Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 41: Which structure is typically resistant to gonococcal infection?

A. Urethra
B. Testis (Correct Answer)
C. Fallopian tube
D. Ampulla of cervix

Explanation: **Explanation:** The correct answer is **Testis (Option B)**. *Neisseria gonorrhoeae* (Gonococcus) has a specific predilection for **columnar and cuboidal epithelium**. The pathogenesis of gonococcal infection involves the attachment of pili to these specific epithelial cells, followed by parasite-specified endocytosis. **Why Testis is resistant:** The testis itself is rarely involved in primary gonococcal infection. While the infection frequently ascends from the urethra to the **epididymis** (causing acute epididymitis), the testis is protected by the blood-testis barrier and lacks the specific mucosal surface required for primary gonococcal colonization. In males, the infection typically manifests as urethritis, which may complicate into epididymitis, but orchitis (inflammation of the testis) is characteristically absent or secondary to severe epididymitis. **Analysis of Incorrect Options:** * **Urethra (A):** This is the most common site of infection in males. It is lined by columnar epithelium (transitioning to stratified squamous at the meatus), making it highly susceptible. * **Fallopian tube (C):** Lined by simple columnar epithelium, it is a primary site for Pelvic Inflammatory Disease (PID) following ascending gonococcal infection. * **Ampulla of cervix (D):** The endocervix (lined by columnar epithelium) is the primary site of infection in females. Note that the adult vagina is resistant due to its stratified squamous epithelium and acidic pH. **High-Yield Clinical Pearls for NEET-PG:** * **Vulnerability:** Gonococcus attacks columnar/cuboidal cells. It spares the adult vagina (stratified squamous) but can cause **vulvovaginitis in prepubertal girls** because their vaginal epithelium is thin and not yet cornified. * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (selective) or Chocolate agar. * **Key Virulence Factor:** **Pili** (fimbriae) are essential for initial attachment and preventing phagocytosis. * **Treatment:** Due to widespread resistance, the current CDC/WHO recommendation is typically a single dose of **Ceftriaxone** (IM).

Question 42: A 10-year-old child presented with severe sore throat and a grey pseudomembrane over the pharynx and tonsils. What is the most likely causative organism?

A. A Gram-positive bacillus (Correct Answer)
B. A Gram-negative bacillus
C. A catalase-positive Gram-positive coccus that grows in clusters
D. A single-stranded positive-sense RNA virus

Explanation: ### Explanation The clinical presentation of a **greyish-white pseudomembrane** over the pharynx and tonsils in a child is the hallmark of **Diphtheria**, caused by ***Corynebacterium diphtheriae***. **1. Why Option A is Correct:** *Corynebacterium diphtheriae* is a **Gram-positive, non-motile, non-spore-forming bacillus**. It is characterized by its "club-shaped" appearance and arrangement in Chinese-letter patterns (cuneiform). The pseudomembrane is formed due to the action of the **Diphtheria toxin**, which causes local tissue necrosis and exudation. **2. Why Other Options are Incorrect:** * **Option B (Gram-negative bacillus):** Organisms like *Haemophilus influenzae* (Epiglottitis) or *Klebsiella* can cause respiratory infections, but they do not produce the characteristic adherent pseudomembrane seen in Diphtheria. * **Option C (Gram-positive cocci in clusters):** This describes *Staphylococcus aureus*. While it can cause pharyngitis, it typically presents with purulent exudates rather than a tough, leathery membrane. * **Option D (ssRNA virus):** This describes viruses like the Influenza virus or Rhinovirus. Viral pharyngitis usually presents with vesicles or diffuse redness, not a thick pseudomembrane. **3. High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Shows **metachromatic granules** (Volutin/Babes-Ernst granules) which stain bluish-black with Albert’s stain. * **Culture Media:** **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Toxin Detection:** The **Elek’s gel precipitation test** is the gold standard for detecting toxigenicity. * **Complications:** The "Bull-neck" appearance due to massive cervical lymphadenopathy and systemic complications like **myocarditis** and **polyneuropathy** (due to toxin absorption). * **Treatment:** Immediate administration of **Diphtheria Antitoxin (DAT)** is the priority.

Question 43: Tetanus is caused by which bacterium?

A. Clostridium perfringens
B. Clostridium tetani (Correct Answer)
C. Staphylococcus aureus
D. Bacillus cereus

Explanation: **Explanation:** **Clostridium tetani** is the causative agent of Tetanus. It is a Gram-positive, anaerobic, spore-forming motile bacillus. The pathogenesis is primarily mediated by **Tetanospasmin**, a potent neurotoxin produced during vegetative growth. This toxin travels via retrograde axonal transport to the spinal cord, where it inhibits the release of inhibitory neurotransmitters (**GABA and Glycine**) from Renshaw cells. This lack of inhibition leads to characteristic spastic paralysis and muscle spasms. **Analysis of Incorrect Options:** * **A. Clostridium perfringens:** Primarily responsible for **Gas Gangrene** (myonecrosis) and food poisoning. It produces alpha-toxin (lecithinase). * **C. Staphylococcus aureus:** A Gram-positive coccus that causes pyogenic infections (abscesses), Osteomyelitis, and Toxin-Mediated diseases like Toxic Shock Syndrome and Scalded Skin Syndrome. * **D. Bacillus cereus:** An aerobic spore-former associated with **food poisoning** (Emetic type from reheated rice and Diarrheal type). **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *C. tetani* spores are terminal and spherical, giving the organism a characteristic **"Drumstick appearance."** * **Clinical Features:** The earliest sign is often **Trismus** (lockjaw), followed by **Risus Sardonicus** (characteristic facial grin) and **Opisthotonus** (arch-like bowing of the back). * **Culture:** On blood agar, it produces a thin film of growth called **swarming**, similar to *Proteus*. * **Management:** Treatment involves wound debridement, metronidazole, and human tetanus immunoglobulin (TIG) to neutralize unbound toxin.

Question 44: Which of the following tests is not used for the diagnosis of syphilis?

A. VDRL
B. TPI
C. Reagin Test
D. Frei Test (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Frei Test**. The **Frei Test** is a delayed hypersensitivity skin test historically used for the diagnosis of **Lymphogranuloma Venereum (LGV)**, caused by *Chlamydia trachomatis* serotypes L1, L2, and L3. It is not used for syphilis. In modern practice, it has largely been replaced by more sensitive methods like Nucleic Acid Amplification Tests (NAAT). **Analysis of other options:** * **VDRL (Venereal Disease Research Laboratory):** This is a non-specific, non-treponemal screening test. It detects **reagin antibodies** (IgM and IgG) produced against cardiolipin-cholesterol-lecithin antigen. It is highly sensitive but lacks specificity. * **TPI (Treponema Pallidum Immobilization):** This is a specific, treponemal test. It is considered the "gold standard" for specificity, where live *T. pallidum* (Nichol's strain) are immobilized by the patient's serum. However, it is technically demanding and rarely used today. * **Reagin Test:** This is a general category of non-treponemal tests (including VDRL and RPR). They detect "reagin," an antibody-like substance that reacts with cardiolipin. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Tests (Non-treponemal):** VDRL, RPR (Rapid Plasma Reagin). Used for monitoring treatment response as titers fall after successful therapy. * **Confirmatory Tests (Treponemal):** FTA-ABS, TPHA, TPI. These remain positive for life, even after treatment. * **False Positives in VDRL (B-O-P-S):** **B**ereliosis (Lyme), **O**ld age, **P**regnancy, **S**ystemic Lupus Erythematosus (SLE)/Malaria. * **Drug of Choice:** Penicillin G remains the treatment of choice for all stages of syphilis.

Question 45: Which of the following is not a pathogenic mycobacteria?

A. M. kansasii
B. M. scrofulaceum
C. M. chelonae
D. M. smegmatis (Correct Answer)

Explanation: **Explanation:** The correct answer is **M. smegmatis**. This question tests the classification of Mycobacteria based on their pathogenicity. **Why M. smegmatis is the correct answer:** *Mycobacterium smegmatis* is a **saprophytic** (commensal) organism. It is a rapid grower (Runyon Group IV) that is part of the normal flora of the human genital tract (found in smegma). While it is acid-fast, it is generally considered **non-pathogenic** to humans, though rare opportunistic infections in severely immunocompromised patients have been reported. In the context of NEET-PG, it is the classic example of a non-pathogenic environmental mycobacterium. **Analysis of Incorrect Options:** * **M. kansasii (Option A):** A slow-growing photochromogen (Runyon Group I). It is a significant pathogen that causes a chronic pulmonary disease clinically indistinguishable from tuberculosis. * **M. scrofulaceum (Option B):** A slow-growing scotochromogen (Runyon Group II). It is a common cause of granulomatous cervical lymphadenitis (scrofula) in children. * **M. chelonae (Option C):** A rapid grower (Runyon Group IV). It is a recognized pathogen associated with skin, soft tissue, and bone infections, often following trauma or surgical procedures (e.g., tattooing or liposuction). **High-Yield Clinical Pearls for NEET-PG:** 1. **Runyon Classification:** Used for Nontuberculous Mycobacteria (NTM). * **Group I (Photochromogens):** Pigment in light (*M. kansasii, M. marinum*). * **Group II (Scotochromogens):** Pigment in dark (*M. scrofulaceum, M. szulgai*). * **Group III (Non-photochromogens):** No pigment (*M. avium-intracellulare/MAC*). * **Group IV (Rapid growers):** Growth < 7 days (*M. fortuitum, M. chelonae, M. abscessus*). 2. **M. marinum:** Causes "Swimming pool granuloma" or "Fish tank granuloma." 3. **M. avium-intracellulare (MAC):** The most common NTM causing systemic infection in HIV/AIDS patients (CD4 count < 50).

Question 46: Metachromatic granules are characteristically found in which of the following microorganisms?

A. Diphtheria (Correct Answer)
B. Mycoplasma
C. Chlamydia
D. Staphylococcus

Explanation: **Explanation:** **1. Why Diphtheria is Correct:** Metachromatic granules, also known as **Volutin or Babes-Ernst granules**, are the characteristic intracellular storage bodies of *Corynebacterium diphtheriae*. These granules are composed of **polymetaphosphate** and serve as energy reserves. They appear "metachromatic" because they take up a different color than the dye used; for instance, they appear reddish-pink when stained with blue dyes like **Albert’s, Neisser’s, or Ponder’s stain**. This characteristic "beaded appearance" is a classic diagnostic feature used to identify the organism. **2. Why Other Options are Incorrect:** * **Mycoplasma:** These are the smallest free-living organisms and lack a cell wall. They do not possess metachromatic granules; their identification relies on a "fried-egg" colony appearance on specialized media. * **Chlamydia:** These are obligate intracellular bacteria characterized by a unique life cycle involving Elementary Bodies (infectious) and Reticulate Bodies (replicative). They do not form polymetaphosphate granules. * **Staphylococcus:** These are Gram-positive cocci in clusters. While they may have storage granules, they are not metachromatic and are not used as a diagnostic hallmark. **High-Yield Clinical Pearls for NEET-PG:** * **Arrangement:** *C. diphtheriae* shows a "Chinese letter" or cuneiform arrangement due to incomplete separation during binary fission (snapping division). * **Stains:** Albert’s stain is the most common (Granules: Bluish-black; Bacilli: Green). * **Culture Media:** Loeffler’s Serum Slope (rapid growth) and Potassium Tellurite Agar (black colonies). * **Other organisms with metachromatic granules:** *Gardnerella vaginalis*, *Alcaligenes*, and *Spirillum volutans*.

Question 47: Which of the following is an acid-fast bacillus?

A. Mycobacterium tuberculosis (Correct Answer)
B. Treponema pallidum
C. Neisseria gonorrhoeae
D. None of the above

Explanation: **Explanation:** The correct answer is **A. Mycobacterium tuberculosis**. **Underlying Concept:** Acid-fastness is a physical property of certain bacteria that makes them resistant to decolorization by acids during staining procedures. This property is due to the presence of high concentrations of **mycolic acids** (long-chain fatty acids) in their cell walls. In the **Ziehn-Neelsen (ZN) stain**, these organisms retain the primary dye (Carbol Fuchsin) even after washing with acid-alcohol, appearing as bright red, beaded rods against a blue background. **Analysis of Options:** * **Mycobacterium tuberculosis:** This is the classic example of an acid-fast bacillus (AFB). Its cell wall is approximately 60% lipid, making it highly hydrophobic and acid-fast. * **Treponema pallidum:** This is a spirochete (the causative agent of Syphilis). It is too thin to be seen under light microscopy and is typically identified using **Dark-ground microscopy** or Silver impregnation stains (e.g., Fontana stain). * **Neisseria gonorrhoeae:** This is a Gram-negative diplococcus. It does not possess mycolic acid and is easily decolorized by acid. **High-Yield Clinical Pearls for NEET-PG:** * **Other Acid-Fast Organisms:** Apart from *Mycobacterium* species, other medically important acid-fast structures include *Nocardia* (weakly acid-fast), *Isospora* and *Cryptosporidium* oocysts, and the head of the human sperm. * **Modifications of ZN Stain:** * *M. tuberculosis:* Uses 20% Sulphuric acid ($H_2SO_4$) as a decolorizer. * *M. leprae:* Uses 5% $H_2SO_4$ (Modified ZN/Kinyoun stain). * *Nocardia:* Uses 1% $H_2SO_4$. * **Culture:** *M. tuberculosis* is typically grown on **Lowenstein-Jensen (LJ) medium**, showing "rough, tough, and buff" colonies.

Question 48: Which of the following are virulence factors for meningococci?

A. Capsule
B. Pili (Correct Answer)
C. Endotoxin
D. Coagulase

Explanation: **Explanation:** *Neisseria meningitidis* (Meningococcus) possesses several virulence factors, but **Pili (Fimbriae)** are considered the most critical initial factor. They mediate the **attachment and colonization** of the organism to the non-ciliated columnar epithelium of the nasopharynx. Without pili, the bacteria cannot establish a foothold to initiate infection or cross the mucosal barrier into the bloodstream. **Analysis of Options:** * **Pili (Correct):** These are essential for adhesion. They also undergo antigenic variation, helping the bacteria evade the host's immune response. * **Capsule:** While the polysaccharide capsule is vital for resisting phagocytosis and is used for serotyping (A, B, C, Y, W-135), it is primarily a survival factor rather than the primary tool for initial attachment. * **Endotoxin:** Meningococci produce **Lipooligosaccharide (LOS)**. While LOS is responsible for the devastating clinical features of meningococcemia (fever, shock, and DIC due to vascular damage), it is a toxin rather than an adhesin. * **Coagulase:** This is a characteristic virulence factor for *Staphylococcus aureus*, not *Neisseria*. **High-Yield Clinical Pearls for NEET-PG:** * **IgA1 Protease:** Another key virulence factor that cleaves mucosal IgA, facilitating colonization. * **Waterhouse-Friderichsen Syndrome:** Hemorrhagic adrenal failure caused by severe meningococcemia. * **Vaccine Note:** The Group B capsule is poorly immunogenic (due to similarity with human neural cell adhesion molecules); therefore, Group B vaccines are protein-based rather than polysaccharide-based. * **Prophylaxis:** Rifampicin is the drug of choice for close contacts; Ceftriaxone is an alternative.

Question 49: What is the most frequent non-sporing anaerobic bacillus isolated from clinical specimens?

A. Bacteroides fragilis (Correct Answer)
B. Clostridium tetani
C. Prevotella melaninogenica
D. Fusobacterium nucleatum

Explanation: **Explanation:** The correct answer is **Bacteroides fragilis**. **Why it is correct:** Anaerobic bacteria are the predominant components of the normal human flora, particularly in the gastrointestinal tract. Among non-sporing anaerobes, the **Bacteroides fragilis group** is the most frequently isolated pathogen from clinical specimens (especially in intra-abdominal infections, peritonitis, and abscesses). Although *B. fragilis* makes up only about 1–2% of the normal colonic flora, it possesses a potent capsular polysaccharide that promotes abscess formation and allows it to outpace other anaerobes in clinical infections. **Why other options are incorrect:** * **Clostridium tetani:** While a significant pathogen, it is a **spore-forming** anaerobic bacillus. The question specifically asks for a non-sporing organism. * **Prevotella melaninogenica:** This is a common non-sporing anaerobe found primarily in the oral cavity and upper respiratory tract. While frequently isolated in dental and pulmonary infections, it is less common than *B. fragilis* in overall clinical isolates. * **Fusobacterium nucleatum:** This is a needle-shaped, non-sporing anaerobe common in the mouth. It is frequently associated with Vincent’s angina and Lemierre’s syndrome but is isolated less frequently than the *B. fragilis* group. **High-Yield Clinical Pearls for NEET-PG:** * **B. fragilis** is famously resistant to penicillin due to beta-lactamase production. The drug of choice is typically **Metronidazole**. * It grows well on **Bile Esculin Agar**, appearing as black colonies (it is bile-tolerant). * Unlike most Gram-negative bacteria, its LPS (endotoxin) has low toxicity because it lacks the typical Lipid A structure. * **Morphology:** It is a pleomorphic, Gram-negative rod with rounded ends, often described as having a "safety-pin" appearance.

Question 50: Izumi fever is caused by which bacterium?

A. Pseudomonas aeruginosa
B. Burkholderia mallei
C. Yersinia pseudotuberculosis (Correct Answer)
D. Pasteurella multocida

Explanation: **Explanation:** **Correct Answer: C. Yersinia pseudotuberculosis** Izumi fever is a febrile illness characterized by fever, abdominal pain, and a scarlatiniform rash. It is caused by **Yersinia pseudotuberculosis**, a Gram-negative coccobacillus. The disease is primarily foodborne or waterborne and is often associated with outbreaks in Japan. The clinical presentation frequently mimics acute appendicitis (pseudoappendicitis) due to mesenteric lymphadenitis, a hallmark of *Yersinia* infections. **Analysis of Incorrect Options:** * **A. Pseudomonas aeruginosa:** Known for causing "Hot tub folliculitis," ecthyma gangrenosum in immunocompromised patients, and opportunistic infections in cystic fibrosis or burn victims. It is not associated with Izumi fever. * **B. Burkholderia mallei:** The causative agent of **Glanders**, a zoonotic disease primarily affecting horses, mules, and donkeys. In humans, it causes localized or systemic pyogenic infections. * **D. Pasteurella multocida:** Typically associated with soft tissue infections following **animal bites** (cats and dogs). It can cause cellulitis or osteomyelitis but does not cause Izumi fever. **High-Yield Clinical Pearls for NEET-PG:** * **Cold Enrichment:** *Yersinia* species (both *Y. enterocolitica* and *Y. pseudotuberculosis*) grow better at lower temperatures ($25-28^\circ\text{C}$) and can survive at $4^\circ\text{C}$. * **Bipolar Staining:** Exhibits a "safety-pin" appearance with Wayson or Giemsa stain. * **Pseudoappendicitis:** Always consider *Yersinia* in a differential diagnosis for a child presenting with right lower quadrant pain and fever where the appendix appears normal during surgery. * **Virulence:** Mediated by **V and W antigens** and Yersinia outer proteins (Yops) via a Type III secretion system.

Practice by Chapter

Staphylococci

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Streptococci and Enterococci

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Bacillus and Clostridium

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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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Mycobacteria

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Spirochetes

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