Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 71: Chlamydia trachomatis is a well-known cause of venereal disease. This organism is also implicated in which of the following?

A. Classic trachoma infection (Correct Answer)
B. Sexually transmitted cardiac disease in adults
C. Perinatal retinitis
D. Middle-ear infection in young children

Explanation: **Explanation:** *Chlamydia trachomatis* is an obligate intracellular bacterium characterized by a unique life cycle involving elementary bodies (infectious) and reticulate bodies (replicative). It is classified into several serovars, which determine its clinical presentation. **1. Why Option A is Correct:** *Chlamydia trachomatis* **serovars A, B, Ba, and C** are the primary causative agents of **Classic Trachoma**. This is a chronic keratoconjunctivitis that remains the world’s leading cause of preventable infectious blindness. Repeated infections lead to conjunctival scarring, trichiasis (inward-turning eyelashes), and eventual corneal opacification. **2. Why the Other Options are Incorrect:** * **Option B:** While *Chlamydia pneumoniae* is associated with atherosclerosis and coronary artery disease, *C. trachomatis* does not cause sexually transmitted cardiac disease. * **Option C:** Perinatal infection with *C. trachomatis* (serovars D-K) typically causes **inclusion conjunctivitis** (ophthalmia neonatorum) and **interstitial pneumonia**, but it is not a recognized cause of retinitis. (Note: CMV is a common cause of viral retinitis). * **Option D:** Middle-ear infections (Otitis media) in children are most commonly caused by *Streptococcus pneumoniae*, *Haemophilus influenzae*, or *Moraxella catarrhalis*, not *Chlamydia*. **High-Yield Clinical Pearls for NEET-PG:** * **Serovars A, B, C:** Trachoma (Endemic blindness). * **Serovars D-K:** Inclusion conjunctivitis, Nongonococcal urethritis (NGU), Cervicitis, and Neonatal pneumonia (staccato cough). * **Serovars L1, L2, L3:** Lymphogranuloma Venereum (LGV) characterized by painful inguinal lymphadenopathy (Groove sign). * **Diagnosis:** **NAAT (Nucleic Acid Amplification Test)** is the gold standard. * **Treatment:** Azithromycin (single dose) or Doxycycline. For Trachoma control, the WHO recommends the **SAFE** strategy (Surgery, Antibiotics, Facial cleanliness, Environmental improvement).

Question 72: Which of the following is NOT true with respect to the genus Neisseria?

A. Gram negative
B. Anaerobic (Correct Answer)
C. Non-sporulating
D. Oxidase positive cocci

Explanation: **Explanation:** The genus *Neisseria* consists of Gram-negative diplococci that are primarily **aerobic** or capnophilic (requiring CO₂). Therefore, the statement that they are "Anaerobic" is incorrect. 1. **Why "Anaerobic" is the correct answer (False statement):** *Neisseria* species (including *N. meningitidis* and *N. gonorrhoeae*) are **obligate aerobes**. They possess an oxidative metabolism and cannot grow under strictly anaerobic conditions. Some species are capnophilic, growing best in an atmosphere enriched with 5–10% CO₂. 2. **Analysis of other options:** * **Gram-negative (True):** They appear as pink/red kidney-bean-shaped diplococci on Gram stain, with adjacent sides flattened. * **Non-sporulating (True):** Like most pathogenic cocci, *Neisseria* do not form spores. * **Oxidase positive (True):** A key biochemical hallmark of the genus *Neisseria* is that they produce the enzyme cytochrome oxidase. They are also **Catalase positive**. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** *N. gonorrhoeae* is fastidious and grows on **Thayer-Martin Medium** (Selective) or Chocolate Agar. * **Fermentation:** *N. **m**eningitidis* ferments **M**altose and Glucose, whereas *N. **g**onorrhoeae* ferments only **G**lucose. * **Virulence:** The **pili** are the most important virulence factor for *N. gonorrhoeae* (attachment), while the **polysaccharide capsule** is critical for *N. meningitidis* (evading phagocytosis). * **Waterhouse-Friderichsen Syndrome:** A severe complication of meningococcemia involving adrenal hemorrhage.

Question 73: A 30-year-old woman with a history of poor obstetric outcomes presents with fever. Blood culture reveals gram-positive, small to medium, pleomorphic coccobacilli occurring in short chains. Direct microscopy of the culture shows tumbling motility. What is the most likely organism?

A. Listeria monocytogenes (Correct Answer)
B. Corynebacterium sp.
C. Enterococcus sp.
D. Erysipelothrix rhusiopathiae

Explanation: ### Explanation **Correct Option: A. Listeria monocytogenes** The clinical presentation and laboratory findings are classic for *Listeria monocytogenes*. * **Morphology:** It is a Gram-positive, non-spore-forming, pleomorphic coccobacillus often appearing in short chains or pairs (resembling *S. pneumoniae*). * **Motility:** The pathognomonic feature is **"tumbling motility"** observed in hanging drop preparations at 20–25°C (due to peritrichous flagella). At 37°C, motility is absent or sluggish. * **Clinical Context:** In adults, it typically affects pregnant women and the immunocompromised. It is a significant cause of **recurrent abortions** and stillbirths (as hinted by "poor obstetric outcomes") and can cause neonatal meningitis. **Why other options are incorrect:** * **B. Corynebacterium sp.:** While Gram-positive and pleomorphic, they are non-motile and typically show a "Chinese-letter" or cuneiform arrangement. * **C. Enterococcus sp.:** These are Gram-positive cocci in pairs/chains. While some species show motility, they do not exhibit the characteristic "tumbling" motion and are not typically associated with recurrent obstetric loss in this manner. * **D. Erysipelothrix rhusiopathiae:** This is a Gram-positive rod associated with skin infections (Erysipeloid) in handlers of animals/fish. It is **non-motile** and produces H₂S on TSI agar. **High-Yield NEET-PG Pearls for Listeria:** 1. **Cold Enrichment:** It can grow at 4°C, a property used for selective isolation from contaminated samples. 2. **Culture:** Shows **"Umbrella-shaped"** growth in semi-solid motility medium. 3. **Hemolysis:** Produces a narrow zone of **beta-hemolysis** on blood agar (CAMP test positive). 4. **Intracellular Pathogen:** It moves cell-to-cell via "actin rockets" (actin polymerization). 5. **Treatment:** Drug of choice is **Ampicillin**. It is inherently resistant to all Cephalosporins.

Question 74: A darkfield microscope is most useful for examining which of the following organisms in a blood sample?

A. Treponema (Correct Answer)
B. Actinomyces
C. Streptococcus
D. Mycobacterium

Explanation: **Explanation:** **1. Why Treponema is the Correct Answer:** *Treponema pallidum* (the causative agent of Syphilis) is a spirochete characterized by its extremely thin diameter (approx. 0.1–0.2 μm), which is below the resolution limit of a standard light microscope. Furthermore, it does not take up traditional aniline dyes (like Gram stain) well. **Darkfield microscopy** works by using a special condenser that prevents direct light from entering the objective; only light reflected/scattered by the organism enters. This makes the thin, silver-white spirochetes appear luminous against a black background, allowing for the visualization of their characteristic morphology and motility (corkscrew movement). **2. Why the Other Options are Incorrect:** * **Actinomyces:** These are Gram-positive, filamentous branching bacteria. They are thick enough to be easily visualized using standard Gram staining and light microscopy. * **Streptococcus:** These are Gram-positive cocci in chains. They are easily seen under a light microscope after Gram staining. * **Mycobacterium:** While these are "acid-fast" and do not stain well with Gram stain due to high lipid content in the cell wall, they are routinely identified using **Ziehl-Neelsen (ZN) staining** or fluorescent microscopy (Auramine-Rhodamine), not darkfield. **3. Clinical Pearls for NEET-PG:** * **Primary Syphilis:** Darkfield microscopy of the exudate from a **chancre** is the gold standard for immediate diagnosis before serology (VDRL/RPR) becomes positive. * **Other Spirochetes:** Darkfield is also used for *Leptospira* (found in blood/urine) and *Borrelia*. * **Silver Staining:** If microscopy is done on tissue sections (biopsy), **Levaditi or Warthin-Starry** silver stains are used to visualize *Treponema*. * **Limitation:** Darkfield cannot be used for oral lesions to diagnose syphilis because commensal oral spirochetes (like *T. denticola*) look identical to *T. pallidum*.

Question 75: Group A Streptococcus is the causative agent of pharyngitis. What is the typical mode of transmission for Group A Streptococcus pharyngitis?

A. Direct contact or droplet spread (Correct Answer)
B. Hematogenous spread
C. Lymphatic spread
D. None of the above

Explanation: **Explanation:** **Group A Streptococcus (GAS)**, or *Streptococcus pyogenes*, is the most common bacterial cause of acute pharyngitis. The primary mode of transmission is through **large respiratory droplets** or **direct person-to-person contact**. When an infected individual coughs, sneezes, or talks, droplets containing the bacteria are expelled and can be inhaled by others or land on mucosal surfaces. Crowded environments like schools, barracks, and daycare centers facilitate this spread. **Analysis of Options:** * **Option A (Correct):** GAS colonizes the upper respiratory tract (throat and nasopharynx). Transmission occurs via respiratory secretions (droplets) or direct contact with infected skin lesions (in the case of impetigo). * **Option B (Incorrect):** Hematogenous spread refers to dissemination via the bloodstream. While GAS can cause bacteremia leading to sepsis or necrotizing fasciitis, this is a *complication* or a different clinical manifestation, not the primary mode of transmission for pharyngitis. * **Option C (Incorrect):** Lymphatic spread involves the movement of pathogens through lymph vessels. While GAS pharyngitis often causes regional lymphadenopathy (swollen cervical nodes), the bacteria do not spread from person to person via the lymphatic system. **High-Yield Clinical Pearls for NEET-PG:** * **Centor Criteria:** Used to clinically diagnose GAS pharyngitis (Fever, Tonsillar exudates, Tender anterior cervical lymphadenopathy, and Absence of cough). * **Virulence Factor:** **M protein** is the chief virulence factor; it is anti-phagocytic and shares structural homology with cardiac myosin (leading to Rheumatic Fever). * **Complications:** Non-suppurative complications include **Acute Rheumatic Fever** (follows only pharyngitis) and **Post-Streptococcal Glomerulonephritis (PSGN)** (follows either pharyngitis or skin infections). * **DOC:** Penicillin remains the drug of choice as no resistance has been documented to date.

Question 76: Which of the following is NOT associated with endocarditis?

A. Staphylococcus aureus
B. Enterococcus
C. HACEK group
D. ESKAPE group (Correct Answer)

Explanation: **Explanation:** In Infective Endocarditis (IE), the causative agents are typically organisms capable of adhering to damaged endocardium or prosthetic valves. The **ESKAPE group** is the correct answer because it is a classification based on **antimicrobial resistance patterns** rather than a specific association with endocarditis. * **Why ESKAPE is the correct answer:** The ESKAPE acronym (*Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa,* and *Enterobacter* species) represents a group of pathogens that "escape" the effects of antibiotics. While individual members (like *S. aureus*) cause IE, the group as a whole is defined by its clinical challenge in nosocomial infections and multi-drug resistance, not as a standard cluster for endocarditis. * **Why other options are incorrect:** * **Staphylococcus aureus:** The most common cause of acute IE globally, especially in IV drug users and those with prosthetic valves. * **Enterococcus:** A common cause of subacute IE, often following urinary tract or GI manipulations. * **HACEK group:** (*Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella*) This group consists of fastidious Gram-negative bacilli known for causing **culture-negative endocarditis**. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of IE (Overall):** *Staphylococcus aureus*. * **Most common cause of Subacute IE:** *Viridans group Streptococci*. * **Most common cause in IV drug users:** *S. aureus* (affects the Tricuspid valve). * **Culture-negative IE:** Most commonly due to prior antibiotic use, but classically associated with HACEK, *Coxiella burnetii* (Q fever), and *Bartonella*. * **Streptococcus bovis (S. gallolyticus):** If found in IE, always screen the patient for **Colorectal Carcinoma**.

Question 77: What are the investigations for a clinically suspected case of tuberculosis?

A. Mantoux test (primarily in children)
B. Sputum for Acid-Fast Bacilli (AFB)
C. Polymerase Chain Reaction (PCR)
D. All of the above (Correct Answer)

Explanation: The diagnosis of Tuberculosis (TB) relies on a combination of immunological, microbiological, and molecular techniques, making **"All of the above"** the correct choice. ### **Detailed Breakdown:** 1. **Mantoux Test (Option A):** This is a Type IV hypersensitivity reaction (Delayed-type) using Purified Protein Derivative (PPD). While it cannot distinguish between active disease and latent infection, it is a crucial screening tool, especially in **pediatric cases** where sputum collection is difficult and the bacterial load is often paucibacillary. 2. **Sputum for AFB (Option B):** This remains the traditional "gold standard" for identifying infectious cases. Using **Ziehl-Neelsen (ZN) staining**, clinicians look for bright red, slightly curved rods. A minimum of two samples (one spot, one early morning) is typically required. 3. **Polymerase Chain Reaction (Option C):** Molecular methods like **CBNAAT (GeneXpert)** have revolutionized TB diagnosis. They offer high sensitivity, rapid results (within 2 hours), and the ability to detect **Rifampicin resistance** simultaneously. ### **NEET-PG High-Yield Pearls:** * **Gold Standard for Diagnosis:** Culture remains the absolute gold standard. **Liquid culture (MGIT)** is faster (1-3 weeks) than solid media like **Lowenstein-Jensen (LJ)** (6-8 weeks). * **CBNAAT:** Under the National TB Elimination Program (NTEP), CBNAAT is now the preferred initial diagnostic test for all suspected TB cases. * **Microscopy Sensitivity:** At least 5,000–10,000 bacilli/ml of sputum are required for a positive ZN stain. * **IGRA (Interferon-Gamma Release Assay):** Unlike Mantoux, IGRA is not affected by prior BCG vaccination, making it more specific for latent TB.

Question 78: A 32-year-old man presented with a painless ulcer with a black eschar on his leg. Smears of exudate show non-motile organisms which are in chains. Spores are absent in the smear. What is the most probable organism?

A. Proteus
B. Diphtheroids
C. Clostridium
D. Bacillus anthracis (Correct Answer)

Explanation: ### Explanation The clinical presentation of a **painless ulcer with a black eschar** is a classic description of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. **Why Bacillus anthracis is correct:** * **Morphology:** *B. anthracis* is a Gram-positive, **non-motile** rod that typically appears in long chains (often described as "bamboo-stick" appearance). * **The "No Spore" Rule:** While *B. anthracis* is a spore-former, it **never forms spores in living tissue** or clinical samples (exudate). Sporulation only occurs under aerobic conditions outside the host (e.g., on culture media or in the environment). * **Clinical Sign:** The "Malignant Pustule" or eschar is characteristic, resulting from the action of the Lethal Toxin (LF) and Edema Toxin (EF). **Why other options are incorrect:** * **Proteus:** These are Gram-negative bacilli known for being **highly motile** (swarming growth). They do not cause eschars. * **Diphtheroids:** These are commensal Gram-positive bacilli that are pleomorphic (club-shaped) and usually arranged in "Chinese letter" patterns, not long chains. * **Clostridium:** While these are Gram-positive rods, they are typically **motile** (except *C. perfringens*) and are **obligate anaerobes**. They cause gas gangrene or tetanus, not a dry black eschar. **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used to visualize the capsule of *B. anthracis* using polychrome methylene blue. * **Medusa Head Appearance:** Characteristic morphology of colonies on agar. * **String of Pearls Reaction:** Seen when grown on agar containing penicillin. * **Selective Media:** PLET Medium (Polymyxin, Lysozyme, EDTA, Thallous acetate).

Question 79: What is the most sensitive investigation for the diagnosis of asymptomatic chlamydial infection?

A. Culture
B. Nucleic acid amplification (Correct Answer)
C. Serology
D. Direct microscopic examination of tissue scraping

Explanation: **Explanation:** **Nucleic Acid Amplification Tests (NAATs)** are currently the **gold standard** and the most sensitive investigation for diagnosing *Chlamydia trachomatis* infections. This is particularly true for asymptomatic cases where the bacterial load may be low. NAATs detect specific DNA or RNA sequences of the pathogen, offering a sensitivity of >90–95%. A major clinical advantage is that they can be performed on non-invasive samples like first-void urine or vaginal swabs, making them ideal for screening. **Why other options are incorrect:** * **Culture:** Historically the "gold standard" due to 100% specificity, it is technically demanding, expensive, and has low sensitivity (60–80%). Since Chlamydia is an obligate intracellular pathogen, it requires live cell lines (e.g., McCoy cells), making it impractical for routine screening. * **Serology:** Detecting antibodies (IgM/IgG) is generally unhelpful for acute or asymptomatic genital infections because it cannot distinguish between a current and a past infection. It is primarily used for diagnosing Lymphogranuloma Venereum (LGV) or neonatal pneumonia. * **Direct Microscopic Examination:** Using Giemsa or Iodine staining to find inclusion bodies (Halberstaedter-Prowazek bodies) has very low sensitivity and is not recommended for genital tract infections. **High-Yield Clinical Pearls for NEET-PG:** * **Sample of choice:** First-void urine (men) and vaginal/cervical swabs (women). * **Drug of choice:** Azithromycin (1g single dose) or Doxycycline (100mg BID for 7 days). * **Co-infection:** Always screen for *Neisseria gonorrhoeae* when Chlamydia is suspected. * **LGV:** Caused by serovars L1, L2, and L3; characterized by the "Groove sign."

Question 80: Which of the following is a tap water scotochromogen?

A. Mycobacterium chelonae
B. Mycobacterium marinum
C. Mycobacterium kansasii
D. Mycobacterium gordonae (Correct Answer)

Explanation: **Explanation:** Non-Tuberculous Mycobacteria (NTM) are classified by the **Runyon Classification** based on their growth rate and pigment production. **Scotochromogens** (Runyon Group II) are slow-growing mycobacteria that produce pigment (yellow-orange) regardless of whether they are grown in the light or dark. **Why Mycobacterium gordonae is correct:** * **Mycobacterium gordonae** is the classic example of a tap water scotochromogen. It is commonly found in environmental water sources and is frequently encountered as a **contaminant** in clinical specimens (sputum or gastric lavage). It rarely causes disease in humans, making its identification crucial to avoid unnecessary treatment. **Analysis of Incorrect Options:** * **A. Mycobacterium chelonae:** This is a **Rapid Grower** (Runyon Group IV). It typically grows within 7 days and is associated with skin and soft tissue infections following trauma or surgery. * **B. Mycobacterium marinum:** This is a **Photochromogen** (Runyon Group I). It produces pigment only when exposed to light. It is known for causing "Swimming Pool Granuloma" or "Fish Tank Granuloma." * **C. Mycobacterium kansasii:** This is also a **Photochromogen** (Runyon Group I). It is the second most common NTM to cause pulmonary disease (resembling TB) after MAC (M. avium complex). **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Group I (Photochromogens):** *M. kansasii, M. marinum.* (Mnemonic: **K**ansas **M**arine). * **Runyon Group II (Scotochromogens):** *M. gordonae, M. scrofulaceum.* * **Runyon Group III (Non-photochromogens):** *M. avium-intracellulare (MAC).* * **Runyon Group IV (Rapid Growers):** *M. fortuitum, M. chelonae, M. abscessus.* * **Buruli Ulcer:** Caused by *M. ulcerans* (Group III). * **Lepra bacilli:** Cannot be grown on artificial media; cultivated on the footpads of mice or nine-banded armadillos.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

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Corynebacterium and Listeria

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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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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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