Methicillin-Resistant Staphylococcus aureus — MCQs
A patient develops an infection of methicillin resistant Staphylococcus aureus. All of the following can be used to treat this infection except
What is the best way to control the MRSA infection in the ward?
Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?
Which of the following drugs is not effective against methicillin-resistant Staphylococcus aureus (MRSA)?
Drug of choice in methicillin-resistant Staphylococcus aureus is:
The best way to prevent MRSA outbreak in indoor patients is:
Which of the following statements regarding resistance of penicillin in Staphylococcus aureus is false?
Staphylococcus Oxacillin resistance is best detected by?
The most common mechanism of resistance to drugs in Staphylococcus is:
Transfer of drug resistance in *Staphylococcus* is by:
Methicillin-Resistant Staphylococcus aureus Indian Medical PG Practice Questions and MCQs
Question 1: A patient develops an infection of methicillin resistant Staphylococcus aureus. All of the following can be used to treat this infection except
- A. Cotrimoxazole
- B. Ciprofloxacin
- C. Cefaclor (Correct Answer)
- D. Vancomycin
Explanation: ***Cefaclor*** - **Cefaclor** is a second-generation **cephalosporin**, which, like all beta-lactam antibiotics, is ineffective against **MRSA** because **MRSA** produces an altered penicillin-binding protein (PBP2a) encoded by the **mecA** gene. - This altered **PBP2a** has a low affinity for **beta-lactam antibiotics**, rendering them inactive. *Cotrimoxazole* - **Cotrimoxazole** (trimethoprim/sulfamethoxazole) is a commonly used and effective oral antibiotic for treating **MRSA** infections, particularly in outpatient settings. - It inhibits **folate synthesis** in bacteria, an essential pathway for their growth and replication. *Ciprofloxacin* - **Ciprofloxacin** is a **fluoroquinolone antibiotic** that can be used to treat certain **MRSA** infections, although resistance can be an issue. [2] - It works by inhibiting bacterial **DNA gyrase** and **topoisomerase IV**, crucial enzymes for DNA replication. [2] *Vancomycin* - **Vancomycin** is a **glycopeptide antibiotic** that is a first-line treatment for serious **MRSA** infections, especially in hospitalized patients. [1] - It works by inhibiting **bacterial cell wall synthesis** at a different site than beta-lactams, making it effective against **MRSA**. [1]
Question 2: What is the best way to control the MRSA infection in the ward?
- A. Fumigation of ward frequently
- B. Washing hand before and after attending patients (Correct Answer)
- C. Wearing masks during invasive procedures in ICU
- D. Vancomycin given empirically to all the patients
Explanation: **Washing hand before and after attending patients** - **Hand hygiene** is the single most effective measure in preventing the transmission of **healthcare-associated infections**, including **MRSA**. - **Healthcare workers' hands** are the primary vehicle for spreading pathogens from one patient to another. *Fumigation of ward frequently* - **Fumigation** is generally not recommended for routine infection control and has limited efficacy against resistant organisms like **MRSA** in this context. - It does not address the primary mode of transmission, which is direct contact via **contaminated hands** or surfaces. *Wearing masks during invasive procedures in ICU is important.* - While important for preventing infections during **invasive procedures** and protecting against **aerosolized pathogens**, masks are not the primary strategy for controlling the spread of **MRSA** in routine ward settings. - **MRSA transmission** is predominantly contact-based, not airborne. *Vancomycin given empirically to all the patients* - **Empirical broad-spectrum antibiotic use** for all patients is a significant driver of **antibiotic resistance**, including **MRSA**. - It should be reserved for patients with suspected or confirmed **MRSA infections** based on clinical criteria and culture results, not as a general preventive measure.
Question 3: Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?
- A. Burns
- B. Tension pneumothorax
- C. Osteomyelitis
- D. Acute cystitis (Correct Answer)
Explanation: ***Acute cystitis*** - **Staphylococcus saprophyticus** is a common cause of **urinary tract infections (UTIs)**, particularly acute cystitis, in young sexually active women. - This bacterium has a high affinity for **uroepithelial cells**, facilitating its colonization and subsequent infection of the bladder. *Tension pneumothorax* - A **tension pneumothorax** is a medical emergency characterized by air accumulation in the pleural space, leading to lung collapse and mediastinal shift. - It is typically caused by trauma or iatrogenic factors, not directly by bacterial infection from either *Staphylococcus saprophyticus* or *Staphylococcus aureus*. *Burns* - Burn wounds are highly susceptible to bacterial colonization and infection, with **Staphylococcus aureus** being a primary pathogen in this context. - *Staphylococcus saprophyticus* is rarely associated with burn wound infections. *Osteomyelitis* - **Osteomyelitis**, an infection of the bone, is most frequently caused by **Staphylococcus aureus** via hematogenous spread or direct inoculation. - *Staphylococcus saprophyticus* is not a common pathogen in osteomyelitis.
Question 4: Which of the following drugs is not effective against methicillin-resistant Staphylococcus aureus (MRSA)?
- A. Ciprofloxacin
- B. Cefaclor (Correct Answer)
- C. Cotrimoxazole
- D. Vancomycin
Explanation: ***Cefaclor*** - **Cefaclor** is a second-generation cephalosporin, and like most beta-lactam antibiotics, it is **ineffective against MRSA** due to the presence of the **mecA gene** which codes for an altered penicillin-binding protein (PBP2a). - MRSA's resistance mechanism renders beta-lactams, including cefaclor, unable to bind effectively to their target, thus inhibiting their antibacterial action. *Cotrimoxazole* - **Cotrimoxazole** (trimethoprim-sulfamethoxazole) is often effective against MRSA, especially for **skin and soft tissue infections**, making it a common choice for outpatient treatment. - It works by inhibiting two consecutive steps in the bacterial **folic acid synthesis pathway**, preventing nucleic acid and protein synthesis. *Ciprofloxacin* - **Ciprofloxacin**, a fluoroquinolone, can be effective against certain strains of MRSA, although **resistance rates have increased**, limiting its broad utility. - Its mechanism involves inhibiting **bacterial DNA gyrase** and topoisomerase IV, essential enzymes for DNA replication. *Vancomycin* - **Vancomycin** is a cornerstone in the treatment of **severe MRSA infections**, particularly in intravenous formulations for systemic infections. - It inhibits **bacterial cell wall synthesis** by binding to the D-Ala-D-Ala terminus of peptidoglycan precursors.
Question 5: Drug of choice in methicillin-resistant Staphylococcus aureus is:
- A. Vancomycin (Correct Answer)
- B. Ampicillin
- C. Ceftriaxone
- D. Ceftazidime
Explanation: ***Vancomycin*** - **Vancomycin** is a **glycopeptide antibiotic** that kills bacteria by inhibiting peptidoglycan synthesis in the bacterial cell wall. - It is considered the **drug of choice** for serious **methicillin-resistant Staphylococcus aureus (MRSA)** infections due to its consistent activity against these resistant strains. *Ampicillin* - **Ampicillin** is a **penicillin-class antibiotic** that inhibits bacterial cell wall synthesis. - It is **ineffective against MRSA** because MRSA strains produce an **altered penicillin-binding protein (PBP2a)** that has low affinity for beta-lactam antibiotics, rendering penicillin-class drugs ineffective. *Ceftriaxone* - **Ceftriaxone** is a **third-generation cephalosporin**, which also inhibits bacterial cell wall synthesis. - Like penicillins, cephalosporins are **generally ineffective against MRSA** due to the presence of altered penicillin-binding proteins (PBP2a) in MRSA. *Ceftazidime* - **Ceftazidime** is another **third-generation cephalosporin** primarily used for its activity against **Pseudomonas aeruginosa**. - It is **not effective against MRSA**, as MRSA expresses PBP2a, rendering it resistant to most beta-lactam antibiotics, including ceftazidime.
Question 6: The best way to prevent MRSA outbreak in indoor patients is:
- A. Practice proper meticulous hand washing (Correct Answer)
- B. Administer vancomycin to all patients
- C. Conduct fumigation of the ward
- D. Use sodium hypochlorite for floor disinfection
Explanation: ***Practice proper meticulous hand washing*** - **Hand hygiene** is the most effective measure to prevent the transmission of **healthcare-associated infections**, including Methicillin-resistant *Staphylococcus aureus* (**MRSA**). - Consistent **hand washing** by healthcare workers removes transient microorganisms and significantly reduces cross-contamination between patients. *Administer vancomycin to all patients* - Administering **vancomycin** to all patients would lead to widespread **antibiotic resistance**, making infections harder to treat. - This practice is inappropriate as **vancomycin** is a broad-spectrum antibiotic and should be reserved for confirmed infections or specific prophylaxis. *Conduct fumigation of the ward* - **Fumigation** is generally not recommended for routine MRSA prevention as it has limited efficacy against bacterial spores and poses risks to staff and patients. - It is an extreme measure reserved for specific, highly contaminated environments and does not address the primary mode of MRSA transmission (person-to-person). *Use sodium hypochlorite for floor disinfection* - While **sodium hypochlorite** is an effective disinfectant, focusing solely on floor disinfection is insufficient for preventing **MRSA outbreaks**. - MRSA primarily spreads through direct contact via contaminated hands, rather than contact with contaminated floors.
Question 7: Which of the following statements regarding resistance of penicillin in Staphylococcus aureus is false?
- A. Methicillin resistance is due to alterations in penicillin-binding proteins (PBPs).
- B. Penicillinase production is mediated by plasmids.
- C. Hospital strains predominantly produce a unique type of penicillinase. (Correct Answer)
- D. Penicillinase production can be transmitted by transduction.
Explanation: ***Hospital strains predominantly produce a unique type of penicillinase*** - This statement is **false** because hospital strains do not produce a truly "unique type" of **penicillinase** compared to community strains. - **Penicillinase (beta-lactamase)** is a common resistance mechanism found across various *S. aureus* strains, not exclusive to hospital environments. *Methicillin resistance is due to alterations in penicillin-binding proteins (PBPs)* - This statement is **true** as **MRSA** resistance involves the **mecA gene** encoding **PBP2a**. - **PBP2a** has low affinity for **beta-lactam antibiotics**, allowing cell wall synthesis despite antibiotic presence. *Penicillinase production is mediated by plasmids* - This statement is **true** because **penicillinase genes** are typically located on **plasmids**. - **Plasmids** facilitate horizontal transfer of resistance genes between bacterial populations. *Penicillinase production can be transmitted by transduction* - This statement is **true** as **transduction** via **bacteriophages** can transfer resistance genes. - **Plasmid-borne penicillinase genes** can spread through this horizontal gene transfer mechanism.
Question 8: Staphylococcus Oxacillin resistance is best detected by?
- A. Cefoxitin disc diffusion (Correct Answer)
- B. Oxacillin disc diffusion
- C. Oxacillin agar
- D. Cefoxitin MIC
Explanation: ***Cefoxitin disc diffusion*** - **Cefoxitin** is a **surrogate marker** for oxacillin resistance due to its superior induction of the **mobilized penicillin-binding protein PBP2a**, which mediates methicillin resistance. - The **disc diffusion method** is a cost-effective and widely used technique to determine antibiotic susceptibility by measuring the **zone of inhibition** around an antibiotic-impregnated disc. *Oxacillin disc diffusion* - While oxacillin is the drug in question, **oxacillin disc diffusion** can sometimes miss heterogeneous resistance, leading to **false susceptibility** results. - The detection of resistance with oxacillin can be technically challenging due to the **slow and heterogeneous expression** of resistance genes. *Oxacillin agar* - **Oxacillin agar screening** is a method for detecting methicillin resistance, but **cefoxitin agar** has been shown to be more sensitive and specific for detecting **MRSA**. - This method requires a specific agar medium and incubation conditions, which might be less convenient than disc diffusion for routine testing. *Cefotixin MIC* - **Cefoxitin Minimum Inhibitory Concentration (MIC)** determination is a quantitative method to determine the lowest concentration of an antibiotic that inhibits bacterial growth. - While accurate, **MIC testing** is typically more labor-intensive and costly than disc diffusion, making it less practical for initial resistance screening in many clinical laboratories.
Question 9: The most common mechanism of resistance to drugs in Staphylococcus is:
- A. Conjugation
- B. Plasmids (Correct Answer)
- C. Transduction
- D. Episomes
Explanation: ***Plasmids*** - **Plasmids** are the most common mechanism for antibiotic resistance in *Staphylococcus* species. - These are extrachromosomal, self-replicating circular DNA molecules that carry resistance genes including **β-lactamase** (penicillin resistance), **mecA gene** (methicillin resistance), and genes for resistance to aminoglycosides, macrolides, and other antibiotics. - Plasmids can be easily transferred between staphylococcal strains through various mechanisms (transduction, transformation, or conjugation), making them the primary vehicle for dissemination of antibiotic resistance. - **Clinical relevance:** MRSA (Methicillin-Resistant *Staphylococcus aureus*) resistance is often plasmid-mediated. *Transduction* - Transduction is a mechanism where bacteriophages transfer bacterial DNA including resistance genes from one bacterium to another. - While important in *Staphylococcus* for transfer of certain virulence factors and some resistance genes, it is a **mechanism of transfer** rather than the primary vehicle carrying resistance. - Less common than plasmid-mediated resistance overall. *Conjugation* - Conjugation involves direct cell-to-cell contact through a pilus for genetic material transfer. - This mechanism is **rare in Gram-positive cocci** like *Staphylococcus* and is more commonly seen in Gram-negative bacteria. - Not a significant mechanism for resistance transfer in staphylococci. *Episomes* - Episomes are genetic elements that can exist as plasmids or integrate into the chromosome. - While they can carry resistance genes, this term is too specific and less commonly used in the context of staphylococcal resistance compared to the broader term "plasmids."
Question 10: 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.
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