Unit 6: Isolation and Characterization of Biofertilizers - Practice Quiz

1 What is the main purpose of isolating beneficial microorganisms from soil or plant samples?

2 Which soil region is closely influenced by plant roots and commonly sampled for beneficial microorganisms?

3 Which method is commonly used to reduce the microbial concentration of a soil suspension before plating?

4 Which laboratory material provides nutrients for microorganisms to grow during isolation?

5 Which technique is commonly used to obtain separate bacterial colonies on solid medium?

6 Why are plant surfaces sterilized before isolating endophytic microorganisms?

7 Which instrument is commonly used to transfer a microbial culture aseptically?

8 What is the purpose of using a selective medium during microbial isolation?

9 Which medium is commonly used for isolating nitrogen-fixing Azotobacter species?

10 What does aseptic handling help prevent during microbial isolation?

11 Which characteristic describes the visible appearance of a microbial colony on agar?

12 Which technique is commonly used to classify bacteria as Gram-positive or Gram-negative?

13 What color do Gram-positive bacterial cells usually appear after Gram staining?

14 Which instrument is used to observe the shape of bacterial cells?

15 Which term describes a spherical bacterial cell?

16 Which test is used to determine whether a microorganism produces the enzyme catalase?

17 Which ability is an important beneficial trait of phosphate-solubilizing microorganisms?

18 A clear zone around a microbial colony on phosphate medium generally indicates what activity?

19 Which plant-growth-promoting substance can be produced by some beneficial rhizobacteria?

20 Why is a pure culture needed for reliable characterization of a beneficial microorganism?

21 Which soil sample is most suitable for isolating phosphate-solubilizing microorganisms associated with crop roots?

22 Why is serial dilution commonly used before plating a soil suspension for microbial isolation?

23 A researcher wants to isolate nitrogen-fixing bacteria that grow independently in soil. Which medium is most appropriate for primary selection?

24 What is the main purpose of surface sterilizing a plant root before isolating endophytic microorganisms?

25 After surface sterilizing a root, colonies grow from the final rinse water plated on nutrient agar. What does this result indicate?

26 Which sampling design would best improve the recovery of diverse beneficial microorganisms from a field?

27 A clear halo forms around a bacterial colony on a calcium-phosphate agar plate. What is the most reasonable preliminary interpretation?

28 Why should morphologically different colonies be subcultured several times during isolation?

29 Which approach is most suitable for isolating rhizobia from nodules of a legume?

30 A soil suspension produces too many colonies to count at the lowest dilution plated. What should the researcher do next?

31 Which combination provides the strongest initial evidence that an isolate could function as a phosphate-solubilizing biofertilizer?

32 An isolate grows well on nitrogen-free medium and gives a positive acetylene reduction test. What does this combination suggest?

33 Why is 16S rRNA gene sequencing commonly used to characterize a bacterial biofertilizer isolate?

34 An isolate produces indole-3-acetic acid only when tryptophan is added to the medium. Which conclusion is most appropriate?

35 Which test would most directly assess whether a bacterial isolate can produce siderophores under iron-limited conditions?

36 A bacterial isolate promotes seedling growth in a growth chamber but not in field soil. Which factor should be investigated first?

37 Why should an uninoculated control be included in a plant-growth-promotion experiment?

38 Two isolates show similar colony morphology, but their biochemical profiles differ substantially. What is the best next step?

39 An isolate shows strong phosphate solubilization in vitro but no improvement in plant phosphorus uptake. What is the most likely explanation?

40 Which characteristic is most important when selecting an isolate for development as a commercial biofertilizer?

41 A researcher plates of a soil suspension whose final dilution relative to the original soil sample is . The plate develops 36 colonies. Assuming each colony originated from one culturable unit, what is the estimated population density?

42 Putative rhizobia are being isolated from root nodules collected in soil containing many morphologically similar bacteria. Which procedure provides the strongest initial enrichment for authentic symbiotic isolates?

43 After surface sterilization of plant tissue for endophyte isolation, which control most directly tests whether epiphytic microorganisms were successfully removed?

44 A pellicle develops below the surface of nitrogen-free semisolid malate medium inoculated with rhizosphere soil. What is the most defensible interpretation before purification?

45 Direct plating of acidic soil repeatedly fails to recover a slow-growing phosphate-solubilizing bacterium because fast-growing heterotrophs overgrow the plates. Which modification best addresses this isolation bias?

46 Wet sieving and sucrose-gradient centrifugation recover few arbuscular mycorrhizal fungal spores from a root-zone soil, although root staining shows substantial colonization. What is the best next isolation strategy?

47 Two soils contain the same number of viable bacterial cells per gram, but one soil yields a much lower plate count because its cells remain in aggregates. Which pretreatment best reduces this source of underestimation while preserving viability?

48 A researcher wants to isolate bacteria specifically associated with the rhizoplane rather than loosely adhering rhizosphere soil or internal root tissues. Which sampling sequence is most appropriate?

49 A plant-free nitrogen medium is inoculated with soil to isolate diazotrophs. Growth occurs after repeated transfer, but the medium contains trace ammonium contamination. Which conclusion remains justified?

50 Root samples have low numbers of a target spore-forming biofertilizer bacterium and high numbers of non-spore-forming competitors. Which pretreatment is most suitable for selective recovery?

51 An isolate forms a large halo on Pikovskaya agar but releases little soluble phosphorus in broth containing the same insoluble phosphate source. Which interpretation best reconciles the results?

52 PCR detects a nifH fragment in a bacterial isolate, but the isolate shows no acetylene reduction under the tested conditions. What is the strongest conclusion?

53 A culture supernatant produces a strong color in the Salkowski assay after growth with tryptophan. Which validation most directly strengthens the claim that the isolate produced indole-3-acetic acid?

54 An isolate grows on minimal medium containing 1-aminocyclopropane-1-carboxylate as the nominal sole nitrogen source. Which experimental design most convincingly demonstrates ACC deaminase-dependent growth?

55 Two bacterial isolates have identical near-full-length 16S rRNA gene sequences but differ consistently in host colonization and phosphorus solubilization. Which characterization strategy best resolves whether they are functionally distinct strains?

56 A siderophore-producing isolate creates an orange zone on chrome azurol S agar. It grows poorly on this medium but strongly promotes plant growth in iron-limited soil. Which metric is least biased by its slow colony expansion?

57 A putative rhizobial isolate nodulates its original legume host, but uninoculated controls also develop nodules when grown in nonsterile soil. Which modification is essential for attributing nodulation to the isolate?

58 An inoculated bacterium increases plant biomass in autoclaved soil but not in untreated soil. Which characterization experiment best tests whether competition from the native microbiome explains this difference?

59 Four isolates are being screened for a saline-soil inoculant. Isolate P fixes the most nitrogen at NaCl but loses activity at NaCl. Isolate Q retains moderate activity through NaCl and colonizes roots at that salinity. Which criterion most strongly supports selecting Q?

60 A commercial-style consortium contains three beneficial bacterial isolates. After storage, one colony type disappears from routine plating, yet a species-specific qPCR assay remains positive. Which explanation must be excluded before claiming that the strain survived functionally?