Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1581: Which of the following are spirochetes?

A. Syphilis, Leptospira, Mycoplasma
B. Leptospira, Mycoplasma, Brucella
C. Mycoplasma, Brucella, Borrelia
D. Syphilis, Leptospira, Borrelia (Correct Answer)

Explanation: **Explanation:** Spirochetes are a distinct group of Gram-negative, motile, spiral-shaped bacteria characterized by their unique **axial filaments (endoflagella)** located in the periplasmic space, which allow them to move in a corkscrew motion. The three medically important genera of spirochetes are **Treponema, Borrelia, and Leptospira.** * **Correct Option (D):** *Syphilis* (caused by *Treponema pallidum*), *Leptospira*, and *Borrelia* (causing Lyme disease and Relapsing fever) all belong to the order Spirochaetales. They share the characteristic thin, helical morphology. **Why other options are incorrect:** * **Mycoplasma (Options A, B, C):** These are the smallest free-living organisms. They are unique because they **lack a cell wall** (making them resistant to beta-lactams) and contain sterols in their cell membrane. They are pleomorphic, not spiral. * **Brucella (Options B, C):** These are small, Gram-negative **coccobacilli**. They are intracellular pathogens primarily causing zoonotic infections (Brucellosis). **NEET-PG High-Yield Pearls:** 1. **Microscopy:** Most spirochetes are too thin to be seen under a light microscope (except *Borrelia*, which can be seen with Giemsa/Wright stain). **Dark-ground microscopy (DGM)** is the gold standard for visualizing *T. pallidum*. 2. **Cultivability:** *Treponema pallidum* cannot be grown on artificial culture media (requires animal inoculation). 3. **Leptospira:** Characterized by "hooked ends" (question-mark shape) and can be cultured on **EMJH or Fletcher’s medium**. 4. **Borrelia:** The largest spirochete; *B. recurrentis* undergoes **antigenic variation**, leading to the characteristic relapsing fever.

Question 1582: The phage-encoded exotoxin of Vibrio cholerae resembles which toxin of E. coli?

A. Heat labile toxin (Correct Answer)
B. Heat stable toxin
C. Shiga-like toxin
D. Verocytotoxin

Explanation: **Explanation:** The correct answer is **Heat labile toxin (LT)**. **1. Why Heat Labile Toxin (LT) is correct:** The *Vibrio cholerae* enterotoxin (Cholera Toxin/CT) and the Heat Labile toxin (LT) of Enterotoxigenic *E. coli* (ETEC) are structurally, functionally, and immunologically nearly identical. * **Mechanism:** Both toxins are A-B subunit toxins. The 'B' subunit binds to the **GM1 ganglioside receptor** on enterocytes, while the 'A' subunit enters the cell to ADP-ribosylate the **Gs protein**. This leads to the permanent activation of **adenylate cyclase**, increasing intracellular **cAMP**. High cAMP levels result in the hypersecretion of water and electrolytes (Cl⁻, Na⁺, K⁺, and HCO₃⁻) into the intestinal lumen, causing profuse watery diarrhea. **2. Why other options are incorrect:** * **Heat stable toxin (ST):** Produced by ETEC, this toxin increases **cGMP** (not cAMP) by activating guanylate cyclase. It is a small peptide, not an A-B toxin. * **Shiga-like toxin (Verocytotoxin):** Produced by EHEC (e.g., O157:H7), this toxin inhibits protein synthesis by removing adenine from the 28S rRNA of the 60S ribosomal subunit. It is associated with Hemolytic Uremic Syndrome (HUS). **3. High-Yield Clinical Pearls for NEET-PG:** * **Genetic Origin:** Cholera toxin is encoded by the **CTXφ (CTX phage)**, a lysogenic bacteriophage. * **Mnemonic for cAMP vs. cGMP:** * **L**abile **A**ctivates **A**denylate cyclase (**cAMP**). * **S**table **A**ctivates **G**uanylate cyclase (**cGMP**). * *(Mnemonic: "cAMP is Labile, cGMP is Stable")* * **Stool Appearance:** *V. cholerae* causes "Rice water stools" with a fishy odor, lacking blood or inflammatory cells.

Question 1583: Which of the following is a late lactose fermenter?

A. E. coli
B. Klebsiella
C. Salmonella
D. Shigella sonnei (Correct Answer)

Explanation: **Explanation:** The classification of Enterobacteriaceae is primarily based on their ability to ferment lactose, which is detected on **MacConkey Agar**. Lactose fermentation requires two essential enzymes: 1. **Lactose Permease:** Transports lactose into the bacterial cell. 2. **$\beta$-galactosidase:** Breaks down lactose into glucose and galactose. **Shigella sonnei** is a classic example of a **Late Lactose Fermenter (LLF)**. These organisms possess the gene for $\beta$-galactosidase but lack or have a sluggish Lactose Permease. Consequently, they take 48–72 hours (instead of the usual 18–24 hours) to ferment lactose and produce pink colonies on MacConkey agar. **Analysis of Incorrect Options:** * **E. coli & Klebsiella:** These are **Rapid Lactose Fermenters (RLF)**. They possess both enzymes and produce bright pink colonies within 18–24 hours. Klebsiella is further distinguished by its mucoid colonies due to a prominent capsule. * **Salmonella:** This is a **Non-Lactose Fermenter (NLF)**. It lacks both enzymes and produces pale, colorless colonies on MacConkey agar. **NEET-PG High-Yield Pearls:** * **Other Late Lactose Fermenters:** *Vibrio cholerae* (specifically the O1 biotype) and *Serratia marcescens*. * **Shigella Classification:** *S. sonnei* belongs to Serogroup D. It is the only Shigella species that is a late lactose fermenter and is also **indole negative** and **ornithine decarboxylase positive**. * **Clinical Context:** *S. sonnei* is the most common cause of shigellosis (bacillary dysentery) in developed countries, whereas *S. flexneri* is more common in developing nations like India.

Question 1584: Which of the following mycobacteria does not produce pigment?

A. M. kansasii
B. M. simiae
C. M. Avium (Correct Answer)
D. M. scrofulaceum

Explanation: ### Explanation The classification of Non-Tuberculous Mycobacteria (NTM) is based on the **Runyon Classification**, which categorizes species according to their growth rate and ability to produce pigment (carotenoids) in the presence or absence of light. **Why M. avium is correct:** *M. avium-intracellulare complex* (MAC) belongs to **Runyon Group III (Non-photochromogens)**. These bacteria are characterized by their inability to produce significant pigment regardless of light exposure. Their colonies typically appear buff, tan, or off-white. **Analysis of Incorrect Options:** * **A. M. kansasii:** This is a **Runyon Group I (Photochromogen)**. It produces a yellow-orange pigment only after exposure to light. * **B. M. simiae:** Also a **Runyon Group I (Photochromogen)**. It is unique among photochromogens for being niacin-positive, often mimicking *M. tuberculosis*. * **D. M. scrofulaceum:** This is a **Runyon Group II (Scotochromogen)**. It produces pigment (usually yellow-orange) in both the light and the dark. **NEET-PG High-Yield Pearls:** 1. **Runyon Classification Summary:** * **Group I (Photochromogens):** Pigment in light only (*M. kansasii, M. marinum, M. simiae*). * **Group II (Scotochromogens):** Pigment in light and dark (*M. scrofulaceum, M. szulgai, M. gordonae*). * **Group III (Non-photochromogens):** No pigment (*MAC, M. ulcerans*). * **Group IV (Rapid Growers):** Growth in <7 days (*M. fortuitum, M. chelonae, M. abscessus*). 2. **Clinical Context:** *M. avium* is the most common NTM causing systemic infection in HIV/AIDS patients (CD4 count <50 cells/mm³). 3. **M. marinum:** Known as "Swimming pool granuloma" or "Fish tank granuloma."

Question 1585: What is the optimal percentage of NaCl for Vibrio cholerae growth?

A. 1% (Correct Answer)
B. 2%
C. 3%
D. 4%

Explanation: **Explanation:** *Vibrio cholerae* is a Gram-negative, comma-shaped bacterium that is **halotolerant** but not strictly halophilic. Unlike other members of the *Vibrio* genus, it does not require high concentrations of salt to grow, but it thrives in slightly alkaline and saline environments. **1. Why 1% is Correct:** The optimal concentration of NaCl for the growth of *V. cholerae* is **0.5% to 1%**. While it can grow in media with 0% NaCl (unlike other *Vibrios*), a 1% concentration provides the ideal osmotic balance for rapid multiplication. This is why standard laboratory media like Peptone Water (which contains 1% NaCl) are used for its cultivation. **2. Why Other Options are Incorrect:** * **Options B, C, and D (2%, 3%, 4%):** These concentrations are too high for *V. cholerae*. While *V. cholerae* can tolerate some salinity, it is inhibited by NaCl concentrations exceeding 6%. In contrast, **halophilic Vibrios** (like *V. parahaemolyticus* and *V. alginolyticus*) require higher salt concentrations (typically 3% or more) for growth and can survive in up to 8-10% NaCl. **High-Yield Clinical Pearls for NEET-PG:** * **Halotolerance:** *V. cholerae* is the only *Vibrio* species that can grow in **0% NaCl** (distinguishing it from halophilic *Vibrios*). * **pH Sensitivity:** It is highly sensitive to acid but grows well in alkaline media (**pH 8.2–9.0**). * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Selective Medium:** **TCBS** (Thiosulfate Citrate Bile Salts Sucrose) agar, where it forms distinctive **yellow colonies** due to sucrose fermentation. * **String Test:** Used for rapid identification; colonies emulsified in 0.5% sodium deoxycholate lose turbidity and form a "string" of DNA.

Question 1586: Which is the most dangerous type of diphtheria?

A. Facial
B. Laryngeal (Correct Answer)
C. Nasal
D. Cutaneous

Explanation: **Explanation:** The correct answer is **Laryngeal Diphtheria** because it poses the highest risk of acute airway obstruction, which is the most common cause of death in diphtheria patients. **1. Why Laryngeal is the most dangerous:** In laryngeal involvement, the characteristic "pseudomembrane" (composed of fibrin, leukocytes, and necrotic epithelial cells) can detach or cause significant edema in the narrow airway. This leads to **asphyxiation** or "mechanical suffocation." Clinically, this manifests as hoarseness, a "brassy" cough, and inspiratory stridor. **2. Analysis of Incorrect Options:** * **Nasal Diphtheria:** Usually the mildest form. It presents with serosanguinous discharge and is associated with low systemic toxicity because the toxin is poorly absorbed from the nasal mucosa. * **Cutaneous Diphtheria:** Presents as non-healing "punched-out" ulcers. While it serves as a reservoir for infection, systemic complications (toxemia) are rare compared to respiratory forms. * **Facial:** This is not a standard clinical classification of diphtheria. While the "Bull-neck" appearance (cervical lymphadenopathy and edema) occurs in severe faucial diphtheria, the immediate life-threatening risk remains airway compromise or myocarditis. **Clinical Pearls for NEET-PG:** * **Pathogenesis:** Caused by *Corynebacterium diphtheriae*. Pathogenicity is due to the **Diphtheria toxin** (an AB toxin) which inhibits protein synthesis by inactivating **EF-2**. * **Most Common Site:** Faucial (Tonsillar/Pharyngeal) diphtheria. * **Most Common Cause of Death:** Myocarditis (toxic effect) or Asphyxia (mechanical effect). * **Diagnosis:** Culture on **Löffler's serum slope** or **Potassium Tellurite agar** (black colonies). Toxin production is confirmed by the **Elek’s gel precipitation test**. * **Treatment:** Immediate administration of **Diphtheria Antitoxin (ADS)** is the priority; antibiotics (Penicillin/Erythromycin) are secondary to stop further toxin production.

Question 1587: A young sexually active woman presents with vaginitis, urethritis, and arthritis. What is the most likely causative organism?

A. Trichomoniasis
B. Candidiasis
C. Chlamydia (Correct Answer)
D. Gardnerella

Explanation: **Explanation:** The clinical triad of **vaginitis, urethritis, and arthritis** in a sexually active woman strongly suggests **Reactive Arthritis** (formerly known as Reiter’s Syndrome), which is most commonly triggered by a genital infection with ***Chlamydia trachomatis*** (Serotypes D-K). 1. **Why Chlamydia is correct:** *Chlamydia trachomatis* is the most common bacterial cause of Sexually Transmitted Infections (STIs). It typically causes cervicitis/vaginitis and urethritis. In a subset of patients, it triggers an immunologic cross-reactivity leading to Reactive Arthritis. This is characterized by the classic mnemonic "Can't see (uveitis), can't pee (urethritis), can't climb a tree (arthritis)." 2. **Why other options are incorrect:** * **Trichomoniasis:** Caused by *Trichomonas vaginalis*, it presents with a foul-smelling, "strawberry cervix" and frothy discharge, but it does not typically cause systemic arthritis. * **Candidiasis:** Presents with thick "cottage-cheese" discharge and intense pruritus; it is not associated with urethritis or reactive arthritis. * **Gardnerella:** Causes Bacterial Vaginosis (BV) characterized by a "fishy odor" and Clue cells. It is a localized vaginal dysbiosis and does not involve the joints. **High-Yield NEET-PG Pearls:** * **Diagnosis:** *Chlamydia* is best diagnosed via **NAAT** (Nucleic Acid Amplification Test). * **Treatment:** The drug of choice is **Doxycycline** (100mg BID for 7 days) or Azithromycin (1g single dose). * **Reactive Arthritis Association:** It is strongly associated with the **HLA-B27** haplotype. * **Other Triggers:** Apart from *Chlamydia*, enteric pathogens like *Shigella*, *Salmonella*, and *Campylobacter* can also cause reactive arthritis.

Question 1588: Which of the following is NOT associated with Chlamydia trachomatis?

A. Endemic trachoma
B. Inclusion conjunctivitis
C. Lymphogranuloma venereum
D. Community-acquired pneumonia (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Community-acquired pneumonia**. While *Chlamydia* species do cause respiratory infections, community-acquired pneumonia (CAP) is specifically associated with ***Chlamydophila pneumoniae*** (TWAR agent) and ***Chlamydophila psittaci*** (Psittacosis). *Chlamydia trachomatis* primarily infects the columnar epithelium of the mucous membranes (eyes and urogenital tract). Although it can cause "Infantile Pneumonia" in neonates (staccato cough) via birth canal transmission, it is not a cause of CAP in adults. **Analysis of Incorrect Options:** * **A. Endemic Trachoma:** Caused by *C. trachomatis* **Serotypes A, B, Ba, and C**. It is a leading cause of preventable blindness worldwide, characterized by follicular conjunctivitis and scarring (Pannus formation). * **B. Inclusion Conjunctivitis:** Caused by *C. trachomatis* **Serotypes D–K**. In adults, it is often a follicular conjunctivitis associated with genital infections (swimming pool conjunctivitis); in neonates, it presents as ophthalmia neonatorum. * **C. Lymphogranuloma Venereum (LGV):** Caused by *C. trachomatis* **Serotypes L1, L2, and L3**. It is a systemic sexually transmitted infection characterized by painless genital ulcers followed by painful inguinal lymphadenopathy (Buboes) and the "Groove sign." **NEET-PG High-Yield Pearls:** * **Serotype Mnemonic:** **A-C** (Blindness/Trachoma), **D-K** (Genital/Neonatal), **L1-L3** (LGV). * **Diagnosis:** **NAAT** (Nucleic Acid Amplification Test) is the gold standard. * **Cytology:** Look for **Halberstaedter-Prowazek (HP) inclusion bodies** (intracytoplasmic) on Giemsa stain. * **Treatment:** Drug of choice is **Azithromycin** (single dose) or Doxycycline. For neonatal pneumonia/conjunctivitis, oral Erythromycin is used.

Question 1589: Which of the following organisms exhibits antigenic variation?

A. Treponema pallidum
B. Neisseria
C. Corynebacterium
D. Borrelia recurrentis (Correct Answer)

Explanation: **Explanation:** **Borrelia recurrentis** is the classic example of a pathogen that utilizes **antigenic variation** to evade the host immune system. This organism causes **Louse-borne Relapsing Fever**. The mechanism involves the programmed rearrangement of genes encoding **Variable Large Proteins (VLP)** on its outer membrane. As the host mounts an antibody response against one antigenic type, a small population of bacteria switches its surface proteins, leading to a new wave of bacteremia and a clinical relapse of fever. **Analysis of Incorrect Options:** * **Treponema pallidum:** Causes syphilis. While it has very few surface proteins ("stealth pathogen") to evade detection, it does not exhibit the rapid, programmed antigenic switching characteristic of *Borrelia*. * **Neisseria:** While *Neisseria gonorrhoeae* exhibits antigenic variation of its **pili** and **Opa proteins**, it is not the classic organism associated with "relapsing" clinical cycles in the context of this specific question type. *Borrelia* is the primary high-yield association for this concept in NEET-PG. * **Corynebacterium diphtheriae:** Its pathogenicity is primarily mediated by the **Diphtheria toxin** (an AB toxin), which inhibits protein synthesis. It does not utilize antigenic variation for survival. **High-Yield Clinical Pearls for NEET-PG:** * **Relapsing Fever:** Louse-borne is caused by *B. recurrentis* (vector: *Pediculus humanus*); Tick-borne is caused by *B. hermsii* (vector: *Ornithodoros* ticks). * **Diagnosis:** Unlike other spirochetes, *Borrelia* can be visualized on a peripheral blood smear using **Giemsa or Wright stain** during the febrile period. * **Jarisch-Herxheimer Reaction:** A common systemic inflammatory response seen shortly after starting antibiotic treatment (usually Penicillin or Tetracycline) for *Borrelia* or *Treponema*.

Question 1590: What is the primary factor contributing to the virulence of gonococci?

A. Pili (Correct Answer)
B. Endotoxin
C. Exotoxin
D. None of the above

Explanation: **Explanation:** The primary virulence factor of *Neisseria gonorrhoeae* (gonococci) is the **Pili** (fimbriae). These hair-like appendages are essential for the initial attachment of the bacteria to the non-ciliated columnar epithelial cells of the human urogenital tract. Without pili, the bacteria cannot colonize the mucosal surface and are easily washed away by urine or vaginal secretions. Furthermore, pili undergo **antigenic and phase variation**, allowing the organism to evade the host’s immune response. **Analysis of Options:** * **Option A (Pili):** Correct. They mediate adherence and inhibit phagocytosis by neutrophils. Non-piliated strains (T3, T4, T5) are generally non-pathogenic, whereas piliated strains (T1, T2) are virulent. * **Option B (Endotoxin):** While gonococci possess **Lipooligosaccharide (LOS)**—a form of endotoxin—it primarily contributes to the inflammatory response and tissue damage (ciliary stasis) rather than being the "primary" factor for establishing infection. * **Option C (Exotoxin):** *Neisseria gonorrhoeae* does not produce any known exotoxins. * **Option D:** Incorrect, as Pili is the established primary factor. **High-Yield Clinical Pearls for NEET-PG:** * **IgA1 Protease:** Another vital virulence factor that cleaves mucosal IgA, facilitating colonization. * **Opa Proteins:** These "Opacity" proteins mediate firmer attachment and cell-to-cell signaling. * **Nutritional Requirement:** Gonococci are fastidious and grow best on **Thayer-Martin Medium** (Chocolate agar with VCN antibiotics). * **Gram Stain:** Characteristically seen as Gram-negative intracellular diplococci (kidney-bean shaped) within polymorphonuclear leukocytes.

Practice by Chapter

Staphylococci

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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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