Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1401: A 36-year-old female presented with fever (105°F) and leukocytosis (15,000/mm³). She also complained of vomiting and diarrhea. She had eaten fried rice at a Chinese restaurant and became ill four days after her menstrual period. She had retained a vaginal tampon. What is the diagnosis?

A. Staphylococcal Scalded Skin Syndrome (SSSS)
B. Toxic Shock Syndrome (Correct Answer)
C. Staphylococcal food poisoning
D. Waterhouse-Friderichsen syndrome

Explanation: ### Explanation **Correct Answer: B. Toxic Shock Syndrome (TSS)** The clinical presentation of high fever, gastrointestinal symptoms (vomiting/diarrhea), and leukocytosis in a female with a history of **retained vaginal tampons** is a classic "textbook" description of **Staphylococcal Toxic Shock Syndrome**. The underlying mechanism involves **TSST-1 (Toxic Shock Syndrome Toxin-1)**, produced by *Staphylococcus aureus*. TSST-1 acts as a **superantigen**, which bypasses normal antigen processing. It binds directly to the MHC Class II of antigen-presenting cells and the Vβ region of T-cell receptors. This leads to a massive, non-specific activation of T-cells and a "cytokine storm" (IL-1, IL-2, TNF-α, and IFN-γ), resulting in multi-organ dysfunction and shock. **Why other options are incorrect:** * **A. SSSS:** Caused by **Exfoliative toxins (A and B)**. It presents with diffuse erythema and skin peeling (Nikolsky sign positive), typically in neonates, not with the systemic GI symptoms and tampon history described. * **C. Staphylococcal food poisoning:** Caused by **Enterotoxins (A-E)**. While the patient ate fried rice (usually associated with *Bacillus cereus*), staphylococcal food poisoning has a very short incubation (1–6 hours) and does not typically cause high fever or leukocytosis. * **D. Waterhouse-Friderichsen syndrome:** Characterized by adrenal hemorrhage and circulatory collapse secondary to *Neisseria meningitidis* septicemia. **NEET-PG High-Yield Pearls:** * **Superantigens:** TSST-1, Staphylococcal Enterotoxins, and Streptococcal Pyrogenic Exotoxin (SpeA/C). * **CDC Criteria for TSS:** Fever >38.9°C, Hypotension, Diffuse macular erythroderma (sunburn-like rash), Desquamation (1–2 weeks after onset), and involvement of ≥3 organ systems. * **Fried Rice Association:** Classically associated with *Bacillus cereus* (emetic toxin), but the tampon history here is the "pathognomonic" distractor/clue for TSS.

Question 1402: What is the location of the receptor for the diphtheria toxin?

A. Cell membrane (Correct Answer)
B. Mucous membrane
C. Nucleus
D. None of the above

Explanation: The correct answer is **A. Cell membrane**. ### **Explanation** Diphtheria toxin, produced by *Corynebacterium diphtheriae*, is a classic **A-B exotoxin**. The mechanism of entry depends entirely on the interaction between the toxin and the host cell surface: 1. **Binding (B-subunit):** The B (binding) subunit of the toxin attaches to a specific receptor on the **host cell membrane**. This receptor is the **Heparin-binding epidermal growth factor (HB-EGF) precursor**. 2. **Internalization:** Once bound, the toxin-receptor complex is internalized via receptor-mediated endocytosis into an endosome. 3. **Action (A-subunit):** After acidification of the endosome, the A (active) subunit is released into the cytosol, where it catalyzes the ADP-ribosylation of **Elongation Factor-2 (EF-2)**, halting protein synthesis and causing cell death. ### **Why other options are incorrect:** * **B. Mucous membrane:** While *C. diphtheriae* colonizes the mucous membranes of the nasopharynx to form a "pseudomembrane," this is the site of infection, not the molecular receptor site for the toxin. The toxin must bind to individual cell membranes to exert its systemic effects. * **C. Nucleus:** The toxin acts in the **cytosol** (on ribosomes/EF-2), not the nucleus. It does not require nuclear entry to inhibit protein synthesis. ### **High-Yield Clinical Pearls for NEET-PG:** * **Receptor Name:** Heparin-binding EGF-like growth factor (HB-EGF). * **Target:** Elongation Factor-2 (EF-2). * **Mechanism:** ADP-ribosylation (similar to *Pseudomonas* Exotoxin A). * **Genetics:** The toxin is encoded by the **tox gene**, which is introduced into *C. diphtheriae* by a lysogenic bacteriophage (**Beta-phage**). * **Diagnosis:** Elek’s gel precipitation test is used to detect toxin production.

Question 1403: A child presents with sepsis. Bacteria isolated showed beta-hemolysis on blood agar, resistance to bacitracin, and a positive CAMP test. What is the most probable organism causing this infection?

A. Streptococcus pyogenes
B. Streptococcus agalactiae (Correct Answer)
C. Enterococcus
D. Streptococcus pneumoniae

Explanation: **Explanation:** The clinical presentation and laboratory findings point directly to **Streptococcus agalactiae (Group B Streptococcus/GBS)**. **1. Why Streptococcus agalactiae is correct:** * **Beta-hemolysis:** GBS typically produces a narrow zone of beta-hemolysis on blood agar. * **Bacitracin Resistance:** Unlike Group A Strep, GBS is resistant to bacitracin (0.04 units). * **CAMP Test Positive:** This is the definitive biochemical marker. GBS produces the "CAMP factor," a diffusible protein that acts synergistically with the beta-lysin of *Staphylococcus aureus* to produce an **arrowhead-shaped** zone of enhanced hemolysis. * **Clinical Context:** GBS is a leading cause of neonatal sepsis and meningitis. **2. Why other options are incorrect:** * **Streptococcus pyogenes (Group A):** While it shows beta-hemolysis, it is characteristically **Bacitracin sensitive** and CAMP negative. * **Enterococcus:** These are usually non-hemolytic (gamma) or alpha-hemolytic. They are identified by their ability to grow in 6.5% NaCl and hydrolyze bile esculin. * **Streptococcus pneumoniae:** This organism shows **alpha-hemolysis** (greenish discoloration) and is bile soluble and optochin sensitive. It is not beta-hemolytic. **High-Yield Clinical Pearls for NEET-PG:** * **Hippurate Hydrolysis:** GBS is also positive for hippurate hydrolysis (converts hippurate to glycine). * **Source:** The primary reservoir is the maternal vagina; screening is done at 35–37 weeks of pregnancy. * **Drug of Choice:** Penicillin G remains the treatment of choice for GBS infections. * **Mnemonic:** **B-ABC** (Group **B** is **A**ntibiotic (**B**acitracin) Resistant and **C**AMP positive).

Question 1404: What is the best site for specimen collection for Neisseria meningitides?

A. Oral swab
B. Nasal swab
C. Nasopharyngeal swab (Correct Answer)
D. Skin lesions

Explanation: **Explanation:** **1. Why Nasopharyngeal Swab is correct:** *Neisseria meningitidis* (Meningococcus) is a transient commensal that colonizes the mucosal surfaces of the **nasopharynx**. In asymptomatic carriers (about 10% of the population), this site serves as the primary reservoir. For diagnostic purposes—especially when screening for carriers or investigating outbreaks—the **nasopharyngeal swab** is the gold standard because it reaches the area with the highest concentration of the organism. The swab should be made of Dacron or calcium alginate (as cotton can be inhibitory) and must be plated immediately on enriched media like Chocolate agar or selective media like Thayer-Martin. **2. Why other options are incorrect:** * **Oral swab:** The oral cavity contains high amounts of commensal flora (e.g., *viridans streptococci*) which can overgrow and mask the presence of *N. meningitidis*. * **Nasal swab:** This site is primarily used for screening *Staphylococcus aureus* carriers. *N. meningitidis* does not typically colonize the anterior nares. * **Skin lesions:** While *N. meningitidis* can be isolated from the purpuric spots or petechiae in cases of meningococcemia, it is not the "best" or primary site for general specimen collection compared to the nasopharynx or CSF. **3. NEET-PG High-Yield Pearls:** * **Transport:** *N. meningitidis* is highly sensitive to cold and drying. Specimens should never be refrigerated. * **Culture Media:** Use **Thayer-Martin Medium** (Selective) or **Chocolate Agar** (Enriched). * **Biochemicals:** It is **Oxidase positive** and ferments both **Glucose and Maltose** (unlike *N. gonorrhoeae*, which ferments only Glucose). * **Clinical Gold Standard:** For acute meningitis, **CSF** is the specimen of choice for microscopy (Gram-negative bean-shaped diplococci) and culture.

Question 1405: Vancomycin is obtained from which organism?

A. Staphylococcus species
B. Aspergillus niger
C. Streptococcus orientalis (Correct Answer)
D. Bacillus anthracis

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Vancomycin is a glycopeptide antibiotic primarily used to treat serious infections caused by Gram-positive bacteria. It was first isolated in 1953 by Edmund Kornfeld (at Eli Lilly) from a soil sample collected in the jungles of Borneo. The producing organism was originally named ***Streptococcus orientalis***. However, due to advances in taxonomy, the organism was later reclassified as ***Amycolatopsis orientalis***. In the context of NEET-PG and classical microbiology textbooks, *Streptococcus orientalis* remains the standard historical answer. **2. Why the Incorrect Options are Wrong:** * **Staphylococcus species (A):** These are the primary *targets* of Vancomycin (especially MRSA), not the source. No clinically used antibiotic is naturally derived from *Staphylococcus*. * **Aspergillus niger (B):** This is a fungus used commercially for the production of citric acid and enzymes like glucoamylase, but it does not produce Vancomycin. * **Bacillus anthracis (C):** This is a highly pathogenic bacterium and the causative agent of Anthrax. While other *Bacillus* species produce antibiotics (e.g., *B. polymyxa* produces Polymyxin B; *B. subtilis* produces Bacitracin), *B. anthracis* does not. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mechanism of Action:** Inhibits bacterial cell wall synthesis by binding to the **D-Ala-D-Ala** terminus of nascent peptidoglycan pentapeptide. * **Spectrum:** Exclusively Gram-positive (too large to pass through the porins of Gram-negative bacteria). * **Red Man Syndrome:** A common infusion-related reaction caused by direct histamine release (not a true IgE allergy); managed by slowing the infusion rate. * **Drug of Choice:** For MRSA (Methicillin-resistant *Staphylococcus aureus*) and orally for *Clostridioides difficile* (pseudomembranous colitis). * **Resistance:** Vancomycin-resistant Enterococci (VRE) occur due to a change in the binding site from D-Ala-D-Ala to **D-Ala-D-Lac**.

Question 1406: All of the following statements are true about diphtheria except?

A. Toxin production depends on lysogenic conversion by beta phage.
B. It is non-sporing, non-motile, and non-capsulated.
C. Toxin detection is done by ELISA test in serum. (Correct Answer)
D. Faucial diphtheria is the commonest type.

Explanation: **Explanation:** The correct answer is **C**, as toxin detection for *Corynebacterium diphtheriae* is primarily performed using the **Elek’s gel precipitation test** (an in vitro immunoprecipitation assay) or PCR for the *tox* gene, rather than a serum ELISA. While ELISA can be used to measure protective antibody levels in a patient's blood, it is not the standard diagnostic method for detecting the toxin produced by the bacteria in a clinical sample. **Analysis of other options:** * **Option A:** This is **true**. Virulence in *C. diphtheriae* is mediated by the production of an exotoxin, which occurs only when the bacterium is infected by a specific temperate bacteriophage (Beta phage) carrying the *tox* gene. This process is known as **lysogenic conversion**. * **Option B:** This is **true**. Morphologically, *C. diphtheriae* are Gram-positive, pleomorphic bacilli (Chinese-letter pattern) that are characteristically non-sporing, non-motile, and non-capsulated. * **Option D:** This is **true**. **Faucial diphtheria** (involving the tonsils and pharynx) is the most frequent clinical presentation, characterized by the pathognomonic "leathery" greyish-white pseudomembrane. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Toxin:** Inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Culture Media:** **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Staining:** Albert’s stain reveals **metachromatic granules** (Volutin/Babes-Ernst granules). * **Schick Test:** Used to determine the immune status (susceptibility) of an individual, not for diagnosis.

Question 1407: Which of the following statements is true about Clostridium tetani?

A. It is Gram-positive.
B. It has a drumstick appearance.
C. It is an obligate anaerobe.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** *Clostridium tetani* is a clinically significant, spore-forming bacterium responsible for tetanus. The correct answer is **D (All of the above)** because it possesses all the classic morphological and physiological characteristics listed. 1. **Gram-positive (Option A):** *C. tetani* is a Gram-positive, slender, motile bacillus. While older cultures may sometimes appear Gram-variable, it is fundamentally classified as a Gram-positive organism. 2. **Drumstick Appearance (Option B):** This is a hallmark feature. The organism produces **terminal, spherical spores** that are wider than the vegetative body. This specific arrangement gives the bacterium a characteristic "drumstick" or "tennis racket" appearance under the microscope. 3. **Obligate Anaerobe (Option C):** Like all members of the genus *Clostridium*, it is a strict anaerobe. It cannot grow in the presence of oxygen and requires a low oxidation-reduction potential, which is why it thrives in deep, necrotic puncture wounds. **High-Yield NEET-PG Pearls:** * **Virulence Factor:** The primary pathogen is **Tetanospasmin**, an A-B type exotoxin (neurotoxin) that blocks the release of inhibitory neurotransmitters (**GABA and Glycine**) from Renshaw cells in the spinal cord. * **Clinical Presentation:** Leads to spastic paralysis, characterized by **Risus sardonicus** (grimace), **Trismus** (lockjaw), and **Opisthotonus** (archback posture). * **Culture:** On blood agar, it produces a thin film of growth called **swarming growth** (due to its peritrichous flagella). * **Sensitivity:** It is highly sensitive to Penicillin and Metronidazole.

Question 1408: A 26-year-old man presents with mild gastroenteritis after consuming raw fish a few days prior. There is no blood or pus in the stool. A culture grown in Wagatsuma agar revealed a causative agent. Which organism is most likely responsible for this illness?

A. Vibrio cholerae
B. Salmonella typhi
C. Shigella sonnei
D. Vibrio parahaemolyticus (Correct Answer)

Explanation: ### Explanation The correct answer is **Vibrio parahaemolyticus**. **1. Why the Correct Answer is Right:** The clinical presentation of mild gastroenteritis following the consumption of **raw seafood/fish** is a classic hallmark of *Vibrio parahaemolyticus*. The definitive diagnostic clue in this question is the growth on **Wagatsuma agar**. *V. parahaemolyticus* produces a thermostable direct hemolysin (TDH), which causes beta-hemolysis on this specific high-salt medium—a phenomenon known as the **Kanagawa Phenomenon**. This test differentiates pathogenic strains from non-pathogenic ones. **2. Why the Incorrect Options are Wrong:** * **Vibrio cholerae:** While it belongs to the same genus, it typically presents with "rice-water stools" and profound dehydration. It does not require high salt for growth (halophilic) and is Kanagawa-negative. * **Salmonella typhi:** This causes Enteric Fever (systemic illness with high fever and bradycardia), not simple gastroenteritis. It is usually transmitted via contaminated water or food (not specifically raw fish) and is diagnosed via blood or stool culture on MacConkey agar. * **Shigella sonnei:** This organism causes bacillary dysentery, characterized by stools containing **blood and pus** (inflammatory diarrhea), which contradicts the "no blood or pus" finding in this case. **3. High-Yield Clinical Pearls for NEET-PG:** * **Halophilic nature:** *V. parahaemolyticus* is halophilic (salt-loving), requiring 1–3% NaCl for growth. * **TCBS Agar:** On Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar, it produces **green colonies** (sucrose non-fermenter), unlike *V. cholerae*, which produces yellow colonies. * **Kanagawa Phenomenon:** This is the most frequently tested association for this organism in postgraduate exams. * **Epidemiology:** It is the leading cause of seafood-associated gastroenteritis worldwide, especially in Japan and coastal regions.

Question 1409: A patient with acute lymphocytic leukemia presents with fever and neutropenia, and develops diarrhea after receiving amoxicillin therapy. Which of the following organisms is most likely the causative agent?

A. Salmonella typhi
B. Clostridium difficile (Correct Answer)
C. Clostridium perfringens
D. Shigella flexneri

Explanation: ### Explanation **Correct Answer: B. Clostridium difficile** **Reasoning:** The clinical presentation of a neutropenic patient developing diarrhea following antibiotic therapy (Amoxicillin) is a classic scenario for **Antibiotic-Associated Diarrhea (AAD)** or **Pseudomembranous Colitis**. * **Pathophysiology:** Broad-spectrum antibiotics (like amoxicillin, clindamycin, or cephalosporins) disrupt the normal colonic flora. This allows *Clostridium difficile*, a gram-positive, spore-forming anaerobic bacillus, to overgrow and release toxins (Toxin A/Enterotoxin and Toxin B/Cytotoxin). * **Risk Factors:** Immunocompromised states (like Leukemia/Neutropenia) and recent antibiotic use are major triggers. **Analysis of Incorrect Options:** * **A. Salmonella typhi:** Causes Enteric fever (systemic illness with high fever and bradycardia). While it can cause "pea-soup" diarrhea, it is not typically triggered by antibiotic use; rather, antibiotics are used to treat it. * **C. Clostridium perfringens:** Primarily associated with gas gangrene (myonecrosis) or self-limiting food poisoning (Type A strains) via contaminated meat. It does not typically cause post-antibiotic diarrhea. * **D. Shigella flexneri:** Causes bacillary dysentery characterized by small-volume bloody stools with mucus and tenesmus. It is transmitted via the fecal-oral route, not induced by antibiotic therapy. **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Oral **Vancomycin** or **Fidaxomicin** (Metronidazole is now considered second-line for initial episodes). * **Diagnosis:** Demonstration of toxins in stool (EIA) or the *tcdB* gene via PCR (NAAT). * **Morphology:** On sigmoidoscopy, look for "yellowish-white plaques" on the colonic mucosa (Pseudomembranes). * **Culture:** Uses **CCFA medium** (Cycloserine-Cefoxitin-Fructose Agar), showing colonies with a "horse-manure" odor and "ground-glass" appearance.

Question 1410: Which of the following statements regarding the Widal test is FALSE?

A. A high titre in the first sample is diagnostic. (Correct Answer)
B. 'H' antigen is specific for serotyping.
C. A highly positive H titre is diagnostic.
D. The titre reaches its maximum in the third week.

Explanation: **Explanation:** The Widal test is a serological test used to detect antibodies against the ‘O’ (somatic) and ‘H’ (flagellar) antigens of *Salmonella typhi* and *paratyphi*. **Why Option A is the Correct (False) Statement:** A single high titre in the first week of illness is **not diagnostic**. In endemic areas like India, many individuals have baseline antibodies due to subclinical infections or prior vaccinations (anamnestic response). Therefore, a **rising titre** (a four-fold increase in samples taken 7–10 days apart) is required for a definitive diagnosis. A single reading can be misleading. **Analysis of Other Options:** * **Option B (True):** The 'H' antigen is species-specific. While 'O' antigens are shared between *S. typhi* and *S. paratyphi*, the 'H' antigen helps differentiate between them (e.g., *S. typhi* has 'd' flagellar antigen). * **Option C (True):** While a rising titre is preferred, in clinical practice, a very high titre (usually >1:160 for 'O' and >1:160 or 1:320 for 'H') in a symptomatic patient is considered highly suggestive of infection. * **Option D (True):** Antibodies start appearing in the second week of fever, but the titre typically peaks during the **third week** of the illness. **Clinical Pearls for NEET-PG:** * **Timeline:** Blood culture is the gold standard in the **1st week**; Widal test is most useful in the **2nd and 3rd weeks**. * **Antigens:** 'O' agglutinins appear early and disappear early (indicates recent infection); 'H' agglutinins appear late and persist longer. * **False Positives:** Can occur in cases of malaria, typhus, or chronic liver disease. * **Prozone Phenomenon:** High antibody concentration can lead to a false-negative result; serial dilution is required.

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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