Microbiologist (Bacteriologist) — Kerala Water Authority — 2024 — 29 Jul 2024 — Provisional Answer Key — Kerala PSC PYQ Practice with Answers

Correct answer (Option C):\nChlorination is one of the most widely used chemical methods for disinfecting drinking water globally. It involves adding chlorine or chlorine compounds (like sodium hypochlorite) to water to kill pathogenic microorganisms, bacteria, and viruses, ensuring the water is safe for public consumption and preventing waterborne diseases.\n\nWhy others are wrong:\nOption A is used mainly for milk and beverages to reduce pathogen load without spoiling flavor. Option B physically removes particles and some microbes but does not chemically destroy hidden viral or bacterial entities unless microscopic membrane pore scales are deployed. Option D is a metabolic process used in food production, not purification.\n\nStudy tip:\nChlorine leaves a residual disinfectant trace in distribution systems, protecting treated water against re-contamination as it travels to consumers.

Correct answer (Option B):\nSterilization is an absolute process that completely eliminates, destroys, or kills all forms of microbial life, including vegetative bacterial cells, fungi, viruses, and highly resistant bacterial endospores. It is achieved using physical or chemical methods like autoclaving, dry heat, radiation, or gaseous sterilants.\n\nWhy others are wrong:\nOption A reduces pathogenic microbial populations on inanimate surfaces but may not destroy resistant endospores. Option C lowers microbial counts to safe public health levels through cleaning. Option D refers to applying chemical agents to living tissue to destroy or inhibit pathogens.\n\nStudy tip:\nRemember that sterilization is binary: an object is either completely sterile or it is not. There is no such thing as partially sterile.

Correct answer (Option C):\nPhase-contrast microscopy utilizes an optical mechanism that converts minute variations in the refractive index and thickness of cellular structures into visible differences in light intensity. This allows the high-contrast observation of living, unstained cells and transparent microorganisms in their natural state.\n\nWhy others are wrong:\nOption A requires staining for transparent objects, which typically kills the specimen. Option B makes specimens appear bright against a dark background using scattered light but is less effective for internal structures. Option D requires tagging the specimens with specific fluorescent dyes.\n\nStudy tip:\nFrits Zernike won the Nobel Prize in Physics in 1953 for inventing phase-contrast microscopy, which revolutionized the study of living cellular dynamics.

Correct answer (Option B):\nThe resolving power, or resolution, of a light microscope is its ability to distinguish two close points as separate entities. According to Abbe's equation, resolution depends on the wavelength of light and the numerical aperture (NA) of the objective lens. Higher NA captures more diffracted light, improving resolution.\n\nWhy others are wrong:\nOption A increases the size of the image but does not add fine structural detail (empty magnification). Option C dictates the wavelength, which influences limits, but the objective lens aperture governs the actual light-gathering capacity. Option D alters specimen visibility and contrast but does not change the physical resolution limits of the optics.\n\nStudy tip:\nUsing immersion oil with a 100× objective lens increases the numerical aperture by matching the refractive index of glass, preventing light refraction.

Correct answer (Option B):\nA chemostat is a specialized bioreactor designed for continuous culture. It maintains a bacterial population in a constant exponential growth phase by continuously pumping fresh nutrient medium into the vessel while simultaneously removing an equal volume of spent culture fluid and excess cells at a constant rate.\n\nWhy others are wrong:\nOption A is a closed system with finite nutrients where bacteria progress through lag, log, stationary, and decline phases. Options C and D are static vessels used for batch growth or isolation, failing to sustain a continuous physiological steady state.\n\nStudy tip:\nIn a chemostat, the growth rate is controlled by manipulating the dilution rate, while the cell density is controlled by the concentration of a limiting nutrient.