Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1591: Trench fever is caused by which organism?

A. Rickettsia prowazekii
B. Coxiella Burnetii
C. Bartonella quintana (Correct Answer)
D. Rickettsia akari

Explanation: **Explanation:** **Bartonella quintana** is the causative agent of **Trench Fever**, a disease historically significant during World War I. It is a Gram-negative, fastidious coccobacillus. The infection is primarily transmitted by the **human body louse** (*Pediculus humanus corporis*). The name "quintana" refers to the characteristic 5-day interval of the recurring fever (also known as "Five-day fever"). **Analysis of Options:** * **A. Rickettsia prowazekii:** Causes **Epidemic Typhus**. Like Trench fever, it is transmitted by the body louse, but it presents with a high continuous fever and a characteristic rash that spreads from the trunk to the extremities. * **B. Coxiella burnetii:** Causes **Q Fever**. It is unique among rickettsial-like organisms because it is transmitted via inhalation of contaminated aerosols (often from livestock) rather than an arthropod vector, and it does not cause a rash. * **D. Rickettsia akari:** Causes **Rickettsialpox**. It is transmitted by the house mouse mite and is characterized by an initial eschar at the bite site followed by a papulovesicular rash. **High-Yield Clinical Pearls for NEET-PG:** * **Bartonella quintana** is also a major cause of **Bacillary Angiomatosis** (vascular skin lesions) and **Culture-negative Endocarditis**, particularly in homeless populations or immunocompromised (HIV) patients. * **Vector Summary:** Trench fever = Body louse; Cat Scratch Disease (*B. henselae*) = Fleas/Cat scratches; Oroya fever (*B. bacilliformis*) = Sandfly. * **Diagnosis:** Best confirmed via serology (IFA) or PCR, as the organism is extremely slow-growing in culture (requires 7–21 days).

Question 1592: Which enzyme produced by Streptococcus species is responsible for breaking down DNA in purulent material, thereby thinning the pus?

A. DNAase (Correct Answer)
B. Streptokinase
C. RNAase
D. C5a peptidase

Explanation: ### Explanation **Correct Option: A. DNAase (Streptodornase)** Streptococcus species, particularly Group A Streptococcus (GAS), produce four distinct deoxyribonucleases (A, B, C, and D). In purulent exudates (pus), the high viscosity is primarily due to the accumulation of DNA released from disintegrated polymorphonuclear leukocytes (neutrophils). **DNAase** liquefies this thick, viscous pus by depolymerizing the DNA, which facilitates the spread of the bacteria through tissues. This property is utilized clinically in "enzymatic debridement." **Analysis of Incorrect Options:** * **B. Streptokinase (Fibrinolysin):** This enzyme activates plasminogen to plasmin, which dissolves fibrin clots. While it aids in bacterial spread by breaking down fibrin barriers, it does not act on DNA or directly thin the pus. * **C. RNAase:** While some strains produce ribonuclease, it does not contribute significantly to the thinning of pus, as the viscosity of purulent material is DNA-dependent. * **D. C5a peptidase:** This enzyme degrades the complement component C5a (a potent chemoattractant). Its primary role is to inhibit the recruitment of phagocytes to the site of infection, serving as an immune evasion mechanism rather than a liquefying agent. **High-Yield Facts for NEET-PG:** * **DNAase B** is the most immunogenic type in humans. * **Clinical Pearl:** The **Anti-DNAase B test** is more sensitive than the ASO (Antistreptolysin O) titer for diagnosing sequelae following **Streptococcal pyoderma** (skin infections/impetigo) and is highly reliable for Post-Streptococcal Glomerulonephritis (PSGN). * **Therapeutic Use:** A mixture of Streptokinase and Streptodornase (DNAase) was historically used to liquefy thick pleural exudates (empyema) and hematomas.

Question 1593: Which of the following bacteria is catalase positive?

A. Staph epidermidis (Correct Answer)
B. Enterococcus
C. Streptococcus
D. Pneumococcus

Explanation: ### Explanation The **Catalase Test** is the primary biochemical tool used to differentiate between the two major families of Gram-positive cocci: **Staphylococcaceae** (Catalase positive) and **Streptococcaceae** (Catalase negative). **1. Why Staph epidermidis is correct:** The enzyme catalase neutralizes hydrogen peroxide ($H_2O_2$) into water and oxygen. All members of the genus *Staphylococcus*, including *S. aureus*, *S. epidermidis*, and *S. saprophyticus*, are **catalase positive**. When $H_2O_2$ is added to a colony of *S. epidermidis*, immediate effervescence (bubble formation) occurs due to the release of oxygen gas. **2. Why the other options are incorrect:** * **Streptococcus (Option C):** This genus is the prototype for **catalase-negative** Gram-positive cocci. * **Pneumococcus (Option D):** *Streptococcus pneumoniae* is a member of the *Streptococcus* genus and is catalase negative. It is further identified by its alpha-hemolytic pattern and optochin sensitivity. * **Enterococcus (Option B):** Formerly classified as Group D Streptococci, *Enterococci* are typically **catalase negative** (though some rare strains may show "pseudo-catalase" activity, for exam purposes, they are categorized as negative). **Clinical Pearls for NEET-PG:** * **Mnemonic:** "Staph is Positive" (Staff are positive people). * **Next Step:** Once a Gram-positive coccus is confirmed as catalase positive, the **Coagulase test** is used to differentiate *S. aureus* (Positive) from Coagulase-Negative Staphylococci or CoNS (e.g., *S. epidermidis*). * **Clinical Context:** *S. epidermidis* is a major component of normal skin flora but is the most common cause of **prosthetic valve endocarditis** and infections related to indwelling catheters/shunts due to its ability to form **biofilms**.

Question 1594: Mycoplasma pneumoniae is characterised by all of the following except?

A. Diagnosis by serum cold agglutinins
B. Treatment with erythromycin
C. Cannot be cultured from sputum (Correct Answer)
D. Raised ESR

Explanation: **Explanation:** The correct answer is **C (Cannot be cultured from sputum)** because *Mycoplasma pneumoniae* **can** be cultured from respiratory secretions, including sputum, although it is clinically challenging. It is a fastidious organism that requires enriched media (like PPLO agar) containing sterols and horse serum. Growth is slow (1–3 weeks), producing characteristic "fried-egg" colonies. In clinical practice, PCR or serology is preferred over culture due to this slow growth. **Analysis of other options:** * **A. Diagnosis by serum cold agglutinins:** This is a classic feature. About 50–70% of patients develop IgM antibodies (cold agglutinins) that agglutinate human Type O RBCs at 4°C. While non-specific, it is a high-yield diagnostic association for *Mycoplasma*. * **B. Treatment with erythromycin:** *Mycoplasma* lacks a cell wall; therefore, beta-lactams are ineffective. Macrolides (Erythromycin, Azithromycin), Tetracyclines, or Fluoroquinolones are the drugs of choice as they inhibit protein or DNA synthesis. * **D. Raised ESR:** Like most infectious and inflammatory processes, *Mycoplasma pneumoniae* (Atypical Pneumonia) typically presents with an elevated Erythrocyte Sedimentation Rate (ESR). **NEET-PG High-Yield Pearls:** * **Smallest free-living organism:** *Mycoplasma* lacks a peptidoglycan cell wall (naturally resistant to Penicillin). * **Cell Membrane:** Unique among bacteria for containing **sterols** (acquired from the growth medium). * **Clinical Presentation:** Known as **"Walking Pneumonia"** because the clinical symptoms (persistent cough, fever) are often milder than the radiological findings (patchy consolidation). * **Complications:** Can cause Stevens-Johnson Syndrome, Raynaud’s phenomenon (due to cold agglutinins), and Bullous Myringitis.

Question 1595: Vine leaf colonies are seen in which of the following organisms?

A. Clostridium perfringens (Correct Answer)
B. Escherichia coli
C. Pseudomonas aeruginosa
D. Enterobacter cloacae

Explanation: **Explanation:** The correct answer is **Clostridium perfringens**. **Clostridium perfringens** is a Gram-positive, anaerobic, spore-forming bacillus. When grown on solid media (such as nutrient agar or blood agar), its colonies exhibit a characteristic morphology described as **"vine leaf"** or **"maple leaf"** appearance. This occurs due to the irregular, spreading edges of the colonies. Additionally, on blood agar, it classically produces a **target hemolysis** (double zone of hemolysis): an inner zone of complete hemolysis due to theta-toxin and an outer zone of incomplete hemolysis due to alpha-toxin (lecithinase). **Analysis of Incorrect Options:** * **Escherichia coli:** Typically produces large, circular, low-convex, and non-mucoid colonies. On MacConkey agar, they are seen as bright pink "lactose-fermenting" colonies. * **Pseudomonas aeruginosa:** Known for producing large, opaque, irregular colonies with a distinct **metallic sheen** and a fruity (grape-like) odor. They often produce pigments like pyocyanin (blue-green). * **Enterobacter cloacae:** Produces mucoid, convex colonies similar to Klebsiella, though usually less viscid. It does not exhibit the vine leaf pattern. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** A rapid biochemical test for *C. perfringens* that detects alpha-toxin (lecithinase) activity on egg yolk agar. * **Stormy Fermentation:** Produced in litmus milk media due to heavy gas production. * **Clinical Conditions:** Most common cause of **Gas Gangrene** (myonecrosis) and a frequent cause of food poisoning. * **Morphology:** It is unique among Clostridia for being **non-motile** and possessing a **capsule**.

Question 1596: Helicobacter pylori resists the acidity of gastric content due to which of the following mechanisms?

A. Urease production (Correct Answer)
B. Vacuolating cytotoxin
C. Secretion of alkaline contents
D. CagA protein

Explanation: **Explanation:** *Helicobacter pylori* is a gram-negative, microaerophilic bacterium that colonizes the human stomach. Its survival in the highly acidic gastric environment (pH ~2.0) is primarily due to the production of the enzyme **Urease**. **1. Why Urease is the Correct Answer:** Urease is the most critical survival factor for *H. pylori*. It catalyzes the hydrolysis of urea into **ammonia (NH₃)** and carbon dioxide (CO₂). The ammonia produced is basic; it reacts with water to form ammonium hydroxide, which creates a **neutralizing "cloud" or alkaline microenvironment** around the bacterium. This protects the organism from the bactericidal effects of gastric acid. **2. Analysis of Incorrect Options:** * **Vacuolating cytotoxin (VacA):** This is an exotoxin that induces vacuoles in gastric epithelial cells and promotes apoptosis. It is a virulence factor but does not neutralize acid. * **Secretion of alkaline contents:** While the microenvironment becomes alkaline, *H. pylori* does not "secrete" alkaline contents in a general sense; it specifically generates ammonia via enzymatic urea breakdown. * **CagA protein:** The Cytotoxin-associated gene A (CagA) is injected into host cells via a Type IV secretion system. It is associated with increased inflammation and a higher risk of gastric cancer (oncoprotein), but it does not play a role in acid resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The **Urea Breath Test (UBT)** and the **Rapid Urease Test (RUT)** on biopsy samples both exploit the organism's urease activity for diagnosis. * **Morphology:** Described as "seagull-wing" shaped or spiral. * **Association:** *H. pylori* is the most common cause of peptic ulcer disease and is a Type 1 Carcinogen (linked to Gastric Adenocarcinoma and MALToma). * **Motility:** It uses lophotrichous flagella to burrow through the mucus layer to reach the neutral pH of the epithelial surface.

Question 1597: What is the causative agent of chancroid?

A. Haemophilus influenzae
B. Haemophilus aegyptius
C. Haemophilus ducreyi (Correct Answer)
D. Haemophilus haemolyticus

Explanation: **Explanation:** **Haemophilus ducreyi** is the causative agent of **chancroid**, a sexually transmitted infection characterized by painful genital ulcers and inguinal lymphadenopathy (buboes). 1. **Why Option C is correct:** *H. ducreyi* is a fastidious, Gram-negative coccobacillus. It causes a "soft chancre," which is clinically distinguished from the "hard chancre" of syphilis by its intense tenderness. A classic diagnostic feature seen on Gram stain is the **"school of fish"** or "railroad track" appearance, representing the parallel arrangement of the bacilli. 2. **Why the other options are incorrect:** * **A. Haemophilus influenzae:** Primarily causes respiratory infections (pneumonia, otitis media) and meningitis (Type B), not genital ulcers. * **B. Haemophilus aegyptius:** Also known as the Koch-Weeks bacillus, it is the causative agent of acute contagious conjunctivitis (pink eye) and Brazilian Purpuric Fever. * **C. Haemophilus haemolyticus:** Generally considered a commensal of the upper respiratory tract and is rarely pathogenic. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "You **do cry** with *H. **ducreyi***" (because the ulcer is painful). * **The "Three P's" of Chancroid:** **P**ainful ulcer, **P**urulent base, and **P**ainful inguinal lymphadenopathy (Buboes). * **Culture Media:** Requires enriched media like **Mueller-Hinton agar** with 5% heated horse blood (Chocolate agar) and Vancomycin to inhibit Gram-positive flora. * **Differential Diagnosis:** Always differentiate from Syphilis (painless) and Herpes Simplex (multiple vesicles).

Question 1598: What is trench fever?

A. Q-fever
B. 5-days fever (Correct Answer)
C. Boutonneuse fever
D. Indian tick typhus

Explanation: **Explanation:** **Trench Fever**, also known as **5-day fever** or His-Werner disease, is caused by the bacterium ***Bartonella quintana***. It gained prominence during World War I among soldiers living in trenches. The disease is transmitted by the **human body louse** (*Pediculus humanus corporis*). It is characterized by a relapsing fever pattern where febrile episodes typically last 1–5 days and recur at intervals of approximately 5 days, hence the name "5-day fever." **Analysis of Options:** * **Option A (Q-fever):** Caused by *Coxiella burnetii*. It is typically transmitted via inhalation of contaminated aerosols from livestock. It does not follow a 5-day relapsing pattern. * **Option C & D (Boutonneuse fever / Indian tick typhus):** Both are caused by *Rickettsia conorii*. These are spotted fevers transmitted by ticks (specifically the dog tick *Rhipicephalus sanguineus*), characterized by a fever and a typical "tache noire" (black eschar) at the bite site. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Human body louse (unlike Cat Scratch Disease, also caused by *Bartonella*, which is transmitted by fleas/scratches). * **Microbiology:** *Bartonella* species are fastidious, Gram-negative coccobacilli. * **Other manifestations of *B. quintana*:** In immunocompromised patients (e.g., HIV), it can cause **Bacillary Angiomatosis** and culture-negative endocarditis. * **Drug of Choice:** Doxycycline is the mainstay of treatment.

Question 1599: Undulant fever is caused by which organism?

A. Salmonella typhi
B. Staphylococcus aureus
C. Brucella melitensis (Correct Answer)
D. Coxiella burnetti

Explanation: **Explanation:** **Brucella melitensis (Correct Answer):** Undulant fever is the classic clinical presentation of **Brucellosis**, a zoonotic infection. The term "undulant" refers to the characteristic wave-like pattern of the fever, which rises during the day and falls (subsides) during the night. *Brucella melitensis* (transmitted via goats/sheep) is the most common and virulent species causing human disease. The bacteria are intracellular, surviving within macrophages of the reticuloendothelial system, leading to chronic symptoms like hepatosplenomegaly, lymphadenopathy, and sacroiliitis. **Analysis of Incorrect Options:** * **Salmonella typhi:** Causes **Enteric (Typhoid) fever**, characterized by a "step-ladder" pattern of fever, bradycardia (Faget’s sign), and rose spots. * **Staphylococcus aureus:** Typically causes acute pyogenic infections, abscesses, or toxic shock syndrome. It is associated with a high-grade continuous or remittent fever, but not undulant fever. * **Coxiella burnetti:** The causative agent of **Q fever**. While it is also a zoonosis, it typically presents as an acute flu-like illness, pneumonia, or culture-negative endocarditis. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Consumption of unpasteurized milk/dairy products or direct contact with infected livestock (occupational hazard for vets/farmers). * **Diagnosis:** **Standard Agglutination Test (SAT)** is the screening test (significant titer >1:160). **Rose Bengal Test** is used for rapid screening. * **Culture:** Requires enriched media (e.g., Castaneda’s biphasic medium). * **Treatment:** WHO recommends **Doxycycline + Rifampicin** for 6 weeks.

Question 1600: Which of the following characteristics differentiates Neisseria meningitidis from Neisseria gonorrhoeae?

A. Oxidase positive
B. Ferments glucose
C. Ferments maltose (Correct Answer)
D. Reduces nitrates

Explanation: **Explanation:** The primary biochemical differentiation between the two clinically significant *Neisseria* species lies in their **carbohydrate utilization (fermentation) patterns**. Both organisms are Gram-negative diplococci, but they differ in their ability to metabolize specific sugars. * **Why Option C is correct:** *Neisseria meningitidis* has the metabolic machinery to ferment both **Glucose and Maltose**. In contrast, *Neisseria gonorrhoeae* can only ferment **Glucose**. A common mnemonic used by medical students is: **M**eningitidis ferments **M**altose and Glucose, while **G**onorrhoeae ferments **G**lucose only. * **Why Option A is incorrect:** Both species are **Oxidase positive**. This is a shared characteristic of the genus *Neisseria* and is used to identify the genus, not to differentiate between species. * **Why Option B is incorrect:** Both species ferment **Glucose**. Since this is a common trait, it cannot be used for differentiation. * **Why Option D is incorrect:** Most pathogenic *Neisseria* species, including both *N. meningitidis* and *N. gonorrhoeae*, are typically **nitrate reduction negative**. **High-Yield Clinical Pearls for NEET-PG:** * **Media:** Both grow on Chocolate agar, but *N. gonorrhoeae* is more fastidious and requires enriched media like **Thayer-Martin Medium** (VCN: Vancomycin, Colistin, Nystatin). * **Capsule:** *N. meningitidis* is **capsulated** (polysaccharide), which is its chief virulence factor and the basis for its vaccine. *N. gonorrhoeae* is **non-capsulated**. * **Portal of Entry:** Meningococcus enters via the nasopharynx (respiratory), while Gonococcus enters via the genitourinary tract.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

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Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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