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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. Protease
B. Cellulase
C. Lipase
D. Amylase

2 Congo Red binds specifically to which type of polysaccharide?

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

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. Cellulose has been degraded by cellulase
B. The medium is contaminated
C. Starch has been produced
D. The colony is dead

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. Carboxymethyl cellulose (CMC)
B. Glycerol
C. Sucrose
D. Glucose

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

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

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

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Molecular DNA screening
B. Qualitative plate-based screening
C. Quantitative spectrophotometric assay
D. Immunological 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. Lower cellulase activity
B. Higher cellulase activity
C. No cellulase activity
D. Higher amylase activity

9 Cellulase enzymes break down cellulose into:

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

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

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

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. Salmonella species
B. Staphylococcus species
C. Trichoderma species
D. Lactobacillus species

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

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Identify strains useful for industrial applications
B. Measure the pH of the medium
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. Dye that stains cellulose
D. Cellulase enzyme

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. Medium acidity
C. Colony age
D. Bacterial motility

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 cellulose-containing agar medium
C. A blood agar medium
D. A pure glucose 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. Does not produce cellulase
B. Produces high cellulase
C. Is a fast grower
D. Produces amylase

18 Cellulose is composed of repeating units of which sugar?

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

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

Screening of cellulase producing microorganisms using Congo Red Method Easy
A. Dependent on advanced sequencing
B. Simple, rapid, and inexpensive
C. Only usable for viruses
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. The center of active colonies
B. The clear halo region
C. Areas with intact cellulose
D. Areas where cellulose is degraded

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

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. Xylan
B. Carboxymethyl cellulose (CMC)
C. Pectin
D. Starch

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. Fix the enzyme so hydrolysis stops immediately
B. Increase dye binding to the remaining cellulose
C. Lyse the bacterial cells to release intracellular enzyme
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 B secretes more active cellulase than isolate A
B. Isolate A grows faster than isolate B
C. Isolate B binds more Congo Red than isolate A
D. Both isolates have identical cellulase activity

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. The smaller colony likely has higher relative cellulase activity
B. Colony size is irrelevant to any comparison
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. Zone size reflects diffusion and hydrolysis rather than exact enzyme concentration in defined units
B. The dye completely inhibits enzyme activity during measurement
C. It can only detect intracellular enzymes and misses secreted ones
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. Incubating at the organism's optimum temperature
B. Using NaCl for destaining
C. Adding CMC to the medium
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. Peptide bonds of the cell wall proteins
B. -1,4-glycosidic linked glucan structures
C. -1,4-glycosidic linkages of starch
D. Phosphodiester bonds of nucleic acids

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. Using a liquid broth instead of agar
B. Increasing agar concentration in the medium
C. Lowering the CMC concentration to zero
D. Removing Congo Red from the protocol

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. Protease activity on gelatin
B. Endoglucanase activity acting on internal -1,4 bonds
C. Amylase activity on starch
D. Lipase activity on triglycerides

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. Uniform red background with no clear zone around the colony
B. Complete decolorization of the entire plate
C. A blue-black halo surrounding the colony
D. A large clear zone equal to that of positive controls

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

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. No colonies can ever grow on the plate
B. Cellulase becomes permanently activated
C. The medium loses all its CMC content
D. The dye may inhibit growth or reduce contrast of hydrolysis zones

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. Isolate Y, because longer incubation means more enzyme
B. Neither can be considered a producer
C. Both are exactly equal regardless of time
D. Isolate X, since it produced the same zone in less time

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

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. Skip the Congo Red staining step
D. Incubate at a temperature that stops growth

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. A blue-black insoluble complex with the dye
B. Crystalline cellulose fibers
C. Shorter oligosaccharides and reducing sugars that no longer bind the dye
D. Intact high-molecular-weight cellulose

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. Autoclave time, dye brand, and room lighting
B. Colony color, plate label, and lid orientation
C. Enzyme activity, incubation time, and substrate concentration
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. Congo Red is enzymatically degraded by extracellular cellulase into a colorless product
B. Congo Red fails to bind hydrolyzed -1,4-glucan chains, so the degraded region remains unstained
C. Cellulase precipitates Congo Red as an insoluble complex around the colony
D. The colony secretes acids that bleach the bound Congo Red dye locally

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. Isolate A has a higher enzymatic index (4.0) than Isolate B (2.4), indicating greater relative cellulase activity
D. Zone size cannot be compared unless incubation times differ

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

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 neutralizes acids so the dye changes color to red
C. It destains excess unbound dye and stabilizes the dye-polysaccharide complex, sharpening zone contrast
D. It converts CMC into a form that binds Congo Red more strongly

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

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. The clearance zone measures cell growth, not enzyme diffusion
C. Large zones always correspond to high specific activity
D. High enzyme diffusion through the agar produced a large zone despite modest total activity

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. Extending incubation and applying the 1 M NaCl destain longer to enhance contrast
B. Increasing Congo Red concentration to 5% to overstain the plate
C. Autoclaving the plate after dye application
D. Removing CMC entirely from the medium

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. Extended -1,4-linked glucan chains with a linear conformation
C. Short mono- and disaccharide units
D. Highly branched -1,6 glycosidic networks

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

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. Average the merged zone diameters as a single value
D. Re-streak at lower colony density and measure zones before they overlap

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 heat-inactivated culture or non-cellulolytic strain that produces no zone on CMC
B. A plate incubated at C only
C. A plate stained without NaCl destaining
D. A plate with no CMC and no organism

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

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

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

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

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

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. The organism cannot grow at C so zones are impossible
D. Congo Red binds CMC only at high temperature

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. NaCl reversed all hydrolysis at the edge
C. Congo Red was applied at the wrong wavelength
D. The colony secreted no enzyme at all

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

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