Bacteriology Practice Questions

Q: Which of the following mycobacteria can cause disease in an HIV+ve patient with a CD4 count of 600/cu.mm?

M. tuberculosis. **Explanation:** The risk and type of opportunistic infections in HIV patients are directly correlated with the **CD4+ T-lymphocyte count**. A CD4 count of 600/cu.mm is relatively high (near the normal range of 500–1500/cu.mm), indicating only mild immunosuppression. **Why M. tuberculosis is correct:** *Mycobacterium tuberculosis* (MTB) is highly virulent. Unlike most opportunistic pathogens, it does not require profound immunosuppression to cause disease. It is the **most common opportunistic infection** in HIV patients worldwide and can occur at **any CD4 count**. When the CD4 count is >500/cu.mm, MTB typically presents with classic pulmonary features (upper lobe cavitations), similar to HIV-negative individuals. **Why the other options are incorrect:** * **B. MAC (Mycobacterium avium complex):** This is a "late-stage" opportunistic infection. It typically occurs only when the CD4 count drops significantly, usually **below 50/cu.mm**. It presents as disseminated disease with fever, night sweats, and lymphadenopathy. * **C & D. M. chelonei and M. fortuitum:** These are Rapidly Growing Mycobacteria (RGM). While they can cause skin and soft tissue infections, they rarely cause systemic disease in HIV patients unless there is severe immunosuppression or direct inoculation (e.g., post-surgical or trauma). **NEET-PG High-Yield Pearls:** 1. **MTB vs. CD4:** MTB is the earliest opportunistic infection in HIV. As CD4 counts decline (<200), the presentation shifts from "typical" (cavitary) to "atypical" (lower lobe infiltrates, no cavities, extrapulmonary spread). 2. **MAC Prophylaxis:** Historically initiated when CD4 <50/cu.mm (usually with Azithromycin), though guidelines now focus on rapid ART initiation. 3. **Chest X-ray:** In an HIV patient with a high CD4 count and TB, the CXR looks "normal/classic." In low CD4 counts, the CXR may show intrathoracic lymphadenopathy or a miliary pattern.

Q: A 3-year-old boy sustained a leg injury while playing. A few days later, crepitations are felt in the injured area. What is the probable causative organism?

Clostridium Welchii. ### Explanation The clinical presentation of an injury followed by the presence of **crepitations** (the sensation of air bubbles under the skin) is a classic hallmark of **Gas Gangrene** (Clostridial Myonecrosis). **1. Why Clostridium Welchii is correct:** *Clostridium welchii* (now commonly known as **Clostridium perfringens**) is the most common cause of gas gangrene. It is an anaerobic, spore-forming Gram-positive bacillus. When introduced into deep, traumatic wounds with low oxygen tension, it produces various toxins (notably **Alpha-toxin/Lecithinase**). These toxins cause tissue necrosis and ferment muscle carbohydrates, leading to the production of gas. This gas accumulates in the tissues, resulting in the characteristic clinical finding of **crepitus**. **2. Why the other options are incorrect:** * **Corynebacterium diphtheriae:** Causes Diphtheria, characterized by a "pseudomembrane" in the throat and exotoxin-mediated myocarditis or neuropathy, not wound gas production. * **Pseudomonas aeruginosa:** Often infects burns or chronic wounds, producing blue-green pigment (pyocyanin) and a fruity odor, but it does not cause gas gangrene or crepitations. * **Clostridium tetani:** While also found in soil and wounds, it causes **Tetanus**. It produces tetanospasmin, leading to muscle spasticity (lockjaw, opisthotonus) rather than tissue necrosis or gas formation. **3. NEET-PG Clinical Pearls:** * **Nagler’s Reaction:** A biochemical test used to identify *C. perfringens* based on its lecithinase activity on egg yolk agar. * **Morphology:** It is described as "box-car shaped" bacilli and is unique among Clostridia for being **non-motile** and **capsulated**. * **Radiology:** X-rays of the affected limb typically show "feathering" patterns due to gas between muscle fascicles. * **Management:** Requires urgent surgical debridement and high-dose Penicillin G.

Q: A box of ham sandwiches with mayonnaise, prepared by a person with a boil on his neck, was left unrefrigerated for on-call interns. Three doctors became violently ill approximately 2 hours after eating the sandwiches. What is the most likely causative agent?

Staphylococcus aureus enterotoxin. **Explanation:** The clinical presentation describes a classic case of **Staphylococcal Food Poisoning**. **Why Option D is Correct:** * **Source:** *Staphylococcus aureus* is a common inhabitant of the skin. The "boil on the neck" of the food handler is the source of contamination. * **Mechanism:** When protein-rich foods (like ham or mayonnaise) are left unrefrigerated, the bacteria multiply and produce **preformed enterotoxins**. These toxins are heat-stable and resistant to gastric enzymes. * **Incubation Period:** This is the highest-yield clue. *S. aureus* has a very short incubation period (**1–6 hours**) because the toxin is already present in the food (intoxication), not produced after ingestion (infection). * **Symptoms:** Rapid onset of projectile vomiting, nausea, and abdominal cramps. **Why Other Options are Incorrect:** * **Option A:** *Clostridium perfringens* typically has a longer incubation period (8–16 hours) and primarily causes watery diarrhea rather than violent vomiting. It is associated with reheated meat/gravy. * **Option B:** Coagulase is an enzyme used for laboratory identification of *S. aureus*, but it is not the virulence factor responsible for food poisoning symptoms; the enterotoxin is. * **Option C:** Penicillinase (beta-lactamase) is an enzyme that confers antibiotic resistance; it has no role in causing acute gastrointestinal illness. **NEET-PG High-Yield Pearls:** 1. **Shortest Incubation:** *S. aureus* (1–6 hrs) and *Bacillus cereus* (emetic type: 1–5 hrs) have the shortest incubation periods. 2. **Heat Stability:** The enterotoxin is stable at 100°C for 30 minutes; reheating food does not prevent the illness. 3. **Mechanism:** The toxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain. 4. **Diagnosis:** Primarily clinical; culture of the food can confirm the presence of the same strain found in the carrier’s lesion.

Q: Which of the following bacteria is NOT typically encapsulated?

Escherichia coli. **Explanation:** The presence of a polysaccharide capsule is a major virulence factor for many bacteria, as it inhibits phagocytosis. While several pathogens are encapsulated, **Escherichia coli** is generally considered **non-encapsulated** in its typical commensal and most pathogenic forms. Although specific strains like the **K1 serotype** (associated with neonatal meningitis) do possess a capsule, E. coli is not classified as a "typically" encapsulated organism in the context of standard microbiology and NEET-PG patterns. **Analysis of Options:** * **Klebsiella pneumoniae:** Known for its prominent, thick polysaccharide capsule that gives its colonies a characteristic **mucoid appearance** on culture media. * **Haemophilus influenzae:** Type b (Hib) is the most virulent strain due to its polyribosylribitol phosphate (PRP) capsule. * **Bacillus anthracis:** A unique exception in bacteriology; it possesses a capsule made of **poly-D-glutamic acid (polypeptide)** rather than polysaccharide. This is a high-yield distinction. **NEET-PG High-Yield Pearls:** 1. **Mnemonic for Encapsulated Organisms:** *"**S**ome **K**illers **H**ave **N**ice **S**hiny **B**odies"* (**S**treptococcus pneumoniae, **K**lebsiella, **H**aemophilus influenzae, **N**eisseria meningitidis, **S**almonella Typhi, **B**acillus anthracis). 2. **Quellung Reaction:** This "capsular swelling" test is used to identify encapsulated bacteria. 3. **Asplenic Patients:** Individuals with functional or anatomical asplenia are at high risk for sepsis from encapsulated organisms (especially *S. pneumoniae*) because the spleen is the primary site for opsonization and clearance of these bacteria.

Q: What is the most common organism responsible for symptomatic native valve infective endocarditis in children?

Staphylococcus aureus. **Explanation:** **Staphylococcus aureus** is currently the most common cause of symptomatic native valve infective endocarditis (IE) in children. While historically Viridans group streptococci (Alpha-hemolytic) were more prevalent, the epidemiology has shifted due to increased use of indwelling central venous catheters and more frequent cardiac interventions. *S. aureus* is highly virulent, often leading to acute, fulminant presentations with rapid valvular destruction and systemic embolization, even in children without prior structural heart disease. **Analysis of Incorrect Options:** * **Alpha-hemolytic streptococci (Viridans group):** These were previously the leading cause of IE in children, typically associated with dental procedures and underlying congenital heart disease (CHD). While still a major cause of subacute IE, they have been surpassed by *S. aureus* in overall frequency. * **Beta-hemolytic streptococci (e.g., Group A):** These are more commonly associated with Acute Rheumatic Fever (ARF) rather than direct infective endocarditis. * **Group D streptococci (e.g., S. bovis/gallolyticus):** These are rare in the pediatric population and are more typically associated with colonic pathology in adults. **High-Yield Pearls for NEET-PG:** * **Most common underlying risk factor:** Congenital Heart Disease (CHD), specifically VSD, TOF, and Left-sided obstructive lesions. * **Most common site:** Mitral valve (overall), but Tricuspid valve in cases of IV drug use or central lines. * **Duke Criteria:** The gold standard for diagnosis (requires 2 Major, 1 Major + 3 Minor, or 5 Minor criteria). * **Prophylaxis:** Indicated only for high-risk cardiac conditions (e.g., prosthetic valves, cyanotic CHD) before dental procedures involving gingival manipulation.

Q: True about the HACEK group of bacteria?

Require CO2 for growth. **Explanation:** The **HACEK group** is a collection of fastidious, slow-growing, **Gram-negative bacilli** that are part of the normal oropharyngeal flora. The group includes: * **H:** *Haemophilus* species (specifically *H. aphrophilus/paraphrophilus*) * **A:** *Aggregatibacter* species * **C:** *Cardiobacterium hominis* * **E:** *Eikenella corrodens* * **K:** *Kingella kingae* **Why Option D is Correct:** HACEK organisms are **capnophilic**, meaning they require an atmosphere enriched with **5–10% carbon dioxide (CO2)** for optimal growth. Because they are fastidious and slow-growing, blood cultures may need to be incubated for an extended period (though modern automated systems usually detect them within 5 days). **Why Other Options are Incorrect:** * **A & C:** HACEK organisms are **Gram-negative** (not positive) and are **facultative anaerobes** or microaerophiles, not obligate anaerobes. * **B:** *Coxiella burnetii* is the causative agent of Q fever and is an obligate intracellular bacterium; it is not part of the HACEK group. **High-Yield Clinical Pearls for NEET-PG:** 1. **Clinical Significance:** They are the most common cause of **culture-negative infective endocarditis** (in the context of subacute presentation) in non-IV drug users. 2. **Eikenella corrodens:** Associated with "clenched fist injuries" (human bites) and produces a characteristic **bleach-like odor** and "pitting" of the agar. 3. **Kingella kingae:** A common cause of septic arthritis and osteomyelitis in children. 4. **Treatment:** Most HACEK organisms are now treated with **Ceftriaxone** (3rd generation cephalosporins) as they may produce beta-lactamases.

Q: Which of the following Corynebacterium species produces a toxin similar to that of diphtheria toxin?

Corynebacterium ulcerans. ### Explanation **Correct Answer: C. Corynebacterium ulcerans** **Why it is correct:** *Corynebacterium ulcerans* and *Corynebacterium pseudotuberculosis* are the only two non-diphtherial species that can carry the **tox gene** (acquired via lysogenic bacteriophages). This gene allows them to produce a toxin that is functionally and immunologically identical to the **Diphtheria Toxin** produced by *C. diphtheriae*. * *C. ulcerans* is a zoonotic pathogen typically transmitted through raw milk or contact with infected farm animals/pets. It can cause a clinical presentation indistinguishable from classical diphtheria, including the formation of a **pharyngeal pseudomembrane** and systemic complications like myocarditis. **Analysis of Incorrect Options:** * **A. Corynebacterium auris:** Primarily associated with ear infections (otitis media); it does not produce diphtheria toxin. * **B. Corynebacterium jeikeium (jikei):** A common opportunistic pathogen in immunocompromised patients (especially those with intravascular catheters). It is notorious for being **multi-drug resistant** but does not produce the toxin. * **C. Corynebacterium striatum:** A commensal of the skin and nasopharynx that can cause device-related infections and endocarditis; it is non-toxigenic. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism of Action:** Diphtheria toxin inhibits protein synthesis by **ADP-ribosylation of Elongation Factor-2 (EF-2)**. 2. **Diagnosis:** To confirm toxin production in *C. ulcerans*, the **Elek Test** or PCR for the *tox* gene is used. 3. **Treatment:** Infections with toxigenic *C. ulcerans* require **Diphtheria Antitoxin (DAT)** in addition to antibiotics (Erythromycin or Penicillin). 4. **Zoonosis:** Unlike *C. diphtheriae* (human-only reservoir), *C. ulcerans* is a **zoonotic** infection.

Q: All of the following are true about Listeria except?

Non-motile. **Explanation:** The correct answer is **C (Non-motile)** because *Listeria monocytogenes* is characteristically **motile**. It exhibits a unique **"tumbling motility"** at 25°C (room temperature) due to peritrichous flagella, while it remains non-motile at 37°C. In semi-solid agar, it produces a classic **"umbrella-shaped"** growth pattern. **Analysis of Options:** * **A. Catalase positive:** This is a true statement. *Listeria* is a Gram-positive bacillus that is catalase-positive, which helps differentiate it from *Streptococcus agalactiae* (GBS), which is catalase-negative but also causes neonatal sepsis. * **B. Anton test positive:** This is true. The Anton test is a diagnostic test where the instillation of the culture into a rabbit's conjunctival sac causes **purulent keratoconjunctivitis**. * **D. Causes chorioamnionitis:** This is true. *Listeria* has a predilection for the placenta. In pregnant females, it can lead to chorioamnionitis, premature labor, abortion, or **Granulomatosis infantiseptica** (disseminated abscesses/granulomas in the fetus). **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-sporing, coccobacilli in short chains. * **Cold Enrichment:** It can grow at temperatures as low as 4°C (psychrophilic), which is why it is associated with refrigerated foods like unpasteurized cheese and cold meats. * **Intracellular Pathogen:** It uses **Listeriolysin O** to escape phagosomes and **ActA protein** for "actin rockets" to move between cells. * **Drug of Choice:** Ampicillin (Note: It is inherently resistant to cephalosporins).

Question 1

Which of the following mycobacteria can cause disease in an HIV+ve patient with a CD4 count of 600/cu.mm?

Accepted Answer

M. tuberculosis

Answer

MAC (Mycobacterium avium complex)

Answer

M. chelonei

Answer

M. fortuitum

Question 2

A 3-year-old boy sustained a leg injury while playing. A few days later, crepitations are felt in the injured area. What is the probable causative organism?

Accepted Answer

Clostridium Welchii

Answer

Corynebacterium Diphtheriae

Answer

Pseudomonas Aeruginosa

Answer

Clostridium Tetani

Question 3

A box of ham sandwiches with mayonnaise, prepared by a person with a boil on his neck, was left unrefrigerated for on-call interns. Three doctors became violently ill approximately 2 hours after eating the sandwiches. What is the most likely causative agent?

Accepted Answer

Staphylococcus aureus enterotoxin

Answer

Clostridium perfringens toxin

Answer

Coagulase from S. aureus in the ham

Answer

Penicillinase given to inactivate penicillin in the pork

Question 4

Which of the following bacteria is NOT typically encapsulated?

Accepted Answer

Escherichia coli

Answer

Klebsiella pneumoniae

Answer

Haemophilus influenzae

Answer

Bacillus anthracis

Question 5

What is the most common organism responsible for symptomatic native valve infective endocarditis in children?

Accepted Answer

Staphylococcus aureus

Answer

Beta-hemolytic streptococci

Answer

Alpha-hemolytic streptococci

Answer

Group D streptococci

Question 6

Which of the following infections does NOT involve lymphatic spread?

Accepted Answer

Erysipelas

Answer

Actinomycosis

Answer

Ludwig's Angina

Answer

Madura foot

Question 7

Bacterial endocarditis is most commonly caused by which organism?

Accepted Answer

Staphylococcus aureus

Answer

alpha-Hemolytic Streptococci

Answer

beta-Hemolytic Streptococci

Answer

Cardiobacterium

Question 8

True about the HACEK group of bacteria?

Accepted Answer

Require CO2 for growth

Answer

Anaerobes

Answer

Includes Coxiella burnetii

Answer

Gram-positive

Question 9

Which of the following Corynebacterium species produces a toxin similar to that of diphtheria toxin?

Accepted Answer

Corynebacterium ulcerans

Answer

Corynebacterium auris

Answer

Corynebacterium jikei

Answer

Corynebacterium striatum

Question 10

All of the following are true about Listeria except?

Accepted Answer

Non-motile

Answer

Catalase positive

Answer

Anton test positive

Answer

Causes chorioamnionitis in pregnant females

Bacteriology — MCQs

Bacteriology — MCQs

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