Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 551: The DT 104 strain belongs to which of the following bacteria?

A. Salmonella Gallinarum
B. Salmonella Typhi
C. Salmonella Enteritidis
D. Salmonella Typhimurium (Correct Answer)

Explanation: **Explanation:** **Salmonella Typhimurium DT 104** (Definitive Type 104) is a specific phage type of *Salmonella enterica* serovar Typhimurium. It gained global clinical significance due to its **multidrug-resistant (MDR)** profile. This strain typically carries chromosomal resistance to five major antibiotics: Ampicillin, Chloramphenicol, Streptomycin, Sulfonamides, and Tetracycline (often referred to by the acronym **ACSSuT**). It is a common cause of zoonotic food poisoning, often transmitted through contaminated meat and dairy products. **Analysis of Incorrect Options:** * **Salmonella Gallinarum (Option A):** This is a non-motile, host-adapted serovar that causes Fowl Typhoid in poultry. It is rarely associated with human infection and does not include the DT 104 strain. * **Salmonella Typhi (Option B):** This is the causative agent of Enteric Fever. While it is highly significant in the Indian context, its classification is based on Vi antigen and phage typing (e.g., Vi phage type A), not the DT series. * **Salmonella Enteritidis (Option C):** Along with Typhimurium, this is a leading cause of non-typhoidal salmonellosis (NTS) worldwide, often linked to poultry and eggs. However, DT 104 is specifically a subtype of Typhimurium, not Enteritidis. **High-Yield Facts for NEET-PG:** * **Resistance Mechanism:** The MDR genes in DT 104 are located on the chromosome in a specific region called **SGI1** (Salmonella Genomic Island 1). * **Kauffman-White Scheme:** Remember that *Salmonella* are classified into serovars based on **O (somatic)** and **H (flagellar)** antigens. * **NTS vs. Typhi:** Non-typhoidal Salmonella (like DT 104) usually cause self-limiting gastroenteritis, whereas *S. Typhi* causes systemic febrile illness. * **Phage Typing:** This is a method used for epidemiological tracking of outbreaks, identifying specific strains within a single serotype.

Question 552: Haemophilus ducreyi causes which of the following conditions?

A. Gonorrhoea
B. Herpes
C. Chancroid (Correct Answer)
D. Granuloma venereum

Explanation: **Explanation:** *Haemophilus ducreyi* is a small, fastidious, Gram-negative coccobacillus that is the causative agent of **Chancroid**, a sexually transmitted infection. **Why Option C is correct:** Chancroid is characterized by the development of **painful** genital ulcers (soft chancre) and painful inguinal lymphadenopathy (buboes). The pain is a hallmark feature that distinguishes it from the painless ulcer of primary syphilis. Microscopically, *H. ducreyi* often exhibits a characteristic **"school of fish"** or "railroad track" appearance due to the parallel arrangement of the bacilli. **Why other options are incorrect:** * **Option A (Gonorrhoea):** Caused by *Neisseria gonorrhoeae*, a Gram-negative diplococcus. It typically presents as urethritis or cervicitis with purulent discharge rather than genital ulcers. * **Option B (Herpes):** Caused by Herpes Simplex Virus (HSV-1 or HSV-2). While it causes painful ulcers, they are typically multiple, vesicular, and superficial, unlike the deeper, necrotic ulcer of Chancroid. * **Option C (Granuloma venereum):** Also known as Donovanosis, it is caused by *Klebsiella granulomatis*. It presents as **painless**, beefy-red, velvety ulcers and is diagnosed by identifying **Donovan bodies** (safety-pin appearance) in tissue smears. **Clinical Pearls for NEET-PG:** * **Mnemonic for Pain:** "Ducreyi makes you **cry**" (because the ulcer is painful). * **Culture Media:** Requires enriched media like **Chocolate agar** supplemented with IsoVitaleX or Vancomycin. * **Treatment:** Azithromycin (1g single dose) or Ceftriaxone is the drug of choice. * **Differential Diagnosis:** Always differentiate from Syphilis (painless, hard chancre) and Lymphogranuloma Venereum (caused by *Chlamydia trachomatis* L1-L3).

Question 553: Where is Helicobacter pylori typically found in the stomach?

A. Epithelial lining
B. Mucous layer (Correct Answer)
C. Lamina propria
D. Submucosa

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, Gram-negative spiral bacterium specifically adapted to survive the harsh acidic environment of the stomach. **Why the Mucous Layer is Correct:** The primary niche for *H. pylori* is the **gastric mucous layer** (specifically the deeper layers overlying the gastric epithelium). The bacterium utilizes its high **urease activity** to create a localized "ammonia cloud" that neutralizes gastric acid. It uses its flagella to migrate through the viscous mucus toward the neutral pH of the epithelial surface. While it adheres to the epithelial cells via adhesins (like BabA), it remains **extracellular** within the mucus and does not typically invade the tissue. **Why Other Options are Incorrect:** * **Epithelial lining:** While *H. pylori* adheres to the surface of epithelial cells, its primary residence and survival zone is the overlying mucus. It does not exist *inside* the lining. * **Lamina propria & Submucosa:** These are deeper histological layers. *H. pylori* is a non-invasive organism; finding it in these layers would imply tissue invasion, which is not a characteristic feature of its pathogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Most commonly found in the **Antrum** of the stomach. * **Shape:** Described as "seagull-wing" or spiral-shaped. * **Key Enzymes:** Urease (basis for the Urea Breath Test), Catalase, and Oxidase positive. * **Virulence Factors:** **CagA** (associated with gastric cancer) and **VacA** (vacuolating cytotoxin). * **Stains:** Silver stains (Warthin-Starry), Giemsa, or Genta stain are preferred for visualization. * **Association:** Strongly linked to Type B Gastritis, Peptic Ulcer Disease, Gastric Adenocarcinoma, and MALT Lymphoma.

Question 554: Which Vibrio species is most halophilic?

A. Vibrio cholerae
B. Vibrio vulnificus
C. Vibrio alginolyticus (Correct Answer)
D. Vibrio parahaemolyticus

Explanation: **Explanation:** The genus *Vibrio* consists of Gram-negative, comma-shaped bacilli that are typically **halophilic** (salt-loving), meaning they require sodium chloride (NaCl) for growth. The degree of salt tolerance varies significantly across species, which is a high-yield point for differentiating them in the laboratory. **Why Vibrio alginolyticus is correct:** *Vibrio alginolyticus* is the **most halophilic** species among those clinically relevant. It is "salt-tolerant" to an extreme degree, capable of growing in media containing NaCl concentrations as high as **8–10%**. In contrast, most other pathogenic vibrios are inhibited at concentrations above 6-8%. **Analysis of Incorrect Options:** * **Vibrio cholerae:** This is the only clinically important species that is **non-halophilic**. It can grow in media with 0% NaCl (like peptone water), which is a key biochemical marker used to distinguish it from other "halophilic vibrios." * **Vibrio parahaemolyticus:** While it is halophilic and requires salt (optimally 3%), it typically grows in a range of 1–7% NaCl. It cannot tolerate the very high concentrations (10%) that *V. alginolyticus* can. * **Vibrio vulnificus:** This is a halophilic species (requiring ~1–3% NaCl) known for causing primary septicemia and necrotizing fasciitis, but its salt tolerance threshold is lower than that of *V. alginolyticus*. **NEET-PG High-Yield Pearls:** 1. **Salt Tolerance Test:** Used to differentiate *Vibrio* species. *V. cholerae* grows at 0% but not 8%; *V. alginolyticus* grows at both 0% (rarely) and 8-10%. 2. **TCBS Agar:** *V. cholerae* and *V. alginolyticus* are **sucrose fermenters** (Yellow colonies), while *V. parahaemolyticus* and *V. vulnificus* are non-sucrose fermenters (Green colonies). 3. **Clinical Association:** *V. alginolyticus* is most commonly associated with superficial infections like **otitis externa** and wound infections following exposure to seawater.

Question 555: Cat scratch disease is characterized by?

A. Caused by a virus
B. Regional lymphadenopathy is prominent (Correct Answer)
C. More common in adults
D. All of the above

Explanation: **Explanation:** **Cat Scratch Disease (CSD)** is a bacterial infection caused by **_Bartonella henselae_**, a fastidious, pleomorphic Gram-negative bacillus. It is typically transmitted to humans through the scratch or bite of an infected domestic cat (the primary reservoir), often involving kittens. 1. **Why Option B is correct:** The hallmark of CSD is **subacute regional lymphadenopathy**. Following an initial skin papule or pustule at the inoculation site, the draining lymph nodes (most commonly axillary, epitrochlear, or cervical) become significantly enlarged, tender, and occasionally suppurative. This is the most prominent clinical feature, occurring in nearly 90% of patients. 2. **Why Option A is incorrect:** CSD is **bacterial**, not viral. While it was historically debated, the causative agent is definitively identified as *Bartonella henselae*. 3. **Why Option C is incorrect:** CSD is significantly **more common in children and adolescents** (under 20 years of age) than in adults, likely due to closer play interaction with kittens. **High-Yield Clinical Pearls for NEET-PG:** * **Stain of Choice:** *Bartonella* is best visualized in tissue sections using the **Warthin-Starry silver stain**. * **Pathology:** Lymph nodes show characteristic **stellate (star-shaped) granulomas** with central necrosis. * **Parinaud Oculoglandular Syndrome:** A specific presentation of CSD involving conjunctivitis and preauricular lymphadenopathy. * **Immunocompromised Patients:** In HIV/AIDS patients, *B. henselae* causes **Bacillary Angiomatosis** (vascular proliferative lesions) rather than typical CSD. * **Treatment:** Most cases are self-limiting, but **Azithromycin** is the drug of choice to reduce lymph node volume.

Question 556: Which of the following statements about Yersinia pestis is true?

A. Gram positive organism
B. Non-motile organism (Correct Answer)
C. Benzyl penicillin is given for prophylaxis
D. All of the above

Explanation: **Explanation:** *Yersinia pestis*, the causative agent of the Plague, is a member of the *Enterobacteriaceae* family. Understanding its unique morphological and clinical characteristics is crucial for NEET-PG. **1. Why Option B is Correct:** Unlike many other members of the *Enterobacteriaceae* family (like *E. coli* or *Salmonella*), **all species of *Yersinia* are non-motile at 37°C.** While *Y. enterocolitica* and *Y. pseudotuberculosis* show motility at 22°C, ***Yersinia pestis* is non-motile at all temperatures.** It lacks flagella, which is a key diagnostic feature used to differentiate it from other enteric pathogens. **2. Why Incorrect Options are Wrong:** * **Option A:** *Yersinia pestis* is a **Gram-negative**, short, ovoid bacillus. It characteristically exhibits **bipolar staining** (safety-pin appearance) when stained with Wayson or Giemsa stain. * **Option C:** Benzyl penicillin is ineffective against *Y. pestis*. For **prophylaxis** (especially post-exposure), the drugs of choice are **Doxycycline** or Ciprofloxacin. For treatment, Aminoglycosides (Streptomycin or Gentamicin) are the traditional gold standards. **High-Yield Clinical Pearls for NEET-PG:** * **Reservoir:** Wild rodents (rats); **Vector:** Rat flea (*Xenopsylla cheopis*). * **Virulence Factors:** Fraction 1 (F1) capsular antigen (antiphagocytic), V and W antigens, and Yops (Yersinia outer proteins). * **Staining:** Safety-pin appearance is due to the accumulation of dye at the poles of the cell. * **Culture:** Shows a characteristic **"Stalactite growth"** in broth and **"Ghee-like"** appearance on agar. * **Public Health:** It is a mandatory reportable disease and a potential bioterrorism agent.

Question 557: What is considered the most serious form of Rickettsial disease?

A. Scrub typhus
B. Q-fever
C. Trench fever
D. Rocky Mountain spotted fever (Correct Answer)

Explanation: **Explanation:** **Rocky Mountain spotted fever (RMSF)**, caused by *Rickettsia rickettsii*, is recognized as the most severe and potentially fatal rickettsial illness. The underlying medical concept is its high degree of **vasculitis**. The bacteria infect the endothelial cells lining the blood vessels, leading to widespread vascular damage, increased permeability, and subsequent organ failure (affecting the lungs, kidneys, and CNS). Without prompt treatment with Doxycycline, the mortality rate can exceed 20-30%. **Analysis of Incorrect Options:** * **Scrub typhus (*Orientia tsutsugamushi*):** While common in the "Tsutsugamushi Triangle" (including India), it is generally less virulent than RMSF. It is characterized by a pathognomonic **eschar** at the mite bite site. * **Q-fever (*Coxiella burnetii*):** This is unique because it does not cause a rash and is often transmitted via inhalation. It usually presents as a self-limiting flu-like illness or pneumonia and is rarely fatal in its acute form. * **Trench fever (*Bartonella quintana*):** Transmitted by the body louse, it causes a relapsing fever. While debilitating, it is historically known for low mortality rates compared to typhus group rickettsiae. **Clinical Pearls for NEET-PG:** * **Drug of Choice:** Doxycycline is the gold standard for all rickettsial diseases, regardless of age. * **Triad of RMSF:** Fever, headache, and a characteristic rash that begins on the **wrists and ankles** (centripetal spread) and involves the **palms and soles**. * **Weil-Felix Test:** A heterophile agglutination test used for diagnosis (though being replaced by IFA). RMSF shows a positive reaction with **OX-19 and OX-2**. * **Vector:** RMSF is transmitted by Hard Ticks (*Dermacentor* species).

Question 558: Streptolysin O is functionally and structurally related to which of the following?

A. Tetanolysin
B. Pneumolysin
C. Streptolysin S (Correct Answer)
D. Clostridium perfringens O toxin

Explanation: **Explanation:** Streptolysin O (SLO) and Streptolysin S (SLS) are the two primary hemolysins produced by *Streptococcus pyogenes* (Group A Streptococcus). While they differ in their oxygen stability, they are functionally related as they both act as **cytolysins** that disrupt host cell membranes to cause hemolysis. * **Why Streptolysin S is correct:** Both SLO and SLS are virulence factors that contribute to the zone of hemolysis on blood agar. SLO is oxygen-labile (active only in anaerobic conditions), while SLS is oxygen-stable (responsible for the surface hemolysis seen on aerobic blood agar plates). They work synergistically to destroy red blood cells, leukocytes, and platelets. **Analysis of Incorrect Options:** * **Tetanolysin (Option A):** Produced by *Clostridium tetani*, it is a cholesterol-dependent cytolysin (CDC) similar to SLO, but it is not the primary functional partner in the context of Streptococcal pathogenesis. * **Pneumolysin (Option B):** Produced by *Streptococcus pneumoniae*. While it belongs to the same family of thiol-activated toxins as SLO, it is structurally distinct and specific to the pneumococcus. * **C. perfringens O toxin (Option D):** Also known as Perfringolysin O. Like SLO, it is a CDC, but it is structurally distinct and associated with gas gangrene rather than Streptococcal infections. **High-Yield Clinical Pearls for NEET-PG:** * **ASO Titer:** SLO is highly antigenic. The **Anti-Streptolysin O (ASO) titer** is a crucial diagnostic marker for post-streptococcal sequelae like Rheumatic Fever (but notably *not* for Glomerulonephritis following skin infections). * **SLS Antigenicity:** Unlike SLO, **Streptolysin S is non-antigenic**, meaning no diagnostic antibodies are formed against it. * **Hemolysis Pattern:** SLO causes "subsurface" hemolysis (stab the agar), while SLS causes "surface" hemolysis.

Question 559: All of the following organisms cause gas gangrene EXCEPT:

A. Clostridium difficile (Correct Answer)
B. Clostridium welchii
C. Clostridium septicum
D. Clostridium perfringens

Explanation: **Explanation:** Gas gangrene (Clostridial Myonecrosis) is a life-threatening muscle infection characterized by rapid tissue destruction and gas production. It is caused by various anaerobic, spore-forming Gram-positive bacilli of the genus *Clostridium*. **Why Option A is Correct:** * **Clostridium difficile:** This organism is primarily a gastrointestinal pathogen. It is the leading cause of **Antibiotic-Associated Diarrhea** and **Pseudomembranous Colitis**. It does not possess the histotoxic toxins (like alpha toxin) required to cause myonecrosis or gas gangrene. **Why the Other Options are Incorrect:** * **Clostridium perfringens (Option D) & Clostridium welchii (Option B):** These are the same organism (*C. welchii* is the older name). It is the **most common cause** of gas gangrene (80-90% of cases). It produces the lethal **Alpha toxin (Lecithinase)**, which destroys cell membranes, leading to necrosis and edema. * **Clostridium septicum (Option C):** This is a major cause of non-traumatic (spontaneous) gas gangrene. It is highly associated with underlying **colonic malignancy** or neutropenia. **NEET-PG High-Yield Pearls:** * **Nagler’s Reaction:** Used for rapid identification of *C. perfringens*; it detects lecithinase activity on egg yolk agar. * **Clinical Sign:** "Crepitus" on palpation due to gas bubbles in the tissues. * **Incubation Period:** Very short (usually 1–7 days). * **Other causative agents:** *C. novyi* (most common in wartime injuries) and *C. histolyticum*. * **Treatment:** Surgical debridement is the mainstay, along with Penicillin and Hyperbaric Oxygen.

Question 560: Which of the following Burkholderia species is nonmotile?

A. B. mallei (Correct Answer)
B. B. pseudomallei
C. B. cepaciae
D. B. bovis

Explanation: **Explanation:** The genus *Burkholderia* consists of Gram-negative bacilli that are generally motile via polar flagella. However, **Burkholderia mallei** is a notable exception and is the only **nonmotile** member of this genus. 1. **Why B. mallei is correct:** Unlike its relatives, *B. mallei* lacks flagella. It is the causative agent of **Glanders**, a primary zoonosis of horses, mules, and donkeys. In humans, it causes localized infections or fatal septicemia. Its lack of motility is a classic laboratory identification feature used to differentiate it from *B. pseudomallei*. 2. **Analysis of Incorrect Options:** * **B. pseudomallei:** This organism is **motile** by means of polar flagella. It causes **Melioidosis** (Whitmore’s disease). It is often identified by its "safety-pin" appearance on staining and wrinkled colonies on agar. * **B. cepacia:** This is a complex of species that are **motile**. It is an important opportunistic pathogen, particularly causing "Cepacia syndrome" (severe necrotizing pneumonia) in patients with **Cystic Fibrosis**. * **B. bovis:** This is a less common species and, like the majority of the genus, possesses motility. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember "**M**allei is **M**otionless." * **B. mallei vs. B. pseudomallei:** Both are potential **biothreat agents** (Category B). * **Strauss Reaction:** Inoculation of *B. mallei* into the testis of a male guinea pig causes acute orchitis (positive Strauss reaction). * **Ashdown’s Medium:** The selective culture medium used for the isolation of *B. pseudomallei*.

Practice by Chapter

Staphylococci

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Streptococci and Enterococci

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Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Mycobacteria

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Spirochetes

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