Bacteriology Practice Questions

Q: An experimental compound prevents the activation of adenyl cyclase and the resulting increase in cyclic AMP. The toxic effects of which of the following bacteria might be prevented with the use of this experimental compound?

Vibrio cholerae. **Explanation:** The correct answer is **Vibrio cholerae**. The pathogenesis of cholera is driven by the **Cholera Toxin (Choleragen)**, an A-B subunit exotoxin. The 'A' subunit catalyzes the ADP-ribosylation of the **Gs regulatory protein**, keeping it in a permanently "on" state. This leads to the constitutive activation of **adenyl cyclase**, resulting in high intracellular levels of **cyclic AMP (cAMP)**. Elevated cAMP triggers the active secretion of chloride ions and inhibits sodium absorption, leading to the characteristic "rice-water" diarrhea. An experimental compound that prevents adenyl cyclase activation would directly block this signaling cascade. **Analysis of Incorrect Options:** * **Corynebacterium diphtheriae:** Produces Diphtheria toxin, which acts by ADP-ribosylation of **Elongation Factor-2 (EF-2)**, inhibiting protein synthesis. It does not involve cAMP. * **Pseudomonas aeruginosa:** Produces **Exotoxin A**, which has an identical mechanism to Diphtheria toxin (inhibition of EF-2). * **Listeria monocytogenes:** Its primary virulence factor is **Listeriolysin O**, a pore-forming toxin that allows the bacteria to escape the phagosome; it does not utilize the adenyl cyclase pathway. **High-Yield Clinical Pearls for NEET-PG:** * **cAMP-mediated toxins:** Remember the mnemonic **"cAMP"** – **C**holera (Vibrio), **A**nthrax (Edema factor), **M**ontezuma’s revenge (ETEC - Heat Labile toxin), and **P**ertussis (Bordetella). * **Mechanism Tip:** While Cholera toxin activates **Gs**, Pertussis toxin inhibits **Gi**; both result in increased cAMP levels. * **ETEC Toxin:** Remember **"Labile in the Air (cAMP), Stable on the Ground (cGMP)"**—Heat-labile (LT) increases cAMP; Heat-stable (ST) increases cGMP.

Q: Which of the following statements is true about cholera vibrios?

Is non-motile.. **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."

Q: Which of the following organisms, when isolated in the blood, requires the synergistic activity of penicillin plus an aminoglycoside for appropriate therapy?

Enterococcus faecalis. **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.

Q: Which of the following strains does not cause hyperendemic trachoma?

Chlamydia trachomatis serovars D. **Explanation:** Trachoma, caused by *Chlamydia trachomatis*, is classified into different clinical presentations based on the specific serovars (strains) involved. **1. Why Option D is Correct:** **Serovars D through K** are primarily associated with **Inclusion Conjunctivitis** (Paratrachoma) and urogenital infections (such as urethritis and cervicitis). These are transmitted via sexual contact or through birth canal exposure in neonates. They do not cause the chronic, scarring, hyperendemic trachoma seen in developing regions. **2. Why Options A, B, and C are Incorrect:** * **Serovars A, B, Ba, and C** are the classic causative agents of **Endemic Trachoma**. * These strains are transmitted via "Eyes, Flies, and Fingers" (the 3 F’s). * They lead to chronic follicular conjunctivitis, which, through repeated reinfection, results in conjunctival scarring, trichiasis, and eventual corneal blindness. In hyperendemic areas, these specific serovars are responsible for high disease prevalence. **Clinical Pearls for NEET-PG:** * **Mnemonic for Serovars:** * **A, B, C:** **A**ssociated with **B**lindness in **C**hildren (Endemic Trachoma). * **D–K:** **D**irect contact/Sexually transmitted (Inclusion Conjunctivitis). * **L1, L2, L3:** **L**ymphogranuloma Venereum (LGV). * **SAFE Strategy (WHO):** **S**urgery (for trichiasis), **A**ntibiotics (Azithromycin), **F**acial cleanliness, **E**nvironmental improvement. * **Hallmark Signs:** Herbert’s pits (limbal scars) and Arlt’s line (palpebral conjunctival scarring) are pathognomonic for Trachoma. * **Drug of Choice:** Single dose of oral Azithromycin (20 mg/kg).

Q: What is the most common causative organism of community-acquired pneumonia?

Streptococcus pneumoniae. **Explanation:** **Streptococcus pneumoniae** (Pneumococcus) remains the **most common cause** of community-acquired pneumonia (CAP) worldwide across all age groups [1]. It is a Gram-positive, lancet-shaped diplococcus [1]. The underlying medical concept involves its high prevalence in the nasopharyngeal flora and its potent virulence factor—the polysaccharide capsule—which allows it to evade phagocytosis and cause typical lobar pneumonia. **Analysis of Incorrect Options:** * **Klebsiella pneumoniae:** While a significant cause of CAP, it is typically associated with specific risk factors such as **chronic alcoholism**, diabetes mellitus, or nursing home residents [1]. It is classic for causing "currant jelly sputum" and upper lobe cavitation [1]. * **Mycoplasma pneumoniae:** This is the most common cause of **atypical pneumonia**, particularly in young adults and school-aged children. It lacks a cell wall and often presents with a "walking pneumonia" clinical picture (milder symptoms, interstitial infiltrates). * **Legionella pneumoniae:** This is an important cause of severe CAP, often linked to contaminated water systems or air conditioning [1]. It is characterized by multisystem involvement, such as hyponatremia and gastrointestinal symptoms (diarrhea). **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Sputum culture and Gram stain, though urinary antigen tests are highly specific for *S. pneumoniae* [1]. * **Radiology:** *S. pneumoniae* typically presents as **lobar consolidation**. * **Vaccination:** Two types are available (PPSV23 and PCV13), recommended for the elderly and immunocompromised to prevent invasive pneumococcal disease. * **Drug of Choice:** Beta-lactams (like Amoxicillin or Ceftriaxone) are first-line, though resistance patterns must be monitored.

Q: Antibiotic-associated diarrhea is most commonly caused by which organism?

Clostridioides difficile. **Explanation:** **1. Why Clostridioides difficile is correct:** *Clostridioides difficile* (formerly *Clostridium*) is the most common identifiable cause of antibiotic-associated diarrhea (AAD) [1]. The underlying mechanism involves the disruption of normal colonic flora by broad-spectrum antibiotics (most commonly Clindamycin, Fluoroquinolones, and Cephalosporins). This allows *C. difficile* to colonize the gut and release **Toxin A (Enterotoxin)** and **Toxin B (Cytotoxin)**, leading to mucosal inflammation and, in severe cases, the formation of yellow-white plaques known as **Pseudomembranous colitis** [1]. **2. Why the other options are incorrect:** * **Vibrio cholerae:** Causes "rice-water" stools through a toxin-mediated mechanism (adenylate cyclase activation) but is associated with contaminated water/food, not antibiotic use. * **Salmonella species:** Typically causes food poisoning or enteric fever. While it causes diarrhea, it is not a sequela of antibiotic-induced flora disruption. * **Klebsiella pneumoniae:** While *K. oxytoca* is specifically linked to antibiotic-associated hemorrhagic colitis, *K. pneumoniae* is primarily a respiratory and urinary pathogen and is not the "most common" cause of AAD. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice:** Oral **Vancomycin** or **Fidaxomicin** are first-line treatments. Metronidazole is now reserved for non-severe cases if other options are unavailable. * **Diagnosis:** The gold standard for toxin identification is the **Cell Cytotoxicity Assay**, but the most common initial tests are **GDH (Glutamate Dehydrogenase) screening** and **NAAT/PCR** for toxin genes [1]. * **Risk Factors:** Advanced age, prolonged hospitalization, and use of **Proton Pump Inhibitors (PPIs)** [1]. * **Alcohol-based rubs** are ineffective against *C. difficile* spores; handwashing with soap and water is mandatory.

Q: What is the antibiotic of choice for streptococcal pharyngitis?

Penicillin. **Explanation:** **1. Why Penicillin is the Correct Answer:** Penicillin (specifically Penicillin V for oral use or Penicillin G Benzathine for IM use) remains the drug of choice for **Streptococcal pharyngitis** caused by *Streptococcus pyogenes* (Group A Streptococcus). The primary medical rationale is that *S. pyogenes* remains **100% universally susceptible** to penicillin; no clinical resistance has been documented to date. Furthermore, penicillin has a narrow spectrum, is cost-effective, and is proven to prevent the most dreaded non-suppurative complication: **Acute Rheumatic Fever (ARF).** **2. Why Other Options are Incorrect:** * **Ampicillin:** While effective, it offers a broader spectrum than necessary. In cases where pharyngitis is actually due to Infectious Mononucleosis (EBV) rather than Strep, ampicillin can trigger a characteristic maculopapular rash. * **Erythromycin:** This is a macrolide used only as an alternative for patients with a **penicillin allergy**. It is not first-line due to increasing patterns of resistance among Group A Strep. * **Vancomycin:** This is a potent "reserve" antibiotic used for serious infections like MRSA or resistant Enterococci. Using it for simple Strep throat is inappropriate and contributes to antibiotic resistance. **3. NEET-PG High-Yield Clinical Pearls:** * **Prevention:** Treatment of Strep pharyngitis prevents **Acute Rheumatic Fever**, but it does **NOT** prevent **Post-Streptococcal Glomerulonephritis (PSGN)**. * **Diagnosis:** The **Centor Criteria** are used clinically to estimate the probability of bacterial pharyngitis. * **Gold Standard:** Throat culture on Blood Agar (showing beta-hemolysis) remains the gold standard for diagnosis. * **Alternative:** For patients with non-anaphylactic penicillin allergy, **First-generation Cephalosporins** (e.g., Cephalexin) are preferred.

Q: Lady Windermere's syndrome is associated with which of the following organisms?

Non-tuberculous mycobacteria. **Explanation:** **Lady Windermere’s Syndrome** is a specific clinical presentation of pulmonary infection caused by **Non-Tuberculous Mycobacteria (NTM)**, most commonly *Mycobacterium avium complex* (MAC) [1]. The syndrome typically occurs in elderly, immunocompetent women who do not have pre-existing lung disease. The underlying pathophysiology is linked to **voluntary cough suppression**. Historically, it was observed in women who, for social reasons, refrained from coughing and expectorating. This leads to the stagnation of secretions in the dependent lobes (specifically the **right middle lobe** or the lingula), creating a nidus for NTM infection. Radiologically, it is characterized by bronchiectasis and tree-in-bud nodules. **Analysis of Incorrect Options:** * **A. Treponema pallidum:** The causative agent of Syphilis; it is associated with aortitis and neurosyphilis, not chronic pulmonary NTM infections. * **B. Listeria monocytogenes:** A gram-positive rod causing meningitis and sepsis, particularly in neonates, the elderly, and the immunocompromised. * **D. Chlamydia trachomatis:** A common cause of STIs and trachoma; it does not cause the chronic bronchiectatic pattern seen in this syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Patient Profile:** Elderly female, thin habitus, often associated with skeletal abnormalities like pectus excavatum or scoliosis. * **Commonest Organism:** *Mycobacterium avium-intracellulare* (MAC) [1]. * **Radiology Hallmark:** Bronchiectasis and nodules specifically involving the **Right Middle Lobe (RML)** or the **Lingula**. * **Treatment:** Long-term multi-drug therapy (typically Macrolides, Ethambutol, and Rifampin).

Question 1

An experimental compound prevents the activation of adenyl cyclase and the resulting increase in cyclic AMP. The toxic effects of which of the following bacteria might be prevented with the use of this experimental compound?

Accepted Answer

Vibrio cholerae

Answer

Corynebacterium diphtheriae

Answer

Pseudomonas

Answer

Listeria monocytogenes

Question 2

Which of the following statements is true about cholera vibrios?

Accepted Answer

Is non-motile.

Answer

Can tolerate a wide range of alkaline pH.

Answer

Cannot be grown in media.

Answer

NaCl stimulates growth.

Question 3

Which of the following organisms, when isolated in the blood, requires the synergistic activity of penicillin plus an aminoglycoside for appropriate therapy?

Accepted Answer

Enterococcus faecalis

Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Answer

Bacteroides fragilis

Question 4

Which of the following strains does not cause hyperendemic trachoma?

Accepted Answer

Chlamydia trachomatis serovars D

Answer

Chlamydia trachomatis serovars A

Answer

Chlamydia trachomatis serovars B

Answer

Chlamydia trachomatis serovars C

Question 5

What is the most common causative organism of community-acquired pneumonia?

Accepted Answer

Streptococcus pneumoniae

Answer

Klebsiella pneumoniae

Answer

Mycoplasma pneumoniae

Answer

Legionella pneumoniae

Question 6

Antibiotic-associated diarrhea is most commonly caused by which organism?

Accepted Answer

Clostridioides difficile

Answer

Vibrio cholerae

Answer

Salmonella species

Answer

Klebsiella pneumoniae

Question 7

What is the antibiotic of choice for streptococcal pharyngitis?

Accepted Answer

Penicillin

Answer

Ampicillin

Answer

Erythromycin

Answer

Vancomycin

Question 8

A couple, who did not know each other very well, dated and had sexual contact. Several weeks later, the man noticed a small, painless vesicle on his penis, which ruptured and then healed. Soon, his inguinal lymph nodes enlarged and discharged pus through multiple sinus tracts. Lymphogranuloma venereum (LGV) is a venereal disease caused by serotype L1, L2, or L3 of Chlamydia trachomatis. The differential diagnosis should include which of the following?

Accepted Answer

Chancroid

Answer

Babesiosis

Answer

Mononucleosis

Answer

Psittacosis

Question 9

Lady Windermere's syndrome is associated with which of the following organisms?

Accepted Answer

Non-tuberculous mycobacteria

Answer

Treponema pallidum

Answer

Listeria monocytogenes

Answer

Chlamydia trachomatis

Question 10

A 56-year-old man presents with intermittent fevers and malaise for the past 2 weeks, with no other localizing symptoms. Two months prior, he underwent valve replacement surgery for a bicuspid aortic valve, receiving a mechanical valve with an uncomplicated postoperative course. On examination, his temperature is 38°C, blood pressure is 124/80 mm Hg, pulse is 72/min, and head and neck are normal. Auscultation reveals a 3/6 systolic ejection murmur, a mechanical second heart sound, and a 2/6 early diastolic murmur. Lungs are clear and skin examination is normal. Three sets of blood cultures and an urgent echocardiogram are performed. Which of the following is the most likely causative organism?

Accepted Answer

Staphylococcus epidermidis

Answer

Staphylococcus aureus

Answer

Streptococcus viridans

Answer

Enterococci

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?