Detection of Antimicrobial Resistance — MCQs
All are true about ESBL except -
The disc diffusion method is also known as?
A diabetic patient developed cellulitis due to S. aureus, which was found to be methicillin resistant on the antibiotic sensitivity testing. All of the following antibiotics will be appropriate except ?
What is the mechanism of resistance in MRSA?
All of the following are true about methicillin resistance in MRSA, except:
Which of the following is NOT a criterion for defining extensively drug-resistant tuberculosis (XDR-TB)?
Burkholderia cepacia is intrinsically resistant to:
Extended-spectrum beta-lactamases (ESBLs) are characterized by activity against all except :
Which antimicrobial resistance mechanism is most commonly associated with extended-spectrum cephalosporin resistance in Neisseria gonorrhoeae?
Organism showing marked resistance to multidrug therapy -
Detection of Antimicrobial Resistance Indian Medical PG Practice Questions and MCQs
Question 1: All are true about ESBL except -
- A. Resistant to carbapenems (Correct Answer)
- B. Classification is based on 3rd generation cephalosporin sensitivity
- C. Cephalosporin sensitivity testing is required to confirm ESBL
- D. Ambler classification is based on molecular structure
Explanation: ***Resistant to carbapenems*** - **ESBL (Extended-Spectrum Beta-Lactamase)**-producing bacteria are typically **susceptible to carbapenems**. Carbapenems are a primary treatment option for serious ESBL infections. - Resistance to carbapenems suggests the presence of other resistance mechanisms, such as **carbapenemases**, not ESBLs. *Classification is based on 3rd generation cephalosporin sensitivity* - ESBLs are specifically defined by their ability to hydrolyze and confer resistance to **extended-spectrum cephalosporins** (e.g., ceftriaxone, ceftazidime) and aztreonam. - This characteristic resistance to third-generation cephalosporins is key to their definition and clinical identification. *Cephalosporin sensitivity testing is required to confirm ESBL* - **Phenotypic confirmatory tests** for ESBLs involve demonstrating increased resistance to an extended-spectrum cephalosporin alone compared to the same cephalosporin combined with a **beta-lactamase inhibitor** like clavulanic acid. - This testing is crucial for accurate detection and guiding appropriate antibiotic therapy. *Ambler classification is based on molecular structure* - The **Ambler classification system** categorizes beta-lactamases into classes A, B, C, and D based on their **amino acid sequence homology** and their active site mechanisms. - This classification helps in understanding the biochemical properties and substrate profiles of different beta-lactamases, including ESBLs.
Question 2: The disc diffusion method is also known as?
- A. Kirby-Bauer (Correct Answer)
- B. VDRL
- C. Dark field microscopy
- D. None of the options
Explanation: ***Kirby-Bauer*** - The **Kirby-Bauer disc diffusion method** is a widely used and standardized technique for determining the **antimicrobial susceptibility** of bacteria. - This method involves placing antibiotic-impregnated discs on an agar plate inoculated with bacteria, and the resulting **zones of inhibition** are measured. *VDRL* - **VDRL (Venereal Disease Research Laboratory)** is a non-treponemal serologic test used for screening and diagnosing **syphilis**, detecting antibodies to cardiolipin. - It is a **flocculation test** that detects reagin antibodies and is not related to antimicrobial susceptibility testing. *Dark field microscopy* - **Dark field microscopy** is a type of light microscopy that illuminates the specimen from the sides, making it appear bright against a dark background. - It is primarily used for visualizing **unstained, live microorganisms**, especially spirochetes like *Treponema pallidum*, and does not involve disc diffusion. *None of the options* - This option is incorrect because the disc diffusion method has a widely recognized alternative name, **Kirby-Bauer**.
Question 3: A diabetic patient developed cellulitis due to S. aureus, which was found to be methicillin resistant on the antibiotic sensitivity testing. All of the following antibiotics will be appropriate except ?
- A. Vancomycin
- B. Teicoplanin
- C. Linezolid
- D. Imipenem (Correct Answer)
Explanation: ***Imipenem*** - **Imipenem** is a carbapenem antibiotic that is effective against many Gram-positive and Gram-negative bacteria, but it is **not active against MRSA (methicillin-resistant *Staphylococcus aureus*)**. - MRSA strains are resistant to all beta-lactam antibiotics, including penicillins, cephalosporins, and carbapenems like imipenem, due to the presence of the **mecA gene** which encodes for an altered penicillin-binding protein (PBP2a). *Vancomycin* - **Vancomycin** is a glycopeptide antibiotic that is a primary choice for treating **MRSA infections**, including cellulitis. - It inhibits cell wall synthesis by binding to the D-Ala-D-Ala precursor, preventing cross-linking, and is specifically active against **Gram-positive bacteria**. *Teicoplanin* - **Teicoplanin** is another glycopeptide antibiotic, similar to vancomycin, and is also considered a suitable agent for treating **MRSA infections**. - It works by inhibiting bacterial cell wall synthesis and has a **longer half-life** than vancomycin, allowing for less frequent dosing. *Linezolid* - **Linezolid** is an oxazolidinone antibiotic known for its activity against **Gram-positive bacteria**, including **MRSA** and vancomycin-resistant enterococci (VRE). - It inhibits protein synthesis by binding to the 50S ribosomal subunit, preventing the formation of the initiation complex.
Question 4: 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 5: All of the following are true about methicillin resistance in MRSA, except:
- A. Resistance is produced as a result of altered PBPs
- B. Resistance may be missed at incubation temperature of 37°C during susceptibility testing
- C. Resistance is primarily mediated/transmitted by plasmids (Correct Answer)
- D. Resistance is associated with increased minimum inhibitory concentrations (MICs) for beta-lactam antibiotics
Explanation: ***Resistance is primarily mediated/transmitted by plasmids*** - Methicillin resistance in MRSA is primarily mediated by the acquisition of the **mecA gene**, which encodes for an altered **penicillin-binding protein (PBP2a)**. - The mecA gene is located on a **staphylococcal chromosomal cassette mec (SCCmec)**, a mobile genetic element integrated into the bacterial chromosome, and **not transmitted via plasmids**. - This is the **false statement** and hence the correct answer to this "except" question. *Resistance is produced as a result of altered PBPs* - This statement is **true** as MRSA acquires the **mecA gene**, which encodes for an altered penicillin-binding protein, **PBP2a**. - **PBP2a** has a low affinity for beta-lactam antibiotics, allowing the bacterium to synthesize its cell wall even in the presence of these drugs. *Resistance may be missed at incubation temperature of 37°C during susceptibility testing* - This statement is **true**; **MRSA expression** can be heterogeneous and temperature-dependent. - Optimal detection of methicillin resistance often requires incubation at **lower temperatures (e.g., 30-35°C)** and/or the addition of salt (2-4% NaCl), as 37°C can sometimes mask the heterogeneous expression of resistance. *Resistance is associated with increased minimum inhibitory concentrations (MICs) for beta-lactam antibiotics* - This statement is **true**; the presence of **PBP2a** results in reduced binding of beta-lactam antibiotics to their target. - This leads to **increased MICs** for methicillin and other beta-lactam antibiotics, defining the resistance phenotype.
Question 6: Which of the following is NOT a criterion for defining extensively drug-resistant tuberculosis (XDR-TB)?
- A. Isoniazid + Rifampicin + Fluoroquinolone
- B. Isoniazid + Rifampicin + Ethambutol + Fluoroquinolone
- C. Fluoroquinolone (Correct Answer)
- D. Isoniazid + Rifampicin + Kanamycin
Explanation: ***Fluoroquinolone*** - Resistance to **fluoroquinolone alone** is NOT a criterion for XDR-TB because XDR-TB requires a **baseline of MDR-TB** (resistance to both rifampicin and isoniazid) plus additional resistances. - XDR-TB definition (WHO 2021): **MDR-TB** + resistance to **any fluoroquinolone** + resistance to **at least one Group A drug** (bedaquiline or linezolid). - Fluoroquinolone resistance in isolation does not meet any of these combined criteria. *Isoniazid + Rifampicin + Fluoroquinolone* - This represents **MDR-TB** (rifampicin + isoniazid resistance) plus **fluoroquinolone resistance**. - This is a partial criterion approaching XDR-TB but still requires additional resistance to at least one Group A drug (bedaquiline or linezolid) for complete XDR-TB classification. - However, this combination includes the essential MDR-TB base and fluoroquinolone component. *Isoniazid + Rifampicin + Ethambutol + Fluoroquinolone* - This includes **MDR-TB** (rifampicin + isoniazid), **fluoroquinolone resistance**, and ethambutol (first-line drug). - While ethambutol resistance alone doesn't define XDR-TB, this combination includes the critical MDR-TB and fluoroquinolone components required for XDR-TB classification. - Similar to above, would need Group A drug resistance for complete XDR-TB. *Isoniazid + Rifampicin + Kanamycin* - This represents **MDR-TB** plus resistance to **kanamycin** (a second-line injectable). - Under previous WHO definitions (pre-2021), injectable resistance was part of XDR-TB criteria. - This combination includes the MDR-TB base essential for any XDR-TB classification, though it lacks fluoroquinolone resistance.
Question 7: Burkholderia cepacia is intrinsically resistant to:
- A. Ciprofloxacin
- B. Ceftriaxone
- C. Polymyxin B (Correct Answer)
- D. Ampicillin
Explanation: ***Polymyxin B*** - *Burkholderia cepacia* demonstrates **classic intrinsic resistance** to **polymyxins (Polymyxin B and Colistin/Polymyxin E)** - **Mechanism**: Modifications in the **lipopolysaccharide (LPS) structure** of the outer membrane, particularly alterations in lipid A, reduce polymyxin binding - This resistance is **chromosomally encoded** and present in all strains, making polymyxins completely ineffective against *B. cepacia* infections - This is the **most characteristic example** of intrinsic resistance in this organism *Ciprofloxacin* - *B. cepacia* is **NOT intrinsically resistant** to ciprofloxacin - Ciprofloxacin may retain activity and is sometimes used in susceptible strains - Resistance can develop through **acquired mechanisms** (efflux pumps, DNA gyrase mutations), but this is not inherent to all strains - Susceptibility testing is essential before use *Ceftriaxone* - *B. cepacia* shows resistance to third-generation cephalosporins like ceftriaxone through **multiple resistance mechanisms** including chromosomal AmpC beta-lactamases and efflux pumps - While this resistance is widespread, it's **less characteristic** as a defining feature compared to polymyxin resistance - The beta-lactam resistance pattern in *B. cepacia* is complex and variable *Ampicillin* - *B. cepacia* is resistant to aminopenicillins like ampicillin through **chromosomal beta-lactamase production** and poor outer membrane permeability - While present in most strains, this resistance mechanism is **common to many Gram-negative bacteria** and not a distinguishing feature - Ampicillin is not considered for *B. cepacia* treatment
Question 8: Extended-spectrum beta-lactamases (ESBLs) are characterized by activity against all except :
- A. Carbapenems (Correct Answer)
- B. Oxyimino-cephalosporins
- C. Penicillins
- D. Cephalosporins
Explanation: ***Carbapenems*** - **Extended-spectrum beta-lactamases (ESBLs)** typically do not hydrolyze **carbapenems**, making these antibiotics generally effective against most ESBL-producing bacteria. - The retention of activity against carbapenems is a key distinction between ESBLs and other beta-lactamases like **carbapenemases**. *Oxyimino-cephalosporins* - ESBLs are specifically named for their ability to hydrolyze and inactivate **oxyimino-cephalosporins**, such as **cefotaxime**, **ceftriaxone**, and **ceftazidime**. - This hydrolysis makes these vital third-generation cephalosporins ineffective for treating infections caused by ESBL-producing organisms. *Penicillins* - ESBLs can effectively hydrolyze and render many **penicillins** inactive, especially those lacking beta-lactamase inhibitors. - This broadens the resistance spectrum beyond just cephalosporins to include common penicillins. *Cephalosporins* - ESBLs primarily confer resistance to a wide range of **cephalosporins**, particularly the **first-, second-, and third-generation agents**. - This resistance is a major clinical challenge, necessitating the use of alternative antibiotic classes.
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.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free