Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 621: Which statement is true regarding the Widal reaction?

A. Antibodies to the H antigen appear first and persist. (Correct Answer)
B. Antibodies to the O antigen appear first and persist.
C. Antibodies to the H and O antigens appear simultaneously.
D. None of the above.

Explanation: The Widal test is a serological test used for the diagnosis of enteric fever (Typhoid and Paratyphoid), detecting antibodies against the somatic (O) and flagellar (H) antigens of *Salmonella Typhi* and *Salmonella Paratyphi*. ### **Explanation of the Correct Option** **Option A is correct.** In the course of enteric fever, **H antibodies (Anti-H)** generally appear earlier than O antibodies. More importantly, H antibodies are IgG in nature and tend to **persist** in the serum for a long duration (months to years) after the infection has subsided. This makes them a marker of past infection or immunization. ### **Explanation of Incorrect Options** * **Option B:** **O antibodies (Anti-O)** are IgM in nature. While they appear early in the acute phase, they disappear rapidly (usually within weeks to a few months). Because they do not persist, their presence is often considered a more reliable indicator of a **recent/active infection**. * **Option C:** The kinetics of antibody production differ. O antigens are less immunogenic than H antigens; therefore, the titers and timing of appearance are not simultaneous. ### **High-Yield NEET-PG Pearls** * **Timing:** The Widal test becomes positive only after the **first week** of fever (highest sensitivity in the 2nd and 3rd weeks). * **Diagnostic Titers:** In endemic areas like India, a single test is significant only if the titer is **>1:80 for O** and **>1:160 for H**. * **Gold Standard:** A **four-fold rise** in titers in paired sera (taken 7–10 days apart) is more diagnostic than a single test. * **Anamnestic Response:** A non-specific rise in H-titers during a different febrile illness (like Malaria or Typhus) due to past exposure to Salmonella. * **Prozone Phenomenon:** High antibody concentrations can sometimes lead to false-negative results.

Question 622: Glanders disease is caused by which organism?

A. Burkholderia mallei (Correct Answer)
B. Diphtheria
C. Mumps
D. Burkholderia pseudomallei

Explanation: **Explanation:** **Glanders** is a serious zoonotic infection primarily affecting horses, mules, and donkeys. It is caused by **Burkholderia mallei**, a gram-negative, non-motile, non-spore-forming bacillus. Humans are accidental hosts, usually infected through direct contact with diseased animals or their secretions. * **Burkholderia mallei (Correct):** It is the causative agent of Glanders. It is unique among the *Burkholderia* species because it is **non-motile**. In humans, it causes localized nodules, pneumonia, or fatal septicemia. * **Burkholderia pseudomallei (Incorrect):** This organism causes **Melioidosis** (also known as Whitmore’s disease). Unlike *B. mallei*, it is **motile** and is found in soil and water (environmental saprophyte). * **Diphtheria (Incorrect):** This is a respiratory or cutaneous infection caused by *Corynebacterium diphtheriae*, a gram-positive, club-shaped bacillus characterized by a pseudomembrane in the throat. * **Mumps (Incorrect):** This is a viral infection caused by the *Mumps virus* (Paramyxoviridae family), typically presenting with parotid gland swelling. **High-Yield Clinical Pearls for NEET-PG:** 1. **Strauss Reaction:** This is a diagnostic test for Glanders where intraperitoneal inoculation of the organism into male guinea pigs results in severe **orchitis** (inflammation of the testes). 2. **Mallein Test:** A delayed hypersensitivity skin test used in veterinary practice to diagnose Glanders (similar to the Tuberculin test). 3. **Biological Warfare:** Both *B. mallei* and *B. pseudomallei* are classified as **Category B bioterrorism agents** due to their high infectivity and potential for aerosol transmission. 4. **Key Distinction:** Remember—**M**allei is **N**on-motile (**MN**), while **P**seudomallei is **P**olar flagellated/motile.

Question 623: Soft chancre is a feature of which of the following?

A. Syphilis
B. Haemophilus ducreyi (Correct Answer)
C. Neisseria gonorrhoeae
D. Lymphogranuloma venereum

Explanation: **Explanation:** **Soft chancre**, also known as **Chancroid**, is caused by the Gram-negative coccobacillus ***Haemophilus ducreyi***. The term "soft" refers to the characteristic painful, non-indurated (soft) ulcer, which distinguishes it from the "hard" (indurated) painless ulcer of syphilis. * **Haemophilus ducreyi (Correct):** It causes painful genital ulcers with ragged, undermined edges and a greyish-yellow exudate. It is often associated with painful, inflammatory inguinal lymphadenopathy (buboes) that may suppurate. * **Syphilis (Incorrect):** Caused by *Treponema pallidum*, it presents as a **Hard Chancre**. This ulcer is typically single, painless, and has a firm, indurated base with clean margins. * **Neisseria gonorrhoeae (Incorrect):** This organism primarily causes urethritis or cervicitis characterized by purulent discharge, rather than genital ulcers. * **Lymphogranuloma venereum (Incorrect):** Caused by *Chlamydia trachomatis* (serotypes L1-L3), it typically presents with a transient, painless primary papule/ulcer that heals quickly, followed by significant, painful regional lymphadenopathy (the "Groove sign"). **High-Yield Clinical Pearls for NEET-PG:** 1. **Microscopy of H. ducreyi:** Shows a characteristic **"School of fish"** or **"Railroad track"** appearance on Gram stain. 2. **Culture:** Requires enriched media like **Chocolate agar** supplemented with IsoVitaleX and Vancomycin. 3. **Mnemonic:** "Ducreyi makes you **cry**" (because the ulcer is painful), whereas Syphilis is "painless." 4. **Treatment:** Azithromycin (1g orally) or Ceftriaxone (250mg IM) are first-line therapies.

Question 624: What is a virulence factor of Group A beta-hemolytic streptococci?

A. Streptococcal pyrogenic exotoxin (Spe) A (Correct Answer)
B. Streptococcal pyrogenic exotoxin (Spe) B
C. Streptococcal pyrogenic exotoxin (Spe) C
D. Lipoteichoic acid

Explanation: **Explanation:** **Group A Streptococcus (GAS)**, or *Streptococcus pyogenes*, possesses a wide array of virulence factors. The correct answer is **Streptococcal pyrogenic exotoxin (Spe) A** because it acts as a potent **superantigen**. Unlike regular antigens, superantigens bypass normal antigen processing and bind directly to the MHC Class II molecules and T-cell receptors (TCR). This leads to a massive, non-specific release of cytokines (Cytokine Storm), resulting in **Streptococcal Toxic Shock Syndrome (STSS)** and necrotizing fasciitis. **Analysis of Options:** * **Spe A (Option A):** This is the primary superantigen associated with severe, invasive GAS infections and the "scarlet fever" rash. It is encoded by a lysogenic bacteriophage. * **Spe B (Option B):** While produced by GAS, Spe B is a **cysteine protease** involved in tissue degradation and skin invasion rather than the classic superantigen-mediated systemic shock. * **Spe C (Option C):** Also a superantigen, but Spe A is clinically more significant and more frequently implicated in the pathogenesis of toxic shock in exam scenarios. * **Lipoteichoic acid (Option D):** This is a structural component of the Gram-positive cell wall used for **adhesion** to host mucosal cells, but it is not considered a "pyrogenic exotoxin" or the primary driver of systemic virulence compared to Spe A. **High-Yield NEET-PG Pearls:** * **M Protein:** The most important virulence factor for GAS; it is anti-phagocytic and responsible for molecular mimicry leading to Rheumatic Fever. * **ASO Titre:** Used to diagnose antecedent pharyngeal infections (Rheumatic Fever), while **Anti-DNase B** is preferred for skin infections (Glomerulonephritis). * **Dick Test:** Historically used to identify susceptibility to Scarlet Fever (caused by Spe A, B, and C). * **Schultz-Charlton Reaction:** A skin blanching test used to diagnose Scarlet Fever rash.

Question 625: Mycoplasma pneumoniae is an infectious agent that:

A. Lacks steroids in its cytoplasmic membrane
B. Contains muramic acid in its cell wall
C. Contains only DNA
D. Causes primary atypical pneumonia (Correct Answer)

Explanation: **Explanation:** *Mycoplasma pneumoniae* is the smallest free-living prokaryote and a significant human pathogen. **Why the correct answer is right:** *Mycoplasma pneumoniae* is the most common cause of **Primary Atypical Pneumonia** (also known as "Walking Pneumonia"). It typically affects older children and young adults. It is termed "atypical" because the clinical presentation (insidious onset, non-productive cough, low-grade fever) and physical findings often appear less severe than the patchy infiltrates seen on a chest X-ray. **Why the other options are wrong:** * **Option A:** Unlike other bacteria, Mycoplasmas **require sterols** (cholesterol) in their cytoplasmic membrane for stability. They do not synthesize sterols but acquire them from the host or culture media (e.g., PPLO broth). * **Option B:** Mycoplasmas **lack a cell wall** entirely. Therefore, they do not contain peptidoglycan components like **muramic acid** or diaminopimelic acid. This makes them naturally resistant to beta-lactam antibiotics (like Penicillins) which target cell wall synthesis. * **Option C:** As true bacteria, Mycoplasmas contain **both DNA and RNA**. Only viruses contain either DNA or RNA (with rare exceptions). **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** Grows on **Eaton’s agar**, producing characteristic **"fried-egg" colonies**. * **Diagnosis:** The **Cold Agglutination Test** (IgM antibodies against I-antigen of RBCs) is a classic bedside test, though non-specific. PCR is now the gold standard. * **Complications:** Can cause Bullous myringitis, Stevens-Johnson Syndrome, and autoimmune hemolytic anemia. * **Treatment:** Macrolides (Azithromycin), Tetracyclines (Doxycycline), or Fluoroquinolones.

Question 626: Which of the following pleomorphic coccobacilli shows the satellite phenomenon on blood agar?

A. Anaerobic staphylococci
B. H. influenzae (Correct Answer)
C. Corynebacterium diphtheria
D. Brucella abortus

Explanation: ### Explanation **Correct Answer: B. *H. influenzae*** **The Concept: Satellitism** *Haemophilus influenzae* is a fastidious, pleomorphic gram-negative coccobacillus that requires two specific growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. While sheep blood agar contains Factor X, it also contains an enzyme (NADase) that destroys Factor V. Therefore, *H. influenzae* cannot grow alone on blood agar. However, when streaked alongside **Staphylococcus aureus**, the staphylococci produce NAD as a metabolic byproduct. *H. influenzae* colonies will then grow as tiny "satellites" immediately surrounding the *S. aureus* streak where the concentration of Factor V is highest. This is known as the **Satellite Phenomenon**. **Analysis of Incorrect Options:** * **A. Anaerobic staphylococci:** These are part of the normal flora (e.g., *Peptostreptococcus*) and do not require external NAD for growth on standard media. * **C. Corynebacterium diphtheriae:** A gram-positive, club-shaped rod. It is typically cultured on **Loeffler’s serum slope** or **Potassium Tellurite agar** (Hoyle’s medium) and does not exhibit satellitism. * **D. Brucella abortus:** A small gram-negative coccobacillus that causes undulant fever. It is highly fastidious but requires enriched media like **Castaneda’s medium** and often CO2, not the satellite phenomenon for identification. **High-Yield NEET-PG Pearls:** * **Chocolate Agar:** Created by heating blood agar to 75°C, which inactivates NADase and releases both Factors X and V, allowing *H. influenzae* to grow without satellitism. * **Culture Media:** *H. influenzae* grows best on **Levinthal’s medium** or **Fildes’ agar**. * **Virulence:** The most invasive strains are encapsulated, specifically **Type b (Hib)**, which contains Polyribosyl Ribitol Phosphate (PRP) in its capsule. * **Clinical:** It is a leading cause of epiglottitis ("thumb sign" on X-ray) and meningitis in unvaccinated children.

Question 627: A 55-year-old man presents with low-grade fever, cough, and increased sputum production for 5 days. He has a history of chronic bronchitis and diabetes. Which of the following structures is NOT used by the causative organism to cause the disease?

A. Pili
B. Outer membrane protein
C. Lipooligosaccharide
D. Capsular polysaccharide (Correct Answer)

Explanation: **Explanation:** The clinical presentation of a 55-year-old diabetic patient with chronic bronchitis suffering from an acute exacerbation points toward **Non-typeable *Haemophilus influenzae* (NTHi)**. While *H. influenzae* type b (Hib) is famous for its polyribosylribitol phosphate (PRP) capsule, NTHi strains are **unencapsulated**. **1. Why "Capsular Polysaccharide" is the correct answer:** Non-typeable *H. influenzae* (NTHi) is the most common cause of acute exacerbations of chronic obstructive pulmonary disease (AECOPD) and chronic bronchitis. Unlike the invasive Hib strains, NTHi lacks a polysaccharide capsule. Therefore, it does not use a capsule for virulence; instead, it relies on surface proteins for mucosal colonization. **2. Analysis of Incorrect Options:** * **Pili (Option A):** NTHi uses pili (fimbriae) to mediate initial attachment to the respiratory epithelial cells. * **Outer Membrane Proteins (Option B):** These proteins (e.g., P2, P5) act as adhesins and are crucial for the organism's attachment and survival within the host's nasopharynx. * **Lipooligosaccharide (LOS) (Option C):** Similar to LPS but lacking the O-antigen, LOS is a major virulence factor that causes inflammatory damage to the respiratory cilia and helps the bacteria evade the host immune response. **Clinical Pearls for NEET-PG:** * **Most common cause of AECOPD:** *H. influenzae* (Non-typeable). * **Culture:** Requires **Factor V (NAD)** and **Factor X (Hemin)**. It shows the "Satellite phenomenon" around *S. aureus* on blood agar. * **Vaccine:** The Hib vaccine targets the **Type b capsule**; it does *not* provide protection against NTHi (the cause of bronchitis/otitis media). * **Morphology:** Gram-negative pleomorphic coccobacilli.

Question 628: What is the most common cause of septicemia in the current scenario?

A. Gram-negative bacteria (Correct Answer)
B. Gram-positive bacteria
C. Fungi
D. Parasites

Explanation: ***Gram-negative bacteria*** - **E. coli**, **Klebsiella pneumoniae**, and **Pseudomonas aeruginosa** are the leading causes of septicemia in both hospital-acquired and community-acquired infections. - **Endotoxin (LPS)** in gram-negative bacterial cell walls triggers massive **inflammatory cascade** leading to severe septic shock and organ dysfunction. *Gram-positive bacteria* - While **Staphylococcus aureus** and **Streptococcus** species can cause septicemia, they are less common than gram-negative pathogens in current epidemiology. - Gram-positive sepsis typically has a **lower mortality rate** and less severe **cytokine storm** compared to gram-negative infections. *Fungi* - **Candida** species and **Aspergillus** primarily cause septicemia in **immunocompromised patients** with prolonged hospitalization or broad-spectrum antibiotic use. - Fungal septicemia has a **slower onset** and represents a smaller percentage of overall septicemia cases compared to bacterial causes. *Parasites* - **Malaria** (Plasmodium species) is the most common parasitic cause of sepsis-like syndrome but is **geographically limited** to endemic areas. - Parasitic septicemia is **extremely rare** in developed countries and represents less than 1% of all septicemia cases globally.

Question 629: The Milk Ring Test is performed to detect which organism in milk?

A. Bordetella
B. Brucella (Correct Answer)
C. Bartonella
D. Salmonella

Explanation: **Explanation:** The **Milk Ring Test (MRT)**, also known as the Abortus Bang Ring Test, is a screening method used to detect **Brucella** antibodies (specifically *Brucella abortus*) in pooled bulk milk samples. **Mechanism:** The test involves adding a hematoxylin-stained (blue-colored) *Brucella* antigen to a sample of whole milk. If antibodies against *Brucella* are present in the milk, they bind to the stained antigen, forming an antigen-antibody complex. These complexes attach to the fat globules in the milk. As the cream rises to the top, it carries the blue complexes with it, forming a **distinct blue ring** at the surface, while the rest of the milk remains white. If the test is negative, the blue color remains dispersed throughout the milk or settles at the bottom. **Analysis of Incorrect Options:** * **Bordetella:** Causes Whooping Cough (Pertussis); it is transmitted via respiratory droplets, not milk. * **Bartonella:** Associated with Cat Scratch Disease (*B. henselae*) or Trench Fever (*B. quintana*); it is transmitted via animal scratches or lice. * **Salmonella:** While *Salmonella* can cause food poisoning via contaminated dairy, it is typically detected via culture or PCR, not the Milk Ring Test. **High-Yield Clinical Pearls for NEET-PG:** * **Brucellosis** is a classic zoonotic infection causing **undulant fever** (fever with evening rises). * **Rose Bengal Plate Test** is the standard screening test for individual human serum. * **Standard Agglutination Test (SAT)** is the most common diagnostic test; a titer of >1:160 is significant. * **Culture:** Castaneda’s medium (biphasic medium) is the traditional gold standard for blood culture. * **Treatment:** Doxycycline + Rifampicin (or Streptomycin) for 6 weeks.

Question 630: What is the rickettsial agent of Epidemic typhus?

A. R. canorii
B. R. typhi
C. R. tsutsugamushi
D. R. prowazekii (Correct Answer)

Explanation: **Explanation:** **Epidemic typhus** is caused by ***Rickettsia prowazekii***. It is a severe, febrile illness historically associated with crowded conditions, war, and famine. The primary reservoir is humans, and the disease is transmitted by the **human body louse** (*Pediculus humanus corporis*). The bacteria are excreted in louse feces and enter the human body through skin abrasions caused by scratching. **Analysis of Options:** * **R. prowazekii (Correct):** The causative agent of Epidemic typhus. A unique feature is its ability to remain latent in the body and recrudesce years later as **Brill-Zinsser disease**. * **R. typhi (Incorrect):** This agent causes **Endemic (Murine) typhus**, which is transmitted to humans by the **rat flea** (*Xenopsylla cheopis*). It is generally milder than the epidemic form. * **R. tsutsugamushi (Incorrect):** Now reclassified as *Orientia tsutsugamushi*, it causes **Scrub typhus**. It is transmitted by the bite of **larval mites (chiggers)** and is characterized by a pathognomonic **eschar** at the bite site. * **R. conorii (Incorrect):** This belongs to the Spotted Fever Group and causes **Indian Tick Typhus** (Boutonneuse fever), transmitted by the brown dog tick. **High-Yield NEET-PG Pearls:** 1. **Vector Mnemonic:** Epidemic = Louse; Endemic = Flea. 2. **Weil-Felix Reaction:** *R. prowazekii* reacts with **OX-19** (Positive) and OX-K (Negative). 3. **Drug of Choice:** **Doxycycline** is the gold standard treatment for all rickettsial infections. 4. **Brill-Zinsser Disease:** A mild, relapsing form of epidemic typhus occurring years after the primary infection without the need for a louse vector.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

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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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