Q.1 Which of the following microorganisms is most commonly responsible for the souring of milk in dairy products?
Lactobacillus bulgaricus
Streptococcus thermophilus
Lactobacillus lactis
Lactobacillus acidophilus
Explanation - Lactobacillus lactis (formerly Lactococcus lactis) is the primary lactic acid bacterium that converts lactose to lactic acid, causing milk souring.
Correct answer is: Lactobacillus lactis
Q.2 Which soil bacterium is known for its ability to fix atmospheric nitrogen and is widely used as a biofertilizer?
Azotobacter vinelandii
Pseudomonas aeruginosa
Bacillus subtilis
Clostridium perfringens
Explanation - Azotobacter vinelandii is a free‑living nitrogen‑fixing bacterium that converts N₂ into ammonia, enhancing soil fertility.
Correct answer is: Azotobacter vinelandii
Q.3 What is the primary mode of action of the biocontrol agent Trichoderma harzianum against plant pathogenic fungi?
Production of antibiotics
Competition for nutrients
Mycoparasitism
Induction of systemic resistance
Explanation - Trichoderma harzianum directly attacks and degrades the hyphae of pathogenic fungi through mycoparasitism, limiting disease spread.
Correct answer is: Mycoparasitism
Q.4 In the context of food microbiology, what does the term ‘hurdle technology’ refer to?
A method of sequencing microbial genomes
The use of multiple preservation factors together
A technique for rapid microbial detection
A type of genetic engineering in microbes
Explanation - Hurdle technology combines several preservation methods (e.g., heat, pH, water activity) to inhibit microbial growth more effectively than any single factor.
Correct answer is: The use of multiple preservation factors together
Q.5 Which of the following is a Gram‑negative bacterium that causes bacterial wilt in many solanaceous crops?
Xanthomonas campestris
Ralstonia solanacearum
Pseudomonas syringae
Erwinia amylovora
Explanation - Ralstonia solanacearum colonizes the plant’s vascular system, causing wilting and eventual death of crops like tomato and potato.
Correct answer is: Ralstonia solanacearum
Q.6 During aerobic plate count (APC) of a food sample, the colonies are incubated at 30 °C for 48 h. Which type of microorganisms are primarily being enumerated?
Thermophilic spore‑formers
Mesophilic aerobic bacteria
Psychrotrophic yeasts
Anaerobic lactic acid bacteria
Explanation - APC at 30 °C targets mesophilic (20–45 °C) aerobic bacteria, which are the dominant spoilage organisms in many foods.
Correct answer is: Mesophilic aerobic bacteria
Q.7 Which of the following enzymes produced by Bacillus subtilis is most important for the degradation of plant cell walls in composting?
Protease
Cellulase
Amylase
Lipase
Explanation - Cellulase breaks down cellulose, a major component of plant cell walls, facilitating organic matter decomposition.
Correct answer is: Cellulase
Q.8 What is the principal advantage of using a closed‑system bioreactor for the production of microbial antibiotics over an open fermentation?
Lower capital cost
Higher contamination risk
Better control of environmental parameters
Simpler downstream processing
Explanation - Closed bioreactors allow precise regulation of temperature, pH, aeration, and sterility, which enhances antibiotic yields and purity.
Correct answer is: Better control of environmental parameters
Q.9 Which mycotoxin is commonly associated with Fusarium spp. infection in cereal grains?
Aflatoxin B1
Ochratoxin A
Deoxynivalenol (DON)
Patulin
Explanation - Fusarium species produce deoxynivalenol, a trichothecene toxin that contaminates wheat, barley, and maize, posing health risks.
Correct answer is: Deoxynivalenol (DON)
Q.10 In the context of microbial inoculants for seed treatment, what does the term ‘rhizosphere competence’ describe?
Ability to produce phytohormones
Capacity to survive in the root zone and colonize roots
Resistance to desiccation
Ability to fix nitrogen
Explanation - Rhizosphere competence refers to a microbe’s ability to persist, compete, and establish on plant roots, ensuring effective inoculation.
Correct answer is: Capacity to survive in the root zone and colonize roots
Q.11 Which of the following detection methods is based on the amplification of specific DNA sequences?
ELISA
PCR
Plate count
Flow cytometry
Explanation - Polymerase Chain Reaction (PCR) amplifies target DNA fragments, enabling rapid and sensitive detection of microorganisms.
Correct answer is: PCR
Q.12 What is the main reason for the use of potassium sorbate as a food preservative?
It reduces water activity
It chelates metal ions
It inhibits mold and yeast growth
It raises pH
Explanation - Potassium sorbate interferes with fungal metabolism, effectively suppressing mold and yeast proliferation in foods.
Correct answer is: It inhibits mold and yeast growth
Q.13 Which bacterial genus is most frequently isolated from the rhizosphere of leguminous plants and forms nodules for nitrogen fixation?
Pseudomonas
Rhizobium
Clostridium
Enterobacter
Explanation - Rhizobium species establish symbiotic nodules on legume roots, converting atmospheric nitrogen into ammonia for plant use.
Correct answer is: Rhizobium
Q.14 In a serial dilution plate method, why is it important to use a sterile diluent for each step?
To increase the concentration of microbes
To ensure accurate dilution factor
To enhance microbial growth
To change the pH of the sample
Explanation - Sterile diluent prevents external contamination and guarantees that each dilution reflects the true reduction in microbial numbers.
Correct answer is: To ensure accurate dilution factor
Q.15 Which of the following is a common indicator organism used to assess the sanitary quality of fresh produce?
Salmonella enterica
Listeria monocytogenes
Escherichia coli (generic)
Staphylococcus aureus
Explanation - Generic E. coli is used as a fecal contamination indicator in produce safety assessments.
Correct answer is: Escherichia coli (generic)
Q.16 What is the primary function of lactic acid bacteria (LAB) in the production of fermented vegetables such as sauerkraut?
Produce ethanol
Generate carbon dioxide
Lower pH through lactic acid production
Synthesize vitamins
Explanation - LAB convert sugars into lactic acid, decreasing pH and creating an environment that preserves the vegetables and inhibits spoilage microbes.
Correct answer is: Lower pH through lactic acid production
Q.17 Which of the following microorganisms is a common cause of post‑harvest rot in citrus fruits?
Penicillium digitatum
Aspergillus niger
Botrytis cinerea
Fusarium oxysporum
Explanation - Penicillium digitatum causes green mold on citrus, leading to significant post‑harvest losses.
Correct answer is: Penicillium digitatum
Q.18 In the context of microbial ecology, what does the term ‘biofilm’ describe?
A collection of spores in soil
A floating colony of algae
A structured community of microbes attached to a surface
A group of viruses infecting bacteria
Explanation - Biofilms consist of microorganisms embedded in extracellular polymeric substances, adhering to biotic or abiotic surfaces.
Correct answer is: A structured community of microbes attached to a surface
Q.19 Which method is most appropriate for enumerating fungal spores in an air sample from a grain storage facility?
Pour plate method
Spread plate method
Impaction onto a PDA agar plate
Most probable number (MPN) technique
Explanation - Air impaction devices collect airborne spores directly onto Potato Dextrose Agar, facilitating fungal enumeration.
Correct answer is: Impaction onto a PDA agar plate
Q.20 What is the main reason why Bacillus thuringiensis (Bt) is considered a safe biopesticide for human consumption?
It produces no toxins
Its toxins are specific to insect gut receptors
It cannot survive in the human gut
It is rapidly degraded by sunlight
Explanation - Bt produces Cry toxins that bind to receptors unique to certain insects, making it non‑toxic to mammals and humans.
Correct answer is: Its toxins are specific to insect gut receptors
Q.21 Which of the following is the most effective method for inactivating bacterial endospores in canned foods?
Pasteurization at 72 °C for 15 s
Boiling at 100 °C for 10 min
Sterilization at 121 °C for 15 min (autoclave)
Freezing at –20 °C for 24 h
Explanation - High‑temperature, high‑pressure steam (121 °C) is required to destroy highly resistant bacterial endospores in canned products.
Correct answer is: Sterilization at 121 °C for 15 min (autoclave)
Q.22 Which plant pathogen is known for causing the ‘fire blight’ disease in apple and pear trees?
Erwinia amylovora
Xanthomonas axonopodis
Pseudomonas syringae
Clavibacter michiganensis
Explanation - Erwinia amylovora infects blossoms and shoots, leading to wilting and a characteristic scorched appearance.
Correct answer is: Erwinia amylovora
Q.23 In dairy fermentation, what role does the enzyme β‑galactosidase (lactase) play?
Breaks down casein proteins
Hydrolyzes lactose into glucose and galactose
Catalyzes the synthesis of lactic acid
Inactivates spoilage bacteria
Explanation - β‑Galactosidase splits lactose into its monosaccharides, making it more digestible for lactose‑intolerant consumers.
Correct answer is: Hydrolyzes lactose into glucose and galactose
Q.24 Which of the following is a common technique for preserving the genetic material of microbial strains for long‑term storage?
Lyophilization (freeze‑drying)
Refrigeration at 4 °C
Subculturing on agar weekly
Incubation at 37 °C
Explanation - Lyophilization removes water while maintaining cell viability, allowing microbes to be stored for years at room temperature.
Correct answer is: Lyophilization (freeze‑drying)
Q.25 Which microbial metabolite is primarily responsible for the distinctive flavor of blue cheese?
Acetaldehyde
Penicillium roqueforti lipases
Lactic acid
Ethanol
Explanation - Lipases from Penicillium roqueforti break down milk fats into free fatty acids, producing the characteristic tangy flavor.
Correct answer is: Penicillium roqueforti lipases
Q.26 What is the principal advantage of using a ‘selective’ agar medium in microbiological analysis of food samples?
It accelerates bacterial growth
It inhibits all microbes except the target group
It changes the colour of colonies for easy counting
It sterilizes the sample
Explanation - Selective media contain agents that suppress unwanted flora, allowing the target microorganisms to grow and be identified.
Correct answer is: It inhibits all microbes except the target group
Q.27 Which of the following microorganisms is used as a starter culture for the production of soy sauce?
Aspergillus oryzae
Saccharomyces cerevisiae
Lactobacillus plantarum
Bacillus subtilis
Explanation - A. oryzae produces proteases and amylases that break down soybeans and wheat, generating the flavour precursors for soy sauce.
Correct answer is: Aspergillus oryzae
Q.28 In the context of agricultural microbiology, what does the term ‘phyllosphere’ refer to?
Microbial communities in the soil
Microbes living inside plant roots
Microorganisms on the aerial parts of plants
Bacterial populations in irrigation water
Explanation - The phyllosphere comprises the leaf and stem surfaces that host diverse bacterial, fungal, and algal communities.
Correct answer is: Microorganisms on the aerial parts of plants
Q.29 Which of the following is the most reliable indicator of effective pasteurization of milk?
Absence of coliforms
Reduction of total aerobic count to <10⁴ CFU/mL
Elimination of *Mycobacterium bovis*
Destruction of *Coxiella burnetii* spores
Explanation - *Mycobacterium bovis* is highly heat‑resistant; its destruction confirms that pasteurization conditions have been met.
Correct answer is: Elimination of *Mycobacterium bovis*
Q.30 Which of the following is a characteristic of psychrotrophic bacteria that makes them problematic in refrigerated foods?
They produce heat‑stable toxins
They grow at temperatures as low as 0 °C
They are resistant to all antibiotics
They form large spores
Explanation - Psychrotrophs can proliferate under refrigeration, leading to spoilage despite low storage temperatures.
Correct answer is: They grow at temperatures as low as 0 °C
Q.31 In the production of bioethanol from lignocellulosic biomass, which microorganism is most commonly engineered for simultaneous saccharification and fermentation (SSF)?
Saccharomyces cerevisiae
Zymomonas mobilis
Clostridium acetobutylicum
Escherichia coli
Explanation - Engineered S. cerevisiae can both hydrolyze cellulose (via expressed cellulases) and ferment resulting sugars to ethanol in a single step.
Correct answer is: Saccharomyces cerevisiae
Q.32 Which of the following compounds is a natural antimicrobial produced by garlic and is used as a food preservative?
Allicin
Cinnamic acid
Nisin
Benzoic acid
Explanation - Allicin, formed when garlic is crushed, exhibits broad‑spectrum antimicrobial activity and can be applied as a natural preservative.
Correct answer is: Allicin
Q.33 What is the primary purpose of using a ‘buffered peptone water’ (BPW) during the pre‑enrichment of food samples for *Salmonella* detection?
To neutralize acids produced by microbes
To provide nutrients and maintain pH stability
To selectively kill Gram‑positive bacteria
To precipitate proteins
Explanation - BPW offers a non‑selective, buffered environment that encourages recovery of stressed *Salmonella* cells before selective enrichment.
Correct answer is: To provide nutrients and maintain pH stability
Q.34 Which bacterial genus is primarily responsible for the nitrogen transformation known as nitrification in soils?
Nitrosomonas and Nitrobacter
Azospirillum and Rhizobium
Bacillus and Clostridium
Pseudomonas and Enterobacter
Explanation - Nitrosomonas oxidize ammonia to nitrite, while Nitrobacter oxidize nitrite to nitrate, completing nitrification.
Correct answer is: Nitrosomonas and Nitrobacter
Q.35 Which of the following statements best describes the term ‘antimicrobial resistance’ (AMR) in the context of agricultural microbiology?
Microbes become immune to all disinfectants
Microbes acquire the ability to survive exposure to specific antimicrobial agents
Microbes lose the ability to produce antibiotics
Microbes can no longer grow in the presence of oxygen
Explanation - AMR refers to genetic or phenotypic changes that enable microbes to withstand drugs designed to inhibit or kill them.
Correct answer is: Microbes acquire the ability to survive exposure to specific antimicrobial agents
Q.36 What is the most common cause of ‘soft rot’ disease in many horticultural crops?
Pseudomonas syringae
Erwinia carotovora (Pectobacterium spp.)
Xanthomonas campestris
Clavibacter michiganensis
Explanation - These bacteria secrete pectolytic enzymes that macerate plant tissue, resulting in watery, soft rot lesions.
Correct answer is: Erwinia carotovora (Pectobacterium spp.)
Q.37 In the context of food safety testing, what does the term ‘limit of detection’ (LOD) refer to?
The highest concentration of a pathogen that can be measured
The lowest concentration of a target that can be reliably distinguished from zero
The average number of colonies on a plate
The time required for a test to give results
Explanation - LOD defines the smallest amount of a microorganism or toxin that a method can detect with confidence.
Correct answer is: The lowest concentration of a target that can be reliably distinguished from zero
Q.38 Which of the following is a characteristic feature of Gram‑positive bacteria that aids in their identification?
Presence of an outer membrane
Thick peptidoglycan layer
Lipopolysaccharide (LPS) in the cell wall
Periplasmic space
Explanation - Gram‑positive bacteria retain crystal violet stain due to a thick peptidoglycan layer, appearing purple under microscopy.
Correct answer is: Thick peptidoglycan layer
Q.39 Which of the following microbial metabolites is widely used as a natural food preservative for its antimicrobial activity against Gram‑positive bacteria?
Nisin
Ethanol
Acetic acid
Propionic acid
Explanation - Nisin, a bacteriocin produced by Lactococcus lactis, is effective against many Gram‑positive spoilage organisms.
Correct answer is: Nisin
Q.40 In the context of seed treatment, what is the main purpose of coating seeds with a formulation containing *Trichoderma* spp.?
To increase seed germination speed
To provide nitrogen fixation
To protect seedlings from soil‑borne fungal pathogens
To improve seed color
Explanation - *Trichoderma* spp. act as biocontrol agents, suppressing pathogenic fungi in the rhizosphere and promoting plant health.
Correct answer is: To protect seedlings from soil‑borne fungal pathogens
Q.41 Which of the following processes best describes the conversion of nitrate to nitrogen gas by soil microbes?
Nitrification
Denitrification
Ammonification
Nitrogen fixation
Explanation - Denitrifying bacteria reduce nitrate (NO₃⁻) to N₂ gas, completing the nitrogen cycle and releasing nitrogen back to the atmosphere.
Correct answer is: Denitrification
Q.42 What is the principal function of the enzyme glucose oxidase in honey?
Breaks down pollen proteins
Produces hydrogen peroxide, providing antimicrobial activity
Converts glucose into fructose
Catalyzes the formation of honey crystals
Explanation - Glucose oxidase oxidizes glucose to gluconic acid and hydrogen peroxide, contributing to honey’s antimicrobial properties.
Correct answer is: Produces hydrogen peroxide, providing antimicrobial activity
Q.43 Which of the following is a major factor influencing the shelf‑life of fresh produce during refrigerated storage?
Ambient light intensity
Water activity (a_w)
Relative humidity inside the package
Carbon dioxide concentration
Explanation - Lower water activity limits microbial growth, extending the shelf‑life of refrigerated fresh produce.
Correct answer is: Water activity (a_w)
Q.44 Which of the following is the most suitable selective medium for isolating *Listeria monocytogenes* from processed meat samples?
MacConkey agar
XLD agar
PALCAM agar
Mannitol Salt agar
Explanation - PALCAM agar contains selective agents and colour indicators that favor *Listeria* growth while suppressing competing flora.
Correct answer is: PALCAM agar
Q.45 During the production of tempeh, which microorganism is primarily responsible for the binding of soybeans into a firm cake?
Rhizopus oligosporus
Lactobacillus plantarum
Saccharomyces cerevisiae
Bacillus subtilis
Explanation - R. oligosporus grows as a mycelial network that binds soybeans together, creating the characteristic texture of tempeh.
Correct answer is: Rhizopus oligosporus
Q.46 Which of the following statements best explains why *Clostridium botulinum* spores are a concern in low‑acid canned foods?
The spores are heat‑labile and can be destroyed by mild heating
They produce a toxin that is stable at high temperatures
They can germinate and produce toxin under anaerobic, low‑acid conditions
They grow rapidly at refrigeration temperatures
Explanation - In low‑acid, oxygen‑free environments, *C. botulinum* spores can germinate and produce botulinum toxin, a severe food‑borne hazard.
Correct answer is: They can germinate and produce toxin under anaerobic, low‑acid conditions
Q.47 Which of the following microorganisms is commonly used as a probiotic in fermented dairy products?
Streptococcus pneumoniae
Lactobacillus rhamnosus
Clostridium difficile
Salmonella enterica
Explanation - L. rhamnosus is a well‑studied probiotic that can survive gastrointestinal transit and confer health benefits to the host.
Correct answer is: Lactobacillus rhamnosus
Q.48 Which of the following is a key factor that determines the efficacy of a microbial inoculant applied to seed surfaces?
The colour of the inoculant formulation
The moisture content of the seed at the time of coating
The presence of UV‑protective compounds
The size of the seed
Explanation - Adequate moisture ensures that the inoculant adheres to the seed and remains viable during storage and early germination.
Correct answer is: The moisture content of the seed at the time of coating
Q.49 In a typical dairy pasteurization process, which temperature‑time combination is commonly used for the High‑Temperature Short‑Time (HTST) method?
63 °C for 30 min
72 °C for 15 s
80 °C for 10 s
90 °C for 5 s
Explanation - HTST pasteurization uses 72 °C for 15 seconds to effectively reduce pathogenic microbes while preserving nutritional quality.
Correct answer is: 72 °C for 15 s
Q.50 Which of the following microorganisms is most commonly associated with the spoilage of canned fruits and vegetables that are stored at room temperature?
Clostridium botulinum
Bacillus cereus
Saccharomyces cerevisiae
Lactobacillus plantarum
Explanation - Inadequately processed low‑acid canned foods can support the growth of *C. botulinum*, leading to toxin production and spoilage.
Correct answer is: Clostridium botulinum
Q.51 Which analytical method is most appropriate for quantifying the amount of lactic acid produced during yogurt fermentation?
Gas chromatography (GC)
High‑performance liquid chromatography (HPLC)
Spectrophotometric assay using pH indicator
Plate count method
Explanation - HPLC provides accurate quantification of organic acids like lactic acid in complex dairy matrices.
Correct answer is: High‑performance liquid chromatography (HPLC)
Q.52 Which of the following microorganisms is the primary cause of ‘soft rot’ in potatoes during storage?
Pseudomonas fluorescens
Erwinia carotovora (Pectobacterium spp.)
Clavibacter michiganensis
Xanthomonas perforans
Explanation - The bacterium secretes pectinases that degrade cell walls, leading to watery, soft decay in stored potatoes.
Correct answer is: Erwinia carotovora (Pectobacterium spp.)
Q.53 In the context of microbial taxonomy, what does the abbreviation ‘sp.’ signify after a genus name?
A specific species identified
A subspecies
An unknown or unspecified species within the genus
A hybrid organism
Explanation - ‘sp.’ indicates that the exact species is not identified, only the genus is known.
Correct answer is: An unknown or unspecified species within the genus
Q.54 Which of the following is a common method used to assess microbial contamination on the surface of fruits and vegetables?
Surface swab followed by enrichment and plating
Grinding the produce and measuring pH
Measuring the weight loss after storage
Testing for pesticide residues
Explanation - Swabbing the surface collects microbes, which can then be enriched and cultured to determine contamination levels.
Correct answer is: Surface swab followed by enrichment and plating
Q.55 Which of the following microorganisms is used as a biological indicator to validate the efficacy of sterilization processes in food processing facilities?
Bacillus subtilis spores
Escherichia coli
Lactobacillus casei
Staphylococcus aureus
Explanation - B. subtilis spores are highly resistant to heat and chemical sterilants, making them suitable for validating sterilization.
Correct answer is: Bacillus subtilis spores
Q.56 What is the primary function of the enzyme pectinase in fruit juice production?
Increase acidity
Break down pectin to clarify juice
Convert sugars to alcohol
Prevent microbial growth
Explanation - Pectinase hydrolyzes pectin, reducing viscosity and improving clarity of fruit juices.
Correct answer is: Break down pectin to clarify juice
Q.57 Which of the following is a major advantage of using solid‑state fermentation (SSF) for the production of fungal enzymes compared to submerged fermentation?
Higher water usage
Lower product yield
Simpler downstream processing and higher enzyme stability
Requirement of sterile liquid media
Explanation - SSF often yields more stable enzymes and reduces the need for extensive filtration steps, lowering costs.
Correct answer is: Simpler downstream processing and higher enzyme stability
Q.58 In the context of post‑harvest disease management, which of the following is a common chemical treatment used to control fungal infections on stored grains?
Carbendazim
Glyphosate
Chlorpyrifos
Atrazine
Explanation - Carbendazim is a fungicide widely applied to grains to suppress mold growth during storage.
Correct answer is: Carbendazim
Q.59 Which of the following microorganisms is a major cause of ‘blossom end rot’ in tomatoes?
Botrytis cinerea
Phytophthora infestans
Calcium deficiency caused by low soil calcium
Pseudomonas syringae
Explanation - While not a microbe, the disease is linked to calcium shortage; however, fungal pathogens can exacerbate the condition. The correct answer focuses on the primary cause—calcium deficiency.
Correct answer is: Calcium deficiency caused by low soil calcium
Q.60 Which of the following best describes the role of *Saccharomyces cerevisiae* in bread making?
Produces lactic acid to flavor the dough
Ferments sugars to carbon dioxide and ethanol, leavening the bread
Breaks down gluten proteins
Inhibits bacterial growth
Explanation - S. cerevisiae metabolizes sugars, releasing CO₂ that expands the dough, and ethanol, which evaporates during baking.
Correct answer is: Ferments sugars to carbon dioxide and ethanol, leavening the bread
Q.61 Which of the following microbial groups is most responsible for the nitrogen loss from agricultural soils via the process of denitrification?
Nitrifying bacteria
Nitrogen‑fixing cyanobacteria
Denitrifying bacteria such as *Pseudomonas* spp.
Ammonia‑oxidizing archaea
Explanation - Denitrifiers convert nitrate to nitrogen gases, leading to loss of plant‑available nitrogen from soils.
Correct answer is: Denitrifying bacteria such as *Pseudomonas* spp.
Q.62 In the production of soy milk, which enzyme is added to improve texture and reduce beany flavor?
Protease
Lipase
Amylase
β‑Glucosidase
Explanation - β‑Glucosidase hydrolyzes glycosidic bonds, reducing beany off‑flavors and improving mouthfeel.
Correct answer is: β‑Glucosidase
Q.63 Which of the following is a hallmark of an effective vaccine strain used for oral delivery of probiotic bacteria?
High resistance to gastric acidity
Production of strong toxins
Requirement of aerobic growth
Inability to colonize the gut
Explanation - Probiotic vaccine strains must survive stomach acidity to reach the intestine where they exert their effect.
Correct answer is: High resistance to gastric acidity
Q.64 Which of the following is a common indicator of successful fermentation in kimchi production?
pH dropping below 4.0
Increase in sugar concentration
Development of a blue colour
Presence of visible mould
Explanation - Lactic acid bacteria lower the pH to below 4.0, ensuring safety and characteristic sour taste in kimchi.
Correct answer is: pH dropping below 4.0
Q.65 Which of the following microorganisms is known for producing the antibiotic penicillin?
Penicillium chrysogenum
Aspergillus niger
Bacillus subtilis
Streptomyces griseus
Explanation - Penicillium chrysogenum (formerly P. notatum) synthesizes penicillin, a β‑lactam antibiotic.
Correct answer is: Penicillium chrysogenum
Q.66 Which of the following is the most common method for detecting the presence of *Salmonella* spp. in a processed food sample?
Direct plating on nutrient agar
Enrichment in selective broth followed by selective agar plating
PCR without prior enrichment
Microscopic observation of gram‑negative rods
Explanation - Enrichment increases the number of *Salmonella* cells, improving detection sensitivity before plating on selective media.
Correct answer is: Enrichment in selective broth followed by selective agar plating
Q.67 Which of the following statements best explains why *Streptomyces* spp. are important in agricultural biotechnology?
They are primary nitrogen fixers
They produce a wide range of antibiotics that suppress soil pathogens
They are the main cause of root rot
They degrade pesticides
Explanation - *Streptomyces* species synthesize antibiotics that can protect crops from bacterial and fungal diseases.
Correct answer is: They produce a wide range of antibiotics that suppress soil pathogens
Q.68 What is the main advantage of using a ‘dual‑culture’ method in the screening of biocontrol agents against a plant pathogen?
It reduces the time needed for incubation
It allows direct observation of antagonistic interactions on the same plate
It eliminates the need for sterile techniques
It increases the growth rate of both organisms
Explanation - Dual‑culture assays let researchers see inhibition zones or overgrowth, indicating biocontrol potential.
Correct answer is: It allows direct observation of antagonistic interactions on the same plate
Q.69 Which of the following microorganisms is a common cause of ‘black rot’ in cruciferous vegetables?
Xanthomonas campestris pv. campestris
Pseudomonas aeruginosa
Botrytis cinerea
Alternaria alternata
Explanation - The bacterium causes V‑shaped leaf lesions that turn black, leading to significant yield loss in brassicas.
Correct answer is: Xanthomonas campestris pv. campestris
Q.70 Which of the following is a common technique for quantifying microbial biomass in soil samples?
Chlorophyll fluorescence
Phospholipid fatty acid (PLFA) analysis
Gas chromatography of CO₂
Spectrophotometric measurement of nitrogen
Explanation - PLFA profiling provides information on the composition and quantity of living microbial cells in soils.
Correct answer is: Phospholipid fatty acid (PLFA) analysis
Q.71 In the context of microbial fermentation, what does the term ‘metabolic shift’ commonly refer to?
A change from aerobic to anaerobic metabolism
An increase in cell size
A transition from lag phase to exponential phase
A switch from growth to production of secondary metabolites
Explanation - During the stationary phase, microbes often redirect resources to produce antibiotics, pigments, or other secondary metabolites.
Correct answer is: A switch from growth to production of secondary metabolites
Q.72 Which of the following is the most appropriate method to determine the water activity (a_w) of a dry food product?
Measuring pH with a pH meter
Using a hygrometer or aaw meter
Conducting a titration with NaOH
Performing a moisture content analysis by oven drying
Explanation - Water activity meters directly measure the vapor pressure of water in the product, providing a_w values.
Correct answer is: Using a hygrometer or aaw meter
Q.73 Which of the following is a major factor contributing to the development of antibiotic resistance in agricultural settings?
Use of UV light for crop protection
Application of sub‑therapeutic levels of antibiotics in animal feed
Implementation of crop rotation
Use of organic fertilizers
Explanation - Low‑dose antibiotic use selects for resistant bacteria, which can spread to the environment and human pathogens.
Correct answer is: Application of sub‑therapeutic levels of antibiotics in animal feed
Q.74 Which of the following microbial genera is most often associated with the production of fermented coffee (café de olla) flavor compounds?
Saccharomyces
Lactobacillus
Acetobacter
Pichia
Explanation - Acetobacter oxidizes ethanol to acetic acid, contributing to the characteristic sour notes in fermented coffee.
Correct answer is: Acetobacter
Q.75 Which of the following statements best describes the purpose of adding *Coccidioides* spores to a vaccine formulation?
To increase the immunogenicity of the vaccine
To act as a preservative
To serve as a nutrient source for the host
To inhibit bacterial contamination
Explanation - Spore‑based adjuvants can enhance the immune response, though *Coccidioides* is not commonly used; this reflects the concept of spore adjuvants.
Correct answer is: To increase the immunogenicity of the vaccine
Q.76 In the context of food spoilage, which of the following metabolites is most commonly associated with the ‘off‑flavour’ of rancid butter?
Acetaldehyde
Butyric acid
Diacetyl
Hexanal
Explanation - Butyric acid, produced by lipolytic bacteria, gives a sharp, rancid odor to butter.
Correct answer is: Butyric acid
Q.77 Which of the following microorganisms is a common cause of bacterial wilt in banana plants?
Ralstonia solanacearum
Fusarium oxysporum f. sp. cubense
Xanthomonas campestris
Pseudomonas syringae
Explanation - The soil‑borne fungus causes Panama disease, leading to wilting and death of banana plants.
Correct answer is: Fusarium oxysporum f. sp. cubense
Q.78 Which of the following is a typical characteristic of a ‘psychrophilic’ microorganism?
Optimal growth at temperatures above 45 °C
Optimal growth at temperatures below 15 °C
Inability to survive freezing temperatures
Requirement of high salt concentrations
Explanation - Psychrophiles thrive in cold environments, such as refrigerated foods and polar ecosystems.
Correct answer is: Optimal growth at temperatures below 15 °C
Q.79 Which of the following is the main advantage of using a ‘biosensor’ for detecting microbial contamination in food processing plants?
Provides real‑time, on‑site detection
Eliminates the need for any sample preparation
Detects all possible pathogens simultaneously
Requires no calibration
Explanation - Biosensors can rapidly signal the presence of specific microbes, enabling immediate corrective actions.
Correct answer is: Provides real‑time, on‑site detection
Q.80 Which of the following microbial processes is primarily responsible for the formation of carbonated beverages?
Lactic acid fermentation
Alcoholic fermentation by yeast
Acetate fermentation
Butyric acid fermentation
Explanation - Saccharomyces cerevisiae ferments sugars to ethanol and CO₂; the CO₂ provides carbonation.
Correct answer is: Alcoholic fermentation by yeast
Q.81 In the context of microbial safety of fresh-cut fruits, which of the following treatments is most effective at reducing *E. coli* O157:H7 without compromising product quality?
Cold water wash
UV-C irradiation
High‑pressure processing (HPP)
Ozone gas treatment
Explanation - UV‑C efficiently inactivates surface bacteria while preserving texture and colour of fresh‑cut produce.
Correct answer is: UV-C irradiation
Q.82 Which of the following is a common indicator of fungal contamination in stored grain silos?
Increased oxygen levels
Elevated moisture content and visible mould growth
Reduced temperature
Higher protein content
Explanation - Moisture promotes fungal proliferation, and mould colonies become visible on grain surfaces.
Correct answer is: Elevated moisture content and visible mould growth
Q.83 Which of the following microorganisms is known for producing the enzyme amylase used industrially to break down starch?
Bacillus amyloliquefaciens
Lactobacillus casei
Pseudomonas putida
Saccharomyces bayanus
Explanation - B. amyloliquefaciens secretes extracellular amylase, widely applied in food and textile industries.
Correct answer is: Bacillus amyloliquefaciens
Q.84 Which of the following best describes the term ‘quorum sensing’ in bacterial populations?
A method of bacterial motility
Cell‑to‑cell communication using signaling molecules to coordinate behavior
A type of bacterial sporulation
A mechanism for DNA repair
Explanation - Quorum sensing allows bacteria to regulate gene expression collectively, influencing virulence, biofilm formation, etc.
Correct answer is: Cell‑to‑cell communication using signaling molecules to coordinate behavior
Q.85 Which of the following microorganisms is commonly used as a starter culture for the production of traditional African fermented millet porridge (ogi)?
Lactobacillus fermentum
Bacillus licheniformis
Saccharomyces cerevisiae
Acetobacter xylinum
Explanation - L. fermentum contributes lactic acid production, giving ogi its sour taste and extending shelf‑life.
Correct answer is: Lactobacillus fermentum
Q.86 In a standard plate count method, what does a colony‑forming unit (CFU) represent?
One viable bacterial cell or a clump of cells that gives rise to a colony
One dead cell
One spore only
One virus particle
Explanation - CFU counts reflect the number of viable microorganisms capable of reproducing on the medium.
Correct answer is: One viable bacterial cell or a clump of cells that gives rise to a colony
Q.87 Which of the following is a primary source of inoculum for the biological control of soil‑borne fungal diseases?
Commercially produced fungal spores
Synthetic chemical fungicides
Irradiated soil
Metallic copper strips
Explanation - Mass‑produced spores of beneficial fungi (e.g., Trichoderma) are applied to soil to suppress pathogens.
Correct answer is: Commercially produced fungal spores
Q.88 Which of the following microorganisms is most likely to cause the ‘spoilage’ of canned fruit juices stored at room temperature?
Lactobacillus plantarum
Clostridium botulinum
Bacillus coagulans
Zygosaccharomyces rouxii
Explanation - Z. rouxii is a osmophilic yeast that tolerates high sugar and low pH, thriving in sweet, acidic beverages.
Correct answer is: Zygosaccharomyces rouxii
Q.89 Which of the following best explains why *Pseudomonas fluorescens* is a concern in refrigerated dairy products?
It forms heat‑stable spores
It produces proteolytic enzymes that cause off‑flavours at low temperatures
It grows only at temperatures above 45 °C
It is a major source of lactic acid
Explanation - P. fluorescens secretes enzymes that degrade proteins, leading to bitter, metallic tastes in chilled dairy.
Correct answer is: It produces proteolytic enzymes that cause off‑flavours at low temperatures
Q.90 Which of the following is a common method for preserving the viability of probiotic cultures during storage?
Lyophilization (freeze‑drying)
Heating at 70 °C for 30 min
Exposure to UV light
Storing at ambient temperature
Explanation - Freeze‑drying removes water while maintaining cell integrity, allowing long‑term storage of probiotics.
Correct answer is: Lyophilization (freeze‑drying)
Q.91 Which of the following microorganisms is a key agent in the production of traditional Japanese miso paste?
Aspergillus oryzae
Bacillus cereus
Candida albicans
Lactobacillus brevis
Explanation - A. oryzae produces proteases and amylases that break down soybeans and wheat, forming the flavor base of miso.
Correct answer is: Aspergillus oryzae
Q.92 Which of the following microorganisms is primarily responsible for the souring of fermented cabbage (sauerkraut)?
Leuconostoc mesenteroides
Clostridium perfringens
Pseudomonas aeruginosa
Staphylococcus epidermidis
Explanation - Leuconostoc initiates lactic acid fermentation, lowering pH and creating the characteristic sour taste.
Correct answer is: Leuconostoc mesenteroides
Q.93 Which of the following is a major health risk associated with the consumption of aflatoxin‑contaminated peanuts?
Acute respiratory distress
Hepatocellular carcinoma
Neurological degeneration
Kidney failure
Explanation - Aflatoxin B1 is a potent hepatocarcinogen linked to liver cancer in humans.
Correct answer is: Hepatocellular carcinoma
Q.94 Which of the following is the primary purpose of adding sodium benzoate to fruit‑based beverages?
To increase sweetness
To inhibit yeast and mold growth
To raise the pH
To enhance colour
Explanation - Sodium benzoate is an effective preservative that suppresses fungal spoilage in acidic drinks.
Correct answer is: To inhibit yeast and mold growth
Q.95 In microbial ecology, what does the term ‘horizontal gene transfer’ (HGT) refer to?
Transfer of genes from parent to offspring
Acquisition of genetic material from unrelated organisms
Mutation caused by radiation
Loss of plasmids during cell division
Explanation - HGT allows microbes to obtain new traits (e.g., antibiotic resistance) from other species via transformation, transduction, or conjugation.
Correct answer is: Acquisition of genetic material from unrelated organisms
Q.96 Which of the following microorganisms is the most common cause of foodborne illness associated with undercooked poultry?
Salmonella enterica
Listeria monocytogenes
Campylobacter jejuni
Staphylococcus aureus
Explanation - C. jejuni is a leading cause of gastroenteritis linked to improperly cooked poultry.
Correct answer is: Campylobacter jejuni
Q.97 Which of the following is the best indicator of successful fermentation of cocoa beans during chocolate production?
Decrease in bean moisture content below 5 %
Increase in bean temperature to 55 °C
Rise in pH above 7.0
Development of a characteristic fruity aroma due to lactic acid bacteria activity
Explanation - Lactic acid bacteria produce volatile compounds that give fermented cocoa beans their unique flavor profile.
Correct answer is: Development of a characteristic fruity aroma due to lactic acid bacteria activity
Q.98 Which of the following microorganisms is widely used as a biocontrol agent against the soil‑borne pathogen *Sclerotinia sclerotiorum*?
Bacillus subtilis
Trichoderma harzianum
Pseudomonas fluorescens
Streptomyces scabies
Explanation - T. harzianum antagonizes S. sclerotiorum through mycoparasitism and production of antifungal metabolites.
Correct answer is: Trichoderma harzianum
Q.99 Which of the following is the most suitable pH range for the growth of most lactic acid bacteria used in food fermentations?
3.0 – 4.5
5.0 – 6.5
7.0 – 8.5
9.0 – 10.5
Explanation - Lactic acid bacteria optimally grow in mildly acidic environments, typically pH 5.0–6.5.
Correct answer is: 5.0 – 6.5
Q.100 Which of the following is a key factor in determining the shelf‑life of fermented dairy products such as kefir?
Amount of added sugar
Initial microbial load and storage temperature
Presence of artificial colourants
Thickness of the packaging material
Explanation - Low initial counts and proper refrigeration slow further microbial activity, extending product shelf‑life.
Correct answer is: Initial microbial load and storage temperature
Q.101 Which of the following microorganisms is known for producing the enzyme phytase, which improves mineral bioavailability in animal feed?
Aspergillus niger
Lactobacillus acidophilus
Bacillus anthracis
Pseudomonas aeruginosa
Explanation - A. niger secretes phytase that hydrolyzes phytic acid, releasing bound phosphorus and minerals.
Correct answer is: Aspergillus niger
Q.102 Which of the following is the main advantage of using ‘protective cultures’ in cheese production?
They accelerate ripening
They inhibit spoilage and pathogenic microbes while allowing desirable fermentation
They increase the fat content of cheese
They change the colour of the cheese
Explanation - Protective cultures, often LAB, produce bacteriocins that suppress unwanted microbes without affecting cheese quality.
Correct answer is: They inhibit spoilage and pathogenic microbes while allowing desirable fermentation
Q.103 Which of the following is a typical characteristic of a ‘spoilage yeast’ in high‑sugar fruit jams?
Inability to grow at low pH
Tolerance to high osmotic pressure (osmophilic)
Requirement of oxygen for growth
Production of lactic acid
Explanation - Osmophilic yeasts, such as Zygosaccharomyces, can grow in the high‑sugar, low‑water‑activity environment of jams.
Correct answer is: Tolerance to high osmotic pressure (osmophilic)
Q.104 Which of the following microorganisms is a well‑known producer of the antibiotic streptomycin?
Streptomyces griseus
Bacillus thuringiensis
Penicillium notatum
Lactobacillus bulgaricus
Explanation - S. griseus synthesizes streptomycin, an aminoglycoside antibiotic effective against many bacteria.
Correct answer is: Streptomyces griseus
Q.105 Which of the following is the primary cause of ‘blackleg’ disease in potatoes?
Pseudomonas syringae
Pectobacterium carotovorum (formerly Erwinia carotovora)
Alternaria solani
Phytophthora infestans
Explanation - The bacterium produces pectolytic enzymes that cause tissue maceration and blackening of potato stems.
Correct answer is: Pectobacterium carotovorum (formerly Erwinia carotovora)
Q.106 Which of the following best explains why *Listeria monocytogenes* can grow at refrigeration temperatures?
It forms heat‑stable spores
It is a psychrotrophic bacterium
It produces toxins that increase temperature
It requires high salt concentrations
Explanation - *Listeria* can proliferate at low temperatures (as low as 0 °C), posing a risk in refrigerated foods.
Correct answer is: It is a psychrotrophic bacterium
Q.107 Which of the following microorganisms is commonly used as a starter for the production of traditional Indian fermented rice beverage ‘bajra‑khar’?
Lactobacillus delbrueckii
Saccharomyces cerevisiae
Zymomonas mobilis
Acetobacter aceti
Explanation - S. cerevisiae ferments sugars to ethanol, creating the alcoholic component of the beverage.
Correct answer is: Saccharomyces cerevisiae
Q.108 Which of the following is a major reason for the use of *Bacillus thuringiensis* (Bt) spores in transgenic crops?
Bt spores enhance plant growth directly
Bt produces Cry toxins that target specific insect pests
Bt spores act as a nitrogen source for the plant
Bt spores increase drought tolerance
Explanation - Cry toxins bind to insect gut receptors, causing mortality while being harmless to mammals.
Correct answer is: Bt produces Cry toxins that target specific insect pests
Q.109 Which of the following is a typical method for detecting the presence of mycotoxins in grain samples?
Plate count on nutrient agar
Enzyme‑linked immunosorbent assay (ELISA)
Gram staining
Oxidase test
Explanation - ELISA uses antibodies to specifically bind and quantify mycotoxins such as aflatoxin or DON.
Correct answer is: Enzyme‑linked immunosorbent assay (ELISA)
Q.110 Which of the following microbial processes is primarily responsible for the formation of carbon dioxide bubbles in sourdough bread?
Lactic acid fermentation by *Lactobacillus* spp.
Alcoholic fermentation by *Saccharomyces* spp.
Acetate fermentation by *Acetobacter* spp.
Butyric acid fermentation by *Clostridium* spp.
Explanation - Yeast converts sugars to ethanol and CO₂; CO₂ expands the dough, giving sourdough its texture.
Correct answer is: Alcoholic fermentation by *Saccharomyces* spp.
Q.111 Which of the following is an example of a ‘prebiotic’ used to stimulate beneficial gut microbes in functional foods?
Inulin
Nisin
Lactoferrin
Erythromycin
Explanation - Inulin is a non‑digestible fructooligosaccharide that promotes growth of beneficial bacteria such as Bifidobacteria.
Correct answer is: Inulin
Q.112 Which of the following microorganisms is the primary cause of ‘powdery mildew’ on grapevines?
Erysiphe necator
Botrytis cinerea
Phytophthora infestans
Pseudomonas syringae
Explanation - E. necator forms a white powdery coating on leaves and fruit, reducing photosynthesis and yield.
Correct answer is: Erysiphe necator
Q.113 Which of the following is the main reason for adding *Saccharomyces cerevisiae* to the fermentation of bioethanol from sugarcane juice?
It produces lactic acid to preserve the juice
It converts sugars directly to ethanol and CO₂ efficiently
It reduces the pH of the medium
It secretes cellulases to break down cellulose
Explanation - S. cerevisiae efficiently ferments glucose and fructose in sugarcane juice into ethanol, making it the workhorse of bioethanol production.
Correct answer is: It converts sugars directly to ethanol and CO₂ efficiently
Q.114 Which of the following is a key benefit of using ‘microbial consortia’ rather than a single strain in composting organic waste?
Reduced oxygen consumption
Faster degradation due to synergistic enzymatic activities
Higher production of methane
Elimination of all pathogens
Explanation - Consortia combine complementary metabolic pathways, accelerating breakdown of complex substrates.
Correct answer is: Faster degradation due to synergistic enzymatic activities
Q.115 Which of the following microorganisms is typically responsible for the ‘ropey’ defect in cheese caused by proteolysis?
Lactobacillus helveticus
Clostridium tyrobutyricum
Streptococcus thermophilus
Penicillium camemberti
Explanation - C. tyrobutyricum produces butyric acid and proteases that cause off‑flavours and texture defects in cheese.
Correct answer is: Clostridium tyrobutyricum
Q.116 Which of the following is a common method to assess the efficacy of a disinfectant against *E. coli* on food contact surfaces?
Measuring pH change after treatment
Determining log reduction in CFU after exposure
Counting spores on agar plates
Measuring temperature rise of the surface
Explanation - Log reduction quantifies the decrease in viable bacteria, indicating disinfectant effectiveness.
Correct answer is: Determining log reduction in CFU after exposure
Q.117 Which of the following microorganisms is a key player in the formation of the characteristic ‘smell’ of ripe cheese due to production of volatile sulfur compounds?
Brevibacterium linens
Lactobacillus plantarum
Saccharomyces cerevisiae
Bacillus subtilis
Explanation - B. linens generates sulfur‑containing volatiles that contribute to the aroma of smear‑ripened cheeses.
Correct answer is: Brevibacterium linens
Q.118 Which of the following is the most common method for detecting *Clostridium botulinum* spores in canned foods?
PCR detection of toxin genes after enrichment
Gram staining of food homogenate
Plate count on nutrient agar
Measuring pH of the food
Explanation - Enrichment followed by PCR allows sensitive detection of botulinum toxin genes, even at low spore levels.
Correct answer is: PCR detection of toxin genes after enrichment
Q.119 Which of the following microorganisms is typically used to produce the enzyme lipase for the dairy industry?
Candida rugosa
Lactobacillus delbrueckii
Streptomyces coelicolor
Pseudomonas aeruginosa
Explanation - C. rugosa secretes lipase that is applied in cheese flavor development and butter processing.
Correct answer is: Candida rugosa
Q.120 Which of the following microorganisms is most associated with the development of ‘off‑flavours’ in fermented fish products due to the production of biogenic amines?
Enterobacteriaceae
Lactobacillus sakei
Pseudomonas spp.
Clostridium botulinum
Explanation - Certain Enterobacteriaceae decarboxylate amino acids, forming histamine and other biogenic amines that cause off‑flavours.
Correct answer is: Enterobacteriaceae
Q.121 Which of the following is a major factor influencing the survival of probiotic bacteria during passage through the human stomach?
Resistance to bile salts
Ability to produce spores
Production of lactic acid
Sensitivity to low pH
Explanation - Probiotic strains must tolerate both gastric acidity and bile salts to reach the intestine alive.
Correct answer is: Resistance to bile salts
Q.122 Which of the following microorganisms is commonly used in the production of the traditional fermented soybean product ‘natto’?
Bacillus subtilis var. natto
Lactobacillus brevis
Saccharomyces cerevisiae
Clostridium perfringens
Explanation - B. subtilis var. natto produces the characteristic sticky polymer (poly‑γ‑glutamic acid) and flavor of natto.
Correct answer is: Bacillus subtilis var. natto
Q.123 Which of the following is a common symptom of aflatoxin exposure in livestock?
Respiratory distress
Liver damage and reduced growth
Neurological seizures
Kidney stones
Explanation - Aflatoxins are hepatotoxic; chronic exposure leads to liver lesions and stunted growth in animals.
Correct answer is: Liver damage and reduced growth
Q.124 Which of the following microorganisms is primarily responsible for the formation of ‘black mold’ on cured meats?
Aspergillus niger
Penicillium nalgiovense
Mucor spp.
Cladosporium fulvum
Explanation - P. nalgiovense forms a protective black coating that inhibits spoilage organisms on dry‑cured meats.
Correct answer is: Penicillium nalgiovense