrict colony expansion.
B. Cells accumulate in grooves, reducing even dilution and impairing isolation.
C. Grooves selectively remove contaminants while retaining the desired organism.
D. The deeper inoculation increases oxygen diffusion and improves 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. Incubate the plate longer so confluent growth separates into colonies.
C. Flame or sterilize the loop between sectors and begin each sector from the preceding edge.
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 edge colonies are necessarily the same organism because they grew on the same medium.
B. The edge colonies may represent contaminants or edge effects and require comparison with controls.
C. The tiny colonies should always be selected because slower growth indicates greater purity.
D. The isolated colonies are necessarily contaminated because all edge colonies are abnormal.

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 guarantees that the selected cells have the fastest growth rate.
B. It prevents all future mutations during subculture.
C. It lowers the chance that cells from neighboring organisms are transferred together.
D. It ensures that the colony contains more cells than an early colony.

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

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

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

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. Surface moisture may allow cells to spread, making apparent isolation unreliable.
C. Condensation permanently sterilizes regions where droplets accumulate.
D. Wet agar selectively prevents contaminants from forming colonies.

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

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. Avoid sterilizing the loop so that cells reach the final sector more efficiently.
C. Use fewer initial loopfuls and extend the later sectors farther into unused agar.
D. Use more inoculum and retrace the first sector during every subsequent pass.

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 original selection likely included more than one organism or the culture was not adequately verified.
B. The streak plate method cannot produce pure cultures under any conditions.
C. The broth caused a mutation that reliably created a second species.
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 ensures that all organisms on the plate express identical colony morphology.
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 sterilizes the area beneath the label and improves bacterial attachment.

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. The loop was cooled too long, causing the control medium to become selective.
C. The streak sectors were too large, forcing contaminants to grow in the control.
D. Excessive exposure allowed airborne or environmental cells to enter the control.
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