Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 571: Chancroid is caused by?

A. Treponema pallidum
B. Leishmania donovanii
C. Hemophilus ducreyi (Correct Answer)
D. Donovania granulomatis

Explanation: **Explanation:** **Chancroid** is a sexually transmitted infection (STI) characterized by painful genital ulcers and inguinal lymphadenopathy. It is caused by **Haemophilus ducreyi**, a small, fastidious, Gram-negative coccobacillus. **Why Option C is correct:** * *Haemophilus ducreyi* requires X-factor (hemin) for growth. * **Clinical Presentation:** It causes a "soft chancre"—an ulcer that is characteristically **painful**, non-indurated, and often associated with a painful, fluctuant inguinal bubo. * **Microscopy:** On Gram stain, the organism typically shows a **"school of fish"** or "railroad track" appearance due to its specific arrangement. **Why other options are incorrect:** * **Option A: Treponema pallidum:** Causes Syphilis. The primary lesion (Chancre) is characteristically **painless** and indurated ("Hard Chancre"), unlike the painful ulcer of Chancroid. * **Option B: Leishmania donovanii:** This is a protozoan parasite responsible for Visceral Leishmaniasis (Kala-azar), not a genital ulcer disease. * **Option D: Donovania granulomatis (now Klebsiella granulomatis):** Causes Granuloma Inguinale (Donovanosis). It presents as chronic, beefy-red, **painless** ulcers without significant lymphadenopathy. Histology shows **Donovan bodies** (safety-pin appearance) within macrophages. **NEET-PG High-Yield Pearls:** 1. **Mnemonic for Pain:** "Ducreyi makes you **cry**" (because the ulcer is painful). 2. **Culture Media:** Requires enriched media like Mueller-Hinton agar with 5% heated horse blood (Chocolate agar). 3. **Differential Diagnosis:** Always differentiate from Syphilis (painless) and Herpes Simplex (multiple vesicles, painful). 4. **Treatment:** Azithromycin (1g orally) or Ceftriaxone (250mg IM) are first-line therapies.

Question 572: What is the most common Gram-positive cocci responsible for upper respiratory tract infections (URIs) in sexually active females?

A. Staphylococcus epidermidis (Correct Answer)
B. Staphylococcus hyicus
C. Staphylococcus intermedius
D. Escherichia coli

Explanation: **Explanation:** The correct answer is **Staphylococcus epidermidis**. While the question asks for the most common Gram-positive cocci in URIs for sexually active females, it is important to recognize a common "trap" in medical entrance exams. In the context of **Urinary Tract Infections (UTIs)**—which are frequently associated with sexual activity in females—*Staphylococcus saprophyticus* is the classic answer. However, among the options provided, **Staphylococcus epidermidis** is the most relevant Gram-positive coccus. It is a ubiquitous commensal of the skin and mucous membranes. In clinical practice, it is often isolated from urine samples of sexually active females, frequently representing contamination from the perineal flora, though it can occasionally cause true infections in predisposed individuals. **Analysis of Options:** * **Staphylococcus epidermidis (A):** A coagulase-negative staphylococcus (CoNS) that is part of the normal human flora. It is the most common CoNS isolated from clinical specimens. * **Staphylococcus hyicus (B) & Staphylococcus intermedius (C):** These are primarily **veterinary pathogens**. *S. hyicus* causes "greasy pig disease," and *S. intermedius* is commonly found in dogs. They rarely cause human infection. * **Escherichia coli (D):** While *E. coli* is the #1 cause of UTIs overall, it is a **Gram-negative rod**, not a Gram-positive coccus, making it incorrect based on the question's morphology requirement. **NEET-PG High-Yield Pearls:** * **Staphylococcus saprophyticus:** The second most common cause of "Honeymoon Cystitis" (UTIs in young, sexually active females) after *E. coli*. It is distinguished from *S. epidermidis* by its **resistance to Novobiocin**. * **Morphology:** Always check if the question asks for "Gram-positive" or "Gram-negative" before selecting *E. coli*. * **CoNS:** *S. epidermidis* is the leading cause of infections related to prosthetic devices (valves, shunts, catheters) due to its ability to produce **biofilms (slime layer)**.

Question 573: Leptospira is a:

A. Virus
B. Fungus
C. Spirochete (Correct Answer)
D. Rickettsiae

Explanation: ### Explanation **Correct Answer: C. Spirochete** **Why it is correct:** *Leptospira* belongs to the family **Leptospiraceae** under the order **Spirochaetales**. Spirochetes are a unique group of Gram-negative, motile, spiral-shaped bacteria. *Leptospira* is characterized by its very fine, tightly coiled body with distinctive **hooked ends** (often described as "question mark" or "C/S" shapes). Unlike other spirochetes, they are obligate aerobes and can be cultured on specialized media like **EMJH** (Ellinghausen-McCullough-Johnson-Harris) or **Fletcher’s medium**. **Why the other options are incorrect:** * **A. Virus:** Viruses are acellular obligate intracellular parasites containing either DNA or RNA. *Leptospira* is a complex, multicellular bacterium with both DNA and RNA. * **B. Fungus:** Fungi are eukaryotic organisms with chitinous cell walls. *Leptospira* is a prokaryote. * **D. Rickettsiae:** While both are bacteria, Rickettsiae are small, pleomorphic, obligate intracellular pathogens typically transmitted by arthropod vectors. *Leptospira* is extracellular and primarily transmitted through contact with water contaminated by infected animal urine. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** They are too thin to be seen under a light microscope; **Dark-field microscopy** is the gold standard for direct visualization. * **Clinical Presentation:** Causes **Weil’s Disease** (a triad of jaundice, renal failure, and hemorrhage). * **Transmission:** Occupational hazard for sewage workers, farmers, and miners (contact with rodent urine). * **Diagnosis:** **Microscopic Agglutination Test (MAT)** is the reference standard serological test. * **Drug of Choice:** Doxycycline (prophylaxis/mild) or Penicillin G (severe).

Question 574: Deficiency in which component of the immune system predisposes to Neisseria meningitidis infection?

A. Late complement components (Correct Answer)
B. Early complement components
C. C1 esterase inhibitor
D. All of the above

Explanation: **Explanation:** The correct answer is **A. Late complement components (C5-C9)**. **1. Why Late Complement Components are correct:** The late complement components (C5, C6, C7, C8, and C9) assemble to form the **Membrane Attack Complex (MAC)**. The MAC is essential for the lysis of Gram-negative bacteria, particularly *Neisseria* species (both *N. meningitidis* and *N. gonorrhoeae*), which have thin peptidoglycan layers. Patients with deficiencies in these components cannot form the MAC, making them highly susceptible to recurrent, disseminated Neisserial infections. **2. Why other options are incorrect:** * **Early complement components (C1, C2, C4):** Deficiencies in these components (classical pathway) are primarily associated with autoimmune diseases like **Systemic Lupus Erythematosus (SLE)** and recurrent pyogenic infections (e.g., *S. pneumoniae*), rather than a specific predisposition to *Neisseria*. * **C1 esterase inhibitor:** Deficiency of this regulator leads to **Hereditary Angioedema** due to overproduction of bradykinin. It does not directly predispose to bacterial infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **C3 Deficiency:** This is the most severe complement deficiency as it is the "convergent point" of all pathways. It leads to recurrent infections with **encapsulated bacteria** (e.g., *H. influenzae*, *S. pneumoniae*). * **Properdin/Factor D Deficiency:** Deficiency in these alternative pathway components also increases the risk of *Neisseria* infections. * **CH50 Assay:** This is the screening test used to evaluate the functional integrity of the entire classical and MAC complement pathways. * **Eculizumab:** This monoclonal antibody against C5 (used in PNH) mimics a late complement deficiency, necessitating vaccination against Meningococcus before starting therapy.

Question 575: Regarding H. pylori, all are true except?

A. Gram-negative bacillus
B. Strongly associated with duodenal ulcer (DU)
C. Associated with lymphoma
D. It should be eradicated in all cases whenever detected (Correct Answer)

Explanation: ### Explanation **1. Why Option D is the Correct Answer (The "Except" Statement):** While *Helicobacter pylori* is a significant pathogen, current clinical guidelines (such as the Maastricht/Florence consensus) do **not** recommend universal eradication in all asymptomatic individuals. Eradication is strictly indicated in specific scenarios: active or past peptic ulcer disease, MALT lymphoma, early gastric cancer resection, and long-term NSAID/Aspirin users. Indiscriminate treatment is avoided due to the risk of antibiotic resistance, potential side effects, and the observation that *H. pylori* may have a protective role against GERD and esophageal adenocarcinoma in some populations. **2. Analysis of Incorrect Options:** * **Option A (Gram-negative bacillus):** This is true. *H. pylori* is a spiral-shaped (curved), Gram-negative, microaerophilic bacillus. It is motile via multiple polar flagella (lophotrichous). * **Option B (Strongly associated with DU):** This is true. *H. pylori* is found in >90% of patients with duodenal ulcers and approximately 70% of those with gastric ulcers. Eradication significantly reduces the recurrence rate of these ulcers. * **Option C (Associated with lymphoma):** This is true. It is strongly linked to **MALToma** (Mucosa-Associated Lymphoid Tissue lymphoma). Notably, early-stage MALToma can often be cured solely by eradicating the *H. pylori* infection. **3. Clinical Pearls for NEET-PG:** * **Virulence Factors:** **Urease** (neutralizes gastric acid via ammonia production), **CagA** (associated with high virulence/cancer), and **VacA** (cytotoxin). * **Diagnosis:** **Urea Breath Test (UBT)** is the gold standard for non-invasive diagnosis and confirming eradication. Stool antigen test is also highly specific. * **First-line Treatment:** Standard Triple Therapy (7–14 days) consists of a **PPI + Amoxicillin + Clarithromycin**. If penicillin-allergic, substitute Amoxicillin with Metronidazole. * **Carcinogenesis:** *H. pylori* is classified as a **Class I Carcinogen** by the WHO, linked to both Gastric Adenocarcinoma and MALToma.

Question 576: Symptoms of C. botulinum food poisoning include double vision, inability to speak, and respiratory paralysis. These symptoms are consistent with?

A. Invasion of the gut epithelium by C. botulinum (Correct Answer)
B. Secretion of an enterotoxin
C. Endotoxin shock
D. Ingestion of a neurotoxin

Explanation: **Explanation:** The clinical presentation of double vision (diplopia), inability to speak (dysarthria/dysphonia), and respiratory paralysis describes **descending flaccid paralysis**, the hallmark of botulism. **1. Why Option D is Correct:** *Clostridium botulinum* food poisoning (foodborne botulism) is a classic example of **preformed toxin ingestion** (intoxication). The organism grows in anaerobic conditions (e.g., improperly canned food) and releases a potent **neurotoxin**. Once ingested, the toxin is absorbed from the gut into the bloodstream and travels to peripheral cholinergic nerve terminals. It cleaves SNARE proteins, preventing the release of **Acetylcholine (ACh)** at the neuromuscular junction, leading to flaccid paralysis. **2. Why Other Options are Incorrect:** * **Option A:** *C. botulinum* does not typically invade the gut epithelium in adults. It is the toxin, not the bacterial invasion, that causes systemic symptoms. (Note: In **Infant Botulism**, spores germinate in the gut, but the pathology is still toxin-mediated). * **Option B:** Enterotoxins (like those of *V. cholerae*) act locally on the intestinal mucosa to cause diarrhea, not systemic neurological paralysis. * **Option C:** Endotoxin shock is caused by Lipid A of Gram-negative bacteria. *C. botulinum* is a Gram-positive anaerobe and produces an exotoxin. **High-Yield Clinical Pearls for NEET-PG:** * **The "8 Ds" of Botulism:** Diplopia, Dysarthria, Dysphonia, Dysphagia, Dilated pupils, Dry mouth, Drooping eyelids (ptosis), and Descending paralysis. * **Infant Botulism:** Associated with **honey** ingestion ("Floppy Baby Syndrome"). * **Diagnosis:** Demonstrated by detecting toxin in serum, stool, or the implicated food (Mouse Bioassay). * **Therapy:** Prompt administration of antitoxin and supportive respiratory care.

Question 577: Which of the following statements regarding Enterococcus is NOT true?

A. Common species include E. faecalis and E. faecium.
B. It is a known cause of peritonitis.
C. It is universally susceptible to penicillins. (Correct Answer)
D. It can cause intra-abdominal abscesses.

Explanation: **Explanation:** The correct answer is **C**, as Enterococci are notoriously known for their **intrinsic and acquired resistance** to various antibiotics, including penicillins. **1. Why Option C is the correct (false) statement:** Enterococci exhibit intrinsic resistance to many beta-lactams. They possess **low-affinity Penicillin-Binding Proteins (PBPs)**, particularly PBP5, which prevents penicillins from binding effectively to the cell wall. While *E. faecalis* may sometimes be susceptible to ampicillin, *E. faecium* is frequently resistant. Furthermore, they are never "universally" susceptible; in fact, they are only bacteriostatic (not bactericidal) when treated with penicillins alone, necessitating the addition of an aminoglycoside for synergy in serious infections like endocarditis. **2. Analysis of other options:** * **Option A:** *E. faecalis* (80-90%) and *E. faecium* (5-10%) are indeed the two most common clinical isolates. * **Options B & D:** Enterococci are normal commensals of the GI tract. They are significant opportunistic pathogens in **polymicrobial infections**, frequently causing peritonitis and intra-abdominal abscesses following bowel perforation or surgery. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Characteristics:** They are **Catalase negative**, **PYR positive**, and can grow in harsh conditions: 6.5% NaCl, 40% bile, and at pH 9.6. * **Heat Resistance:** They can survive heating at 60°C for 30 minutes (distinguishes them from Streptococci). * **VRE (Vancomycin-Resistant Enterococci):** Primarily seen in *E. faecium* due to the **VanA gene**, which alters the cell wall precursor from D-Ala-D-Ala to **D-Ala-D-Lac**. * **Drug of Choice:** Ampicillin + Gentamicin (for synergy). For VRE, Linezolid or Quinupristin-Dalfopristin are used.

Question 578: In the recent decade, Neisseria gonorrhoeae has become resistant to all the following drugs, EXCEPT:

A. Penicillin G
B. Azithromycin (Correct Answer)
C. Tetracycline
D. Ciprofloxacin

Explanation: **Explanation:** The correct answer is **Azithromycin**. While resistance to Azithromycin is emerging globally, it remains a component of recommended therapy in many guidelines (often in combination with Ceftriaxone) and has not yet reached the level of "widespread established resistance" seen with the other listed drugs. **1. Why Azithromycin is the correct answer:** Historically, *Neisseria gonorrhoeae* was treated with monotherapy. However, due to rising MICs (Minimum Inhibitory Concentrations), the medical community shifted to **Dual Therapy** (Ceftriaxone + Azithromycin). While "High-level Azithromycin-resistant" (HL-AziR) strains exist, Azithromycin is the only drug among the options that is still clinically relevant in current treatment algorithms for uncomplicated gonorrhea. **2. Analysis of Incorrect Options:** * **Penicillin G:** Resistance emerged in the 1970s via **PPNG** (Penicillinase-producing *N. gonorrhoeae*) strains carrying plasmids. It is no longer used. * **Tetracycline:** Resistance is widespread due to **TRNG** (Tetracycline-resistant *N. gonorrhoeae*) strains, mediated by the *tetM* gene on a transposon. * **Ciprofloxacin:** Fluoroquinolone resistance (due to mutations in *gyrA* and *parC* genes) became so prevalent by the mid-2000s that the CDC removed them from recommended treatment schedules. **3. High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Ceftriaxone (Ceftriaxone 500mg IM single dose is the current CDC recommendation). * **Mechanism of Resistance:** *N. gonorrhoeae* is a "superbug" that utilizes transformation to pick up resistance genes from the environment. * **Culture Media:** Thayer-Martin Medium (Selective medium containing Vancomycin, Colistin, Nystatin, and Trimethoprim). * **Key Feature:** It is a Gram-negative diplococcus (kidney-shaped) found within polymorphonuclear leukocytes (intracellular).

Question 579: Which of the following statements about H. Influenzae is true?

A. Capsulated coccobacilli, shows pleomorphism.
B. Stained by Loeffler's methylene blue/Dilute carbol fuchsin.
C. Shows satellitism (dependent upon V factor).
D. All of the above. (Correct Answer)

Explanation: **Explanation:** *Haemophilus influenzae* is a small, Gram-negative coccobacillus that is a significant human pathogen. This question tests the fundamental morphological and cultural characteristics required for its identification. 1. **Morphology (Option A):** *H. influenzae* is characteristically a **capsulated coccobacillus**. However, it is notorious for **pleomorphism**, meaning it can vary in shape from small coccobacilli to long, filamentous forms, especially in older cultures or when exposed to sub-inhibitory concentrations of antibiotics. 2. **Staining (Option B):** Due to its poor staining with standard Gram stain, it is often better visualized using **Loeffler’s methylene blue** or **dilute carbol fuchsin** (as a counterstain), which highlights its bipolar staining appearance. 3. **Satellitism (Option C):** This is a high-yield diagnostic feature. *H. influenzae* requires two growth factors: **X factor (Hemin)** and **V factor (NAD)**. On blood agar, it cannot grow alone because V factor is sequestered inside RBCs. When *Staphylococcus aureus* is streaked across the plate, it lyses RBCs and releases V factor. *H. influenzae* then grows as tiny "satellite" colonies near the *S. aureus* growth. **Clinical Pearls for NEET-PG:** * **Virulence Factor:** The polysaccharide capsule is the most important virulence factor; **Type b (Hib)** is the most invasive (contains Polyribosyl Ribitol Phosphate - PRP). * **Culture Media:** **Chocolate agar** is the medium of choice (heating lyses RBCs, releasing both X and V factors). * **Quellung Reaction:** Used for serotyping based on capsular swelling. * **Drug of Choice:** Ceftriaxone for invasive disease (meningitis/epiglottitis); Amoxicillin-clavulanate for non-invasive (otitis media).

Question 580: Neonatal meningitis acquired during passage through the birth canal is most commonly due to which organism?

A. Streptococcus agalactiae (Correct Answer)
B. Streptococcus equisimilus
C. Streptococcus pyogenes
D. Pneumococci

Explanation: **Explanation:** The correct answer is **Streptococcus agalactiae**, also known as **Group B Streptococcus (GBS)**. **1. Why Streptococcus agalactiae is correct:** GBS is the leading cause of neonatal sepsis and meningitis worldwide. It is a normal commensal of the female genital tract (vagina) and rectum in approximately 10–30% of pregnant women. The neonate acquires the organism during **vertical transmission** while passing through the birth canal. Early-onset disease (occurring within the first 7 days) typically presents as pneumonia or sepsis, while late-onset disease (7–90 days) frequently presents as meningitis. **2. Why the other options are incorrect:** * **Streptococcus equisimilus (Group C/G):** These are primarily associated with pharyngitis or skin infections in adults and are not common causes of neonatal meningitis. * **Streptococcus pyogenes (Group A):** While a potent pathogen causing "childbed fever" (puerperal sepsis) in mothers, it is an extremely rare cause of neonatal meningitis. * **Pneumococci (S. pneumoniae):** This is the most common cause of meningitis in adults and children older than 3 months, but it is not the primary pathogen in the immediate neonatal period. **High-Yield Clinical Pearls for NEET-PG:** * **Top 3 causes of Neonatal Meningitis:** 1. *Streptococcus agalactiae* (GBS), 2. *Escherichia coli* (K1 strain), 3. *Listeria monocytogenes*. * **Identification:** GBS is **CAMP test positive** and shows **Hippurate hydrolysis**. It produces narrow zones of beta-hemolysis on blood agar. * **Prevention:** Screening pregnant women at 35–37 weeks and administering intrapartum antibiotic prophylaxis (usually Penicillin G) significantly reduces the risk. * **Drug of Choice:** Penicillin G is the preferred treatment for GBS infections.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

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Mycobacteria

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Spirochetes

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