76 practice questions on Microbes in Human Welfare , sorted Easy → Hard. Try each one first, then open its answer page for the worked explanation. Want the full theory first? Read the Microbes in Human Welfare notes .
Sewage Treatment Flow Primary physical filtering + sedimentation → Secondary (aerobic) aeration tank, flocs consume organic matter → Activated sludge settles out; part reused as inoculum → Sludge digester ANAEROBIC → biogas (CH₄) Note the switch: aerobic microbes in the tank, anaerobic microbes in the digester Sewage passes through physical removal (primary), then aerobic biological treatment where microbes form flocs that consume organic matter (secondary); the settled activated sludge is then digested anaerobically to produce biogas - a deliberate switch from aerobic to anaerobic conditions between stages.
Easy - 25 questions Q1.
The large holes (eyes) in Swiss cheese are produced by the gas released by which bacterium?
A Propionibacterium sharmaniiB Lactobacillus acidophilusC Streptococcus lactisD Acetobacter acetiShow answer & explanation →
Q2.
The dough used to make bread rises mainly because of which gas released during fermentation?
A OxygenB Carbon dioxideC NitrogenD MethaneShow answer & explanation →
Q3.
Which microorganism is used for the large-scale production of the antibiotic penicillin in industry?
A Aspergillus nigerB Trichoderma polysporumC Penicillium chrysogenumD Monascus purpureusShow answer & explanation →
Q4.
During secondary treatment of sewage, the effluent is passed into large tanks known as:
A Settling tanksB Sludge digestersC Primary clarifiersD Aeration tanksShow answer & explanation →
Q5.
Toddy, a traditional beverage of southern India, is made by fermenting the sap of which source?
A PalmsB GrapesC BarleyD RiceShow answer & explanation →
Q6.
Which microorganism is responsible for the conversion of milk into curd?
A Saccharomyces cerevisiaeB LactobacillusC Aspergillus nigerD Penicillium notatumShow answer & explanation →
Q7.
Which organism is used for fermentation in bread making?
A LactobacillusB Aspergillus nigerC Saccharomyces cerevisiaeD RhizobiumShow answer & explanation →
Q8.
Penicillin, the first antibiotic, was discovered from which fungus?
A Aspergillus nigerB Trichoderma polysporumC Penicillium notatumD Monascus purpureusShow answer & explanation →
Q9.
Which bacterium is used as a biocontrol agent against caterpillars and other insect pests?
A Rhizobium, a nitrogen-fixing root bacteriumB Bacillus thuringiensisC Azotobacter, a free-living nitrogen fixerD Pseudomonas, a common soil bacteriumShow answer & explanation →
Q10.
Rhizobium bacteria fix nitrogen in symbiotic association with which type of plants?
A Paddy plantsB LegumesC Wheat plantsD FernsShow answer & explanation →
Q11.
Which step of sewage treatment involves the use of aerobic microorganisms?
A Primary treatmentB Tertiary treatmentC Secondary treatmentD ChlorinationShow answer & explanation →
Q12.
BOD stands for which of the following?
A Biological Oxygen DemandB Biochemical Oxygen DeficiencyC Bacterial Oxygen DemandD Biological Oxidation DepthShow answer & explanation →
Q13.
Which cyanobacterium forms a symbiotic association with the water fern Azolla?
A NostocB AnabaenaC SpirulinaD OscillatoriaShow answer & explanation →
Q14.
Citric acid is commercially produced using which microorganism?
A LactobacillusB SaccharomycesC Aspergillus nigerD StreptococcusShow answer & explanation →
Q15.
Which gas is the major component of biogas produced in sludge digesters?
A Carbon dioxideB Hydrogen sulfideC MethaneD NitrogenShow answer & explanation →
Q16.
Mycorrhiza is a symbiotic association between fungi and which part of a plant?
A LeavesB RootsC StemsD FlowersShow answer & explanation →
Q17.
Which microorganism produces the immunosuppressant drug cyclosporin A?
A Monascus purpureusB Aspergillus nigerC Trichoderma polysporumD Penicillium chrysogenumShow answer & explanation →
Q18.
What is the primary treatment of sewage mainly concerned with?
A Biological degradation of organic matter using activated sludgeB Removal of suspended solids by settling and screeningC Chlorination of treated water before final dischargeD Addition of coagulant chemicals to settle out solidsShow answer & explanation →
Q19.
Azotobacter is a bacterium that fixes nitrogen in which manner?
A Symbiotically in root nodulesB Freely in the soilC In partnership with legumesD Inside leaf cellsShow answer & explanation →
Q20.
Which microorganism is used as a biofertilizer to provide phosphorus to plants?
A RhizobiumB GlomusC AzotobacterD AnabaenaShow answer & explanation →
Q21.
The enzyme streptokinase used as a clot buster is produced by which organism?
A StreptococcusB AspergillusC PenicilliumD LactobacillusShow answer & explanation →
Q22.
Statins, which lower blood cholesterol, are produced by which microorganism?
A Trichoderma polysporumB Aspergillus nigerC Monascus purpureusD Penicillium notatumShow answer & explanation →
Q23.
Which type of microorganism belongs to the genus Nucleopolyhedrovirus used in biocontrol?
A BacteriumB FungusC BaculovirusD ProtozoanShow answer & explanation →
Q24.
Lactic acid fermentation is carried out by which microorganism?
A Aspergillus nigerB Saccharomyces cerevisiaeC LactobacillusD RhizobiumShow answer & explanation →
Q25.
Which organism is used to produce toddy, a traditional fermented drink?
A LactobacillusB Saccharomyces cerevisiaeC Aspergillus nigerD NostocShow answer & explanation →
Medium - 26 questions Q26.
Distillation is required to produce beverages like whisky and rum but not for wine and beer. What does this indicate?
A Wine and beer are not fermented at allB Whisky and rum have higher alcohol contentC Whisky and rum use bacteria instead of yeastD Wine and beer are distilled twiceShow answer & explanation →
Q27.
Methanogens such as Methanobacterium are found in the rumen of cattle and also in which human-made setup?
A Trickling filtersB Activated sludge tanksC Biogas plantsD Primary settling tanksShow answer & explanation →
Q28.
A farmer finds that his sewage treatment plant discharge has sharply reduced the dissolved oxygen in a stream. This is most likely because the effluent still had a high:
A Chlorine levelB Nitrogen contentC pH valueD BOD valueShow answer & explanation →
Q29.
Why is Trichoderma considered a useful biocontrol agent in agriculture?
A It controls several plant pathogens in rootsB It fixes atmospheric nitrogen in soilC It increases phosphorus in leavesD It produces antibiotics for humansShow answer & explanation →
Q30.
Ladybird and dragonflies are promoted in fields as biocontrol agents because they:
A Fix atmospheric nitrogen in the soilB Prey on pests like aphids and mosquitoesC Decompose crop residuesD Pollinate the flowersShow answer & explanation →
Q31.
The cry proteins (delta endotoxins) of Bacillus thuringiensis are stored in the bacterium as which structure?
A Flagella, used for bacterial motilityB Plasmids, small circular DNA moleculesC Spores and crystalsD Capsules, the protective outer layerShow answer & explanation →
Q32.
In sewage treatment, the term floc refers to which structure?
A A free-floating cluster of photosynthetic algal cellsB Masses of bacteria held together by fungal filamentsC A perforated filter bed used in the primary settling tankD Activated sludge specifically recycled from secondary settling tanksShow answer & explanation →
Q33.
Which enzyme produced by microorganisms is widely used in detergent formulations to remove oil stains?
A StreptokinaseB PectinaseC LipaseD AmylaseShow answer & explanation →
Q34.
What does a higher BOD value in a water sample indicate?
A The water is unusually clean and contains abundant dissolved oxygenB Greater level of organic pollution requiring more oxygenC The presence of dissolved heavy metals such as lead and mercuryD An unusually low level of bacterial metabolic activity in the sampleShow answer & explanation →
Q35.
Which free-living nitrogen-fixing bacterium is closely associated with the rhizosphere of grasses and cereals?
A RhizobiumB NostocC AzospirillumD AnabaenaShow answer & explanation →
Q36.
Cyclosporin A is used primarily for which medical purpose?
A As an antibiotic against bacterial infectionsB To lower blood cholesterol levelsC To prevent organ rejection after transplantsD As a clot-dissolving agentShow answer & explanation →
Q37.
Which microorganism acts as a biocontrol agent against plant pathogens by competing for nutrients and space?
A Bacillus thuringiensisB TrichodermaC PseudomonasD AzotobacterShow answer & explanation →
Q38.
In the production of cheese, the characteristic large holes in Swiss cheese are made by which organism?
A Penicillium roquefortiB Propionibacterium shermaniiC Lactobacillus helveticusD Streptococcus thermophilusShow answer & explanation →
Q39.
Roquefort cheese gets its characteristic flavour from which microorganism?
A LactobacillusB Saccharomyces cerevisiaeC Penicillium roquefortiD Aspergillus nigerShow answer & explanation →
Q40.
Biogas produced in sewage sludge digesters consists of which combination of gases?
A Methane and oxygenB Methane, CO<sub>2</sub>, and H<sub>2</sub>SC Hydrogen and nitrogenD Carbon monoxide and methaneShow answer & explanation →
Q41.
Pectinases and proteases produced by microbes are used in which industrial application?
A Production of antibioticsB Clarification of fruit juicesC Sewage treatmentD Nitrogen fixationShow answer & explanation →
Q42.
Which species of Pseudomonas is used as a bioherbicide against the weed Phalaris minor?
A Pseudomonas fluorescensB Pseudomonas putidaC Pseudomonas aeruginosaD Pseudomonas syringaeShow answer & explanation →
Q43.
Mycorrhizal fungi of the genus Glomus provide which benefit besides phosphorus absorption?
A Direct atmospheric nitrogen fixation within the root cortexB Resistance to root pathogens and tolerance to salinityC Production of plant growth hormones as their sole additional benefitD Conversion of soil urea into ammonia for plant uptakeShow answer & explanation →
Q44.
Which cyanobacterium is found free-living in paddy fields and fixes atmospheric nitrogen?
A Anabaena and NostocB Spirulina mainlyC Chlorella mainlyD Oscillatoria mainlyShow answer & explanation →
Q45.
The activated sludge produced during secondary sewage treatment is used for what purpose?
A Directly applied as fertilizer without any further treatment stepsB Pumped into anaerobic sludge digesters to produce biogasC Discharged directly into nearby rivers and streamsD Used to make biochar through pyrolysis treatmentShow answer & explanation →
Q46.
Which property makes baculoviruses especially suitable as biocontrol agents?
A They are broad-spectrum agents capable of killing nearly all insect pestsB They are species-specific and do not harm non-target organismsC They remain heat-stable and persist for years within soil reservoirsD They produce potent chemical toxins that diffuse through plant tissueShow answer & explanation →
Q47.
The process by which sewage effluent is released into water bodies after treatment is regulated by measuring which parameter?
A Mainly pH levels measured before dischargeB BOD reduction to acceptable levelsC Chlorine concentration measured before dischargeD Turbidity alone measured before dischargeShow answer & explanation →
Q48.
Streptokinase is classified as which type of enzyme based on its function?
A A lipase enzyme that breaks down dietary fats into fatty acidsB Thrombolytic enzyme that dissolves blood clotsC A protease enzyme commonly incorporated into laundry detergentsD An amylase enzyme that hydrolyzes starch into simple sugarsShow answer & explanation →
Q49.
Which component of Bacillus thuringiensis is responsible for its insecticidal activity?
A Flagellar proteins used for bacterial movementB Cry proteins encoded by cry genesC Capsular polysaccharides forming a protective layerD Endospore coat proteins protecting the dormant sporeShow answer & explanation →
Q50.
Which type of treatment in sewage management is associated with the term effluent?
A The raw, untreated sewage entering the treatment plant initiallyB The liquid that passes out after primary and secondary treatmentC The settled sludge collected from the bottom of the settling tanksD Biogas collected from the anaerobic sludge digestersShow answer & explanation →
Q51.
Biofertilizers differ from chemical fertilizers in what fundamental way?
A Biofertilizers contain synthetic nitrogen compounds manufactured industriallyB Biofertilizers use living microorganisms to enrich soil nutrients naturallyC Biofertilizers act considerably faster than conventional chemical fertilizersD Biofertilizers are effective exclusively within tropical soil conditionsShow answer & explanation →
Hard - 25 questions Q52.
In an aeration tank the activated sludge is allowed to settle, after which a small part of it is pumped back into the fresh effluent. What is the purpose of this returned sludge?
A To increase the BOD of the effluentB To supply oxygen to the tankC To act as inoculum of useful microbesD To remove heavy metals chemicallyShow answer & explanation →
Q53.
Cyclosporin A and statins are both microbial products used as medicines, but they differ in action. Which pairing is correct?
A Cyclosporin A lowers cholesterol; statins suppress immunityB Cyclosporin A is an antibiotic; statins are immunosuppressantsC Both lower blood cholesterol levelsD Cyclosporin A suppresses immunity; statins lower cholesterolShow answer & explanation →
Q54.
Statins lower blood cholesterol by acting competitively against which enzyme of the cholesterol synthesis pathway?
A The key enzyme of cholesterol synthesisB DNA polymerase enzyme of the liver cellsC Pepsin in the stomachD ATP synthase of mitochondriaShow answer & explanation →
Q55.
In a biogas plant, spent slurry from the digester is removed through an outlet regularly. Why is this slurry valued by farmers?
A It can be refermented to make more gasB It is used as excellent manureC It is a source of drinking waterD It is rich in methane gasShow answer & explanation →
Q56.
The toxin in Bacillus thuringiensis exists as inactive protoxin crystals inside the bacterium but kills insect larvae after being eaten. Which conditions convert it into the active toxin?
A The acidic pH of the human stomachB Exposure to sunlight on leavesC The alkaline pH of the insect gutD The anaerobic conditions in soilShow answer & explanation →
Q57.
The cry proteins of Bacillus thuringiensis become active toxins only after being ingested by an insect. What activates them in the insect gut?
A Acidic pH of the insect gut directly dissolves the protective protein crystals over timeB Alkaline pH of the insect gut along with gut proteases dissolve the crystalsC Insect digestive enzymes acting at a neutral gut pH alone, independent of pH levelD High temperature within the insect gut directly denatures the protective outer coatShow answer & explanation →
Q58.
In the Anabaena-Azolla symbiosis used in paddy fields, what is the specific role of each partner?
A Azolla fixes atmospheric N<sub>2</sub> directly while Anabaena supplies it with photosynthates insteadB Anabaena fixes N<sub>2</sub> providing it to Azolla which provides carbon through photosynthesisC Both Azolla and Anabaena independently fix nitrogen on their own without cooperating at allD Azolla supplies dissolved soil minerals while Anabaena instead supplies water uptake supportShow answer & explanation →
Q59.
Statins produced by Monascus purpureus lower cholesterol by which specific mechanism?
A Increasing hepatic bile acid production specifically to eliminate excess circulating cholesterolB Competitively inhibiting HMG-CoA reductase, a key enzyme in cholesterol biosynthesisC Blocking intestinal absorption of dietary cholesterol directly from ingested food sourcesD Enhancing LDL receptor expression mainly within liver cell membranes specificallyShow answer & explanation →
Q60.
During secondary sewage treatment, what is the significance of reducing BOD before discharge?
A To prevent offensive odors mainly, with little concern for aquatic oxygen levels overallB High BOD effluent would consume oxygen in receiving water bodies, causing death of aquatic lifeC To make the discharged water colorless mainly for aesthetic purposes and appearanceD To eliminate most pathogenic microorganisms present in the treated wastewater streamShow answer & explanation →
Q61.
Penicillium chrysogenum is now preferred over Penicillium notatum for industrial penicillin production. What is the main reason?
A P. chrysogenum produces a noticeably different type of penicillin compound entirelyB P. chrysogenum produces significantly higher yields of penicillinC P. chrysogenum is considerably easier to identify under standard laboratory conditionsD P. notatum was found to be mildly toxic to humans under certain conditionsShow answer & explanation →
Q62.
Which characteristic of mycorrhizal associations with Glomus makes them more efficient than plant roots alone at absorbing phosphorus?
A Mycorrhizal fungi produce phosphatase enzymes mainly, with little other contributionB The extraradical hyphae vastly increase the absorptive surface area beyond the root hair zoneC Mycorrhizal fungi convert organic phosphorus to plant-available forms mainly, rarely otherwiseD Glomus secretes phosphorus-mobilizing acids mainly in alkaline soils, rarely in acidic onesShow answer & explanation →
Q63.
The sludge digester in sewage treatment operates under anaerobic conditions. Which group of microorganisms is primarily responsible for methane production?
A Aerobic heterotrophsB Sulfate-reducing bacteriaC Methanogenic archaeaD Lactic acid bacteriaShow answer & explanation →
Q64.
Baculoviruses used in biocontrol have a narrow host range. This property is advantageous because it means they are what?
A Highly effective against essentially all agricultural pest species simultaneously and equallyB Safe for use in integrated pest management without harming beneficial insects, birds, and mammalsC Able to persist for considerably longer periods within the surrounding environment overallD Capable of being manufactured more cheaply than synthetic chemical pesticides generallyShow answer & explanation →
Q65.
In the context of biocontrol, Trichoderma species act against plant pathogens through which combination of mechanisms?
A Mainly through the production of antibiotic compounds released continuously into the surrounding soil environmentB Through mycoparasitism, antibiosis, competition for nutrients and space, and induced systemic resistanceC Mainly by outcompeting pathogens for available physical space within the surrounding soil matrixD Through secretion of broad-spectrum toxins that indiscriminately kill all other soil microbes present nearbyShow answer & explanation →
Q66.
Why are microbes considered essential for the efficient operation of biogas plants that use agricultural waste?
A Microbes mainly provide passive structural support within the digester tank, with little biochemical contribution in most cases under typical conditionsB Anaerobic microbes including bacteria and archaea hydrolyze, ferment, and ultimately convert organic waste to methane through a multi-stage processC Microbes present in the digester produce mainly carbon dioxide gas, with little methane generated at any stage according to standard textbooksD Mainly generally aerobic microbes are utilized throughout the entire biogas production process inside the digester in general practice as frequently describedShow answer & explanation →
Q67.
The heterocysts found in cyanobacteria like Anabaena are specialized cells for what purpose, and why are they necessary?
A Heterocysts are specialized photosynthetic cells that primarily function to generate extra oxygen for the surrounding filamentB Heterocysts are nitrogen-fixing cells with thick walls that exclude O<sub>2</sub>, protecting the oxygen-sensitive nitrogenase enzymeC Heterocysts store starch reserves specifically to sustain the cyanobacterial organism through periods of drought stressD Heterocysts secrete specific growth-regulating hormones that control the overall growth rate of the cyanobacterial filamentShow answer & explanation →
Q68.
The immunosuppressive action of cyclosporin A involves inhibiting which cellular pathway in T lymphocytes?
A The mTOR signaling pathway that governs T-cell growth and proliferationB The calcineurin-NFAT pathway, preventing cytokine gene transcriptionC The JAK-STAT pathway responsible for interferon-mediated signalingD The NF-kB pathway that mediates the broader inflammatory responseShow answer & explanation →
Q69.
In food fermentation, what is the precise role of lactic acid produced by Lactobacillus during curd formation?
A Lactic acid adds extra nutritional protein content directly to the finished curd product in most textbook accountsB Lactic acid lowers pH, causing casein proteins to denature and coagulate, forming the gel structure of curdC Lactic acid acts mainly as a preservative by killing most contaminating bacteria present during normal conditionsD Lactic acid contributes a distinct sweetness to the final curd product over time as generally observedShow answer & explanation →
Q70.
What distinguishes the mode of action of streptokinase from other fibrinolytic agents in clinical use?
A Streptokinase cleaves fibrin strands directly and immediately, rarely involving plasminogen in the process in typical laboratory settingsB Streptokinase forms a complex with plasminogen, inducing a conformational change that activates it to plasmin, which then lyses fibrin clotsC Streptokinase is clinically effective mainly on blood clots that are less than roughly one hour old under usual circumstances according to most researchersD Streptokinase inhibits thrombin activity directly mainly to prevent the formation of new blood clots in the majority of cases studied as widely reportedShow answer & explanation →
Q71.
In the context of biofertilizers, what is the term for the process by which Rhizobium infects legume roots and how do the plant and bacterium mutually signal each other?
A Infection occurs entirely at random throughout the root system, without any specific chemical signaling exchanged between the two partners at allB Nod factors produced by Rhizobium in response to flavonoids secreted by the legume root trigger root hair curling and infection thread formationC The legume root itself produces antibiotic compounds specifically designed to attract Rhizobium bacteria from the surrounding soilD Rhizobium bacteria mechanically penetrate root cells directly, without using any form of chemical signaling whatsoever between the partnersShow answer & explanation →
Q72.
Aspergillus niger produces citric acid through which metabolic pathway and under what culture conditions?
A Through the glyoxylate cycle under aerobic, high-sugar, low-pH conditionsB Through fermentation under anaerobic conditions at neutral pHC Through beta-oxidation of fatty acids under aerobic conditionsD Through the pentose phosphate pathway under nitrogen-rich conditionsShow answer & explanation →
Q73.
The genus Nucleopolyhedrovirus infects insects through which route, and what happens to infected cells?
A Through direct penetration of the insect skin surface, causing rapid and explosive lysis of host cells over time in standard practice under most conditions encounteredB Through ingestion of contaminated plant material, with the virus replicating in the nucleus forming polyhedral inclusion bodies that cause liquefaction of the insectC Through entry via the respiratory tract of the insect, leading to progressive damage of the central nervous system over time as frequently observed in practiceD Through transovarial infection of the developing egg, causing mainly relatively minor developmental defects in many documented cases according to conventional understandingShow answer & explanation →
Q74.
What is the key difference between the nitrogen fixation by Azotobacter and Rhizobium in terms of oxygen sensitivity and their respective solutions?
A Both of these soil bacteria are sometimes thought to rely on a largely identical molecular mechanism to shield their nitrogenase enzyme from oxygen damage in routine practice overall in most casesB Azotobacter uses high respiration rates and an alginate capsule to maintain low internal O<sub>2</sub>, while Rhizobium relies on leghaemoglobin (plant-produced) in nodules to buffer O<sub>2</sub> supply to bacteroidsC Mainly Rhizobium shows meaningful sensitivity to oxygen, while Azotobacter requires little protective mechanism by comparison under typical conditions according to standard textbooks in general practiceD Azotobacter instead relies specifically on specialized heterocysts, while Rhizobium relies mainly on the outer nodule cortex layer for protection as frequently described in most textbook accountsShow answer & explanation →
Q75.
The microorganism used to convert milk into curd is:
A LactobacillusB RhizobiumC PenicilliumD AspergillusShow answer & explanation →
Q76.
The antibiotic penicillin is obtained from the fungus:
A PenicilliumB LactobacillusC RhizobiumD SaccharomycesShow answer & explanation →