Bacteriology Practice Questions

Q: Which of the following conditions is associated with antibodies against phenolic glycolipid (PGL-1)?

M. leprae. **Explanation:** The correct answer is **M. leprae**. **Phenolic Glycolipid-1 (PGL-1)** is a unique, species-specific surface antigen found in the cell wall of *Mycobacterium leprae*. It plays a critical role in the pathogenesis of Leprosy by facilitating the invasion of Schwann cells through binding to the laminin-α2 chain. 1. **Why M. leprae is correct:** PGL-1 is highly immunogenic. Patients with Leprosy, particularly those on the multibacillary (lepromatous) end of the spectrum, develop high titers of IgM antibodies against PGL-1. Serological assays (like ELISA) detecting these antibodies are used as biomarkers for bacterial load and to monitor treatment efficacy. 2. **Why other options are incorrect:** * **M. tuberculosis:** While it shares the *Mycobacterium* genus, its primary cell wall lipids are cord factor and sulfatides; it lacks PGL-1. * **Borrelia:** This spirochete is associated with antibodies against OspC or VlsE (Lyme disease), not phenolic glycolipids. * **Brucella:** Diagnosis relies on antibodies against the lipopolysaccharide (LPS) of the cell wall (Standard Agglutination Test). **NEET-PG High-Yield Pearls:** * **Schwann Cell Tropism:** PGL-1 is the specific molecule that allows *M. leprae* to infect peripheral nerves, leading to the characteristic anesthesia seen in Leprosy. * **Diagnostic Utility:** Anti-PGL-1 titers are highest in **Lepromatous Leprosy (LL)** and lowest/absent in Tuberculoid Leprosy (TT) due to the difference in humoral vs. cell-mediated immunity. * **Other specific components:** Remember that *M. leprae* cannot be cultured in vitro; it is grown in the footpads of mice or the nine-banded armadillo.

Q: Chinese letter configuration is seen in which bacterium?

Cdiphtheriae. **Explanation:** The characteristic **"Chinese letter"** or **"cuneiform"** arrangement is a hallmark feature of ***Corynebacterium diphtheriae***. This configuration occurs because the bacteria undergo incomplete separation during binary fission. When the cells divide, they remain attached at specific points, snapping into angular positions that resemble letters like **V, L, or Y**, or Chinese characters. **Why the correct answer is right:** * **C. diphtheriae:** These are Gram-positive, pleomorphic, club-shaped bacilli. Apart from the Chinese letter arrangement, they also demonstrate **metachromatic granules** (Volutin or Babes-Ernst granules), which are best visualized using special stains like Albert’s or Neisser’s stain. **Why the other options are incorrect:** * **Cl. tetani:** Known for its **"drumstick"** appearance due to terminal, spherical spores that are wider than the bacillus body. * **Cl. perfringens:** Typically appears as large, thick, Gram-positive capsulated bacilli with blunt ends, often described as **"box-car"** shaped. * **Strept. salivarius:** These are Gram-positive cocci that grow in chains; they do not exhibit angular arrangements or pleomorphism. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** Loeffler’s Serum Slope (rapid growth) and Potassium Tellurite Agar (black colonies). * **Toxin Detection:** Elek’s Gel Precipitation Test is the gold standard for detecting toxigenicity. * **Schick Test:** Used to determine the immune status of an individual against diphtheria. * **Mechanism of Action:** Diphtheria toxin inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Q: A 70-year-old patient presents with high-grade fever, dry cough, and abdominal pain. Sputum sample collected from the patient shows Gram-negative organisms that are able to grow only on charcoal yeast extract medium. What is the most likely organism?

Legionella. ### **Explanation** The correct answer is **Legionella pneumophila**. **1. Why Legionella is correct:** The clinical presentation of high-grade fever, dry cough (atypical pneumonia), and **extrapulmonary symptoms** like abdominal pain (or diarrhea/confusion) is classic for Legionnaires' disease. The definitive microbiological clue is the organism's growth requirement: *Legionella* is a fastidious Gram-negative rod that **requires L-cysteine and iron** for growth. **Buffered Charcoal Yeast Extract (BCYE) agar** is the specific selective medium used, where the charcoal acts to neutralize toxic fatty acids and peroxides. **2. Why the other options are incorrect:** * **Haemophilus influenzae:** While it causes pneumonia, it requires **Factors X (hemin) and V (NAD)** for growth and is typically cultured on **Chocolate Agar**, not charcoal medium. * **Listeria monocytogenes:** This is a **Gram-positive** bacillus (unlike the organism in the question). It exhibits "tumbling motility" and grows well on standard media like blood agar, often showing narrow-zone beta-hemolysis. * **Moraxella catarrhalis:** This is a Gram-negative **diplococcus** (not a rod) that grows easily on blood and chocolate agar. It is a common cause of COPD exacerbations but does not require BCYE agar. **3. NEET-PG High-Yield Pearls:** * **Stain:** *Legionella* stains poorly on Gram stain; **Silver (Dieterle) stain** is preferred. * **Diagnosis:** The **Urinary Antigen Test** is the most rapid initial test (detects Serogroup 1). * **Hyponatremia:** A very common laboratory finding in *Legionella* infections (SIADH-like picture). * **Transmission:** Associated with **aerosols from water sources** (AC cooling towers, showers, nebulizers); no person-to-person transmission. * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin).

Q: In a splenectomized patient, which of the following organisms is least likely to cause infection?

Staph. aureus. **Explanation:** The spleen plays a critical role in the body’s immune defense, specifically through the filtration of blood and the production of antibodies (IgM) by splenic B-cells. Its most vital function in this context is the clearance of **encapsulated bacteria** via opsonization. **Why Staph. aureus is the correct answer:** *Staphylococcus aureus* is a Gram-positive coccus that is **not** typically encapsulated in the clinical sense that requires splenic clearance. The risk of Overwhelming Post-Splenectomy Infection (OPSI) is primarily associated with organisms that possess a polysaccharide capsule. Since the spleen is the primary site for the destruction of these opsonized encapsulated organisms, its absence does not significantly predispose a patient to *Staph. aureus* infections compared to the general population. **Why the other options are incorrect:** * **A, B, and C (Pneumococci, Klebsiella, H. influenzae):** These are all **encapsulated organisms**. * *Streptococcus pneumoniae* (Pneumococcus) is the most common cause of OPSI (50-90% of cases). * *Haemophilus influenzae* type b and *Neisseria meningitidis* are the other two "classic" encapsulated pathogens. * *Klebsiella pneumoniae* is also an encapsulated Gram-negative rod that poses an increased risk in asplenic individuals. **High-Yield Clinical Pearls for NEET-PG:** * **OPSI (Overwhelming Post-Splenectomy Infection):** Characterized by a prodrome of fever and malaise, rapidly progressing to septic shock and DIC. * **Most Common Organism:** *Streptococcus pneumoniae*. * **Vaccination Protocol:** Patients should ideally be vaccinated **2 weeks before** elective splenectomy or **2 weeks after** emergency splenectomy. * **Essential Vaccines:** Pneumococcal, Meningococcal, and *H. influenzae* type b (Hib). * **Peripheral Smear Findings:** Look for **Howell-Jolly bodies** (nuclear remnants), Pappenheimer bodies, and Heinz bodies.

Q: Which of the following microorganisms is a known cause of necrotizing enterocolitis?

Clostridium perfringens type C. **Explanation:** **Clostridium perfringens type C** is the correct answer because it is the specific causative agent of **Enteritis Necroticans** (also known as *Pigbel*). This condition is characterized by patchy, necrotizing inflammation of the small intestine, particularly the jejunum. The pathogenesis is driven by the production of the **Beta-toxin**, a potent necrotizing and lethal toxin. In individuals with low dietary protein intake or those consuming foods containing trypsin inhibitors (like sweet potatoes), the Beta-toxin is not inactivated by pancreatic trypsin, leading to severe mucosal destruction and gas gangrene of the bowel wall. **Analysis of Incorrect Options:** * **Clostridium botulinum:** Causes botulism, a paralytic illness mediated by a neurotoxin that inhibits acetylcholine release at the neuromuscular junction. It does not cause primary necrotizing bowel disease. * **Clostridium tetani:** Produces tetanospasmin, which blocks inhibitory neurotransmitters (GABA/glycine) in the spinal cord, leading to spastic paralysis and "lockjaw." * **Clostridium perfringens type A:** While it is the most common cause of gas gangrene (myonecrosis) and food poisoning (via an enterotoxin), it is not the primary agent associated with the specific clinical entity of necrotizing enteritis/Pigbel. **NEET-PG High-Yield Pearls:** * **Beta-toxin** is the hallmark of *C. perfringens* Type C; it is highly sensitive to trypsin. * **Nagler’s Reaction:** Used to identify *C. perfringens* by detecting **Alpha-toxin** (Lecithinase) activity on egg yolk agar. * **Stormy Fermentation:** A characteristic laboratory finding in litmus milk culture due to vigorous gas production by *C. perfringens*. * **Note:** While *C. perfringens* Type C causes Enteritis Necroticans, neonatal Necrotizing Enterocolitis (NEC) is often multifactorial, though *C. perfringens* and *C. butyricum* are frequently implicated.

Q: What is the causative organism of hemolytic uremic syndrome?

Shigella. **Explanation:** **Hemolytic Uremic Syndrome (HUS)** is a clinical triad consisting of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. **Why Shigella is the correct answer:** The primary causative agent among the options is **Shigella dysenteriae type 1**. It produces the potent **Shiga toxin (Stx)**. This toxin binds to the Gb3 receptors on glomerular endothelial cells, leading to endothelial damage, microthrombi formation, and subsequent renal failure. While *Enterohemorrhagic E. coli* (EHEC/O157:H7) is the most common cause of HUS globally, *Shigella dysenteriae* is a classic and frequently tested cause in the context of bacillary dysentery. **Why the other options are incorrect:** * **ETEC (Option A):** Causes traveler’s diarrhea via heat-labile (LT) and heat-stable (ST) toxins. It does not produce Shiga toxin and is not associated with HUS. * **Salmonella (Option B):** Typically causes enteric fever or gastroenteritis. While it can cause systemic illness, it does not produce the specific toxins required to trigger HUS. * **Pseudomonas (Option D):** An opportunistic pathogen causing pneumonia, UTIs, and sepsis (especially in burns or cystic fibrosis), but it is not an etiological agent for HUS. **NEET-PG High-Yield Pearls:** * **Most common cause overall:** Enterohemorrhagic *E. coli* (EHEC), specifically the O157:H7 strain (produces Shiga-like toxin/Verotoxin). * **The Toxin:** Shiga toxin inhibits protein synthesis by damaging the **28S ribosomal RNA** (60S subunit). * **Clinical Tip:** Antibiotics are generally **avoided** in EHEC-related diarrhea as they may increase toxin release and worsen the risk of HUS. * **Schistocytes:** A peripheral smear in HUS will characteristically show fragmented RBCs (schistocytes) due to mechanical destruction in microvasculature.

Question 1

For a bacterium to be seriously considered in the etiology of dental caries, it must:

Accepted Answer

Exist regularly in the dental plaque

Answer

Produce extracellular amylopectins

Answer

Be lethal for gnotobiotic animals

Answer

Produce intracellular dextrans

Question 2

Which of the following conditions is associated with antibodies against phenolic glycolipid (PGL-1)?

Accepted Answer

M. leprae

Answer

M. tuberculosis

Answer

Borrelia

Answer

Brucella

Question 3

Chinese letter configuration is seen in which bacterium?

Accepted Answer

Cdiphtheriae

Answer

Cltetani

Answer

Clperfringens

Answer

Strept salivarius

Question 4

Which of the following is NOT a characteristic microscopic appearance of Bacillus anthracis?

Accepted Answer

Bamboo stick appearance

Answer

Medusa head appearance

Answer

String of pearl reaction

Answer

Inverted fir tree appearance

Question 5

A 70-year-old patient presents with high-grade fever, dry cough, and abdominal pain. Sputum sample collected from the patient shows Gram-negative organisms that are able to grow only on charcoal yeast extract medium. What is the most likely organism?

Accepted Answer

Legionella

Answer

Haemophilus influenzae

Answer

Listeria monocytogenes

Answer

Moraxella catarrhalis

Question 6

In a splenectomized patient, which of the following organisms is least likely to cause infection?

Accepted Answer

Staph. aureus

Answer

Pneumococci

Answer

Klebsiella

Answer

H. influenzae

Question 7

Which of the following microorganisms is a known cause of necrotizing enterocolitis?

Accepted Answer

Clostridium perfringens type C

Answer

Clostridium botulinum

Answer

Clostridium tetani

Answer

Clostridium perfringens type A

Question 8

Which of the following is false regarding H. pylori?

Accepted Answer

With chronic infection, the urease breath test can become negative.

Answer

H. pylori infection remains lifelong if untreated.

Answer

Endoscopy is diagnostic.

Answer

Toxigenic strains usually cause ulcers.

Question 9

What is the causative organism of hemolytic uremic syndrome?

Accepted Answer

Shigella

Answer

Enterotoxigenic Escherichia coli (ETEC)

Answer

Salmonella

Answer

Pseudomonas

Question 10

A 35-year-old patient complains of abdominal cramps along with profuse diarrhea. The treating physician wants to process the stool specimen for isolation of Campylobacter jejuni. Which of the following is the method of choice for the culture of stool?

Accepted Answer

Culture on Skirrow's medium incubated at 42°C under micro-aerophilic conditions.

Answer

Culture on TCBS medium incubated at 37°C under aerobic conditions.

Answer

Culture on MacConkey's medium incubated at 42°C under anaerobic conditions.

Answer

Culture on Wilson and Blair's medium incubated at 37° under microaerophilic conditions.

Bacteriology — MCQs

Bacteriology — MCQs

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