Bacteriology Practice Questions

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).

Q: A patient presented with pyrexia of unknown origin. Blood culture in special laboratory media was positive for gram-negative coccobacilli which were oxidase positive. Which one of the following is the likely organism grown in culture?

Brucella spp.. ### Explanation **Correct Option: D. Brucella spp.** The clinical presentation of Pyrexia of Unknown Origin (PUO) combined with the laboratory findings of **Gram-negative coccobacilli** that are **oxidase positive** strongly points toward *Brucella*. * **Microbiology:** *Brucella* are small, aerobic, non-motile, Gram-negative coccobacilli. They are catalase and oxidase positive. * **Culture:** They are fastidious and require "special laboratory media" (e.g., Castaneda’s biphasic medium) for growth. Blood culture is the gold standard for diagnosing Brucellosis, especially during the febrile phase. **Analysis of Incorrect Options:** * **A. Pasteurella spp.:** While these are Gram-negative coccobacilli and oxidase positive, they are typically associated with animal bites (cats/dogs) and cause cellulitis or abscesses rather than classic PUO. They show characteristic bipolar staining ("safety-pin appearance"). * **B. Francisella spp.:** *Francisella tularensis* causes Tularemia. Although it is a Gram-negative coccobacillus, it is **oxidase negative**. It also requires cysteine-enriched media (like BCYE) for growth. * **C. Bartonella spp.:** These are Gram-negative bacilli but are typically **oxidase negative**. They are associated with Cat Scratch Disease or Trench Fever, not the classic undulant fever pattern of PUO seen in Brucellosis. **NEET-PG High-Yield Pearls:** * **Undulant Fever:** The classic temperature pattern associated with Brucellosis. * **Castaneda’s Medium:** The specific biphasic medium used to reduce the risk of laboratory-acquired infections (a significant hazard with *Brucella*). * **Rose Bengal Plate Test (RBPT):** A rapid screening test for Brucellosis. * **Standard Agglutination Test (SAT):** Detects antibodies; a titer of 1:160 or more is significant. * **Drug of Choice:** Doxycycline + Rifampicin (or Streptomycin).

Q: Which one of the following factors released by heating a suspension of sheep erythrocytes is required for the growth of Haemophilus Influenzae in chocolate agar?

Nicotinamide adenine dinucleotide (NAD). ### Explanation *Haemophilus influenzae* is a fastidious organism that requires two specific growth factors found in blood: **Factor X (Hemin)** and **Factor V (NAD)**. **Why Option B is Correct:** While sheep blood agar contains both factors, Factor V (NAD) is trapped inside the erythrocytes. Furthermore, sheep blood contains **NADases** (enzymes) that can inactivate Factor V. Heating the blood to approximately 75–80°C to create **Chocolate Agar** serves two purposes: 1. It lyses the red blood cells, **releasing NAD (Factor V)** into the medium. 2. It inactivates the heat-labile NADases, ensuring the NAD remains available for the bacteria. Therefore, the specific factor released and made available by heating is NAD. **Analysis of Incorrect Options:** * **A & C (Hemin/Hemoglobin):** Factor X (Hemin) is derived from hemoglobin. While essential for *H. influenzae*, it is heat-stable and already available in basic blood agar; heating is not specifically required to "release" it for growth in the same way it is for NAD. * **D (Hemolysin):** This is an exotoxin produced by certain bacteria to lyse RBCs; it is not a growth factor required by *Haemophilus*. **High-Yield NEET-PG Pearls:** * **Satellitism:** *H. influenzae* can grow on sheep blood agar near colonies of *Staphylococcus aureus* because *S. aureus* produces NAD as a metabolic byproduct. * **Culture Media:** Chocolate agar is the medium of choice. For selective isolation, **Levinthal’s medium** or **Fildes’ agar** can be used. * **Species Differentiation:** *H. influenzae* requires both X and V; *H. parainfluenzae* requires only V; *H. ducreyi* (Chancroid) requires only X.

Q: All the following species of Haemophilus show satellitism, except?

H. ducreyi. **Explanation:** The phenomenon of **satellitism** occurs when certain *Haemophilus* species grow on Blood Agar only in the vicinity of colonies of *Staphylococcus aureus*. This happens because *S. aureus* synthesizes and releases **Factor V (NAD)** into the medium, while the Blood Agar itself provides **Factor X (Hemin)**. **Why H. ducreyi is the correct answer:** *Haemophilus ducreyi* is unique among the *Haemophilus* species because it **requires only Factor X** for growth and is **Factor V independent**. Since it does not require Factor V, it does not exhibit the satellitism phenomenon. **Analysis of Incorrect Options:** * **H. influenzae:** A classic "X and V factor dependent" organism. It shows robust satellitism around *S. aureus* on Blood Agar. * **H. aegypticus:** Closely related to *H. influenzae* (biotype III), it also requires both Factors X and V, thus showing satellitism. * **H. haemolyticus:** This species is also dependent on both Factors X and V, exhibiting satellitism and often causing confusion with *S. pyogenes* due to its hemolytic properties. **High-Yield NEET-PG Pearls:** * **Factor X:** Hemin (Heat stable). * **Factor V:** NAD - Nicotinamide Adenine Dinucleotide (Heat labile). * **H. ducreyi Clinical Link:** Causes **Chancroid** (Soft, painful chancre). On Gram stain, it shows a characteristic **"School of fish"** or "Railroad track" appearance. * **Culture Media:** While satellitism is seen on Blood Agar, the gold standard for *H. influenzae* is **Chocolate Agar** (where RBCs are lysed to release both X and V factors).

Question 1

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 2

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

Accepted Answer

Clostridioides difficile

Answer

Vibrio cholerae

Answer

Salmonella species

Answer

Klebsiella pneumoniae

Question 3

What is the antibiotic of choice for streptococcal pharyngitis?

Accepted Answer

Penicillin

Answer

Ampicillin

Answer

Erythromycin

Answer

Vancomycin

Question 4

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 5

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 6

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

Question 7

A patient presented with pyrexia of unknown origin. Blood culture in special laboratory media was positive for gram-negative coccobacilli which were oxidase positive. Which one of the following is the likely organism grown in culture?

Accepted Answer

Brucella spp.

Answer

Pasteurella spp.

Answer

Francisella spp.

Answer

Bartonella spp.

Question 8

A man presents to a sexually transmitted infection clinic with urethritis and urethral discharge. Gram stain shows numerous pus cells but no microorganisms. The culture is negative on routine laboratory media. What is the most likely causative agent?

Accepted Answer

Chlamydia trachomatis

Answer

Haemophilus ducreyi

Answer

Treponema pallidum

Answer

Neisseria gonorrhoeae

Question 9

Which one of the following factors released by heating a suspension of sheep erythrocytes is required for the growth of Haemophilus Influenzae in chocolate agar?

Accepted Answer

Nicotinamide adenine dinucleotide (NAD)

Answer

Hemin

Answer

Hemoglobin

Answer

Hemolysin

Question 10

All the following species of Haemophilus show satellitism, except?

Accepted Answer

H. ducreyi

Answer

H. influenzae

Answer

H. aegypticus

Answer

H. haemolyticus

Bacteriology — MCQs

Bacteriology — MCQs

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