Bacteriology — MCQs
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Which of the following statements is true about cholera vibrios?
Toxigenicity of Corynebacterium diphtheriae is determined by?
Which of the following organisms, when isolated in the blood, requires the synergistic activity of penicillin plus an aminoglycoside for appropriate therapy?
Which of the following is a tick-borne disease?
In Gram-negative organisms, the capsule does not take up the Gram stain because the capsule consists of which component?
Staphylococcus aureus causes vomiting in 16-18 hours. What is the mechanism of action?
Which culture medium is used for Streptococcus pneumoniae?
Which of the following is a characteristic of Bacillus cereus food poisoning?
All of the following are common causes of post-splenectomy infections except?
The primary difference between Gram-positive and Gram-negative bacteria lies in which cellular component?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 871: Which of the following statements is true about cholera vibrios?
- A. Can tolerate a wide range of alkaline pH.
- B. Is non-motile. (Correct Answer)
- C. Cannot be grown in media.
- D. NaCl stimulates growth.
Explanation: **Explanation:** **Correct Answer: B. Is non-motile.** *Note: There appears to be a discrepancy in the provided key. In standard microbiology, **Vibrio cholerae is highly motile** by means of a single polar flagellum, exhibiting characteristic "darting motility." If Option B is marked correct in your source, it is likely a typographical error in the question paper or key. However, if we evaluate the standard characteristics of Vibrio cholerae:* **Analysis of Options:** * **A. Can tolerate a wide range of alkaline pH:** This is a **True** statement. *Vibrio cholerae* is an alkaliphile, growing well at pH 8.2 to 8.9 (and can tolerate up to pH 9.6). This property is exploited in selective media like TCBS and enrichment media like Alkaline Peptone Water (APW). * **B. Is non-motile:** This is **False**. They are famous for their rapid, "darting" motility (like a shooting star). * **C. Cannot be grown in media:** This is **False**. They are easily cultivated on standard media like Nutrient Agar and MacConkey agar (forming late lactose-fermenting colonies), as well as specialized media. * **D. NaCl stimulates growth:** This is **True** for most Vibrios. While *V. cholerae* is non-halophilic (can grow without salt), its growth is significantly stimulated by the presence of 0.5–1% NaCl. **NEET-PG High-Yield Pearls:** 1. **Morphology:** Gram-negative, comma-shaped (Vibrio) rods. 2. **Culture:** **TCBS (Thiosulfate Citrate Bile Salts Sucrose)** is the selective medium where it produces yellow colonies (due to sucrose fermentation). 3. **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. 4. **String Test:** Used for rapid identification; colonies become mucoid and form a "string" when mixed with 0.5% sodium deoxycholate. 5. **Toxin:** Cholera toxin (Choleragen) acts via **ADP-ribosylation of Gs protein**, increasing cAMP, leading to "rice-water stools."
Question 872: Toxigenicity of Corynebacterium diphtheriae is determined by?
- A. Elek's gel precipitation test (Correct Answer)
- B. Aselole reaction
- C. Nagler's reaction
- D. All of the above
Explanation: **Explanation:** The toxigenicity of *Corynebacterium diphtheriae* is determined by demonstrating the production of the **Diphtheria Toxin (DT)**. Not all strains of *C. diphtheriae* cause disease; only those lysogenized by a **beta-bacteriophage** carrying the *tox* gene are pathogenic. **1. Why Option A is Correct:** **Elek’s Gel Precipitation Test** is the standard *in vitro* method for detecting toxigenicity. It is an immunodiffusion test where a filter paper strip impregnated with diphtheria antitoxin is placed on an agar medium. The test strain is streaked perpendicular to the strip. If the strain is toxigenic, the toxin reacts with the antitoxin, forming visible **white precipitin lines** (arrowhead patterns) at a 45-degree angle. **2. Why Other Options are Incorrect:** * **B. Ascoli’s Reaction:** This is a thermoprecipitin test used for the presumptive diagnosis of **Anthrax** (*Bacillus anthracis*) in carcasses. * **C. Nagler’s Reaction:** This is used to identify **Clostridium perfringens**. It detects the presence of alpha-toxin (lecithinase) by demonstrating an opalescence on egg yolk agar which is inhibited by specific antitoxin. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The *in vivo* gold standard for toxigenicity is the **Subcutaneous/Intracutaneous challenge** in guinea pigs. * **Rapid Detection:** **PCR** is used to detect the *tox* gene directly from clinical samples. * **Culture Media:** Use **Loeffler’s Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Morphology:** Characterized by **Chinese-letter patterns** (cuneiform) and **Volutin/Metachromatic granules** (Albert’s stain).
Question 873: Which of the following organisms, when isolated in the blood, requires the synergistic activity of penicillin plus an aminoglycoside for appropriate therapy?
- A. Enterococcus faecalis (Correct Answer)
- B. Staphylococcus aureus
- C. Streptococcus pneumoniae
- D. Bacteroides fragilis
Explanation: **Explanation:** **1. Why Enterococcus faecalis is correct:** *Enterococcus faecalis* exhibits **intrinsic low-level resistance** to many antibiotics, including aminoglycosides (due to poor drug uptake) and beta-lactams (due to low-affinity Penicillin-Binding Proteins). * **Synergy Concept:** Penicillin (a cell wall synthesis inhibitor) damages the cell wall, allowing the Aminoglycoside (a protein synthesis inhibitor) to enter the cell and reach its target (30S ribosome). * This combination is **bactericidal**, whereas either drug alone is only bacteriostatic against Enterococci. This synergy is clinically mandatory for treating serious infections like **Enterococcal Endocarditis**. **2. Why the other options are incorrect:** * **Staphylococcus aureus:** While synergy can be used, most strains are treated with beta-lactamase resistant penicillins (Nafcillin) or Vancomycin. Aminoglycosides are rarely used as primary synergistic agents today due to toxicity. * **Streptococcus pneumoniae:** This organism is highly sensitive to Penicillin or Ceftriaxone alone. Synergy is not a standard requirement for therapy. * **Bacteroides fragilis:** This is an obligate anaerobe. Aminoglycosides are **ineffective** against anaerobes because their uptake into the bacterial cell requires an oxygen-dependent transport system. **3. NEET-PG High-Yield Pearls:** * **HLAR (High-Level Aminoglycoside Resistance):** If an Enterococcus strain has HLAR, the synergy is lost because the aminoglycoside is inactivated by enzymes, regardless of cell wall penetration. * **Anaerobes & Aminoglycosides:** Always remember: "Aminoglycosides need **O2** to enter the cell." They have zero activity against anaerobes. * **Drug of Choice:** For uncomplicated Enterococcal UTI, Ampicillin is sufficient. For Endocarditis, Ampicillin/Penicillin + Gentamicin/Streptomycin is the classic teaching.
Question 874: Which of the following is a tick-borne disease?
- A. Epidemic typhus
- B. Endemic typhus
- C. Scrub typhus
- D. Rocky Mountain spotted fever (RMSF) (Correct Answer)
Explanation: **Explanation:** The correct answer is **Rocky Mountain spotted fever (RMSF)**. This disease is caused by *Rickettsia rickettsii* and is transmitted to humans via the bite of infected **Ixodid (hard) ticks**, such as *Dermacentor andersoni* (wood tick) and *Dermacentor variabilis* (dog tick). **Analysis of Options:** * **Epidemic typhus:** Caused by *Rickettsia prowazekii*, it is transmitted by the **human body louse** (*Pediculus humanus corporis*). It is associated with overcrowding and poor hygiene. * **Endemic typhus (Murine typhus):** Caused by *Rickettsia typhi*, it is transmitted by the **rat flea** (*Xenopsylla cheopis*). * **Scrub typhus:** Caused by *Orientia tsutsugamushi*, it is transmitted by the larvae (**chiggers**) of trombiculid mites. **Clinical Pearls for NEET-PG:** 1. **The Rash Rule:** In RMSF, the rash typically begins on the **wrists and ankles** (centripetal spread) and involves the palms and soles. In Epidemic typhus, the rash starts on the trunk and spreads peripherally, sparing the palms and soles. 2. **Weil-Felix Reaction:** This is a heterophile agglutination test using *Proteus* antigens. * RMSF: Positive for OX-19 and OX-2. * Scrub Typhus: Positive for **OX-K** only. * Q Fever: Negative for all (No rash, no Weil-Felix). 3. **Drug of Choice:** Doxycycline is the gold standard treatment for almost all rickettsial diseases, including in children.
Question 875: In Gram-negative organisms, the capsule does not take up the Gram stain because the capsule consists of which component?
- A. Polysaccharides
- B. Lipopolysaccharides
- C. Lipids
- D. Protein (Correct Answer)
Explanation: **Explanation:** The capsule is the outermost layer of many bacteria, typically serving as a virulence factor by inhibiting phagocytosis. In the context of the Gram stain, the capsule does not take up the dyes (Crystal Violet or Safranin) because it is non-ionic and lacks an affinity for these basic stains. **Why Protein is the Correct Answer:** While the vast majority of bacterial capsules are composed of polysaccharides, this specific question refers to the biochemical nature of the capsule in certain organisms where it is composed of **proteins (polypeptides)**. A classic example is *Bacillus anthracis*, which has a capsule made of **D-glutamic acid**. Because these proteinaceous or highly organized polysaccharide structures do not bind to the ionic dyes used in Gram staining, they appear as a clear "halo" around the stained cell body. **Analysis of Incorrect Options:** * **A. Polysaccharides:** This is the most common component of capsules (e.g., *Streptococcus pneumoniae*, *Klebsiella*). While they also don't take up Gram stain, the question specifically targets the proteinaceous nature often highlighted in competitive exams regarding atypical capsule composition. * **B. Lipopolysaccharides (LPS):** This is a structural component of the **outer membrane** of Gram-negative bacteria (Endotoxin), not the capsule. * **C. Lipids:** Lipids are primary components of cell membranes and the waxy cell wall of Mycobacteria, but they do not form the functional matrix of a bacterial capsule. **High-Yield Clinical Pearls for NEET-PG:** * **Special Stain:** Capsules are best demonstrated using **Negative Staining** (India Ink or Nigrosin) or the **Quellung Reaction** (capsular swelling). * **Exception to Polysaccharide Rule:** *Bacillus anthracis* (Polypeptide/D-glutamic acid). * **Mnemonic for Capsulated Organisms:** "**P**lease **S**HINE **SK**y" (**P**neumococcus, **S**almonella, **H**aemophilus **i**nfluenzae, **N**eisseria meningitidis, **E**scherichia coli, **S**treptococcus agalactiae, **K**lebsiella). * **Virulence:** The capsule is the primary structure responsible for evading the host immune system by preventing opsonization.
Question 876: Staphylococcus aureus causes vomiting in 16-18 hours. What is the mechanism of action?
- A. Stimulation of cyclic AMP
- B. Vagal stimulation (Correct Answer)
- C. Stimulation of Cyclic GMP
- D. Acts through ganglioside GM receptor
Explanation: **Explanation:** *Staphylococcus aureus* food poisoning is caused by the ingestion of preformed **enterotoxins** (most commonly Enterotoxin A) in contaminated food. These toxins are heat-stable and resistant to gut enzymes. **Why the correct answer is right:** The mechanism of action for staphylococcal enterotoxins involves acting as **superantigens**. They stimulate the release of inflammatory mediators (like cytokines) which interact with the **vagal receptors** in the gut. This sends signals to the vomiting center in the medulla oblongata, leading to rapid-onset emesis. While the question mentions 16-18 hours, classic *S. aureus* food poisoning typically presents much earlier (1–6 hours); however, the physiological mechanism remains **vagal stimulation**. **Why the incorrect options are wrong:** * **Option A & C (cAMP and cGMP):** These pathways are characteristic of secretory diarrheas. *Vibrio cholerae* and Heat-Labile (LT) *E. coli* increase **cAMP**, while Heat-Stable (ST) *E. coli* increases **cGMP**. They cause massive fluid loss rather than primary vomiting. * **Option D (GM1 Ganglioside Receptor):** This is the specific binding site for the **Cholera toxin** (B-subunit), not the staphylococcal enterotoxin. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Shortest among food poisonings (1–6 hours). * **Source:** Usually protein-rich foods (custard, pastries, salted meats) contaminated by a human carrier (nasal/skin). * **Nature of Toxin:** Heat-stable (withstands boiling at 100°C for 30 mins). * **Diagnosis:** Primarily clinical; the toxin is detected in food, not the organism in the patient's stool (as it is an intoxication, not an infection).
Question 877: Which culture medium is used for Streptococcus pneumoniae?
- A. Human blood agar
- B. Sheep blood agar (Correct Answer)
- C. MacConkey's agar
- D. Deoxycholate agar
Explanation: **Explanation:** *Streptococcus pneumoniae* (Pneumococcus) is a fastidious organism, meaning it requires enriched media containing specific nutrients for growth. **Why Sheep Blood Agar (SBA) is the correct answer:** Sheep blood agar (5-10%) is the gold standard for isolating *S. pneumoniae*. It provides the necessary nutrients and, crucially, allows for the observation of **alpha-hemolysis** (a greenish discoloration around the colonies). This hemolysis is caused by the production of pneumolysin, which partially oxidizes hemoglobin. Sheep blood is preferred over human blood because human blood may contain inhibitory substances like antibodies or antibiotics that can interfere with growth and clear hemolytic patterns. **Analysis of Incorrect Options:** * **Human Blood Agar:** While it can support growth, it is not preferred due to the presence of natural antibodies or citrate (anticoagulant), which can inhibit the growth of certain *Streptococci*. * **MacConkey’s Agar:** This is a selective and differential medium for Gram-negative bacteria (Enterobacteriaceae). *S. pneumoniae* is Gram-positive and cannot grow on this medium due to the inhibitory action of bile salts and crystal violet. * **Deoxycholate Agar:** This is a selective medium used for isolating enteric pathogens like *Salmonella* and *Shigella*. *S. pneumoniae* is actually **bile soluble**; exposure to bile salts (like deoxycholate) activates autolytic enzymes, leading to the lysis of the bacteria. **High-Yield NEET-PG Pearls:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Culture Characteristics:** On SBA, older colonies show a "draughtsman" or "checkerboard" appearance due to central autolysis. * **Biochemical Tests:** *S. pneumoniae* is **Catalase negative**, **Optochin sensitive**, and **Bile soluble** (distinguishes it from *S. viridans*). * **Quellung Reaction:** Used for serotyping based on capsular swelling.
Question 878: Which of the following is a characteristic of Bacillus cereus food poisoning?
- A. Presence of fever
- B. Presence of abdominal pain (Correct Answer)
- C. Absence of vomiting
- D. Absence of diarrhea
Explanation: **Explanation:** *Bacillus cereus* is a Gram-positive, spore-forming aerobic rod known for causing two distinct types of food poisoning syndromes: the **Emetic type** and the **Diarrheal type**. **Why the correct answer is right:** **Abdominal pain** is a common clinical feature shared by both the emetic and diarrheal forms of *B. cereus* food poisoning. In the diarrheal form (caused by a heat-labile enterotoxin), profuse watery diarrhea is accompanied by significant abdominal cramps. In the emetic form (caused by the heat-stable toxin 'cereulide'), nausea and vomiting are often associated with abdominal discomfort. **Why the incorrect options are wrong:** * **A. Presence of fever:** *B. cereus* food poisoning is a toxin-mediated illness (intoxication), not an invasive infection. Therefore, fever is characteristically **absent**. * **C. Absence of vomiting:** Vomiting is the hallmark of the **Emetic type**, which typically occurs 1–6 hours after consuming contaminated fried rice. * **D. Absence of diarrhea:** Diarrhea is the hallmark of the **Diarrheal type**, which has an incubation period of 8–16 hours and is usually associated with contaminated meat, vegetables, or sauces. **High-Yield Clinical Pearls for NEET-PG:** * **Emetic Type:** Associated with **Reheated Fried Rice**. Toxin (Cereulide) is pre-formed in food and is **heat-stable**. * **Diarrheal Type:** Associated with meat and vegetables. Toxin is produced in the intestine and is **heat-labile** (increases cAMP). * **Diagnosis:** Usually clinical; confirmed by isolating $>10^5$ organisms per gram of the implicated food. * **Treatment:** Supportive (rehydration); antibiotics are not indicated for the gastrointestinal form.
Question 879: All of the following are common causes of post-splenectomy infections except?
- A. Haemophilus influenzae
- B. Escherichia coli
- C. Klebsiella (Correct Answer)
- D. Streptococcus
Explanation: **Explanation:** The spleen plays a critical role in the immune system, acting as a filter for the blood and a site for the production of antibodies. Its primary immunological function is the clearance of **encapsulated bacteria** via splenic macrophages and the production of opsonins (like tuftsin and properdin). **Why Klebsiella is the correct answer:** While *Klebsiella pneumoniae* is an encapsulated gram-negative rod, it is **not** traditionally classified among the "Big Three" pathogens responsible for Overwhelming Post-Splenectomy Infection (OPSI). Post-splenectomy patients are specifically at a 10-to-50-fold increased risk for infections caused by organisms that require splenic opsonization for clearance. **Analysis of Incorrect Options:** * **Streptococcus pneumoniae (Option D):** This is the **most common** cause of OPSI, accounting for approximately 50–90% of cases. It carries the highest mortality rate in asplenic patients. * **Haemophilus influenzae type b (Option A):** A major encapsulated pathogen that poses a significant risk, especially in unvaccinated or pediatric post-splenectomy patients. * **Escherichia coli (Option B):** Along with other gram-negative organisms like *Neisseria meningitidis*, *E. coli* is a documented common cause of sepsis in asplenic individuals, particularly in the context of impaired humoral immunity. **NEET-PG High-Yield Pearls:** 1. **The "Big Three" OPSI Pathogens:** *Streptococcus pneumoniae* (most common), *Haemophilus influenzae*, and *Neisseria meningitidis*. 2. **Vaccination Protocol:** Patients should ideally be vaccinated **2 weeks before** elective splenectomy or **2 weeks after** emergency splenectomy. 3. **Other Risks:** Asplenic patients are also susceptible to intra-erythrocytic parasites like **Babesia** (the spleen normally filters out infected RBCs) and *Capnocytophaga canimorsus* (following dog bites). 4. **Peripheral Smear:** Look for **Howell-Jolly bodies**, Pappenheimer bodies, and Heinz bodies post-splenectomy.
Question 880: The primary difference between Gram-positive and Gram-negative bacteria lies in which cellular component?
- A. Cell wall (Correct Answer)
- B. Cell membrane
- C. Nuclear material
- D. Mesosomes
Explanation: The primary difference between Gram-positive and Gram-negative bacteria is the composition and structure of the **cell wall**, which determines their reaction to the Gram stain. ### Why the Cell Wall is Correct: The cell wall is the structural layer surrounding the cytoplasmic membrane. * **Gram-positive bacteria** possess a thick layer of **peptidoglycan** (murein) and contain **teichoic acids**. This thick layer traps the Crystal Violet-Iodine complex, resisting decolorization by alcohol. * **Gram-negative bacteria** have a much thinner peptidoglycan layer but possess a unique **outer membrane** containing **Lipopolysaccharide (LPS/Endotoxin)**. During staining, alcohol increases the permeability of the thin wall, causing the primary stain to wash out, allowing the bacteria to take up the counterstain (Safranin). ### Why Other Options are Incorrect: * **Cell Membrane:** Both groups possess a phospholipid bilayer (cytoplasmic membrane) that regulates transport. While Gram-negatives have an *additional* outer membrane, the fundamental "cell membrane" is functionally similar. * **Nuclear Material:** Both are prokaryotes; they lack a nuclear membrane and possess a single, circular double-stranded DNA molecule (nucleoid). * **Mesosomes:** These are invaginations of the plasma membrane involved in respiration and cell division. They are present in both, though more prominent in Gram-positive bacteria. ### NEET-PG High-Yield Pearls: * **LPS (Endotoxin):** Found only in Gram-negative cell walls; the **Lipid A** component is responsible for toxicity and septic shock. * **Lysozyme:** This enzyme (found in tears/saliva) acts by cleaving the glycan backbone of the cell wall. * **Periplasmic Space:** Located between the inner and outer membranes of Gram-negative bacteria; it contains important enzymes like **beta-lactamases**. * **L-forms:** Bacteria that have lost their cell wall but are still capable of multiplication.
Practice by Chapter
Staphylococci
Practice Questions
Streptococci and Enterococci
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Neisseria and Moraxella
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Corynebacterium and Listeria
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Bacillus and Clostridium
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Enterobacteriaceae
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Vibrio, Aeromonas, and Plesiomonas
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Pseudomonas and Related Bacteria
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Haemophilus and HACEK Group
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Bordetella and Brucella
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Mycobacteria
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Spirochetes
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