General Microbiology — MCQs

General Microbiology Indian Medical PG Practice Questions and MCQs

Question 71: The Phenol test or Reidel-Walker test is done to determine what?

A. Hardness of water
B. Chlorine demand
C. Quality of a disinfectant
D. Efficacy of a disinfectant (Correct Answer)

Explanation: **Explanation:** The **Rideal-Walker (RW) test** is a standardized laboratory method used to determine the **efficacy of a disinfectant** by comparing its germicidal power to that of pure phenol. **1. Why the correct answer is right:** The test calculates the **Phenol Coefficient**. A specific microorganism (usually *Salmonella typhi*) is exposed to varying dilutions of the test disinfectant and phenol under controlled conditions. The phenol coefficient is derived by dividing the highest dilution of the disinfectant that kills the organism in 7.5 minutes (but not in 5 minutes) by the corresponding dilution of phenol. A coefficient >1 indicates the disinfectant is more effective than phenol. **2. Why the incorrect options are wrong:** * **Hardness of water (A):** This is measured using EDTA titration (complexometric method) to determine calcium and magnesium ion concentration. * **Chlorine demand (B):** This is determined using **Horrock’s Apparatus**, which calculates the amount of bleaching powder required to disinfect a specific volume of water. * **Quality of a disinfectant (C):** While related, "efficacy" is the precise pharmacological/microbiological term for the killing power measured by this test. "Quality" is a broader term involving stability, shelf-life, and safety. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Chick-Martin Test:** A modification of the RW test that uses organic matter (like dried feces) to simulate real-world conditions, making it more practical. * **In-use Test (Kelsey-Maurer Test):** Used to check if the disinfectant solution currently being used in hospital wards/theatres is contaminated or has lost its potency. * **Standard Organisms used:** *Salmonella typhi*, *Staphylococcus aureus*, and *Pseudomonas aeruginosa*. * **Limitation:** The RW test does not account for the presence of organic matter, which often neutralizes disinfectants in clinical settings.

Question 72: Which of the following microorganisms is described as "strictly intracellular but not cultivable"?

A. Mycobacterium leprae (Correct Answer)
B. Mycobacterium tuberculosis
C. Treponema pallidum
D. Salmonella

Explanation: **Explanation:** The correct answer is **Mycobacterium leprae**. **1. Why Mycobacterium leprae is correct:** *M. leprae*, the causative agent of Leprosy (Hansen’s disease), is a unique pathogen because it is an **obligate intracellular** organism that has never been successfully grown on artificial (cell-free) culture media or tissue culture. It lacks many essential genes for independent metabolism (reductive evolution). In the laboratory, it can only be cultivated in living animals, specifically the **footpads of mice** (Shepard’s method) or **nine-banded armadillos**. **2. Analysis of Incorrect Options:** * **Mycobacterium tuberculosis:** While it is a facultative intracellular pathogen, it is **cultivable** on artificial media like Lowenstein-Jensen (LJ) medium or liquid systems like BACTEC. * **Treponema pallidum:** This organism is the causative agent of Syphilis. It is indeed **non-cultivable** on artificial media; however, it is **extracellular** in nature (found in interstitial spaces), not strictly intracellular. * **Salmonella:** This is a **facultative intracellular** bacterium that grows easily on standard laboratory media like Blood Agar or MacConkey Agar. **3. NEET-PG High-Yield Pearls:** * **Generation Time:** *M. leprae* has the longest generation time among bacteria (~12–13 days), contributing to the long incubation period of leprosy. * **Staining:** It is Acid-Fast (Ziehl-Neelsen stain) but is **less acid-fast** than *M. tuberculosis*; hence, 5% sulfuric acid is used for decolorization instead of 20%. * **Target Cells:** It shows a unique tropism for **Schwann cells** (leading to nerve damage) and macrophages. * **Other Non-cultivable organisms:** *Tropheryma whipplei* and *Treponema pallidum* are also non-cultivable on artificial media, but *M. leprae* is the classic "intracellular" example.

Question 73: LED media is preferred over MacConkey media for the isolation of bacteria from urine specimens because it:

A. is a differential medium
B. inhibits the swarming of Proteus species
C. promotes the growth of Pseudomonas species
D. promotes the growth of Staphylococcus aureus and Candida species (Correct Answer)

Explanation: **CLED (Cystine-Lactose-Electrolyte-Deficient) Agar** is the standard non-inhibitory culture medium used for routine urine cultures. It is preferred over MacConkey agar primarily because of its **broader growth spectrum**. ### Why Option D is Correct: While MacConkey agar is selective for Gram-negative bacilli and inhibits most Gram-positive organisms and fungi, **CLED agar is non-selective**. It supports the growth of common urinary pathogens that MacConkey might miss, specifically **Staphylococcus aureus, Enterococci, and Candida species**. In clinical practice, these organisms are significant causes of UTIs (especially hospital-acquired or catheter-associated), making CLED a more reliable primary screening tool. ### Why Other Options are Incorrect: * **Option A:** Both CLED and MacConkey are differential media (using lactose fermentation to distinguish colonies). This is not a reason to prefer one over the other. * **Option B:** While CLED **does** inhibit the swarming of *Proteus* (due to its electrolyte-deficient nature), MacConkey agar also inhibits *Proteus* swarming (due to its bile salt content). Therefore, this is not a unique advantage of CLED. * **Option C:** Both media support the growth of *Pseudomonas*, though its characteristic pigment (pyocyanin) is often more visible on CLED. ### NEET-PG High-Yield Pearls: * **Electrolyte Deficiency:** The lack of electrolytes (specifically NaCl) in CLED is what prevents the swarming of *Proteus*. * **Indicator:** CLED uses **Bromothymol Blue** as a pH indicator (Lactose fermenters appear yellow; non-lactose fermenters appear blue/green). * **Cystine:** Added to support the growth of cystine-dependent "dwarf colony" coliforms. * **Quantitative Culture:** CLED is ideal for the "Calibrated Loop" method to determine significant bacteriuria (Kass criteria: >10⁵ CFU/mL).

Question 74: What procedure is used for the analysis of RNA?

A. Northern blot (Correct Answer)
B. Southern blot
C. Western blot
D. Eastern blot

Explanation: **Explanation:** The correct answer is **Northern blot**. This technique is a laboratory method used to detect specific **RNA** molecules among a mixture of RNA. It involves the use of electrophoresis to separate RNA samples by size, followed by detection with a hybridization probe complementary to part of or the entire target sequence. **Analysis of Options:** * **Southern blot:** Developed by Edwin Southern, this technique is used for the detection of specific **DNA** sequences. (Mnemonic: **S**outhern = **D**NA). * **Western blot:** This procedure is used to detect specific **proteins** in a tissue homogenate or extract. It uses gel electrophoresis to separate native or denatured proteins. (Mnemonic: **W**estern = **P**rotein). * **Eastern blot:** This is an extension of the western blot used to analyze **post-translational modifications** of proteins, such as lipids, phosphomoieties, and glycoconjugates. **High-Yield NEET-PG Pearls:** To remember these easily, use the **SNOW DROP** mnemonic: * **S**outhern — **D**NA * **N**orthern — **R**NA * **O** — (nothing) * **W**estern — **P**rotein **Clinical Relevance:** * **Western Blot** was historically the "gold standard" confirmatory test for **HIV** (detecting antibodies against gp120, gp41, and p24), though it has largely been replaced by 4th generation immunoassays and NAT. * **Northern Blotting** is essential in studying gene expression by measuring mRNA levels, which helps in understanding disease pathogenesis and cellular responses to treatment.

Question 75: Which of the following is used as an antibacterial agent?

A. Lytic bacteriophage
B. Lysogenic bacteriophage (Correct Answer)
C. Plasmid
D. Transposon

Explanation: **Explanation:** The correct answer is **Lysogenic bacteriophage**. This concept revolves around **Lysogenic Conversion**, where a temperate (lysogenic) bacteriophage infects a bacterium and integrates its genome into the bacterial chromosome as a prophage. This process can introduce new genetic traits to the host bacterium, including the production of potent exotoxins. **Why Lysogenic Bacteriophage is correct:** Several medically important bacterial toxins are encoded by lysogenic phages rather than the bacterial chromosome itself. Without the presence of these specific phages, the bacteria remain non-pathogenic. A classic mnemonic for toxins acquired via lysogenic conversion is **ABCDES**: * **A:** Group A *Streptococcus* (Pyrogenic exotoxin/Scarlet fever) * **B:** *Botulinum* toxin * **C:** *Cholera* toxin * **D:** *Diphtheria* toxin * **E:** *E. coli* (Shiga-like toxin/VTEC) * **S:** *Shiga* toxin **Analysis of Incorrect Options:** * **Lytic bacteriophage:** These phages follow the lytic cycle, leading to the immediate multiplication of viruses and the death (lysis) of the host cell. While they kill bacteria, they are not the primary mechanism for toxin production (virulence). * **Plasmid:** These are extrachromosomal DNA molecules. While they often carry antibiotic resistance genes (R-plasmids), they are distinct from bacteriophages. * **Transposon:** Known as "jumping genes," these are mobile genetic elements that move within or between DNA molecules. They contribute to genetic diversity but are not independent viral agents. **NEET-PG High-Yield Pearls:** * **Diphtheria toxin** is the classic example of lysogenic conversion; only strains of *C. diphtheriae* infected by the **Beta-phage** cause disease. * **Transduction** is the process of genetic transfer mediated by bacteriophages (Generalized = Lytic; Specialized = Lysogenic).

Question 76: The "inveed fir tree" appearance on a gelatin stab is characteristic of which organism?

A. Mycoplasma
B. Bacillus anthracis (Correct Answer)
C. Clostridium
D. Bacteroides

Explanation: **Explanation:** The **"inverted fir tree"** appearance is a classic laboratory finding for **Bacillus anthracis**. This phenomenon occurs when the organism is inoculated into a **gelatin stab culture**. *B. anthracis* is non-motile and possesses weak proteolytic activity. As it grows along the line of the puncture, liquefaction starts at the top and radiates downwards, with the maximum growth occurring near the surface (where oxygen is available), creating the characteristic tapering, downward-branching appearance resembling an upside-down fir tree. **Analysis of Options:** * **A. Mycoplasma:** These are the smallest free-living organisms and lack a cell wall. On solid media (like PPLO agar), they produce a characteristic **"fried egg" colony** appearance, not a fir tree pattern. * **C. Clostridium:** While some species of *Clostridium* (like *C. perfringens*) can liquefy gelatin, they are obligate anaerobes. They would not show the oxygen-dependent "inverted" growth pattern seen in the aerobic *B. anthracis*. * **D. Bacteroides:** These are Gram-negative anaerobic bacilli. They are typically cultured on specialized media like BBE (Bacteroides Bile Esculin) agar and do not exhibit this specific growth morphology. **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used for presumptive diagnosis of *B. anthracis* using polychrome methylene blue to visualize the **M’Fadyean capsule** (composed of D-glutamic acid). * **Medusa Head Colonies:** On blood agar, *B. anthracis* produces large, non-hemolytic, greyish-white colonies with wavy margins (resembling locks of hair). * **String of Pearls Reaction:** Growth on agar containing low concentrations of penicillin leads to the formation of large, spherical bacilli resembling a string of pearls. * **Bamboo Stick Appearance:** On Gram stain, the bacilli appear in long chains with squared-off ends.

Question 77: Which of the following methods is used for the sterilization of fibre optics?

A. Glutaraldehyde (Correct Answer)
B. Chlorine
C. Autoclave
D. Phenol

Explanation: **Explanation:** The correct answer is **Glutaraldehyde**. Fibre optic instruments, such as endoscopes, bronchoscopes, and cystoscopes, are delicate and heat-sensitive. They cannot withstand the high temperatures of an autoclave or the corrosive nature of many common disinfectants. **1. Why Glutaraldehyde is correct:** Glutaraldehyde (specifically a 2% buffered solution, often known by the brand name Cidex) is a high-level disinfectant and chemical sterilant. It works by alkylating amino, carboxyl, and hydroxyl groups, which halts protein synthesis. It is the preferred agent for fibre optics because it is **non-corrosive** to metals, does not damage lenses or rubber/plastic components, and is effective against a broad spectrum of pathogens, including spores (when immersed for 10 hours). **2. Why other options are incorrect:** * **Chlorine:** While effective for water treatment and surface disinfection (e.g., HIV/HBV spills), it is highly corrosive to metals and can damage the delicate components of fibre optic scopes. * **Autoclave:** This uses moist heat at high pressure (121°C). The intense heat and pressure would melt the adhesives and damage the delicate optical fibres and lenses. * **Phenol:** Phenols are primarily used for disinfecting floors and surfaces. They are protoplasmic poisons that are too toxic for medical instruments and can damage synthetic materials. **Clinical Pearls for NEET-PG:** * **Exposure Time:** For high-level disinfection (killing most bacteria/viruses), 20 minutes of immersion in 2% glutaraldehyde is required. For **sterilization** (killing spores), **10 hours** is necessary. * **Shelf Life:** Once activated by adding an alkalizing agent, the solution is stable for **14 days**. * **Alternative:** **Ortho-phthalaldehyde (OPA)** is increasingly preferred over glutaraldehyde as it is more stable, faster-acting, and less irritating to the skin and eyes.

Question 78: What is the culture media of choice for growing most fungi?

A. Dorsett egg media
B. LJ media
C. Loeffler serum slope media
D. Sabouraud's agar (Correct Answer)

Explanation: **Explanation:** **Sabouraud’s Dextrose Agar (SDA)** is the standard medium used for the primary isolation and cultivation of most fungi. Its effectiveness lies in its specific composition: * **Low pH (around 5.4 - 5.6):** This acidic environment inhibits the growth of most bacteria while allowing fungi to flourish. * **High Glucose Concentration:** It contains 4% dextrose, which provides a rich energy source for fungal growth. * **Selectivity:** It can be further modified with antibiotics like chloramphenicol (to inhibit bacteria) or cycloheximide (to inhibit saprophytic fungi) for more selective isolation. **Analysis of Incorrect Options:** * **Dorsett Egg Media:** Used primarily for the cultivation of *Mycobacterium tuberculosis*. It is an egg-based medium used as an alternative to LJ media. * **LJ (Lowenstein-Jensen) Media:** The gold standard solid medium for the growth of *Mycobacterium tuberculosis*. It contains malachite green to inhibit contaminating flora. * **Loeffler Serum Slope Media:** Specifically used for the cultivation of *Corynebacterium diphtheriae*. It enhances the development of characteristic metachromatic granules. **NEET-PG High-Yield Pearls:** * **Modified SDA:** When SDA is supplemented with antibiotics, it is often referred to as "Mycosel" or "Mycobiotic" agar. * **Incubation:** Fungi are typically incubated at **25°C (Room Temperature)** for molds and **37°C** for yeast/dimorphic fungi. * **Alternative Media:** For fastidious fungi or to study specific morphologies, **Corn Meal Agar** (for *C. albicans* chlamydospore production) or **Potato Dextrose Agar (PDA)** are used.

Question 79: What is the best method of sterilization for dusting powder?

A. Autoclaving
B. Hot air oven (Correct Answer)
C. Inspissation
D. Tyndallisation

Explanation: **Explanation:** The correct method for sterilizing dusting powder is the **Hot Air Oven** (Dry Heat Sterilization). **Why Hot Air Oven is correct:** Dusting powders, along with oils, grease, fats, and sharp instruments, are best sterilized by dry heat. The primary reason is that dry heat has high penetrating power for anhydrous substances. Unlike moist heat, dry heat does not cause clumping or moisture-related degradation of the powder, ensuring it remains free-flowing and chemically stable. It acts by causing oxidative damage to microbial proteins and electrolytes. The standard cycle is **160°C for 2 hours**. **Why other options are incorrect:** * **Autoclaving (Moist Heat):** This is unsuitable for powders because steam cannot penetrate the substance effectively. Furthermore, the moisture causes the powder to cake, clump, and lose its physical properties. * **Inspissation:** This is a method used to sterilize media containing high amounts of protein (e.g., Lowenstein-Jensen or Loeffler’s serum slope) by heating at 80-85°C for 3 successive days. It is insufficient for powders. * **Tyndallisation:** Also known as intermittent sterilization, it uses moist heat at 100°C for 20 minutes on three consecutive days. It is used for heat-sensitive media containing sugar or gelatin, not for powders. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Control:** The biological indicator for Hot Air Oven is *Bacillus atrophaeus* (formerly *B. subtilis* var. *niger*). * **Sharp Instruments:** Hot air oven is preferred over autoclaving for sharps (like scalpels) to prevent rusting and dulling of the edges. * **Glassware:** It is the method of choice for glass syringes, petri dishes, and flasks.

Question 80: In which phase of the bacterial growth curve are toxins released?

A. Lag phase
B. Log phase
C. Stationary phase (Correct Answer)
D. Decline phase

Explanation: ### Explanation The correct answer is **Stationary phase**. **1. Why the Stationary Phase is Correct:** The stationary phase occurs when the rate of bacterial growth equals the rate of bacterial death due to the depletion of nutrients and the accumulation of toxic metabolic byproducts. During this phase, bacteria undergo significant physiological shifts to survive stress. This is the primary stage where **secondary metabolites**, such as **exotoxins** and **antibiotics**, are produced and released. Additionally, for certain bacteria, the initiation of sporulation occurs during this phase. **2. Why Other Options are Incorrect:** * **Lag Phase:** This is a period of intense metabolic activity and enzyme synthesis but **no increase in cell number**. Bacteria are adapting to their new environment; no toxins are released here. * **Log (Exponential) Phase:** This is the phase of rapid cell division where generation time is constant. While bacteria are most metabolically active and susceptible to antibiotics (like Penicillin), toxin production is generally minimal compared to the stationary phase. * **Decline (Death) Phase:** This phase is characterized by a decrease in the number of viable bacteria. While some intracellular toxins (endotoxins) may be released due to cell lysis, the active physiological secretion of toxins is a hallmark of the stationary phase. **3. NEET-PG High-Yield Clinical Pearls:** * **Morphology:** Bacteria show maximum uniformity in size and shape during the **Log phase**. * **Antibiotic Sensitivity:** Bactericidal drugs (e.g., Beta-lactams) are most effective during the **Log phase** because they target actively dividing cells. * **Spores:** Sporulation typically begins at the end of the Log phase or during the **Stationary phase**. * **Gram Stain:** Best results are obtained from cultures in the **Log phase**; older cultures in the stationary phase may show variable staining.

Practice by Chapter

History and Scope of Microbiology

Practice Questions

Classification of Microorganisms

Practice Questions

Bacterial Morphology and Structure

Practice Questions

Bacterial Physiology and Metabolism

Practice Questions

Bacterial Genetics

Practice Questions

Microbial Growth and Nutrition

Practice Questions

Sterilization and Disinfection

Practice Questions

Bacterial Identification Methods

Practice Questions

Normal Microbiota and Pathogenicity

Practice Questions

Antimicrobial Susceptibility Testing

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free