Methicillin resistance in Staphylococcus species is most commonly due to: A. MecA gene B. MecB gene C. MecC gene D. MecD gene
Reading File
Finding Sources
The answer is confirmed across multiple authoritative textbooks. Here is the full breakdown:
Answer: A. MecA gene
Mechanism
Methicillin (and oxacillin) resistance in Staphylococcus species is most commonly caused by the mecA gene, which encodes PBP2a (Penicillin-Binding Protein 2a). PBP2a has extremely low affinity for all beta-lactam antibiotics, allowing cell wall synthesis to continue even when normal PBPs are inhibited. This is the basis of MRSA (Methicillin-Resistant S. aureus).
Key points:
- The mecA gene is carried on a mobile genetic element called SCCmec (Staphylococcal Cassette Chromosome mec)
- It confers resistance not just to methicillin, but to all beta-lactams including penicillins, cephalosporins, and carbapenems
- Cefoxitin disk diffusion is used as a surrogate marker to screen for mecA-mediated resistance in the lab
Why the other options are wrong
| Option | Gene | Notes |
|---|---|---|
| A. mecA | Encodes PBP2a | Most common cause - the correct answer |
| B. mecB | Found in Macrococcus caseolyticus | Rare, unrelated to classic MRSA |
| C. mecC | A rare variant found in S. aureus in livestock/wildlife | Causes atypical MRSA, not detected by standard mecA PCR |
| D. mecD | Extremely rare variant | Not clinically significant |
Sources
- Henry's Clinical Diagnosis and Management by Laboratory Methods: "Most commonly, oxacillin resistance in Staphylococcus spp. is due to production of an altered PBP protein PBP2a, a product of the mecA gene"
- Miller's Review of Orthopaedics 9th Ed.: "Penicillin-binding protein 2a (mecA gene) makes S. aureus methicillin-resistant (MRSA)"
- Harrison's Principles of Internal Medicine 22E (2025): lists mecA among the key antibiotic resistance genes
Vancomycin resistance in Enterococcus species is mediated by the: A. Accumulation of random point mutations B. Acquisition of the Van operon C. Acquisition of the MecA gene D. All of the above.
Finding Sources
Finding Sources
Confirmed across multiple major textbooks. Here is the complete explanation:
Answer: B. Acquisition of the Van operon
Mechanism of Resistance
Vancomycin works by binding to the D-Ala-D-Ala terminal dipeptide of peptidoglycan precursors (lipid II), blocking cell wall synthesis. VRE (Vancomycin-Resistant Enterococcus) escapes this by acquiring the van gene cluster (operon), which reprograms the terminal dipeptide:
| Genotype | Substitution | Effect |
|---|---|---|
| vanA | D-Ala-D-Ala → D-Ala-D-Lac | 1000x reduced vancomycin binding; resistance to both vancomycin AND teicoplanin |
| vanB | D-Ala-D-Ala → D-Ala-D-Lac | Resistance to vancomycin only; teicoplanin remains active |
| vanC | D-Ala-D-Ala → D-Ala-D-Ser | Intrinsic, low-level resistance (in E. gallinarum, E. casseliflavus) |
Key Facts
- The vanA and vanB genes are acquired via mobile genetic elements - plasmids and transposons (e.g., Tn1546) - making them transmissible between organisms
- vanA is the most common and clinically significant type, predominantly found in E. faecium and E. faecalis
- High-level resistance occurs via acquisition of mobile transposable elements carrying these van genes
- Molecular assays (PCR) targeting vanA and vanB are used for rapid VRE detection directly from rectal swabs and blood specimens
Why the other options are wrong
- A. Random point mutations - NOT the mechanism for vancomycin resistance in Enterococcus. Point mutations are relevant for other resistance types (e.g., rifampin resistance via rpoB mutations in M. tuberculosis)
- C. mecA gene - This is specific to methicillin resistance in Staphylococcus, not vancomycin resistance in Enterococcus
- D. All of the above - Incorrect, since A and C do not apply here
Sources
- Tietz Textbook of Laboratory Medicine, 7th Ed.: "Vancomycin resistance in the enterococci is due to modification of the vancomycin bacterial target. The machinery to do this is encoded by a van gene cluster"
- Henry's Clinical Diagnosis and Management by Laboratory Methods: "The vanA and vanB genes...are acquired, plasmid-borne genes that can create infection control problems"
- Murray & Nadel's Textbook of Respiratory Medicine: "vanA and vanB for vancomycin resistance in Enterococcus"
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