Vancomycin-Resistant Enterococci — MCQs
An otherwise healthy patient who has just received a prosthetic aortic valve develops postoperative fever. Blood cultures are done and she is placed on broad-spectrum antibiotics. Two days later she is still febrile and clinically deteriorating. Which of the following organisms is the most probable etiologic agent?
What is the therapy of choice for pseudomembranous enterocolitis?
Cholera is caused by?
Vertical transmission of resistance to ciprofloxacin occurs :-
What is the mechanism of resistance in MRSA?
What is the treatment of choice for a gonococcal infection that is resistant to penicillin?
A patient admitted to an ICU is on a central venous line for the last one week. He is on ceftazidime and amikacin. After 7 days of antibiotics, he develops a spike of fever, and his blood culture is positive for gram-positive cocci in chains, which are catalase negative. Following this, vancomycin was started, but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing the infection is:
Transfer of drug resistance in *Staphylococcus* is by:
Which antimicrobial resistance mechanism is most commonly associated with extended-spectrum cephalosporin resistance in Neisseria gonorrhoeae?
Organism showing marked resistance to multidrug therapy -
Vancomycin-Resistant Enterococci Indian Medical PG Practice Questions and MCQs
Question 1: An otherwise healthy patient who has just received a prosthetic aortic valve develops postoperative fever. Blood cultures are done and she is placed on broad-spectrum antibiotics. Two days later she is still febrile and clinically deteriorating. Which of the following organisms is the most probable etiologic agent?
- A. Nocardia asteroides
- B. Actinomyces israelii
- C. Histoplasma capsulatum
- D. Candida albicans (Correct Answer)
Explanation: ***Candida albicans*** - *Candida albicans* is a common cause of **early prosthetic valve endocarditis**, especially in patients with prolonged hospitalization, broad-spectrum antibiotic use, or central venous catheters [1]. - The patient's clinical deterioration despite broad-spectrum antibacterial antibiotics suggests a **fungal etiology**, as bacteria are typically covered by such therapy [2]. *Nocardia asteroides* - *Nocardia asteroides* typically causes **pulmonary or cutaneous infections** in immunocompromised individuals, less commonly endocarditis. - It would usually respond to specific antibiotics like **trimethoprim-sulfamethoxazole**, and its presentation as prosthetic valve endocarditis is rare. *Actinomyces israelii* - *Actinomyces israelii* causes **actinomycosis**, characterized by chronic, slowly progressive infections with abscesses and draining sinuses, often following oral or abdominal trauma. - While it can cause endocarditis, it is less common in the context of acute prosthetic valve infection and would likely present with a more indolent course. *Histoplasma capsulatum* - *Histoplasma capsulatum* is a **dimorphic fungus** endemic to the Ohio and Mississippi River valleys, primarily causing pulmonary infections. - Disseminated histoplasmosis with endocarditis is rare and typically occurs in immunocompromised patients, not usually in an otherwise healthy individual post-surgery.
Question 2: What is the therapy of choice for pseudomembranous enterocolitis?
- A. Penicillin
- B. Ampicillin
- C. Erythromycin
- D. Vancomycin (Correct Answer)
Explanation: ***Vancomycin*** - **Oral vancomycin** is indicated for pseudomembranous enterocolitis, particularly for severe or recurrent cases, as it achieves high luminal concentrations in the colon to target *C. difficile* [1]. - Vancomycin works by inhibiting **bacterial cell wall synthesis**, effectively eradicating the toxigenic *C. difficile* strains responsible for the condition [1]. *Penicillin* - **Penicillin** is ineffective against *C. difficile* because *C. difficile* is a Gram-positive anaerobic bacterium producing toxins, and penicillin does not have the appropriate spectrum of activity. - In fact, many cases of pseudomembranous enterocolitis are triggered by prior **antibiotic use**, including penicillins, which disrupt the normal gut flora [2]. *Ampicillin* - Similar to penicillin, **ampicillin** is a broad-spectrum penicillin derivative and is not considered a treatment for *C. difficile* infection [3]. - Ampicillin can commonly be one of the **antibiotics that precipitates** the development of pseudomembranous enterocolitis by altering the normal gut microbiota [2]. *Erythromycin* - **Erythromycin**, a macrolide antibiotic, is not effective against *C. difficile* and is not used in the treatment of pseudomembranous enterocolitis. - Like other broad-spectrum antibiotics, erythromycin can **disrupt the normal gut flora**, potentially contributing to the overgrowth of *C. difficile* [2].
Question 3: Cholera is caused by?
- A. Enterococcus
- B. E.coli
- C. Vibrio cholerae O1 (Correct Answer)
- D. Vibrio parahemolyticus
Explanation: ***Vibrio cholerae O1*** - **Cholera** is primarily caused by **toxigenic strains** of *Vibrio cholerae*, with serogroup **O1** being the most common and historically significant cause of epidemics and pandemics. - This bacterium produces **cholera toxin**, which leads to profuse watery diarrhea by acting on the intestinal epithelial cells. *Enterococcus* - **Enterococcus** species are common inhabitants of the human gastrointestinal tract and are often associated with **nosocomial infections**, such as urinary tract infections, endocarditis, and bacteremia. - They do not typically cause the severe, watery diarrhea characteristic of cholera. *Vibrio parahemolyticus* - *Vibrio parahemolyticus* is a common cause of **foodborne illness**, particularly associated with consuming **raw or undercooked seafood**. - It causes a form of gastroenteritis characterized by abdominal pain, watery diarrhea, nausea, vomiting, and fever, but it is distinct from cholera. *E.coli* - While various strains of **Escherichia coli (E. coli)** can cause diarrheal diseases (e.g., enterotoxigenic *E. coli* causing traveler's diarrhea, enterohemorrhagic *E. coli* causing bloody diarrhea), **E. coli** is not the causative agent of cholera. - The pathogenesis and specific toxins produced differ significantly from those of *Vibrio cholerae*.
Question 4: Vertical transmission of resistance to ciprofloxacin occurs :-
- A. Transduction
- B. Mutation (Correct Answer)
- C. Conjugation
- D. Transformation
Explanation: ***Mutation*** - Vertical transmission of resistance occurs when a bacterium with a **resistance gene**, such as one conferring resistance to **ciprofloxacin**, replicates and passes that gene to its progeny. - This typically happens through **spontaneous mutations** in the bacterial genome that alter the drug's target or uptake, and these mutations are then inherited by subsequent generations. *Transduction* - **Transduction** involves the transfer of genetic material, including resistance genes, via **bacteriophages** (viruses that infect bacteria). - This is a **horizontal gene transfer** mechanism, not a vertical one. *Conjugation* - **Conjugation** is the direct transfer of genetic material, often in the form of **plasmids**, from one bacterium to another through direct cell-to-cell contact. - This is a form of **horizontal gene transfer**, not vertical transmission. *Transformation* - **Transformation** is the uptake of **free DNA** from the environment by a bacterium, which can then integrate this DNA into its own genome. - This mechanism is also a type of **horizontal gene transfer**, not vertical transmission of resistance to daughter cells during replication.
Question 5: What is the mechanism of resistance in MRSA?
- A. PBP2a alteration (Correct Answer)
- B. Efflux pump activation
- C. Porins modification
- D. Beta-lactamase production
Explanation: ***PBP2a alteration*** - Methicillin-resistant Staphylococcus aureus (MRSA) acquires the **mecA gene**, which encodes for a modified penicillin-binding protein, **PBP2a**. - **PBP2a** has a low affinity for **beta-lactam antibiotics**, allowing the bacteria to synthesize its cell wall even in the presence of these drugs. *Efflux pump activation* - Efflux pumps are mechanisms used by bacteria to actively pump out various antibiotics from their cells, leading to resistance. - While efflux pumps contribute to resistance against other antibiotics, they are **not the primary mechanism** of methicillin resistance in MRSA. *Porins modification* - Porins are channels in the outer membrane of Gram-negative bacteria that allow the passage of hydrophilic molecules, including some antibiotics. - Modification of porins is a common resistance mechanism in **Gram-negative bacteria** but is not relevant to MRSA, which is Gram-positive. *Beta-lactamase production* - Beta-lactamases are enzymes that **hydrolyze the beta-lactam ring** of antibiotics like penicillin, rendering them inactive. - While many Staphylococcus aureus strains produce beta-lactamase (penicillinase) causing resistance to penicillins, MRSA's resistance to methicillin and other broader-spectrum beta-lactams is primarily due to **PBP2a alteration**, not just beta-lactamase production.
Question 6: What is the treatment of choice for a gonococcal infection that is resistant to penicillin?
- A. Penicillin
- B. Ceftazidime
- C. Ceftriaxone (Correct Answer)
- D. Spectinomycin
Explanation: ***Ceftriaxone*** - **Ceftriaxone** is the recommended first-line treatment for uncomplicated gonococcal infections, especially given increasing resistance to other antibiotics like penicillin. - It is a **third-generation cephalosporin** that targets the bacterial cell wall synthesis effectively. *Penicillin* - **Penicillin** is no longer the recommended treatment for gonorrhea due to widespread resistance, primarily mediated by **beta-lactamase production** by *Neisseria gonorrhoeae*. - Treating with penicillin when resistance is present would lead to treatment failure and continued transmission. *Ceftazidime* - **Ceftazidime** is a third-generation cephalosporin with activity against gram-negative organisms. - However, its activity against *Neisseria gonorrhoeae* is not considered first-line or superior to ceftriaxone, which has better pharmacokinetic properties for treating gonorrhea. *Spectinomycin* - **Spectinomycin** is an alternative treatment for gonococcal infections, particularly in individuals with severe allergy to cephalosporins or in regions where ceftriaxone resistance is emerging. - However, it is not the treatment of choice in regions where ceftriaxone is effective and available.
Question 7: A patient admitted to an ICU is on a central venous line for the last one week. He is on ceftazidime and amikacin. After 7 days of antibiotics, he develops a spike of fever, and his blood culture is positive for gram-positive cocci in chains, which are catalase negative. Following this, vancomycin was started, but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing the infection is:
- A. Staphylococcus aureus
- B. Viridans streptococci
- C. Enterococcus faecalis (Correct Answer)
- D. Coagulase negative Staphylococcus
Explanation: ***Enterococcus faecalis*** - The organism is a **gram-positive cocci in chains** and is **catalase negative**, which is consistent with *Enterococcus*. - **Vancomycin resistance** in *Enterococcus* (VRE) is a significant clinical problem, explaining the persistent positive culture despite vancomycin therapy. *Staphylococcus aureus* - *Staphylococcus aureus* is a **catalase-positive** organism, which contradicts the patient's culture results. - While it can be vancomycin-resistant (VRSA), the initial catalase test rules it out. *Viridans streptococci* - *Viridans streptococci* are typically **susceptible to vancomycin**, making a persistent positive culture after 2 weeks of therapy unlikely unless there's a serious underlying issue like endocarditis with large vegetations or an undrained abscess. - They are also **catalase-negative**, but the vancomycin resistance points away from this option. *Coagulase negative Staphylococcus* - **Coagulase-negative Staphylococci** (e.g., *Staphylococcus epidermidis*) are **catalase-positive**, which is inconsistent with the patient's culture results. - While they can cause central line infections and be vancomycin-resistant, the catalase test rules out this group.
Question 8: Transfer of drug resistance in *Staphylococcus* is by:
- A. Conjugation
- B. Transfection
- C. Transformation
- D. Transduction (Correct Answer)
Explanation: ***Transduction*** - **Transduction** is a common mechanism for the transfer of antibiotic resistance genes in *Staphylococcus aureus*, particularly for methicillin resistance (*mecA* gene). - This process involves **bacteriophages (viruses)** infecting bacterial cells and transferring genetic material, including resistance genes, from one bacterium to another. *Conjugation* - **Conjugation** involves direct cell-to-cell contact between bacteria, typically through a **pilus**, to transfer plasmids containing resistance genes. - While conjugation can occur in staphylococci, it is less common for widespread drug resistance dissemination than transduction, especially for *mecA*. *Transfection* - **Transfection** is the process of introducing **foreign nucleic acids (DNA or RNA)** into eukaryotic cells, often used in molecular biology research. - This term is not typically used to describe natural gene transfer mechanisms between bacteria. *Transformation* - **Transformation** involves the uptake of **naked DNA** from the environment by a bacterial cell. - While *Staphylococcus aureus* can undergo transformation, it is generally less efficient and frequent than transduction for the acquisition of significant resistance traits, especially in clinical settings.
Question 9: Which antimicrobial resistance mechanism is most commonly associated with extended-spectrum cephalosporin resistance in Neisseria gonorrhoeae?
- A. PenA mosaic alleles (Correct Answer)
- B. mtrR promoter mutation
- C. 23S rRNA methylation
- D. TetM plasmid
Explanation: ***PenA mosaic alleles*** - **PenA mosaic alleles** are altered forms of the gene encoding **penicillin-binding protein 2 (PBP2)**, which is the primary target of cephalosporin antibiotics in *Neisseria gonorrhoeae*. These mosaic alleles result from recombination events with homologous genes from commensal *Neisseria* species. - The altered PBP2 has **reduced affinity for cephalosporins**, making the bacteria resistant to this class of antibiotics, including extended-spectrum cephalosporins. *mtrR promoter mutation* - A **mutation in the *mtrR* promoter** typically leads to overexpression of the **MtrCDE efflux pump**, which pumps out various antimicrobial agents, including some macrolides, disinfectants, and bile salts. - While it contributes to multidrug resistance, its primary role is not in mediating high-level resistance to extended-spectrum cephalosporins in *N. gonorrhoeae*. *23S rRNA methylation* - **23S rRNA methylation** is a common mechanism of resistance to **macrolide antibiotics** (e.g., azithromycin), which bind to the 50S ribosomal subunit. - This mechanism interferes with macrolide binding to the ribosome, but it does not directly affect the activity of cephalosporins, which target bacterial cell wall synthesis. *TetM plasmid* - The **TetM plasmid** confers resistance to **tetracycline antibiotics** by protecting the bacterial ribosome from their action. TetM is a ribosomal protection protein. - This plasmid is a well-known mechanism of tetracycline resistance in many bacteria, including *N. gonorrhoeae*, but it is not involved in resistance to cephalosporins.
Question 10: Organism showing marked resistance to multidrug therapy -
- A. Haemophilus ducreyi
- B. Calymmatobacterium granulomatosis
- C. Treponema pallidum
- D. Gonococci (Correct Answer)
Explanation: ***Gonococci*** - **Gonococci (Neisseria gonorrhoeae)** increasingly show **resistance to multiple antibiotics**, including penicillin, tetracycline, macrolides, and some cephalosporins, making treatment challenging. - The Centers for Disease Control and Prevention (CDC) recommends **dual therapy with ceftriaxone and azithromycin** to overcome rising resistance. *Haemophilus ducreyi* - **Haemophilus ducreyi**, the causative agent of **chancroid**, is typically susceptible to macrolides and cephalosporins, with **less reported multidrug resistance** compared to gonococci. - Single-dose therapy with **azithromycin or ceftriaxone** is usually effective. *Calymmatobacterium granulomatosis* - Now known as **Klebsiella granulomatis**, this organism causes **donovanosis (granuloma inguinale)**, and it is generally sensitive to **doxycycline**, **azithromycin**, or ciprofloxacin. - While prolonged treatment may be needed, **widespread multidrug resistance** is not characteristic. *Treponema pallidum* - **Treponema pallidum**, which causes **syphilis**, remains exquisitely susceptible to **penicillin**, which is the gold standard treatment. - There is **no significant reported multidrug resistance** to penicillin, although macrolide resistance has emerged in some regions.
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