Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1651: Which of the following is NOT true about Helicobacter pylori?

A. It splits urea to produce ammonia for survival.
B. It is associated with an increased risk of gastric cancer.
C. It presents as Gram-negative curved rods.
D. The Cag-A gene is not associated with an increased risk of duodenal ulcers. (Correct Answer)

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, Gram-negative spiral bacterium that colonizes the gastric mucosa. Understanding its virulence factors is crucial for NEET-PG. **1. Why Option D is the correct answer (The False Statement):** The **CagA (Cytotoxin-associated gene A)** is the most important virulence marker of *H. pylori*. Strains carrying the CagA gene are significantly more aggressive and are strongly associated with an increased risk of severe inflammation, **duodenal ulcers**, and gastric adenocarcinoma. Therefore, stating it is *not* associated with duodenal ulcers is factually incorrect. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** *H. pylori* produces a potent **Urease** enzyme. This enzyme splits urea into ammonia and CO₂, creating an alkaline "ammonia cloud" that neutralizes gastric acid, allowing the bacteria to survive in the acidic stomach environment. * **Option B:** It is classified as a **Type 1 Carcinogen** by the WHO. Chronic infection leads to mucosal atrophy and intestinal metaplasia, increasing the risk of gastric adenocarcinoma and MALT lymphoma. * **Option C:** Morphologically, *H. pylori* are **Gram-negative, curved or spiral-shaped rods** (often described as "seagull-wing" shaped) with multiple polar flagella. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Endoscopic biopsy followed by a **Rapid Urease Test (RUT)** or Histopathology (Warthin-Starry silver stain). * **Non-invasive Test of Choice:** Urea Breath Test (using C13 or C14 isotopes). * **Treatment:** Standard Triple Therapy (PPI + Amoxicillin + Clarithromycin). * **MALT Lymphoma:** *H. pylori* is unique because treating the infection can lead to the regression of the associated malignancy (MALToma).

Question 1652: Q-fever is caused by which bacterium?

A. Rochaemelia Quintana
B. Coxiella burnetii (Correct Answer)
C. Mycoplasma hominis
D. None of the above

Explanation: **Explanation:** **Coxiella burnetii (Option B)** is the causative agent of **Q-fever**. Although it was historically classified under the family Rickettsiaceae, it is now recognized as a distinct genus. It is an obligate intracellular, Gram-negative bacterium that is highly resistant to environmental stressors due to its ability to form **spore-like variants**. **Why the other options are incorrect:** * **Rochaemelia quintana (Option A):** Now known as *Bartonella quintana*, this organism causes **Trench Fever**, which is transmitted by the human body louse. * **Mycoplasma hominis (Option C):** This is a cell-wall-deficient bacterium associated with urogenital tract infections, pelvic inflammatory disease (PID), and postpartum fever, but not Q-fever. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Unlike other Rickettsial diseases, Q-fever is **not** transmitted by an arthropod vector. It is primarily a **zoonosis** transmitted via inhalation of contaminated aerosols from birth products (placenta), urine, or feces of infected livestock (sheep, goats, cattle). * **Diagnosis:** The **Weil-Felix test is negative** in Q-fever (a common MCQ trap). Diagnosis is usually confirmed via Serology (IFA). * **Antigenic Variation:** It undergoes **Phase Variation**. Phase I is the highly infectious, "smooth" form found in nature; Phase II is the "rough" form produced after repeated laboratory subcultures. * **Clinical Presentation:** Presents as an atypical pneumonia or hepatitis (acute) or culture-negative endocarditis (chronic). * **Treatment:** Doxycycline is the drug of choice.

Question 1653: Neisseria infection is associated with which of the following conditions?

A. Deficiency of early complements
B. Deficiency of late complements (Correct Answer)
C. There is no such association
D. Any complement deficiency can be associated

Explanation: **Explanation:** The correct answer is **B. Deficiency of late complements.** **Underlying Medical Concept:** The complement system is a vital component of innate immunity. The "late" or "terminal" complement components (**C5, C6, C7, C8, and C9**) assemble to form the **Membrane Attack Complex (MAC)**. The MAC is essential for the lysis of Gram-negative bacteria, particularly those with thin peptidoglycan layers like *Neisseria* species (*N. meningitidis* and *N. gonorrhoeae*). Patients with deficiencies in these terminal components cannot form the MAC effectively, making them highly susceptible to recurrent, invasive infections by *Neisseria*. **Analysis of Options:** * **Option A (Early Complements):** Deficiencies in early components (C1, C2, C4) are primarily associated with **immune complex diseases** like Systemic Lupus Erythematosus (SLE) and pyogenic infections (e.g., *S. pneumoniae*), but not specifically with *Neisseria*. C3 deficiency is severe and leads to susceptibility to various encapsulated bacteria. * **Option C & D:** These are incorrect because there is a well-documented, specific clinical association between the terminal pathway and *Neisseria* susceptibility. **NEET-PG High-Yield Pearls:** * **C5-C9 Deficiency:** Classic association with recurrent **Meningococcal** meningitis and disseminated **Gonococcal** infections. * **C1, C2, C4 Deficiency:** Most common presentation is **SLE-like autoimmune disease**. * **C3 Deficiency:** The most severe complement deficiency; presents with recurrent infections by **encapsulated bacteria** (e.g., *H. influenzae*, *S. pneumoniae*). * **Properdin/Factor D Deficiency:** Also associated with *Neisseria* infections due to impairment of the alternative pathway. * **CH50 Assay:** Used to screen for total complement activity; it will be low/zero in terminal component deficiencies.

Question 1654: Orientia Tsutsugamushi causes which of the following diseases?

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

Explanation: **Explanation:** **Correct Answer: C. Scrub typhus** *Orientia tsutsugamushi* is the causative agent of **Scrub typhus**. It belongs to the family Rickettsiaceae but is genetically distinct from the genus *Rickettsia* (lacking a lipopolysaccharide cell wall). It is transmitted to humans through the bite of the larval stage (chigger) of **Leptotrombidium mites**. Clinically, it is characterized by high fever, lymphadenopathy, and a pathognomonic **black eschar** at the site of the mite bite. **Analysis of Incorrect Options:** * **A. Epidemic typhus:** Caused by ***Rickettsia prowazekii*** and transmitted by the **human body louse**. It typically occurs in crowded, unsanitary conditions. * **B. Endemic typhus:** Also known as Murine typhus, it is caused by ***Rickettsia typhi*** and transmitted by **rat fleas** (*Xenopsylla cheopis*). * **C. Q fever:** Caused by ***Coxiella burnetii***. Unlike other rickettsial diseases, it does not cause a rash and is usually transmitted via inhalation of contaminated aerosols from livestock. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Chigger-borne (Mite). * **Diagnosis:** The **Weil-Felix test** shows agglutination with **OX-K** antigens (negative for OX-19 and OX-2). * **Drug of Choice:** **Doxycycline** is the gold standard treatment for all rickettsial infections, including Scrub typhus. * **Reservoir:** The mite acts as both the vector and the reservoir (via transovarial transmission).

Question 1655: Chlamydial infection is associated with which of the following?

A. Coronary heart disease
B. Lymphogranuloma venereum (LGV)
C. Ophthalmia neonatorum
D. All of the above (Correct Answer)

Explanation: **Explanation:** The genus *Chlamydia* consists of obligate intracellular bacteria that cause a wide spectrum of human diseases. The correct answer is **"All of the above"** because different species and serovars of Chlamydia are implicated in each of these conditions. 1. **Coronary Heart Disease (CHD):** *Chlamydia pneumoniae* has been linked to atherosclerosis and CHD. Seroepidemiological studies and the detection of the organism within atherosclerotic plaques suggest that chronic inflammation triggered by this bacterium may contribute to the pathogenesis of coronary artery disease. 2. **Lymphogranuloma Venereum (LGV):** This is a sexually transmitted infection caused by *Chlamydia trachomatis* **serovars L1, L2, and L3**. It is characterized by primary genital lesions followed by painful regional lymphadenopathy (buboes) and potential proctitis. 3. **Ophthalmia Neonatorum:** *Chlamydia trachomatis* (**serovars D-K**) is a leading cause of neonatal conjunctivitis, transmitted from the mother’s infected birth canal. It typically presents 5–14 days after birth, later than gonococcal ophthalmia. **High-Yield Clinical Pearls for NEET-PG:** * **Trachoma:** Caused by *C. trachomatis* serovars **A, B, Ba, and C** (leading cause of preventable blindness). * **Inclusion Conjunctivitis/Urethritis:** Caused by serovars **D-K**. * **Diagnosis:** **Nucleic Acid Amplification Test (NAAT)** is the gold standard. * **Microscopy:** Look for **Halberstaedter-Prowazek (HP) inclusion bodies** (intracytoplasmic) on Giemsa stain. * **Treatment:** Azithromycin (single dose) or Doxycycline (7 days) are the drugs of choice. For neonatal ophthalmia, oral Erythromycin is preferred to prevent subsequent pneumonia.

Question 1656: Which E. coli pathotype is depicted in the image?

A. Enterotoxigenic E. coli (ETEC)
B. Enteropathogenic E. coli (EPEC)
C. Enterohemorrhagic E. coli (EHEC)
D. All of the above (Correct Answer)

Explanation: ***All of the above*** - **ETEC**, **EPEC**, and **EHEC** are all recognized **diarrheogenic E. coli pathotypes** that cause distinct forms of gastrointestinal illness. - Each pathotype has unique **virulence mechanisms** and **clinical presentations**, making them all legitimate E. coli variants associated with diarrheal diseases. *Enterotoxigenic E. coli (ETEC)* - Causes **traveler's diarrhea** through production of **heat-labile (LT)** and **heat-stable (ST) toxins** that increase cAMP/cGMP levels. - Results in **watery diarrhea** without invasion or inflammation of intestinal mucosa. *Enteropathogenic E. coli (EPEC)* - Causes **infantile diarrhea** through **attaching and effacing (A/E) lesions** that destroy intestinal microvilli. - Leads to **watery diarrhea** in children under 2 years, particularly in developing countries. *Enterohemorrhagic E. coli (EHEC)* - Produces **Shiga toxins (Stx1/Stx2)** causing **bloody diarrhea** and potential **hemolytic uremic syndrome (HUS)**. - Associated with **hemorrhagic colitis** and severe complications including kidney failure and neurological symptoms.

Question 1657: Which of the following organisms is catalase positive, coagulase positive, non-motile, facultative anaerobe, and does not form spores?

A. Streptococcus viridians
B. Pneumococcus
C. Staphylococcus aureus (Correct Answer)
D. Pseudomonas aeruginosa

Explanation: ### Explanation The question describes the classic biochemical and morphological profile of **Staphylococcus aureus**. **1. Why Staphylococcus aureus is correct:** * **Catalase Positive:** Staphylococci produce catalase, which differentiates them from Streptococci. * **Coagulase Positive:** This is the definitive test for *S. aureus*, distinguishing it from Coagulase-Negative Staphylococci (CoNS) like *S. epidermidis*. * **Morphology & Physiology:** It is a Gram-positive coccus (non-motile, non-spore-forming) and a facultative anaerobe, meaning it can grow in both aerobic and anaerobic conditions. **2. Why the other options are incorrect:** * **Streptococcus viridans & Pneumococcus (S. pneumoniae):** Both belong to the *Streptococcus* genus, which is characteristically **catalase-negative**. Furthermore, *S. pneumoniae* is bile soluble and optochin sensitive, unlike Staphylococci. * **Pseudomonas aeruginosa:** This is a Gram-negative bacilli. While it is catalase positive, it is an **obligate aerobe** (not facultative) and is highly **motile** via polar flagella. **3. NEET-PG High-Yield Pearls:** * **Gold Standard Test:** The **tube coagulase test** detects "free coagulase" and is the definitive test for *S. aureus*. * **Culture:** On Blood Agar, it typically shows **beta-hemolysis**. On Mannitol Salt Agar (MSA), it ferments mannitol, turning the medium yellow. * **Protein A:** A key virulence factor of *S. aureus* that binds to the Fc portion of IgG, inhibiting phagocytosis. * **Common Infections:** It is the most common cause of osteomyelitis, septic arthritis (in general population), and acute bacterial endocarditis.

Question 1658: Which of the following is NOT true about ASO titre?

A. May be positive in normal individuals.
B. Is a major Jones criterion. (Correct Answer)
C. May be negative in post-streptococcal glomerulonephritis.
D. May not be elevated in the presence of carditis.

Explanation: **Explanation:** The ASO (Antistreptolysin O) titre is a serological marker used to detect a recent infection with Group A Streptococcus (GAS). **Why Option B is the correct answer (The "False" statement):** In the **Revised Jones Criteria** for Acute Rheumatic Fever (ARF), an elevated ASO titre (or other streptococcal antibodies) is considered **evidence of a preceding streptococcal infection**, which is a mandatory requirement for diagnosis. However, it is **not** a Major or Minor criterion. Major criteria include Carditis, Polyarthritis, Chorea, Erythema marginatum, and Subcutaneous nodules (mnemonic: **J♥NES**). **Analysis of other options:** * **Option A:** ASO titres can be positive in normal individuals (healthy carriers) because the "normal" range depends on age and geographical prevalence of streptococcal infections. * **Option C:** ASO titres may be negative in **Post-Streptococcal Glomerulonephritis (PSGN)**, especially if the preceding infection was a skin infection (impetigo). In skin infections, the ASO response is poor because cholesterol in the skin inhibits Streptolysin O. Anti-DNase B is a better marker for skin-related PSGN. * **Option D:** ASO titres peak 3–5 weeks after infection. If a patient presents with isolated carditis or chorea months after the initial infection, the ASO titre may have already declined to normal levels. **NEET-PG High-Yield Pearls:** * **Standard Value:** A titre >200 units is generally considered significant in adults. * **Best marker for Pyoderma/Impetigo:** Anti-DNase B (ASO is unreliable here). * **Mechanism:** Streptolysin O is oxygen-labile and highly antigenic; ASO antibodies neutralize its hemolytic activity. * **False Positives:** Can occur in liver disease (due to hypercholesterolemia) and bacterial contamination of serum.

Question 1659: Which one of the following organisms is the most frequent cause of acute pyogenic meningitis in adults?

A. Streptococcus pneumoniae (Correct Answer)
B. Neisseria meningitidis
C. Haemophilus influenzae
D. Listeria monocytogenes

Explanation: **Explanation:** Acute pyogenic meningitis is a medical emergency characterized by the inflammation of the meninges due to bacterial infection. The etiology varies significantly based on the patient's age group. **1. Why Streptococcus pneumoniae is correct:** *Streptococcus pneumoniae* (Pneumococcus) is currently the **most common cause** of community-acquired bacterial meningitis in **adults** (especially those >20 years) and children over 5 years. It accounts for nearly 50% of all cases. Risk factors include pneumonia, otitis media, alcoholism, and splenectomy. It is associated with the highest mortality and morbidity rates among the pyogenic causes. **2. Why the other options are incorrect:** * **Neisseria meningitidis:** While it is a leading cause in children and young adults (ages 2–20), it is now second to *S. pneumoniae* in the general adult population. It often occurs in outbreaks (dorms, military barracks) and is characterized by a petechial rash. * **Haemophilus influenzae type b (Hib):** Historically the leading cause in children, its incidence has plummeted globally due to the success of the **Hib vaccine**. It is now a rare cause in adults. * **Listeria monocytogenes:** This is an important cause in specific populations: **neonates, the elderly (>65 years), and immunocompromised individuals.** It is not the most frequent cause in the general adult population. **High-Yield Clinical Pearls for NEET-PG:** * **Neonates (<1 month):** Most common causes are *Group B Streptococcus* (S. agalactiae), *E. coli*, and *Listeria*. * **CSF Findings in Pyogenic Meningitis:** Elevated PMNs (neutrophils), markedly low glucose (<40 mg/dL), and high protein (>100 mg/dL). * **Empiric Treatment:** Usually involves Ceftriaxone + Vancomycin (add Ampicillin if *Listeria* is suspected). * **Dexamethasone:** Administered before or with the first dose of antibiotics to reduce neurological complications, specifically in *S. pneumoniae* infections.

Question 1660: Hansen's bacillus is cultured in which of the following?

A. LJ medium
B. Robertson’s cooked meat medium
C. Foot pad of mice (Correct Answer)
D. Sabouraud’s agar

Explanation: **Explanation:** **Hansen’s bacillus (*Mycobacterium leprae*)**, the causative agent of leprosy, is unique because it is an **obligate intracellular pathogen** that has never been grown on artificial (in vitro) culture media. This is due to its massive reductive evolution and loss of essential metabolic genes. 1. **Why Option C is correct:** Since *M. leprae* cannot grow on agar, researchers use **animal models** for cultivation. The **mouse footpad** (Shepard’s method) is the standard model because the bacteria prefer cooler temperatures (approx. 30°C) found in the extremities. For large-scale production of the bacilli (e.g., for lepromin antigen), the **nine-banded armadillo** is used as it develops systemic infection. 2. **Why other options are incorrect:** * **Option A (LJ Medium):** Lowenstein-Jensen medium is the classic egg-based medium used for *Mycobacterium tuberculosis*, but it does not support the growth of *M. leprae*. * **Option B (RCM Medium):** Robertson’s Cooked Meat medium is used for the cultivation of **anaerobes** (e.g., *Clostridium* species). * **Option C (Sabouraud’s Agar):** SDA is a selective medium used for the cultivation of **fungi**. **High-Yield Clinical Pearls for NEET-PG:** * **Generation Time:** *M. leprae* is the slowest-growing human pathogen, with a doubling time of approximately **12–13 days**. * **Staining:** It is Acid-Fast (Ziehl-Neelsen stain) but **less acid-fast** than *M. tuberculosis*; hence, 5% sulfuric acid is used for decolonization instead of 20%. * **Target Cells:** It has a specific tropism for **Schwann cells** and macrophages. * **Viability:** The **Morphologic Index (MI)** measures the percentage of uniformly stained (viable) bacilli in a smear, used to monitor treatment response.

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

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