Bacteriology Practice Questions

Q: Which carbohydrate does gonococcus ferment?

Glucose. **Explanation:** The differentiation of pathogenic *Neisseria* species is primarily based on their carbohydrate fermentation patterns (acid production from sugars). *Neisseria gonorrhoeae* (Gonococcus) is biochemically characterized by its ability to ferment **only Glucose**. **1. Why Glucose is correct:** *Neisseria gonorrhoeae* utilizes glucose oxidatively to produce acid, but it lacks the enzymes necessary to ferment other sugars. A helpful mnemonic for NEET-PG is: **G**onococcus ferments **G**lucose only. **2. Analysis of Incorrect Options:** * **Maltose:** This is the key differentiating factor. While *N. meningitidis* (Meningococcus) ferments both **G**lucose and **M**altose (**M**eningococcus = **M**altose), *N. gonorrhoeae* cannot ferment maltose. * **Sucrose & Fructose:** These sugars are typically fermented by non-pathogenic, commensal *Neisseria* species (e.g., *N. sicca*). Pathogenic species like Gonococcus and Meningococcus are sucrose-negative. **Clinical Pearls for NEET-PG:** * **Culture Media:** The gold standard for isolation is **Thayer-Martin medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin). * **Gram Stain:** Appears as Gram-negative, kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Oxidase/Catalase:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Key Distinction:** Unlike Meningococcus, Gonococcus does **not** have a polysaccharide capsule and does **not** ferment maltose.

Q: Crepitus in a wound is produced by which of the following microorganisms?

Clostridium welchii. **Explanation:** **Correct Answer: C. Clostridium welchii (Clostridium perfringens)** The presence of **crepitus** (a crackling sensation felt on palpation) indicates the presence of gas within the soft tissues. This is the hallmark clinical sign of **Gas Gangrene** (Clostridial Myonecrosis). * **Mechanism:** *Clostridium welchii* is an anaerobic, Gram-positive spore-forming bacillus. In an anaerobic environment (like deep crush injuries), it ferments carbohydrates in the muscle tissue, producing significant amounts of insoluble gas (CO₂ and H₂). This gas dissects through tissue planes, leading to crepitus, severe pain, and rapid tissue necrosis. The primary virulence factor responsible for tissue destruction is the **Alpha toxin** (Lecithinase). **Why other options are incorrect:** * **A. Staphylococcus aureus:** Typically causes pyogenic (pus-forming) infections like abscesses or cellulitis. While it can cause necrotizing fasciitis in rare cases, it is not a primary gas-producer. * **B. Clostridium tetani:** While also an anaerobe, it does not cause tissue necrosis or gas production. It produces **Tetanospasmin**, a neurotoxin that leads to spastic paralysis (Lockjaw). * **C. Pseudomonas:** Known for causing opportunistic infections in burns and producing a characteristic blue-green pigment (Pyocyanin). It does not typically produce gas in tissues. **NEET-PG High-Yield Pearls:** * **Nagler’s Reaction:** Used for rapid identification of *C. perfringens* (detects lecithinase activity on egg yolk agar). * **Morphology:** Described as "Box-car shaped" bacilli; notably **non-motile** (unlike most Clostridia). * **Stormy Fermentation:** Characteristic reaction in litmus milk. * **Target Hemolysis:** Double zone of hemolysis on blood agar (inner complete zone by theta toxin, outer partial zone by alpha toxin).

Q: What is the most common opportunistic infection in AIDS patients?

Tuberculosis. **Explanation:** In the context of the HIV/AIDS pandemic, **Tuberculosis (TB)**, caused by *Mycobacterium tuberculosis*, is the most common opportunistic infection (OI) and the leading cause of mortality worldwide. While HIV-infected individuals are susceptible to a wide range of pathogens, TB is unique because it can occur at any CD4 T-cell count, although the risk increases significantly as the count drops. In India and other developing nations, TB remains the most frequent co-infection encountered in clinical practice. **Analysis of Options:** * **Tuberculosis (A):** Correct. It is the most common OI globally and in India. It often presents atypically in advanced AIDS (extrapulmonary or disseminated forms). * **Pneumocystis jirovecii pneumonia (C):** This was historically the most common opportunistic infection in the Western world before the widespread use of prophylactic Co-trimoxazole. It typically occurs when the CD4 count falls below 200 cells/µL. * **Cryptococcosis (B):** This is the most common fungal meningitis in AIDS patients, usually occurring at CD4 counts <100 cells/µL, but it is less frequent than TB. * **Histoplasmosis (D):** This is a regional fungal infection (endemic in specific river valleys) and is not as globally prevalent as TB or PCP. **High-Yield Clinical Pearls for NEET-PG:** * **Most common OI overall:** Tuberculosis. * **Most common fungal OI:** Candidiasis (Oral Thrush). * **Most common CNS mass lesion:** Toxoplasmosis. * **Most common cause of blindness:** CMV Retinitis (CD4 <50 cells/µL). * **Screening:** All HIV-positive patients should be screened for TB using the four-symptom complex (cough, fever, night sweats, and weight loss).

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

Heat labile toxin. **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.

Q: Which of the following is a late lactose fermenter?

Shigella sonnei. **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.

Q: Which of the following mycobacteria does not produce pigment?

M. Avium. ### 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."

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

Chlamydia. **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.

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

Community-acquired pneumonia. **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.

Q: Which of the following organisms exhibits antigenic variation?

Borrelia recurrentis. **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*.

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

Pili. **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.

Question 1

Which carbohydrate does gonococcus ferment?

Accepted Answer

Glucose

Answer

Maltose

Answer

Sucrose

Answer

Fructose

Question 2

Crepitus in a wound is produced by which of the following microorganisms?

Accepted Answer

Clostridium welchii

Answer

Staphylococcus aureus

Answer

Clostridium tetani

Answer

Pseudomonas

Question 3

What is the most common opportunistic infection in AIDS patients?

Accepted Answer

Tuberculosis

Answer

Cryptococcosis

Answer

Pneumocystis jirovecii pneumonia

Answer

Histoplasmosis

Question 4

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

Accepted Answer

Heat labile toxin

Answer

Heat stable toxin

Answer

Shiga-like toxin

Answer

Verocytotoxin

Question 5

Which of the following is a late lactose fermenter?

Accepted Answer

Shigella sonnei

Answer

E. coli

Answer

Klebsiella

Answer

Salmonella

Question 6

Which of the following mycobacteria does not produce pigment?

Accepted Answer

M. Avium

Answer

M. kansasii

Answer

M. simiae

Answer

M. scrofulaceum

Question 7

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

Accepted Answer

Chlamydia

Answer

Trichomoniasis

Answer

Candidiasis

Answer

Gardnerella

Question 8

Which of the following is NOT associated with Chlamydia trachomatis?

Accepted Answer

Community-acquired pneumonia

Answer

Endemic trachoma

Answer

Inclusion conjunctivitis

Answer

Lymphogranuloma venereum

Question 9

Which of the following organisms exhibits antigenic variation?

Accepted Answer

Borrelia recurrentis

Answer

Treponema pallidum

Answer

Neisseria

Answer

Corynebacterium

Question 10

What is the primary factor contributing to the virulence of gonococci?

Accepted Answer

Pili

Answer

Endotoxin

Answer

Exotoxin

Answer

None of the above

Bacteriology — MCQs

Bacteriology — MCQs

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