General Microbiology — MCQs

General Microbiology Indian Medical PG Practice Questions and MCQs

Question 81: What is the diluent used for trimethoprim in antibiotic disc for sensitivity testing?

A. Sodium hydroxide solution
B. Sodium bicarbonate
C. Ethanol
D. Acetic acid (Correct Answer)

Explanation: ### Explanation The correct answer is **Acetic acid**. **1. Why Acetic Acid is Correct:** In antimicrobial susceptibility testing (AST), the preparation of antibiotic stock solutions requires specific solvents and diluents based on the drug's solubility and stability. **Trimethoprim** is a weak base that is poorly soluble in water but highly soluble in acidic environments. Therefore, **glacial acetic acid** (or 0.05 N HCl) is used as the primary solvent to ensure the drug dissolves completely before being diluted to the required concentration for disc impregnation or MIC (Minimum Inhibitory Concentration) testing. **2. Analysis of Incorrect Options:** * **Sodium hydroxide (NaOH) / Sodium bicarbonate:** These are alkaline solvents. They are typically used for drugs that are acidic in nature, such as certain sulfonamides or beta-lactams (e.g., Ampicillin), but would not effectively dissolve trimethoprim. * **Ethanol:** While ethanol is used as a solvent for certain water-insoluble drugs like Erythromycin or Chloramphenicol, it is not the standard diluent for Trimethoprim in standardized CLSI (Clinical & Laboratory Standards Institute) protocols. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Co-trimoxazole Ratio:** In disc diffusion (Kirby-Bauer method), the ratio of Trimethoprim to Sulfamethoxazole is **1:19** (1.25 µg TMP + 23.75 µg SMX). * **Media Choice:** Mueller-Hinton Agar (MHA) is the standard. However, for Trimethoprim testing, the media must be low in **thymidine**; high levels of thymidine can antagonize the drug, leading to false resistance. * **Mechanism of Action:** Trimethoprim inhibits **dihydrofolate reductase**, preventing the conversion of dihydrofolate to tetrahydrofolate. * **Solvent Memory Tip:** Remember "T-A" (Trimethoprim-Acetic Acid) and "S-B" (Sulfonamides-Base/NaOH).

Question 82: Which of the following cellular organisms will not have a cell wall?

A. Prokaryotes
B. Eukaryotes
C. Protozoa (Correct Answer)
D. Fungi

Explanation: **Explanation:** The presence or absence of a cell wall is a fundamental characteristic used to classify microorganisms. A cell wall is a rigid layer located outside the cell membrane that provides structural support and protection. **Why Protozoa is the Correct Answer:** Protozoa are unicellular **eukaryotic** organisms that lack a rigid cell wall. Instead, they are bounded by a flexible cell membrane (plasmalemma) or a thin, elastic proteinaceous layer called a **pellicle**. The absence of a cell wall allows many protozoa (like *Amoeba*) to change shape and ingest food through phagocytosis, which would be impossible if a rigid wall were present. **Analysis of Incorrect Options:** * **Prokaryotes (A):** Most prokaryotes (Bacteria) possess a complex cell wall made of **peptidoglycan** (murein). *Exception:* Mycoplasma is the only genus of bacteria that naturally lacks a cell wall. * **Eukaryotes (B):** This is a broad category. While animals and protozoa lack cell walls, other eukaryotes like plants (cellulose) and fungi (chitin) possess them. Therefore, "Eukaryotes" as a whole is an incorrect generalization. * **Fungi (D):** Fungi possess a rigid cell wall primarily composed of **Chitin** (a polymer of N-acetylglucosamine), along with glucans and proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Mycoplasma:** The only bacteria lacking a cell wall; hence, they are naturally resistant to Beta-lactam antibiotics (like Penicillin) which target cell wall synthesis. * **Sterols:** While most prokaryotes lack sterols in their cell membrane, *Mycoplasma* contains them (acquired from the host). * **Fungal Wall Target:** Echinocandins (e.g., Caspofungin) act by inhibiting the synthesis of β-(1,3)-D-glucan, a key component of the fungal cell wall. * **L-forms:** These are bacteria that have lost their cell wall due to adverse conditions but can still replicate.

Question 83: Which of the following statements regarding disinfectants is FALSE?

A. Gluteraldehyde is sporicidal.
B. Hypochlorites are virucidal.
C. Ethylene oxide is an intermediate-level disinfectant. (Correct Answer)
D. Phenol usually requires organic matter to be effective.

Explanation: **Explanation:** The correct answer is **C** because **Ethylene oxide (EtO)** is not an intermediate-level disinfectant; it is a **high-level disinfectant** and, more accurately, a chemical **sterilant**. It is a gaseous agent used for heat-sensitive materials (like endoscopes and plastic syringes) because it can kill all forms of microbial life, including highly resistant bacterial spores, by alkylating proteins and nucleic acids. **Analysis of other options:** * **A. Glutaraldehyde is sporicidal:** This is true. Glutaraldehyde (e.g., Cidex) is a high-level disinfectant used for "cold sterilization" of endoscopes. It requires a long contact time (usually 3–10 hours) to achieve sporicidal action. * **B. Hypochlorites are virucidal:** This is true. Sodium hypochlorite is highly effective against a wide range of viruses, including HIV and Hepatitis B. It is the disinfectant of choice for cleaning blood spills. * **D. Phenol usually requires organic matter to be effective:** This is a slightly nuanced point. While phenol is one of the few disinfectants that *retains* activity in the presence of organic matter (unlike alcohols or halogens), it does not "require" it to work. However, in the context of NEET-PG, the false statement is clearly Option C, as EtO's classification as a sterilant is a fundamental microbiology fact. **NEET-PG High-Yield Pearls:** * **Sterilization vs. Disinfection:** Sterilization kills **all** microbes including spores; Disinfection does not necessarily kill spores. * **Chick-Martin Test / Rideal-Walker Coefficient:** Used to determine the efficacy of disinfectants compared to Phenol. * **Prions:** The most resistant infectious agents; they require autoclaving at 134°C for 18 minutes or 1N NaOH for 1 hour. * **Plasma Sterilization:** Uses Hydrogen peroxide vapor; it is the modern replacement for EtO as it leaves no toxic residue.

Question 84: For uniform staining reaction, morphology, and biochemical activity, it is advisable to study bacterial cultures during which phase of growth?

A. Lag phase
B. Death phase
C. Stationary phase
D. Logarithmic phase (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Logarithmic phase** (also known as the Exponential phase). **Why it is correct:** During the Logarithmic phase, bacteria divide at their maximum possible rate under the given conditions. This phase is characterized by high metabolic activity and rapid cell division. Because the cells are actively synthesizing new cell walls and proteins, they exhibit **uniform morphology** and **consistent staining characteristics** (e.g., Gram-positive bacteria are most reliably Gram-positive during this stage). Biochemical activity is also at its peak and most predictable, making this the ideal time for laboratory identification and antibiotic sensitivity testing. **Why other options are incorrect:** * **Lag phase:** This is a period of adaptation where bacteria increase in size and synthesize enzymes but do not divide. Morphology is inconsistent as cells are "gearing up" for growth. * **Stationary phase:** As nutrients deplete and toxic metabolites accumulate, the growth rate equals the death rate. Bacteria may develop **involution forms** (irregular shapes), produce spores, or show variable staining (e.g., Gram-variable), making them unreliable for study. * **Death phase:** In this phase, the number of dying cells exceeds new cells. Cellular autolysis occurs, leading to distorted morphology and loss of staining integrity. **NEET-PG High-Yield Pearls:** * **Generation Time:** The time taken for a bacterial population to double; it is calculated during the **Log phase**. * **Antibiotic Sensitivity:** Bactericidal antibiotics (like Penicillin) that act on the cell wall are **most effective** during the Log phase because the cells are actively dividing. * **Sporulation:** Occurs at the end of the Log phase or during the **Stationary phase** as a survival mechanism. * **Secondary Metabolites:** Production of antibiotics and exotoxins typically begins at the end of the Log phase or during the Stationary phase.

Question 85: Which of the following methods can be used to disinfect sputum?

A. Autoclave
B. Boiling
C. Cresol
D. All the above (Correct Answer)

Explanation: **Explanation:** The disinfection of sputum is a critical practice in hospital infection control, particularly for preventing the transmission of *Mycobacterium tuberculosis*. Sputum is a proteinaceous, viscous substance that can protect embedded pathogens from heat and chemicals; therefore, robust sterilization or high-level disinfection methods are required. * **Autoclave (Steam under pressure):** This is the most effective method for sterilizing contaminated laboratory waste and sputum containers. It ensures the destruction of all vegetative forms and highly resistant spores by utilizing moist heat at 121°C for 15–20 minutes. * **Boiling:** Boiling for 20–30 minutes is a reliable method for disinfecting sputum in resource-limited settings. While it may not achieve absolute sterilization (as some spores survive), it effectively kills *M. tuberculosis* and most vegetative pathogens. * **Cresol (and other Phenolics):** Chemical disinfection using 5% Cresol or 5% Phenol for 24 hours is a standard procedure for sputum. These chemicals act by disrupting cell membranes and precipitating proteins, effectively penetrating the organic matter in sputum. **Clinical Pearls for NEET-PG:** * **Burning/Incineration:** This is the **best and most preferred method** for the final disposal of sputum and its containers to ensure complete destruction. * **Pre-treatment:** In clinical practice, sputum is often treated with **1% Sodium Hypochlorite** or **5% Cresol** before disposal. * **Chlorhexidine:** Note that *M. tuberculosis* is resistant to many common disinfectants like chlorhexidine; hence, phenolics or alcohols are preferred. * **High-yield:** For the disinfection of **spills** of sputum on surfaces, 1% Sodium Hypochlorite is the agent of choice.

Question 86: Which of the following is a strictly aerobic organism?

A. Staphylococci
B. Streptococci
C. Pseudomonas (Correct Answer)
D. E. coli

Explanation: **Explanation:** The classification of bacteria based on oxygen requirements is a high-yield topic in Microbiology. Bacteria are categorized into obligate aerobes, obligate anaerobes, facultative anaerobes, and microaerophiles. **Correct Option: C. Pseudomonas** *Pseudomonas aeruginosa* is a classic example of an **obligate (strict) aerobe**. These organisms lack the fermentation pathways required to produce energy in the absence of oxygen; they rely solely on aerobic respiration using oxygen as the terminal electron acceptor. This explains why *Pseudomonas* typically causes infections in oxygen-rich environments, such as the lungs (especially in Cystic Fibrosis patients) and skin (burn wounds). **Incorrect Options:** * **A & B (Staphylococci and Streptococci):** These are **facultative anaerobes**. They prefer using oxygen for aerobic respiration (as it yields more ATP) but possess the metabolic machinery to switch to fermentation or anaerobic respiration when oxygen is unavailable. * **D (E. coli):** As a member of the *Enterobacteriaceae* family, *E. coli* is also a **facultative anaerobe**. This allows it to thrive both in the oxygen-depleted environment of the human gut and in aerobic laboratory cultures. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Oblate Aerobes:** "**N**agging **P**ests **M**ust **B**reathe" (**N**eisseria, **P**seudomonas, **M**ycobacterium tuberculosis, **B**acillus). * **Biochemical marker:** Oblate aerobes possess enzymes like **Superoxide Dismutase (SOD)** and **Catalase** to detoxify reactive oxygen species. * *Pseudomonas* is notorious for producing a grape-like odor and blue-green pigments (Pyocyanin and Pyoverdin) in culture.

Question 87: The Haemophilus influenzae vaccine contains which of the following?

A. Lipopolysaccharide (LPS)
B. Live attenuated H. influenzae
C. Polypeptide antigens containing D-glutamate
D. Polyribitol phosphate antigens (Correct Answer)

Explanation: **Explanation:** The *Haemophilus influenzae* type b (Hib) vaccine is a **conjugate vaccine** targeting the primary virulence factor of the bacterium: its polysaccharide capsule. **Why Option D is Correct:** The capsule of *H. influenzae* type b is composed of **Polyribitol Phosphate (PRP)**, a polymer of ribitol, ribose, and phosphate. While PRP is highly immunogenic in adults, it is a T-independent antigen that does not elicit a strong immune response in children under two years of age. To overcome this, the PRP is **conjugated to a carrier protein** (such as Tetanus toxoid or Diphtheria CRM197). This converts the immune response to T-cell dependent, inducing high-affinity IgG antibodies and long-term mucosal immunity (memory). **Why Other Options are Incorrect:** * **Option A (LPS):** While Hib possesses lipooligosaccharide (LOS), it is an endotoxin and is not used in vaccines due to its high toxicity and poor immunogenicity. * **Option B (Live attenuated):** There is no live-attenuated vaccine for Hib. The vaccine is a subunit (capsular) vaccine. * **Option C (D-glutamate):** This refers to the polypeptide capsule of ***Bacillus anthracis***. Most bacterial capsules are polysaccharides; *B. anthracis* is a high-yield exception. **High-Yield Clinical Pearls for NEET-PG:** * **Target Population:** The Hib vaccine has significantly reduced the incidence of **epiglottitis** and **meningitis** in children. * **Schedule:** Usually given at 6, 10, and 14 weeks as part of the **Pentavalent vaccine** (DPT + HepB + Hib) in India. * **Nontypable H. influenzae:** The Hib vaccine does **not** protect against nontypable (unencapsulated) strains, which are common causes of otitis media and sinusitis.

Question 88: Ideally, an antibiotic should focus on a microbial target not found in mammalian cells. By this standard, which of the following antibiotic agents would be expected to be most toxic to humans?

A. Penicillin
B. Mitomycin (Correct Answer)
C. Cephalosporin
D. Bacitracin

Explanation: **Explanation:** The core principle of antimicrobial chemotherapy is **selective toxicity**, which means a drug should be lethal to the microorganism without harming the host. This is best achieved by targeting structures or metabolic pathways present in microbes but absent in humans. **Why Mitomycin is the correct answer:** Mitomycin (specifically Mitomycin C) acts by cross-linking DNA, which inhibits DNA synthesis. Unlike cell wall inhibitors, its target—**DNA**—is universal to both microbial and mammalian cells. Because it lacks selective toxicity, it is highly toxic to human cells. Consequently, Mitomycin is not used as an antibacterial agent; instead, it is utilized as a **chemotherapeutic (anti-cancer) drug** to kill rapidly dividing human tumor cells. **Why the other options are incorrect:** * **Penicillin & Cephalosporins (Beta-lactams):** These inhibit the synthesis of **peptidoglycan**, a structural component of the bacterial cell wall. Since mammalian cells do not possess a cell wall or peptidoglycan, these drugs have a high therapeutic index and are generally non-toxic to host cells. * **Bacitracin:** This agent inhibits the recycling of the lipid carrier (bactoprenol) required for peptidoglycan synthesis. Like beta-lactams, it targets the bacterial cell wall, a structure absent in humans, making it selectively toxic. **High-Yield NEET-PG Pearls:** * **Highest Selective Toxicity:** Drugs targeting the **cell wall** (e.g., Penicillins) are the least toxic to humans. * **Moderate Selective Toxicity:** Drugs targeting **ribosomes** (e.g., Aminoglycosides, Tetracyclines) exploit the difference between bacterial 70S and mammalian 80S ribosomes, though some toxicity occurs due to similarities with mitochondrial ribosomes. * **Lowest Selective Toxicity:** Drugs targeting **DNA or cell membranes** (e.g., Mitomycin, Polymyxins, Amphotericin B) tend to be more toxic because these targets are shared by humans.

Question 89: What is the preferred medium for the growth of anaerobic bacteria?

A. Chocolate agar (Correct Answer)
B. Löwenstein-Jensen medium
C. Blood agar
D. Robson cooked meat medium

Explanation: **Explanation:** The growth of anaerobic bacteria requires a medium with a low oxidation-reduction (redox) potential. **Chocolate agar** is the preferred choice among the given options because it is an enriched medium containing lysed red blood cells. The heating process used to create chocolate agar releases intracellular nutrients like **Factor V (NAD)** and **Factor X (Hemin)**. Hemin, in particular, acts as a vital growth factor for many fastidious anaerobes. When supplemented with vitamin K and hemin, it becomes an excellent non-selective medium for recovering a wide range of anaerobic pathogens. **Analysis of Incorrect Options:** * **Löwenstein-Jensen (LJ) medium:** This is a selective medium specifically designed for the growth of *Mycobacterium tuberculosis*. It contains egg yolk, malachite green (to inhibit contaminants), and glycerol. * **Blood agar:** While it supports many bacteria, it is generally used for aerobic and facultative anaerobic organisms. Without specific enrichment or reducing agents, it is less effective than chocolate agar for fastidious anaerobes. * **Robertson’s Cooked Meat (RCM) medium:** This is a classic **liquid enrichment/transport medium** for anaerobes. The unsaturated fatty acids in the meat pieces act as reducing agents. While it is excellent for *maintaining* anaerobes, it is not the primary solid medium used for initial colony isolation and identification. **NEET-PG High-Yield Pearls:** * **Gold Standard:** The most common solid medium for anaerobes in clinical labs is **Schaedler Agar** or **Brucella Blood Agar** supplemented with Vitamin K1 and Hemin. * **Indicator:** **Resazurin** or **Methylene blue** are used in anaerobic jars to confirm an anaerobic environment (they turn colorless when oxygen is absent). * **Key Anaerobe:** *Bacteroides fragilis* is the most common anaerobe isolated from clinical infections; it grows well on **Bile Esculin Agar (BBE)**.

Question 90: Which bacteria initially colonize the oral cavity at birth?

A. Streptococcus salivarius (Correct Answer)
B. Streptococcus mutans
C. Streptococcus sanguis
D. Streptococcus mitis

Explanation: **Explanation:** The oral cavity of a newborn is sterile at birth but is rapidly colonized within hours by microbes from the mother’s vaginal tract, skin, and saliva. **1. Why Streptococcus salivarius is correct:** *Streptococcus salivarius* is the **pioneer species** of the oral cavity. It is a Gram-positive, facultative anaerobe that preferentially adheres to the epithelial surfaces of the tongue and buccal mucosa. Since a newborn lacks teeth, the oral environment consists entirely of mucosal surfaces, making *S. salivarius* the dominant initial colonizer (often appearing within 12–18 hours of birth). **2. Why the other options are incorrect:** * **Streptococcus mutans & Streptococcus sanguis:** These are "tooth-dependent" organisms. They require hard, non-shedding surfaces (enamel) to form biofilms. Therefore, they do not colonize the mouth until **primary tooth eruption** (around 6 months of age). *S. mutans* is the primary agent of dental caries. * **Streptococcus mitis:** While *S. mitis* is a member of the viridans group found in the oral cavity, it typically colonizes slightly later than *S. salivarius* and is more associated with the dental plaque and buccal mucosa once the microbial community begins to diversify. **NEET-PG High-Yield Pearls:** * **Pioneer Species:** *S. salivarius* (Oral cavity), *Bifidobacterium* (Gut of breastfed infants). * **Dental Caries:** *S. mutans* is the most common cause (produces dextran from sucrose). * **Subacute Bacterial Endocarditis (SBE):** *S. sanguis* and *S. mitis* are frequent culprits following dental procedures. * **Niche Succession:** The oral microbiome shifts from aerobic/facultative (infancy) to include more anaerobes (like *Porphyromonas gingivalis*) as the gingival crevice develops with teething.

Practice by Chapter

History and Scope of Microbiology

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Classification of Microorganisms

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Bacterial Morphology and Structure

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Bacterial Physiology and Metabolism

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Bacterial Genetics

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Microbial Growth and Nutrition

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Sterilization and Disinfection

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Bacterial Identification Methods

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Normal Microbiota and Pathogenicity

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Antimicrobial Susceptibility Testing

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