Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 381: What is the most important serotype of E. coli causing hemolytic uremic syndrome?

A. O 157:H7 of EHEC (Correct Answer)
B. O 107:H7 of EIEC
C. O 157:H7 of ETEC
D. O 109:H7 of EAEC

Explanation: **Explanation:** **1. Why Option A is Correct:** Hemolytic Uremic Syndrome (HUS) is primarily caused by **Enterohemorrhagic E. coli (EHEC)**, specifically the serotype **O157:H7**. The pathogenesis involves the production of **Shiga-like toxins (Stx1 and Stx2)**, also known as Verotoxins. These toxins enter the bloodstream and bind to **Gb3 receptors**, which are highly expressed on glomerular endothelial cells. This leads to endothelial damage, microvascular thrombosis, and the classic HUS triad: **Microangiopathic Hemolytic Anemia (MAHA), Thrombocytopenia, and Acute Renal Failure.** **2. Why Other Options are Incorrect:** * **Option B (EIEC):** Enteroinvasive E. coli causes a dysentery-like illness (similar to Shigellosis) by invading the colonic mucosa but does not typically produce Shiga-like toxins or cause HUS. * **Option C (ETEC):** Enterotoxigenic E. coli is the leading cause of **Traveler’s Diarrhea**. It produces Heat-labile (LT) and Heat-stable (ST) toxins, resulting in watery diarrhea without systemic complications like HUS. * **Option D (EAEC):** Enteroaggregative E. coli causes persistent watery diarrhea. While certain rare hybrid strains (like O104:H4) have caused HUS outbreaks, O109:H7 is not a recognized significant serotype for this condition. **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture:** EHEC (O157:H7) is unique because it **does not ferment Sorbitol**. It is identified using **Sorbitol MacConkey (SMAC) Agar**, where it forms colorless colonies. * **Source:** Most commonly transmitted via undercooked ground beef (hamburger meat) or unpasteurized milk. * **Treatment Warning:** Antibiotics are generally **avoided** in EHEC infections as they may increase toxin release and worsen the risk of HUS. * **Morphology:** On a peripheral smear of a HUS patient, look for **Schistocytes** (fragmented RBCs).

Question 382: Weil-Felix reaction is positive with OXK antigen in which condition?

A. Trench fever
B. Scrub typhus (Correct Answer)
C. Endemic typhus
D. Epidemic typhus

Explanation: The **Weil-Felix reaction** is a heterophile agglutination test used for the presumptive diagnosis of Rickettsial diseases. It relies on the cross-reactivity between antibodies produced during a Rickettsial infection and the somatic (O) antigens of certain strains of *Proteus vulgaris* (OX19, OX2) and *Proteus mirabilis* (OXK). ### **Explanation of Options** * **Scrub Typhus (Correct):** Caused by *Orientia tsutsugamushi*, this condition is unique because it is the only Rickettsial infection that reacts exclusively with the **OXK** antigen. It shows no reaction with OX19 or OX2. * **Epidemic Typhus:** Caused by *Rickettsia prowazekii*, it typically shows a strongly positive reaction with **OX19** and a weak/negative reaction with OX2 and OXK. * **Endemic (Murine) Typhus:** Caused by *Rickettsia typhi*, it follows the same pattern as Epidemic typhus, reacting strongly with **OX19**. * **Trench Fever:** Caused by *Bartonella quintana*. This organism does not belong to the Rickettsia genus and is **Weil-Felix negative**. ### **High-Yield NEET-PG Pearls** * **The "K" Rule:** Remember **K** for **K**rub (Scrub) Typhus. * **Rocky Mountain Spotted Fever (RMSF):** Shows a positive reaction with both **OX19 and OX2**. * **Q Fever:** Caused by *Coxiella burnetii*; it is characteristically **Weil-Felix negative**. * **Rickettsialpox:** Caused by *Rickettsia akari*; it is also **Weil-Felix negative**. * **Vector for Scrub Typhus:** Larval stage (chigger) of trombiculid mites. Look for the characteristic **eschar** (black necrotic scab) at the bite site in clinical vignettes.

Question 383: Rigidity of facial muscles, 'Risus sardonicus', is associated with which condition?

A. Tetany
B. Tetanus (Correct Answer)
C. Leprosy
D. Actinomycosis

Explanation: **Explanation:** **Tetanus** is the correct answer. It is caused by the neurotoxin **tetanospasmin** produced by *Clostridium tetani*. The toxin acts by blocking the release of inhibitory neurotransmitters (GABA and Glycine) from Renshaw cells in the spinal cord. This leads to unchecked excitatory impulses, resulting in generalized muscle rigidity and spasms. * **Risus sardonicus** (Sardonic smile) is a characteristic clinical sign caused by the sustained spasm of the facial muscles (specifically the *frontalis* and *orbicularis oris*). * **Trismus** (Lockjaw) is often the presenting symptom due to masseter muscle involvement. **Analysis of Incorrect Options:** * **Tetany:** Caused by hypocalcemia, it presents with carpopedal spasm and neuromuscular irritability (Chvostek’s and Trousseau’s signs), but not the classic "sardonic smile" of tetanus. * **Leprosy:** While it can cause facial nerve palsy (leading to sagging or inability to close eyes), it does not cause spastic rigidity or Risus sardonicus. * **Actinomycosis:** Known for causing "lumpy jaw" with chronic granulomatous lesions and sulfur granules, it involves local swelling rather than generalized muscle spasms. **High-Yield Clinical Pearls for NEET-PG:** * **Opisthotonus:** A backward arching of the spine caused by powerful extensor muscle spasms. * **Mechanism:** Retrograde axonal transport of the toxin to the CNS. * **Management:** Wound debridement, Metronidazole (preferred over Penicillin as Penicillin is a GABA antagonist), and Tetanus Immunoglobulin (TIG). * **Neonatal Tetanus:** Known as "8th-day disease," usually due to unsterile umbilical cord cutting.

Question 384: What is the mode of action of Clostridium botulinum toxin?

A. Presynaptic block (Correct Answer)
B. Postsynaptic block
C. Competitive inhibition of acetylcholine
D. Destruction of acetylcholine

Explanation: **Explanation:** **Clostridium botulinum** produces a potent neurotoxin (Botulinum toxin) that causes flaccid paralysis. The toxin is a zinc-dependent endopeptidase that targets the neuromuscular junction. 1. **Why Option A is Correct:** Botulinum toxin acts at the **presynaptic nerve terminal**. It cleaves **SNARE proteins** (such as synaptobrevin, SNAP-25, and syntaxin), which are essential for the fusion of synaptic vesicles with the neuronal membrane. By preventing this fusion, the toxin inhibits the release of **Acetylcholine (ACh)** into the synaptic cleft. Since the blockage occurs before the synapse, it is a **presynaptic block**. 2. **Why Other Options are Incorrect:** * **Option B:** Postsynaptic blocks occur when the signal transmission is interrupted *after* the synapse (e.g., Curare or Myasthenia Gravis). * **Option C:** Competitive inhibition involves molecules competing for the ACh receptor (e.g., d-tubocurarine). Botulinum toxin does not interact with the receptor itself. * **Option D:** The toxin does not destroy the neurotransmitter; it simply prevents its release from the nerve ending. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Characterized by **symmetric descending flaccid paralysis**, starting with cranial nerves (diplopia, dysphagia, ptosis). * **Infant Botulism:** Associated with **honey** consumption (ingestion of spores); presents as "Floppy Baby Syndrome." * **Food-borne Botulism:** Ingestion of pre-formed toxin (often from canned foods). * **Therapeutic Uses:** Botox is used for achalasia cardia, strabismus, blepharospasm, and cosmetic procedures. * **Key Distinction:** While Botulinum causes flaccid paralysis (blocks ACh), **Tetanus toxin** causes spastic paralysis (blocks GABA/Glycine release in the spinal cord).

Question 385: Which of the following is NOT a diagnostic test for H. Pylori?

A. Urea-breath test
B. Urea-breath test
C. Gastric biopsy with Giemsa stain
D. SAFA test (Correct Answer)

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, spiral-shaped bacterium primarily associated with peptic ulcer disease and gastric carcinoma. Diagnostic methods are classified into **Invasive** (requiring endoscopy) and **Non-invasive** tests. **Why SAFA is the correct answer:** The **SAFA (Soluble Antigen Fluorescent Antibody) test** is a serological test historically used for the diagnosis of **Leprosy** (detecting antibodies against *M. leprae*), not *H. pylori*. Therefore, it is the odd one out. **Analysis of other options:** * **Urea Breath Test (UBT):** This is the **non-invasive gold standard** for diagnosing active infection and confirming eradication. It relies on the potent **urease enzyme** of *H. pylori*, which splits ingested labeled urea ($^{13}C$ or $^{14}C$) into ammonia and labeled $CO_2$, the latter being detected in expired breath. * **Gastric Biopsy with Giemsa Stain:** This is an **invasive** test. Histopathological examination of an endoscopic biopsy is highly sensitive. While H&E stain is common, **Giemsa, Warthin-Starry silver stain, and Acridine Orange** are superior for visualizing the spiral morphology of the bacilli. **NEET-PG High-Yield Pearls:** * **Most Sensitive/Specific Non-invasive Test:** Urea Breath Test (UBT). * **Screening/Epidemiological Test:** Stool Antigen Test or Serology (ELISA for IgG). Note: Serology cannot distinguish between past and current infection. * **Invasive Gold Standard:** Endoscopic biopsy followed by **Rapid Urease Test (RUT)** or Histology. * **Culture:** Most specific but difficult; requires special media like **Skirrow’s medium** or Chocolate agar. * **Virulence Factors:** **CagA** (associated with cancer) and **VacA** (vacuolating cytotoxin).

Question 386: A 24-year-old female presents with fever, malaise, and a dry, nonproductive cough. She also reports headache, muscle aches, and leg pain. Laboratory findings are significant for elevated cold agglutinins. Which of the following microorganisms is most likely responsible for her symptoms?

A. Haemophilus influenzae
B. Klebsiella pneumoniae
C. Legionella pneumophila
D. Mycoplasma pneumoniae (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Mycoplasma pneumoniae** The clinical presentation of a young adult with a gradual onset of constitutional symptoms (fever, headache, malaise), a persistent nonproductive cough, and "walking pneumonia" (symptoms more severe than physical findings) is classic for **Atypical Pneumonia**. The definitive laboratory clue here is the **elevation of cold agglutinins** (IgM antibodies that agglutinate RBCs at 4°C). *Mycoplasma pneumoniae* expresses an I-antigen on its surface that induces the production of these autoantibodies, a high-yield association for NEET-PG. **Analysis of Incorrect Options:** * **A. Haemophilus influenzae:** Typically causes typical community-acquired pneumonia (CAP) characterized by productive cough and lobar consolidation on X-ray, common in COPD patients. It does not trigger cold agglutinins. * **B. Klebsiella pneumoniae:** Associated with "currant jelly" sputum and seen frequently in alcoholics or diabetics. It causes necrotizing lobar pneumonia, not atypical symptoms. * **C. Legionella pneumophila:** Another cause of atypical pneumonia, but it is classically associated with high fever, hyponatremia, and gastrointestinal symptoms (diarrhea). It does not produce cold agglutinins. **High-Yield Clinical Pearls for NEET-PG:** * **Smallest free-living organism:** *Mycoplasma* lacks a cell wall (naturally resistant to Beta-lactams). * **Culture:** Requires sterols; grows on **PPLO agar** (Eaton’s agar) showing characteristic **"fried-egg" colonies**. * **Complications:** Stevens-Johnson Syndrome, Raynaud’s phenomenon (due to cold agglutinins), and Bullous myringitis. * **Drug of Choice:** Macrolides (Azithromycin) or Tetracyclines (Doxycycline).

Question 387: Which of the following are non-invasive tests for Helicobacter pylori?

A. Rapid urease test
B. Urease breath test
C. Stool antigen assay (Correct Answer)
D. Stomach aspiration culture

Explanation: **Explanation:** The diagnosis of *Helicobacter pylori* is categorized into **Invasive** (requiring endoscopy) and **Non-invasive** (not requiring endoscopy) methods. **Why the Correct Answer is Right:** * **Stool Antigen Assay (Option C):** This is a **non-invasive** test that detects the presence of *H. pylori* antigens directly in the feces using monoclonal antibodies (ELISA). It is highly sensitive and specific, making it an excellent choice for both initial diagnosis and confirming eradication after treatment. **Why Other Options are Incorrect:** * **Rapid Urease Test (RUT) (Option A):** While it detects the urease enzyme produced by the bacteria, it is **invasive** because it requires a gastric mucosal biopsy obtained via upper GI endoscopy. * **Urease Breath Test (UBT) (Option B):** *Wait, a clarification:* In many standard classifications, the Urea Breath Test is **also** a non-invasive test. However, in the context of this specific question where only one option is marked correct, the Stool Antigen Assay is a definitive non-invasive marker for active infection. (Note: In NEET-PG, if both UBT and Stool Antigen are options, UBT is often considered the "Gold Standard" non-invasive test for confirming eradication). * **Stomach Aspiration Culture (Option D):** This is an **invasive** procedure requiring the collection of gastric juices or tissue, typically via endoscopy, to culture the organism. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** Endoscopic biopsy followed by Histopathology (Warthin-Starry silver stain or Giemsa stain). * **Gold Standard Non-invasive Test:** Urea Breath Test (using C13 or C14 isotopes). * **Most Sensitive Culture Media:** Skirrow’s medium or Chocolate agar. * **Virulence Factors:** **CagA** (associated with gastric cancer) and **VacA** (vacuolating cytotoxin). * **Urease Production:** *H. pylori* produces abundant urease to neutralize gastric acid by creating an "ammonia cloud," allowing survival in the low pH of the stomach.

Question 388: Which species of Staphylococcus is susceptible to novobiocin?

A. Staphylococcus aureus
B. Staphylococcus epidermidis (Correct Answer)
C. Staphylococcus hemolyticus
D. Staphylococcus saprophyticus

Explanation: This question tests the ability to differentiate between Coagulase-Negative Staphylococci (CoNS) using biochemical markers, a high-yield topic for NEET-PG. ### **Explanation** The **Novobiocin Susceptibility Test** is the primary laboratory method used to differentiate Coagulase-Negative Staphylococci. 1. **Staphylococcus epidermidis (Correct):** It is the most common CoNS isolated from clinical specimens. It is **susceptible** to novobiocin (zone of inhibition >16 mm). It is a major component of normal skin flora but can cause infections related to prosthetic devices and catheters due to its ability to form biofilms. 2. **Staphylococcus aureus (Incorrect):** While *S. aureus* is susceptible to novobiocin, it is **Coagulase-positive**. The novobiocin test is specifically used to categorize CoNS; therefore, *S. aureus* is not the primary focus of this diagnostic algorithm. 3. **Staphylococcus saprophyticus (Incorrect):** This species is characteristically **resistant** to novobiocin. This is a key diagnostic feature used to identify it as a common cause of urinary tract infections (UTIs) in young, sexually active females ("honeymoon cystitis"). 4. **Staphylococcus hemolyticus (Incorrect):** Like *S. epidermidis*, it is a CoNS, but it often shows variable or intermediate patterns and is less commonly the focus of this specific high-yield distinction compared to the *epidermidis-saprophyticus* duo. ### **NEET-PG High-Yield Pearls** * **Mnemonic for Novobiocin:** *"Staph **S**aprophyticus is **S**econdary (Resistant), Staph **E**pidermidis is **E**ntirely sensitive."* * **Biofilm Production:** *S. epidermidis* produces an extracellular polysaccharide (slime) that allows it to adhere to prosthetic valves and indwelling catheters. * **Urease:** *S. saprophyticus* is urease-positive, which contributes to its pathogenicity in the urinary tract. * **Catalase Test:** All *Staphylococcus* species are Catalase-positive, distinguishing them from *Streptococcus* species.

Question 389: What is the drug of choice for Mycoplasma pneumoniae?

A. Penicillin
B. Tetracycline
C. Cefuroxime
D. Erythromycin (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Erythromycin**. **1. Why Erythromycin is correct:** *Mycoplasma pneumoniae* is unique because it **lacks a cell wall** (it only has a triple-layered sterol-containing cell membrane). Erythromycin, a macrolide, acts by inhibiting protein synthesis (binding to the 50S ribosomal subunit) rather than targeting the cell wall. Macrolides (Erythromycin, Azithromycin, or Clarithromycin) are the drugs of choice for treating "Atypical Pneumonia" caused by Mycoplasma. **2. Why other options are incorrect:** * **A. Penicillin & C. Cefuroxime:** These are Beta-lactam antibiotics. Their mechanism of action involves inhibiting cell wall synthesis (peptidoglycan cross-linking). Since *Mycoplasma* lacks a cell wall, it is **intrinsically resistant** to all Beta-lactams, including penicillins and cephalosporins. * **B. Tetracycline:** While Tetracyclines (like Doxycycline) are effective against *Mycoplasma*, they are generally considered second-line or alternative treatments, especially in children and pregnant women, due to side effects. Macrolides remain the primary recommendation in standard textbooks for NEET-PG. **3. High-Yield Clinical Pearls for NEET-PG:** * **Atypical Pneumonia:** Characterized by "walking pneumonia" where clinical symptoms (dry cough, low-grade fever) are milder than the extensive infiltrates seen on X-ray. * **Diagnosis:** Cold agglutinin test (IgM antibodies) is a classic bedside test; PCR is now the gold standard. * **Culture:** Requires special media (PPLO broth/Horse serum) and shows characteristic **"Fried Egg" colonies**. * **Eaton’s Agent:** Another name for *Mycoplasma pneumoniae*.

Question 390: A cholera patient with a stool output of 1000-1500 mL/day is classified as having which type of cholera?

A. Cholera mitis
B. Cholera dumdum
C. Cholera magna
D. Cholera gravis (Correct Answer)

Explanation: **Explanation:** The classification of Cholera is based on the severity of fluid loss and the resulting clinical presentation. **1. Why Cholera Gravis is correct:** Cholera is characterized by profuse "rice-water" stools caused by the action of the cholera toxin (choleragen) on the intestinal mucosa. **Cholera gravis** is the most severe form of the disease. It is defined by a stool output exceeding **1000 mL/day** (often reaching 500–1000 mL/hour in extreme cases). This rapid loss of fluid and electrolytes leads to severe dehydration, hypovolemic shock, and potentially death within hours if not treated with aggressive rehydration. **2. Why other options are incorrect:** * **Cholera mitis:** This refers to the mildest form of the disease, where the patient experiences only mild diarrhea and minimal dehydration. Stool output is significantly lower than 1000 mL/day. * **Cholera dumdum:** This is a fictitious or non-standard clinical term. While "Dum-dum fever" is a historical synonym for Visceral Leishmaniasis (Kala-azar), it has no clinical relevance to the classification of Cholera. * **Cholera magna:** This is an archaic term sometimes used to describe severe cases, but it is not the standard clinical classification used in modern medical examinations or textbooks like Ananthanarayan. **High-Yield Clinical Pearls for NEET-PG:** * **Causative Agent:** *Vibrio cholerae* (Serogroups O1 and O139 are the primary pathogens). * **Mechanism:** Toxin-mediated (AB toxin) activation of **adenylate cyclase**, leading to increased **cAMP** levels, which results in the hypersecretion of water and electrolytes. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium. * **Enrichment Media:** Alkaline Peptone Water (APW) or Monsur’s Taurocholate Tellurite Peptone Water. * **Selective Media:** TCBS (Thiosulfate Citrate Bile salts Sucrose) agar, where it forms characteristic **yellow colonies**.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

Want unlimited practice?

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

Start For Free