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Unit 10: Microbial biotechnology II - Practice Quiz

BTY555 — Biotechnology Laboratory-I 60 Questions
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1 The Congo Red method is primarily used to screen microorganisms that produce which enzyme?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Lipase
B. Amylase
C. Protease
D. Cellulase

2 Congo Red binds specifically to which type of polysaccharide?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Simple sugars like glucose
B. Lipid molecules
C. -1,4-linked polysaccharides like cellulose
D. -1,4-linked starch only

3 In the Congo Red assay, a clear zone around a microbial colony indicates:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. The medium is contaminated
B. Starch has been produced
C. The colony is dead
D. Cellulose has been degraded by cellulase

4 Which carbon source is typically added to the agar medium for cellulase screening by the Congo Red method?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Glucose
B. Glycerol
C. Sucrose
D. Carboxymethyl cellulose (CMC)

5 After incubation, the plates are usually flooded with a solution of:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Iodine solution
B. Methylene blue
C. Congo Red dye
D. Gram's crystal violet

6 Which solution is commonly used to wash or destain the plates after Congo Red staining?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. 1 M NaCl solution
B. Acetic acid
C. Distilled water only
D. Ethanol

7 The Congo Red method is an example of which type of screening?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Qualitative plate-based screening
B. Immunological screening
C. Quantitative spectrophotometric assay
D. Molecular DNA screening

8 A larger clear zone around a colony in the Congo Red assay generally suggests:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. No cellulase activity
B. Higher cellulase activity
C. Lower cellulase activity
D. Higher amylase activity

9 Cellulase enzymes break down cellulose into:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Fatty acids
B. Amino acids
C. Glucose units
D. Nucleotides

10 Why does the intact cellulose region appear red after staining?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. The bacteria produce a red pigment
B. The dye reacts with glucose
C. Congo Red binds to undegraded cellulose
D. Cellulose reflects red light naturally

11 The clear zone formed around a cellulase-producing colony is commonly called a:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Growth zone
B. Colony margin
C. Inhibition ring
D. Halo or clearance zone

12 Which of the following microorganisms is a well-known cellulase producer often screened using this method?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Trichoderma species
B. Lactobacillus species
C. Staphylococcus species
D. Salmonella species

13 The main purpose of screening cellulase-producing microorganisms is to:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Measure the pH of the medium
B. Identify strains useful for industrial applications
C. Kill unwanted bacteria
D. Produce antibiotics

14 Congo Red is best described as a:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Growth nutrient
B. Selective antibiotic
C. Cellulase enzyme
D. Dye that stains cellulose

15 The ratio of clear zone diameter to colony diameter is often used to express:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Relative cellulase activity
B. Bacterial motility
C. Medium acidity
D. Colony age

16 The Congo Red method requires the microorganisms to be grown on:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. A nutrient broth only
B. A pure glucose medium
C. A blood agar medium
D. A cellulose-containing agar medium

17 If no clear zone forms around a colony after Congo Red staining, it indicates that the organism:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Is a fast grower
B. Produces high cellulase
C. Produces amylase
D. Does not produce cellulase

18 Cellulose is composed of repeating units of which sugar?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Ribose
B. Fructose
C. Glucose
D. Galactose

19 The Congo Red screening method is popular because it is:

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Only usable for viruses
B. Dependent on advanced sequencing
C. Simple, rapid, and inexpensive
D. Highly complex and costly

20 In the Congo Red assay, which region of the plate remains stained red?

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Areas where cellulose is degraded
B. Areas with intact cellulose
C. The center of active colonies
D. The clear halo region

21 In the Congo Red method for screening cellulase producers, a clear zone (halo) appears around a colony because the dye:

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Binds strongly to intact -1,4-glucan chains but not to the hydrolyzed products
B. Stains only the microbial colony and leaves the medium unstained
C. Diffuses away from regions of high enzyme concentration during incubation
D. Reacts chemically with cellulase enzyme to form a colored complex

22 Which substrate is most commonly incorporated into the agar medium for Congo Red-based cellulase screening?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Carboxymethyl cellulose (CMC)
B. Starch
C. Pectin
D. Xylan

23 After staining CMC plates with Congo Red, a wash with NaCl is performed. The primary purpose of this NaCl wash is to:

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Increase dye binding to the remaining cellulose
B. Lyse the bacterial cells to release intracellular enzyme
C. Fix the enzyme so hydrolysis stops immediately
D. Destain unbound dye and enhance contrast of the clear zones

24 Two isolates, A and B, form clear zones of and diameter respectively around colonies of equal size. What is the most reasonable interpretation?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Isolate A grows faster than isolate B
B. Both isolates have identical cellulase activity
C. Isolate B secretes more active cellulase than isolate A
D. Isolate B binds more Congo Red than isolate A

25 The ratio commonly used to compare cellulolytic potential among isolates is the Hydrolysis Capacity (HC) value, calculated as:

Screening of cellulase producing microorganisms using Congo Red Method Medium
A.
B.
C.
D.

26 A student observes a large colony with only a very small clear zone, while a smaller colony shows a large zone. Using the HC value, which conclusion is correct?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Colony size is irrelevant to any comparison
B. The smaller colony likely has higher relative cellulase activity
C. Zone size alone determines the better producer
D. The larger colony always has higher enzyme activity

27 Why is Congo Red staining considered a qualitative-to-semi-quantitative screening method rather than a precise assay of cellulase units?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. It can only detect intracellular enzymes and misses secreted ones
B. Zone size reflects diffusion and hydrolysis rather than exact enzyme concentration in defined units
C. The dye completely inhibits enzyme activity during measurement
D. It requires purified enzyme to produce any visible zone

28 Which of the following would most likely produce a false-negative (no clear zone) result during Congo Red screening of a true cellulase producer?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Adding CMC to the medium
B. Incubating at the organism's optimum temperature
C. Using NaCl for destaining
D. Insufficient incubation time before staining

29 The chemical basis for Congo Red binding to cellulose (CMC) is best described as an interaction with:

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Phosphodiester bonds of nucleic acids
B. Peptide bonds of the cell wall proteins
C. -1,4-glycosidic linked glucan structures
D. -1,4-glycosidic linkages of starch

30 During screening, a researcher wants to reduce diffusion of secreted enzyme so that zones stay compact and comparable. Which adjustment helps most?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Removing Congo Red from the protocol
B. Using a liquid broth instead of agar
C. Lowering the CMC concentration to zero
D. Increasing agar concentration in the medium

31 In a plate assay, which enzyme activity is primarily responsible for creating the clear zone on CMC agar?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Amylase activity on starch
B. Lipase activity on triglycerides
C. Endoglucanase activity acting on internal -1,4 bonds
D. Protease activity on gelatin

32 A control plate inoculated with a known non-cellulolytic bacterium is included in the screening. What result is expected after Congo Red staining?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. A blue-black halo surrounding the colony
B. A large clear zone equal to that of positive controls
C. Complete decolorization of the entire plate
D. Uniform red background with no clear zone around the colony

33 Why is Congo Red screening often preferred as an initial step before quantitative enzyme assays in a cellulase discovery workflow?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. It allows rapid, low-cost visual screening of many isolates on plates before detailed assays are run on the best candidates
B. It gives the exact international units of enzyme activity directly
C. It only works with genetically engineered strains
D. It replaces the need for any downstream enzyme quantification

34 If Congo Red is added directly into the molten agar before pouring (instead of flooding after growth), a common drawback is that:

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Cellulase becomes permanently activated
B. The medium loses all its CMC content
C. The dye may inhibit growth or reduce contrast of hydrolysis zones
D. No colonies can ever grow on the plate

35 Two isolates give the same clear zone diameter, but isolate X reached that zone after and isolate Y after of incubation. Which is the more active cellulase producer?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Neither can be considered a producer
B. Isolate X, since it produced the same zone in less time
C. Both are exactly equal regardless of time
D. Isolate Y, because longer incubation means more enzyme

36 Which of the following best explains why NaCl, rather than water, is preferred for the destaining wash?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Salt disrupts weak dye interactions and stabilizes specific Congo Red-cellulose binding for clearer zones
B. Salt dissolves the agar completely to reveal colonies
C. Water cannot remove any Congo Red at all
D. Salt converts Congo Red into a colorless product

37 A fungal isolate produces a diffuse, poorly defined clear zone that overlaps with neighboring colonies. The best corrective action for reliable screening is to:

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Remove CMC from the medium entirely
B. Streak isolates farther apart or use spot inoculation with fewer colonies per plate
C. Incubate at a temperature that stops growth
D. Skip the Congo Red staining step

38 The clear zone in the Congo Red method indicates the region where CMC has been converted primarily into:

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Crystalline cellulose fibers
B. Shorter oligosaccharides and reducing sugars that no longer bind the dye
C. Intact high-molecular-weight cellulose
D. A blue-black insoluble complex with the dye

39 Which combination of factors most directly influences the diameter of the clear zone observed in this screening method?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A. Colony color, plate label, and lid orientation
B. Enzyme activity, incubation time, and substrate concentration
C. Autoclave time, dye brand, and room lighting
D. Media pH indicator, agar shape, and inoculum color

40 A colony shows a clear zone diameter of and a colony diameter of . What is its Hydrolysis Capacity (HC) value?

Screening of cellulase producing microorganisms using Congo Red Method Medium
A.
B.
C.
D.

41 In the Congo Red screening assay, a clear halo forms around a cellulolytic colony after staining. What is the fundamental molecular basis for this zone of clearance?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. The colony secretes acids that bleach the bound Congo Red dye locally
B. Cellulase precipitates Congo Red as an insoluble complex around the colony
C. Congo Red is enzymatically degraded by extracellular cellulase into a colorless product
D. Congo Red fails to bind hydrolyzed -1,4-glucan chains, so the degraded region remains unstained

42 Two isolates produce identical colony diameters (5 mm) but different clearance zones: Isolate A has a 20 mm zone, Isolate B has a 12 mm zone. Which conclusion about relative cellulolytic activity is most defensible?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Isolate B is more active because a smaller zone reflects concentrated enzyme
B. Both are equal because colony diameters are identical
C. Zone size cannot be compared unless incubation times differ
D. Isolate A has a higher enzymatic index (4.0) than Isolate B (2.4), indicating greater relative cellulase activity

43 A student uses starch-agar plates instead of CMC-agar for Congo Red cellulase screening and obtains large clearance zones from many colonies. What is the correct interpretation?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. No zones should form because Congo Red does not bind starch
B. The zones confirm cellulase because Congo Red is specific to any hydrolysis
C. The zones reflect amylase activity on starch, not cellulase, giving false positives
D. Starch enhances cellulase secretion, so results are valid

44 Why is a NaCl (typically 1 M) wash applied after flooding CMC plates with Congo Red solution?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. It kills the microbial colonies to stop further hydrolysis
B. It converts CMC into a form that binds Congo Red more strongly
C. It neutralizes acids so the dye changes color to red
D. It destains excess unbound dye and stabilizes the dye-polysaccharide complex, sharpening zone contrast

45 An isolate shows a distinct clearance zone at pH 5.0 CMC plates but almost none at pH 8.0 plates. What does this most strongly indicate?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Alkaline pH permanently denatures Congo Red dye
B. The organism grows only at pH 8.0 and dies at pH 5.0
C. Congo Red does not bind CMC at alkaline pH, so zones vanish artificially
D. The secreted cellulase has an acidic pH optimum, reducing activity under alkaline conditions

46 A researcher observes a large clearance zone but subsequent enzyme assays show low CMCase activity. Which explanation best reconciles this discrepancy?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Congo Red produces zones only for high-activity enzymes, so the assay is wrong
B. High enzyme diffusion through the agar produced a large zone despite modest total activity
C. Large zones always correspond to high specific activity
D. The clearance zone measures cell growth, not enzyme diffusion

47 In a Congo Red overlay method, gram-positive filamentous actinomycetes give faint, irregular zones. What experimental adjustment most improves zone clarity?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Autoclaving the plate after dye application
B. Removing CMC entirely from the medium
C. Extending incubation and applying the 1 M NaCl destain longer to enhance contrast
D. Increasing Congo Red concentration to 5% to overstain the plate

48 Congo Red binds preferentially to polysaccharides with which structural feature, explaining its use for cellulose-type substrates?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Sulfated glycosaminoglycan backbones only
B. Short mono- and disaccharide units
C. Highly branched -1,6 glycosidic networks
D. Extended -1,4-linked glucan chains with a linear conformation

49 Why can the Congo Red plate assay underestimate the activity of organisms producing predominantly exoglucanases (cellobiohydrolases)?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Congo Red binds only to cellobiose, inflating exoglucanase zones
B. Exoglucanases cannot diffuse through agar at all
C. Exoglucanases degrade Congo Red directly, masking the zone
D. Exoglucanases act on chain ends releasing cellobiose, causing less disruption of Congo Red binding than endoglucanases cleaving internal bonds

50 A plate incubated too long shows merged clearance zones from adjacent colonies. What is the best corrective measure for reliable enzymatic index calculation?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Add more CMC to slow hydrolysis
B. Increase Congo Red concentration to separate the zones
C. Re-streak at lower colony density and measure zones before they overlap
D. Average the merged zone diameters as a single value

51 Which control is most essential to confirm that clearance zones in a Congo Red assay result from enzymatic hydrolysis rather than a physical artifact?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. A plate stained without NaCl destaining
B. A plate with no CMC and no organism
C. A heat-inactivated culture or non-cellulolytic strain that produces no zone on CMC
D. A plate incubated at C only

52 An isolate grown on glucose-supplemented CMC plates shows no clearance zone, but the same isolate on CMC-only plates shows a large zone. What best explains this?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Glucose binds Congo Red and prevents zone formation
B. Glucose denatures Congo Red at incubation temperature
C. CMC-only plates lack nutrients, forcing false zones
D. Glucose repressed cellulase synthesis via catabolite repression, so little enzyme was produced

53 When comparing enzymatic indices across a large screen, why is the ratio (zone/colony diameter) preferred over absolute zone diameter alone?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. It eliminates the need for a substrate in the medium
B. It normalizes for differences in colony size and biomass, allowing fairer comparison of relative activity
C. It corrects for Congo Red concentration differences between plates
D. It converts diffusion into an absolute enzyme concentration

54 A lab reports that Congo Red at high concentration itself inhibits colony growth on CMC plates. What is the recommended protocol modification to avoid biasing screening results?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Double the Congo Red concentration to overcome inhibition
B. Omit CMC so growth is unaffected
C. Grow colonies first, then flood mature plates with Congo Red rather than incorporating dye into the medium
D. Incubate plates in the dye solution from inoculation onward

55 Two students obtain different zone sizes for the same isolate; one used 0.1% CMC and the other 1.0% CMC. Which statement correctly interprets this?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Lower CMC always gives smaller zones regardless of enzyme
B. Higher CMC concentration typically yields sharper, sometimes smaller zones because more substrate must be hydrolyzed to clear dye binding
C. Congo Red does not bind at 1.0% CMC, so no zone forms
D. CMC concentration has no effect on zone size

56 In quantitative follow-up, why should a Congo Red plate hit be confirmed with a reducing-sugar assay (e.g., DNS method) before declaring a strain a strong cellulase producer?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. DNS replaces the need for CMC substrate entirely
B. The DNS method measures Congo Red binding directly
C. Plate zones cannot detect any enzyme, so DNS is the only valid test
D. The plate assay only indicates relative activity and diffusion; DNS quantifies actual released reducing sugars from CMC hydrolysis

57 A thermophilic isolate shows minimal zones at C incubation but large zones at C. What does this reveal about the screening design?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Higher temperature always increases dye diffusion, unrelated to enzyme
B. Incubation temperature must match the enzyme's thermal optimum, or activity is underestimated
C. Congo Red binds CMC only at high temperature
D. The organism cannot grow at C so zones are impossible

58 Why might a truly cellulolytic organism produce a clear zone that is not visibly bounded by a sharp edge even after NaCl destaining?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. A gradient of partial hydrolysis at the zone periphery creates diffuse dye binding rather than an abrupt boundary
B. The colony secreted no enzyme at all
C. Congo Red was applied at the wrong wavelength
D. NaCl reversed all hydrolysis at the edge

59 A metagenomic library of environmental clones is screened on CMC-Congo Red plates. What is the primary advantage of this functional screening over sequence-based prediction of cellulase genes?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. It guarantees the exact enzyme kinetics of each clone
B. It requires no substrate in the medium
C. It only detects previously characterized cellulase genes
D. It detects functionally active cellulases regardless of sequence novelty, capturing enzymes with no homology to known genes

60 During screening, a plate shows uniform faint pink coloration with no discrete zones, though colonies grew well. Which troubleshooting step addresses the most likely cause?

Screening of cellulase producing microorganisms using Congo Red Method Hard
A. Remove CMC from the recipe to see zones
B. Repeat with adequate Congo Red staining time and proper NaCl destaining, since insufficient staining or destaining flattens contrast
C. Lower the incubation temperature to C
D. Add glucose to induce cellulase
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Unit 9