Bacteriology Practice Questions

Q: A 32-year-old woman presents to the emergency department with severe, watery diarrhea, vomiting, and intense thirst. She also experiences oliguria, muscle cramps, and weakness. Physical examination reveals marked loss of tissue turgor, sunken eyes, and wrinkling of the skin of her fingers. Laboratory studies show hemoconcentration and severe metabolic acidosis with potassium depletion. An organism cultured from the patient's stool is characterized as an oxidase-positive, gram-negative curved rod with polar flagella that shows "shooting star" mobility and grows best on TCBS medium. Which of the following is the most likely pathogen?

Vibrio cholerae. **Explanation** The clinical presentation of profuse, watery diarrhea ("rice-water stools"), severe dehydration (sunken eyes, loss of turgor, "washerwoman’s hands" wrinkling), and metabolic derangements is classic for **Cholera**. **Why Vibrio cholerae is correct:** * **Morphology:** It is a Gram-negative, comma-shaped (curved) rod. * **Motility:** It exhibits characteristic **"shooting star" motility** due to a single polar flagellum. * **Biochemical/Culture:** It is **oxidase-positive** (distinguishing it from Enterobacteriaceae). It grows on **TCBS (Thiosulfate-Citrate-Bile salts-Sucrose) agar**, producing large yellow colonies due to sucrose fermentation. * **Pathophysiology:** The symptoms are mediated by the **Cholera toxin (Choleragen)**, which increases intracellular cAMP, leading to the hypersecretion of water and electrolytes into the intestinal lumen. **Why other options are incorrect:** * **A. Pseudomonas:** While oxidase-positive and motile, it typically causes respiratory or wound infections, not profuse watery diarrhea. * **B. Salmonella:** These are Gram-negative rods but are **oxidase-negative** and do not show shooting star motility. They typically cause enteric fever or inflammatory diarrhea. * **C. Shigella:** These are **non-motile**, oxidase-negative rods that cause bacillary dysentery (bloody/mucoid stools), not massive watery diarrhea. **High-Yield Pearls for NEET-PG:** * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium. * **Enrichment Media:** Alkaline Peptone Water (APW) or Monsur’s Taurocholate Tellurite Peptone Water. * **String Test:** Positive for *V. cholerae* (colonies lose morphology and become mucoid when mixed with sodium deoxycholate). * **Hanging Drop Preparation:** Best method to visualize "shooting star" motility.

Q: Which of the following is true about Actinomyces?

Gram-positive. **Explanation:** **Actinomyces** are a genus of **Gram-positive**, non-acid-fast, anaerobic to microaerophilic filamentous bacteria. They are often mistaken for fungi due to their branching appearance, but they are true bacteria as they lack a nuclear membrane and respond to antibiotics. * **Why Option A is correct:** Actinomyces species (most commonly *A. israelii*) stain purple on Gram stain, identifying them as Gram-positive bacilli with characteristic branching filaments. * **Why Option B is incorrect:** The most common site of infection is the **cervicofacial region** ("lumpy jaw"), often following dental procedures or poor oral hygiene. Brain abscesses are rare and usually occur via hematogenous spread. * **Why Option C is incorrect:** The drug of choice for Actinomycosis is **Penicillin G** (administered for a prolonged duration). Tetracycline is an alternative for penicillin-allergic patients but is not the primary choice. * **Why Option D is incorrect:** Actinomyces are **commensals** of the oral cavity, gastrointestinal tract, and female genital tract. Infection is endogenous, occurring when the mucosal barrier is breached (e.g., trauma or surgery), not via inhalation. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** These are characteristic yellowish colonies found in abscess pus; they are not actually made of sulfur but are masses of filamentous bacteria. * **Pelvic Actinomycosis:** Strongly associated with the long-term use of **Intrauterine Contraceptive Devices (IUCDs)**. * **Mnemonic:** "SNAP" — **S**ulfa for **N**ocardia, **A**ctinomyces gets **P**enicillin.

Q: A patient presents with sulphur granules discharging from a sinus. This finding is suggestive of infection with which of the following?

Mycetoma. **Explanation:** The presence of **sulphur granules** in a discharging sinus is a classic clinical hallmark of **Mycetoma** (specifically Actinomycetoma). These granules are not actually composed of sulphur; they are organized micro-colonies of the causative bacteria (like *Nocardia* or *Actinomadura*) or fungi (Eumycetoma) embedded in a matrix of calcium phosphate and host tissue. **Why Mycetoma is correct:** Mycetoma is a chronic granulomatous infection of the subcutaneous tissue, typically involving the foot (Madura foot). It is characterized by a triad of: 1. Localized swelling (tumefaction) 2. Multiple interconnecting sinus tracts 3. Discharge containing macroscopic grains (sulphur granules) **Why other options are incorrect:** * **Staphylococcus:** Typically causes acute pyogenic infections (abscesses, boils) with creamy yellow pus, but does not produce organized granules or chronic sinus tracts. * **Haemophilus ducreyi:** Causes **Chancroid**, characterized by painful genital ulcers and inguinal lymphadenopathy (buboes), not chronic discharging sinuses with granules. * **Sporotrichosis:** Known as "Rose gardener’s disease," it typically presents with nodular lesions following a linear lymphatic distribution (sporotrichoid spread). It does not produce sulphur granules. **High-Yield Clinical Pearls for NEET-PG:** * **Actinomycosis vs. Mycetoma:** While both produce sulphur granules, *Actinomyces israelii* (anaerobic) usually causes cervicofacial "lumpy jaw," whereas Mycetoma (aerobic) typically affects the lower extremities. * **Granule Color:** In Mycetoma, the color of the granule can hint at the etiology: * **Yellow/White:** *Nocardia* or *Actinomadura madurae*. * **Black:** Fungal (Eumycetoma) like *Madurella mycetomatis*. * **Red:** *Actinomadura pelletieri*. * **Diagnosis:** Crushing the granule and performing a Gram stain or KOH mount is the initial diagnostic step.

Q: Which drug resistance differentiates non-fermenting bacteria such as Pseudomonas and Burkholderia?

Resistance to polymyxin B. **Explanation:** The differentiation between non-fermenting Gram-negative bacilli often relies on their intrinsic antimicrobial susceptibility patterns. The correct answer is **Resistance to polymyxin B**. **1. Why Polymyxin B is the correct answer:** * ***Pseudomonas aeruginosa*** is typically **sensitive** to Polymyxins (Polymyxin B and Colistin). These drugs act by disrupting the bacterial outer membrane. * ***Burkholderia cepacia*** complex is **intrinsically resistant** to Polymyxins. This resistance is due to a modified lipopolysaccharide (LPS) in their cell wall that contains 4-amino-4-deoxy-L-arabinose, which reduces the binding affinity of the cationic polymyxin molecules. This biochemical marker is a classic laboratory differentiator between the two genera. **2. Why other options are incorrect:** * **Ampicillin:** Most non-fermenters, including both *Pseudomonas* and *Burkholderia*, are intrinsically resistant to ampicillin due to beta-lactamase production or efflux pumps. It does not help in differentiation. * **Ceftazidime:** This is a third-generation cephalosporin often used to treat *both* infections. While resistance can be acquired, it is not a primary differentiating feature. * **Ciprofloxacin:** Both organisms can show varying degrees of sensitivity or acquired resistance to fluoroquinolones; it is not a reliable taxonomic marker. **Clinical Pearls for NEET-PG:** * **Burkholderia cepacia:** Highly associated with "Cepacia syndrome" (rapid pulmonary decline) in **Cystic Fibrosis** patients. * **Drug of Choice for Burkholderia:** Trimethoprim-sulfamethoxazole (Co-trimoxazole) is the preferred treatment, unlike *Pseudomonas* where aminoglycosides or antipseudomonal penicillins are used. * **Other Polymyxin-resistant non-fermenters:** *Burkholderia pseudomallei* (Melioidosis) and *Proteus* species (though *Proteus* is a fermenter).

Q: Mycoplasma is intrinsically resistant to which class of antibiotics?

Ceftazidime. **Explanation:** The correct answer is **Ceftazidime**. **1. Why Ceftazidime is correct:** *Mycoplasma pneumoniae* is unique among bacteria because it **lacks a peptidoglycan cell wall**; it is bounded only by a cholesterol-containing cell membrane. **Ceftazidime** is a third-generation cephalosporin (Beta-lactam), a class of antibiotics that works exclusively by inhibiting cell wall synthesis (targeting Penicillin-Binding Proteins). Since Mycoplasma has no cell wall to target, it is **intrinsically resistant** to all Beta-lactams, including penicillins, cephalosporins, and carbapenems. **2. Why the other options are incorrect:** * **A. Aminoglycosides:** These inhibit the 30S ribosomal subunit. While not the first-line treatment for Mycoplasma, they possess some in-vitro activity and do not face intrinsic resistance based on the cell wall structure. * **C. Macrolides (e.g., Azithromycin):** These inhibit the 50S ribosomal subunit. They are the **drugs of choice** for Mycoplasma infections because they target protein synthesis, which is independent of the cell wall. * **D. Doxycycline (Tetracyclines):** These inhibit the 30S ribosomal subunit. They are effective alternatives for treating Mycoplasma, especially in adults. **NEET-PG High-Yield Pearls:** * **Cell Wall Deficient:** Mycoplasma is the smallest free-living organism and does not stain on Gram stain (requires Giemsa or Silver stain). * **Culture:** Shows a characteristic **"Fried Egg" appearance** on Eaton’s agar. * **Clinical:** It is a common cause of **Atypical Pneumonia** ("Walking Pneumonia") and is associated with **Cold Agglutinins** (IgM antibodies). * **Resistance Rule:** Any bacteria lacking a cell wall (e.g., *Ureaplasma*, *Mycoplasma*) is inherently resistant to all Cell Wall Synthesis Inhibitors (Vancomycin, Bacitracin, and Beta-lactams).

Q: What is the pathogenesis of diphtheria attributed to?

Exotoxin. **Explanation:** The pathogenesis of Diphtheria, caused by *Corynebacterium diphtheriae*, is primarily mediated by a potent **Exotoxin**. **Why Exotoxin is correct:** The diphtheria toxin is an **A-B type exotoxin** encoded by the *tox* gene, which is introduced into the bacterium by a lysogenic bacteriophage (Beta-phage). * **Fragment B** (Binding) attaches to the heparin-binding epidermal growth factor receptor on host cells. * **Fragment A** (Active) enters the cytosol and catalyzes the ADP-ribosylation of **Elongation Factor-2 (EF-2)**. This inhibits protein synthesis, leading to cell death and the formation of the characteristic greyish-white "pseudomembrane" in the pharynx. **Why other options are incorrect:** * **Invasion:** *C. diphtheriae* is non-invasive. It remains localized to the mucosal surface of the upper respiratory tract; the systemic manifestations (myocarditis, polyneuritis) are due to the hematogenous spread of the toxin, not the bacteria. * **Endotoxin:** Endotoxins are components of the cell wall of Gram-negative bacteria (LPS). *C. diphtheriae* is a Gram-positive bacillus and does not produce endotoxin. * **Capsule:** While some bacteria use capsules to evade phagocytosis (e.g., *S. pneumoniae*), *C. diphtheriae* is non-capsulated. **High-Yield NEET-PG Pearls:** * **Schick Test:** Used to demonstrate the immune status of an individual against diphtheria. * **Elek’s Gel Precipitation Test:** The gold standard *in vitro* test to detect the toxigenicity of the strain. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Morphology:** Chinese-letter arrangement (cuneiform) with metachromatic granules (Volutin/Babes-Ernst granules) seen on Albert’s stain.

Q: What is the mechanism of action of tetanospasmin?

Inhibition of GABA release. ### **Explanation** **Mechanism of Action (Correct Answer: A)** Tetanospasmin is a potent neurotoxin produced by *Clostridium tetani*. It is an A-B toxin that acts by cleaving **synaptobrevin** (a SNARE protein). The toxin undergoes retrograde axonal transport to reach the cell bodies of inhibitory interneurons (Renshaw cells) in the spinal cord. Here, it prevents the exocytosis of inhibitory neurotransmitters, primarily **GABA (Gamma-Aminobutyric Acid)** and **Glycine**. The loss of central inhibition leads to unchecked excitatory nerve impulses, resulting in the characteristic spastic paralysis and muscle spasms of tetanus. **Analysis of Incorrect Options:** * **B. Inhibition of cAMP:** This is not associated with tetanus. Conversely, toxins like *Vibrio cholerae* (Cholera toxin) and *ETEC* (LT) act by **increasing** cAMP levels. * **C. Inactivation of Ach receptors:** This describes the mechanism of **Myasthenia Gravis** (autoantibodies against receptors). In contrast, *Clostridium botulinum* toxin inhibits the **release** of Acetylcholine (Ach) at the neuromuscular junction, leading to flaccid paralysis. * **D. Inhibition of cGMP:** This is incorrect. Toxins like *ETEC* (ST) act by increasing cGMP levels. **High-Yield Clinical Pearls for NEET-PG:** * **Retrograde Transport:** Tetanospasmin travels from the wound site to the CNS via dynein-mediated retrograde axonal transport. * **Clinical Triad:** Trismus (lockjaw), Risus sardonicus (grimace), and Opisthotonus (archback). * **Botulinum vs. Tetanus:** Botulinum toxin causes **flaccid** paralysis (peripheral action); Tetanospasmin causes **spastic** paralysis (central action). * **Management:** Treatment involves wound debridement, Metronidazole (preferred over Penicillin), and Human Tetanus Immunoglobulin (TIG) to neutralize unbound toxin.

Q: What is the antibiotic of choice for Stenotrophomonas maltophilia infection?

Trimethoprim-sulfamethoxazole (TMP-SMX). **Explanation:** *Stenotrophomonas maltophilia* is a non-fermenting, Gram-negative bacillus increasingly associated with nosocomial infections, particularly in immunocompromised patients and those on long-term mechanical ventilation. **1. Why Trimethoprim-sulfamethoxazole (TMP-SMX) is correct:** TMP-SMX is the **drug of choice** because *S. maltophilia* is inherently resistant to a vast majority of broad-spectrum antibiotics, including most carbapenems. TMP-SMX demonstrates the most consistent in-vitro activity and clinical efficacy against this pathogen. It works by inhibiting sequential steps in the bacterial folic acid synthesis pathway. **2. Why the other options are incorrect:** * **Ampicillin & Penicillin (Options A & C):** *S. maltophilia* is intrinsically resistant to almost all beta-lactams. It produces two potent chromosomal beta-lactamases: **L1** (a metallo-beta-lactamase that hydrolyzes carbapenems) and **L2** (a serine cephalosporinase). * **Ciprofloxacin (Option D):** While some strains may show susceptibility to fluoroquinolones (like Levofloxacin), resistance develops rapidly during monotherapy. It is not considered the primary treatment of choice compared to TMP-SMX. **Clinical Pearls for NEET-PG:** * **Carbapenem Resistance:** A high-yield fact is that *S. maltophilia* is **intrinsically resistant to Imipenem/Meropenem** due to the L1 metallo-beta-lactamase. * **Risk Factors:** Prior use of broad-spectrum antibiotics (especially carbapenems) and presence of indwelling devices (CVCs). * **Alternative Therapy:** For patients allergic to sulfa drugs, **Levofloxacin**, **Minocycline**, or **Ceftazidime-avibactam** are considered alternative options. * **Culture Characteristics:** It is oxidase-negative and catalase-positive.

Q: In mycetoma, which organism produces pink to red granules?

Actinomadura pelletri. **Explanation:** Mycetoma is a chronic, granulomatous infection of the subcutaneous tissue characterized by a triad of localized swelling, underlying bone destruction, and the discharge of grains (granules) through sinus tracts. The **color and consistency of these granules** are high-yield diagnostic clues for identifying the causative agent. **Why Actinomadura pelletieri is correct:** * **Actinomadura pelletieri** is an aerobic actinomycete (Actinomycetoma) known for producing characteristic **small, firm, pink to red granules**. This is a classic "spotter" for NEET-PG. **Analysis of Incorrect Options:** * **Madurella (e.g., M. mycetomatis):** This is the most common cause of Eumycetoma (fungal). It produces **black granules** (Madura foot). * **Aspergillus:** While it can cause opportunistic infections, it is an uncommon cause of mycetoma. When it does occur, granules are typically **white to yellowish**. * **Actinomadura madurae:** This actinomycete produces **large, cream-colored to white/yellowish granules**, not red. **High-Yield Clinical Pearls for NEET-PG:** 1. **Granule Color Coding:** * **Red:** *Actinomadura pelletieri*. * **Black:** *Madurella mycetomatis*, *Exophiala jeanselmei*. * **Yellow/White:** *Nocardia brasiliensis*, *Actinomadura madurae*, *Streptomyces somaliensis*. 2. **Actinomycetoma vs. Eumycetoma:** Actinomycetoma (bacterial) usually has a more rapid progression and responds to antibiotics (Welsh regimen), whereas Eumycetoma (fungal) is slower and requires surgical debridement plus antifungals. 3. **Diagnosis:** The most definitive diagnosis is made by histopathological examination of the granules using **Gomori Methenamine Silver (GMS)** or **PAS stains**.

Question 1

A 50-year-old man presents with multiple abscesses and discharging sinuses on his left foot. X-ray of the foot shows destructive changes in the underlying bone. Which of the following is the most likely causative organism?

Accepted Answer

Nocardia

Answer

Sporothrix schenkii

Answer

Histoplasma capsulatum

Answer

Cryptococcus neoformans

Question 2

A 32-year-old woman presents to the emergency department with severe, watery diarrhea, vomiting, and intense thirst. She also experiences oliguria, muscle cramps, and weakness. Physical examination reveals marked loss of tissue turgor, sunken eyes, and wrinkling of the skin of her fingers. Laboratory studies show hemoconcentration and severe metabolic acidosis with potassium depletion. An organism cultured from the patient's stool is characterized as an oxidase-positive, gram-negative curved rod with polar flagella that shows "shooting star" mobility and grows best on TCBS medium. Which of the following is the most likely pathogen?

Accepted Answer

Vibrio cholerae

Answer

Pseudomonas

Answer

Salmonella

Answer

Shigella

Question 3

Which of the following is true about Actinomyces?

Accepted Answer

Gram-positive

Answer

Most common site is brain

Answer

Tetracycline is the drug of choice

Answer

Portal of entry is inhalation

Question 4

A patient presents with sulphur granules discharging from a sinus. This finding is suggestive of infection with which of the following?

Accepted Answer

Mycetoma

Answer

Staphylococcus

Answer

Haemophilus ducreyi

Answer

Sporotrichosis

Question 5

Which drug resistance differentiates non-fermenting bacteria such as Pseudomonas and Burkholderia?

Accepted Answer

Resistance to polymyxin B

Answer

Resistance to ampicillin

Answer

Resistance to ceftazidime

Answer

Resistance to ciprofloxacin

Question 6

Mycoplasma is intrinsically resistant to which class of antibiotics?

Accepted Answer

Ceftazidime

Answer

Aminoglycosides

Answer

Macrolides

Answer

Doxycycline

Question 7

What is the pathogenesis of diphtheria attributed to?

Accepted Answer

Exotoxin

Answer

Invasion

Answer

Endotoxin

Answer

Capsule

Question 8

What is the mechanism of action of tetanospasmin?

Accepted Answer

Inhibition of GABA release

Answer

Inhibition of cAMP

Answer

Inactivation of Ach receptors

Answer

Inhibition of cGMP

Question 9

What is the antibiotic of choice for Stenotrophomonas maltophilia infection?

Accepted Answer

Trimethoprim-sulfamethoxazole (TMP-SMX)

Answer

Ampicillin

Answer

Penicillin

Answer

Ciprofloxacin

Question 10

In mycetoma, which organism produces pink to red granules?

Accepted Answer

Actinomadura pelletri

Answer

Madurella

Answer

Aspergillus

Answer

Actinumadura madurae

Bacteriology — MCQs

Bacteriology — MCQs

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