Bacteriology Practice Questions

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**.

Q: A patient with conjunctival infection, which led to corneal perforation, was positive for Gram-negative coccoid appearance on Gram stain. Further investigation showed small translucent colonies, which are oxidase positive. What is the most probable causative organism?

Neisseria gonorrhoeae. ### Explanation The clinical presentation of a severe conjunctival infection leading to **corneal perforation** is a hallmark of **Hyperacute Bacterial Conjunctivitis**, most commonly caused by *Neisseria gonorrhoeae*. **Why the correct answer is right:** * **Microscopy:** *Neisseria gonorrhoeae* are Gram-negative, kidney-shaped diplococci (coccoid appearance). * **Biochemical Profile:** They are **Oxidase positive** and Catalase positive. * **Culture:** They produce small, translucent, or grayish-white colonies on enriched media like Chocolate agar or Thayer-Martin medium. * **Pathogenesis:** Unlike most bacteria, *N. gonorrhoeae* can penetrate an **intact corneal epithelium**, leading to rapid ulceration and perforation if not treated aggressively. **Analysis of Incorrect Options:** * **Moraxella catarrhalis:** While it is a Gram-negative coccus and oxidase-positive, it typically causes respiratory infections or otitis media. In the eye, it causes milder angular blepharoconjunctivitis, not rapid corneal perforation. * **Pseudomonas aeruginosa:** A common cause of aggressive corneal ulcers (especially in contact lens users), but it is a **Gram-negative rod**, not a coccus. * **Acinetobacter species:** These are Gram-negative coccobacilli and are notably **Oxidase negative**, which contradicts the findings in the question. **High-Yield NEET-PG Pearls:** * **Organisms that penetrate intact cornea:** *N. gonorrhoeae, N. meningitidis, Corynebacterium diphtheriae,* and *Listeria monocytogenes*. * **Ophthalmia Neonatorum:** *N. gonorrhoeae* presents within 2–5 days of birth (hyperacute); *Chlamydia trachomatis* presents later (5–14 days). * **Drug of Choice:** Systemic Ceftriaxone is mandatory for gonococcal conjunctivitis due to the risk of globe perforation and systemic spread.

Q: Asymptomatic gonorrhoea in females is typically due to infections of which site?

Endocervix. **Explanation:** The **endocervix** is the primary site of infection for *Neisseria gonorrhoeae* in females. This is because the organism has a specific predilection for **columnar epithelium**, which lines the endocervical canal. In approximately 50–70% of cases, endocervical gonorrhea remains **asymptomatic**, acting as a major reservoir for transmission and increasing the risk of unrecognized progression to Pelvic Inflammatory Disease (PID). **Analysis of Options:** * **Vagina (Incorrect):** The adult vagina is lined by stratified squamous epithelium, which is resistant to gonococcal infection. Vaginal involvement (vulvovaginitis) is typically only seen in prepubertal girls where the epithelium is thin. * **Urethra (Incorrect):** While the urethra is frequently co-infected (causing dysuria), it is not the primary site of asymptomatic colonization in females. * **Fundus (Incorrect):** The fundus refers to the upper part of the uterus. While infection can ascend to the endometrium, the primary and most common site of initial infection is the cervix. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin). * **Most Sensitive Test:** Nucleic Acid Amplification Test (NAAT). * **Microscopy:** Gram-negative kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Fitz-Hugh-Curtis Syndrome:** A complication of gonococcal PID involving peri-hepatitis ("violin-string" adhesions). * **Treatment:** Current CDC/WHO guidelines recommend Ceftriaxone (IM) as the mainstay of therapy.

Q: What is true about Mycoplasma?

Requires cholesterol for growth. **Explanation:** **Mycoplasma** are the smallest free-living organisms. The defining characteristic of Mycoplasma is the **complete absence of a cell wall**. 1. **Why Option C is Correct:** Since Mycoplasma lack a cell wall, their cell membrane is their only protective barrier. To provide structural integrity and stability to this membrane, they incorporate **sterols (cholesterol)**. Unlike most bacteria, Mycoplasma cannot synthesize sterols and must acquire them from the external environment (e.g., PPLO agar supplemented with horse serum). 2. **Why Other Options are Incorrect:** * **Option A:** Mycoplasma are **extracellular** pathogens (except *M. penetrans*). They typically adhere to the respiratory or urogenital epithelium. * **Option B:** Penicillin acts by inhibiting cell wall synthesis (peptidoglycan). Because Mycoplasma **lack a cell wall**, they are **innately resistant** to all beta-lactam antibiotics (Penicillins, Cephalosporins). * **Option D:** As mentioned, they have **no cell wall**, making them highly pleomorphic (able to take many shapes) and resistant to Gram staining. **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** They produce characteristic **"Fried Egg" colonies** on agar. * **Diagnosis:** *M. pneumoniae* causes **Atypical Pneumonia** (Walking Pneumonia). It is associated with **Cold Agglutinins** (anti-I antibodies) which can cause autoimmune hemolytic anemia. * **Treatment:** Drug of choice is **Macrolides** (Azithromycin) or Tetracyclines (Doxycycline). * **Staining:** They do not Gram stain; **Giemsa or Diene’s stain** is used instead.

Question 1

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 2

Mycoplasma is intrinsically resistant to which class of antibiotics?

Accepted Answer

Ceftazidime

Answer

Aminoglycosides

Answer

Macrolides

Answer

Doxycycline

Question 3

What is the pathogenesis of diphtheria attributed to?

Accepted Answer

Exotoxin

Answer

Invasion

Answer

Endotoxin

Answer

Capsule

Question 4

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 5

What is the antibiotic of choice for Stenotrophomonas maltophilia infection?

Accepted Answer

Trimethoprim-sulfamethoxazole (TMP-SMX)

Answer

Ampicillin

Answer

Penicillin

Answer

Ciprofloxacin

Question 6

In mycetoma, which organism produces pink to red granules?

Accepted Answer

Actinomadura pelletri

Answer

Madurella

Answer

Aspergillus

Answer

Actinumadura madurae

Question 7

A patient with conjunctival infection, which led to corneal perforation, was positive for Gram-negative coccoid appearance on Gram stain. Further investigation showed small translucent colonies, which are oxidase positive. What is the most probable causative organism?

Accepted Answer

Neisseria gonorrhoeae

Answer

Moraxella catarrhalis

Answer

Pseudomonas aeruginosa

Answer

Acinetobacter actinatus

Question 8

Donovan bodies are best described as:

Accepted Answer

Monocytes that contain the organism, Calymmatobacterium granulomatis

Answer

Leukocytes that contain the organism, Calymmatobacterium granulomatis

Answer

Eosinophils that contain the organism, Calymmatobacterium granulomatis

Answer

Monocytes that contain the organism, Haemophilus influenza

Question 9

Asymptomatic gonorrhoea in females is typically due to infections of which site?

Accepted Answer

Endocervix

Answer

Vagina

Answer

Urethra

Answer

Fundus

Question 10

What is true about Mycoplasma?

Accepted Answer

Requires cholesterol for growth

Answer

Obligate intracellular organism

Answer

Penicillin is an effective treatment

Answer

Has a thick cell wall

Bacteriology — MCQs

Bacteriology — MCQs

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