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Microbes in Human Welfare Set-3 MCQ Online Practice Test | Class 12 | Quiz Tweek

Microbes in Human Welfare Set-3

Practice Important MCQs for Microbes in Human Welfare — Biology, Class 12.

This chapter explains how microbes affect human life—both as harmful agents (causing diseases and contaminating water) and as beneficial tools (in sewage treatment, biogas production, antibiotics, and vaccines). Board and competitive exams commonly test key processes like BOD reduction, sewage treatment stages, antibiotic/immune mechanisms, microbial survival during storage, and ARG behavior under disinfection; hence strong conceptual clarity here is essential.

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Q1. A sewage sample has an influent biochemical oxygen demand (BOD) of $300\,\text{mg L}^{-1}$ . Primary treatment removes $25\%$ of the BOD and secondary treatment (activated sludge) removes $90\%$ of the BOD remaining after primary treatment. Calculate the BOD of the effluent after secondary treatment (in $\text{mg L}^{-1}$ ).

(A)

$75\,\text{mg L}^{-1}$

(B)

$150\,\text{mg L}^{-1}$

(C)

$22.5\,\text{mg L}^{-1}$

(D)

$45\,\text{mg L}^{-1}$

Q2. In industrial ethanol production by Saccharomyces cerevisiae the overall fermentation stoichiometry is $C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2$ . If a fermenter feeds $180\,\text{kg}$ of glucose per hour and the process achieves $80\%$ conversion efficiency, estimate the volume of ethanol produced per hour. (Molar masses: glucose $=180\,\text{g mol}^{-1}$ , ethanol $=46\,\text{g mol}^{-1}$ ; density of ethanol $=0.789\,\text{kg L}^{-1}$ .)

(A)

$93.3\,\text{L}$

(B)

$116.7\,\text{L}$

(C)

$58.4\,\text{L}$

(D)

$139.9\,\text{L}$

Q3. The capsular polysaccharide of some bacterial pathogens is a poor immunogen in infants; conjugating this polysaccharide to a protein carrier improves vaccine efficacy. Which of the following best explains the immunological mechanism and an additional advantage conferred by conjugation?

(A)

Conjugation increases polysaccharide size causing stronger B-cell receptor cross-linking and therefore a robust T cell–independent $IgM$ response that provides long-term immunity.

(B)

Conjugation promotes uptake by antigen-presenting cells and presentation of carrier-derived peptides on MHC II, recruiting T-helper cells that provide B-cell help leading to isotype switching to $IgG$ and generation of memory B cells.

(C)

Conjugation masks antigenic variability of the polysaccharide so that complement-mediated lysis is enhanced but T-cell involvement is unnecessary.

(D)

Conjugation simply increases antigen dose and elicits a stronger innate immune response, which reduces the need for booster doses.

Q4. Antibiotic production by Streptomyces spp. is typically associated with secondary metabolism and is enhanced under particular fermentation conditions. Which operational strategy is most likely to increase antibiotic yield in an industrial batch culture?

(A)

Keep abundant readily utilizable carbon and nitrogen throughout the run to maximize exponential growth and antibiotic output.

(B)

Operate a continuous high-dilution culture to maintain cells at their maximum specific growth rate.

(C)

Adjust fermentation pH to strongly acidic values to denature primary metabolic enzymes and redirect resources to antibiotic biosynthesis.

(D)

Introduce controlled limitation of key nutrients (e.g., phosphate or nitrogen) or restrict carbon supply to promote transition to stationary phase and induction of secondary metabolism (antibiotic biosynthesis).

Q5. A probiotic powder initially contains $10^{10}$ CFU g $^{-1}$ . During storage the viable count falls by $5\%$ per day (daily surviving fraction $=0.95$ ). Using $N=N_0\times 0.95^t$ , what is the maximum storage time (in whole days, rounded down) after which the product still meets the label claim of at least $10^{9}$ CFU g $^{-1}$ ?

(A)

$30\ \text{days}$

(B)

$40\ \text{days}$

(C)

$44\ \text{days}$

(D)

$60\ \text{days}$

Q6. In a household biogas plant 1000 kg of cow dung with 8% total solids (TS) is fed to the digester. If the average methane yield is $0.05\ \mathrm{m^3\;CH_4}$ per kg TS, what is the approximate volume of methane produced? (Use $V_{CH_4}=\text{TS mass}\times Y_{CH_4}$ .)

(A)

$2\ \mathrm{m^3}$

(B)

$40\ \mathrm{m^3}$

(C)

$4\ \mathrm{m^3}$

(D)

$0.4\ \mathrm{m^3}$

Q7. Raw sewage has an initial BOD of $240\ \mathrm{mg\;L^{-1}}$ . A primary clarifier removes $30\%$ of BOD and the secondary biological treatment removes $80\%$ of the remaining BOD. Using $B_{\text{final}}=B_0(1-r_1)(1-r_2)$ , what is the effluent BOD and does it meet the discharge standard of $<30\ \mathrm{mg\;L^{-1}}$ ?

(A)

$33.6\ \mathrm{mg\;L^{-1}}$ ; No, it does not meet the standard.

(B)

$21.0\ \mathrm{mg\;L^{-1}}$ ; Yes, it meets the standard.

(C)

$70.0\ \mathrm{mg\;L^{-1}}$ ; No, it does not meet the standard.

(D)

$48.0\ \mathrm{mg\;L^{-1}}$ ; No, it does not meet the standard.

Q8. In an industrial antibiotic fermentation the broth at harvest contains $3\ \mathrm{g\;L^{-1}}$ antibiotic in a $5000\ \mathrm{L}$ fermenter. Downstream recovery yields $60\%$ of the antibiotic mass and the purified product is formulated at $20\ \mathrm{g\;L^{-1}}$ . Using $M_{\text{total}}=C\times V$ and $V_{\text{purified}}=\dfrac{M_{\text{total}}\times\text{recovery}}{C_{\text{final}}}$ , what volume of purified antibiotic solution is obtained?

(A)

$750\ \mathrm{L}$

(B)

$150\ \mathrm{L}$

(C)

$225\ \mathrm{L}$

(D)

$450\ \mathrm{L}$

Q9. Assertion (A): Application of treated sewage sludge (biosolids) as a soil amendment increases soil organic carbon and improves water‑holding capacity.
Reason (R): Anaerobic digestion of sewage sludge generates biogas; capture and utilization of this biogas reduces greenhouse gas emissions compared to uncontrolled disposal.

(A)

Both A and R are true and R is the correct explanation of A.

(B)

Both A and R are true but R does not explain A.

(C)

A is true but R is false.

(D)

A is false but R is true.

Q10. Hospital wastewater containing antibiotics is treated by primary sedimentation, activated sludge and final chlorination. Culture‑based counts of antibiotic‑resistant bacteria (ARB) drop by 99% after chlorination, but quantitative PCR shows only a 20% reduction in antibiotic resistance gene (ARG) copy numbers. Which of the following best explains this observation and suggests an effective mitigation step?

(A)

Chlorination inactivates culturable cells but can leave extracellular DNA (eDNA) containing ARGs intact; eDNA can be taken up by competent environmental bacteria (natural transformation). Therefore, tertiary treatments that degrade nucleic acids (e.g., UV irradiation or advanced oxidation) should be included to reduce ARG dissemination.

(B)

Chlorination increases ARG copy numbers because it induces prophage activation in surviving bacteria that package and release ARGs as phage particles; hence simply increasing chlorine dose will solve the problem.

(C)

The qPCR result is misleading because qPCR detects only RNA transcripts from active genes; switching to culture‑based methods for regulatory decisions will accurately reflect ARG risk.

(D)

Chlorination transforms plasmid‑borne ARGs into chromosomal integrons making them more persistent; therefore adding excess organic carbon to the aeration tank will dilute ARG concentration.

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