Vancomycin Resistance
Susceptibility Testing
When possible, antibacterial substances, such as vancomycin, are tested for their effectiveness against various infectious pathogens. These test results allow clinicians to choose the antibiotic likely to result in the most effective treatment of a particular bacterial infection. For instance, one such susceptibility test provides minimum inhibitory concentration (MIC) values that can then be used to identify a pathogenic bacterial strain as susceptible, intermediate, or resistant to a certain antibiotic (see Table 6).
Table 6. Minimum inhibitory concentrations (MIC) for IV vancomycin that would classify the pathogenic bacterial strain as susceptible, intermediate, or resistant for different pathogens (FDA, 2017). These concentrations may not be the latest, approved by the US FDA.
| Pathogen | MIC (µg/mL) for Susceptible (S) strains | MIC (µg/mL) for Intermediate (I) strains | MIC (µg/mL) for Resistant (R) strains |
|---|---|---|---|
| Enterococci | ≤ 4 | 8 to 16 | ≥ 32 |
| Staphylococcus aureus | ≤ 2 | 4-8 | ≥ 16 |
Resistance mechanism(s)
Vancomycin resistance occurs when the antibiotic is not able to treat certain bacterial infections because the pathogens causing these infections have developed mechanisms to prevent the drug from functioning. The main mechanisms of bacterial resistance against the drug include:
* Antibiotic target alteration
* Antibiotic efflux
Antibiotic Target Alteration
Vancomycin works by inhibiting late-stage peptidoglycan synthesis in gram-positive bacteria. It binds to the D-alanyl-D-alanine C-terminus of the lipid II pentapeptide group with high affinity and prevents the formation of cross-links in the growing matrix of peptidoglycan outside of the cell (Courvalin, 2006). Learn more about cell wall biosynthesis.
Bacteria resistant to vancomycin use various approaches to modify the drug target and prevent its interactions with the drug in the bacteria. The van resistance gene clusters (van operons) have been shown to play a key role (ARO:3000234). These inducible gene clusters encode enzymes that remodel the bacterial cell wall, changing the D-Ala-D-Ala target to D-Ala-D-lactate or D-Ala-D-Ser. Their acquisition confers moderate- to high-level resistance to vancomycin and other glycopeptide antibiotics. Once acquired, these gene clusters may be found either on plasmids or on the chromosome. Learn more about the van operon and the protein products coded by them.
Learn about other examples of antibiotic resistance due to substitution of the original target.
Antibiotic Efflux
Some resistant bacteria can transport vancomycin out of the cell through efflux pumps. Examples of such efflux pumps are included in Table 7 (CARD, ARO:0000028).
Table 7: Examples of efflux pumps that render cells vancomycin resistant.
| Cause of Resistance | Description |
|---|---|
| VRA-F | This is an ATP-binding cassette (ABC) antibiotic efflux pump, when mutated (e.g., K84E) can lead to vancomycin resistance (Cao et al., 2024) |
| mlaFEDB | This is an ABC antibiotic efflux pump involved in a phospholipid transport pathway that maintains lipid asymmetry in the outer membrane. (ARO:3005058) |
| YajC | This resistance-nodulation-cell division (RND) antibiotic efflux pump interacts with the AcrAB-TolC efflux pump to ensure bacterial cell survival even in the presence of linezolid, rifampicin, and vancomycin. (ARO:3005040) |
Learn more about efflux pumps.
Back to the article on vancomycin.
References
Cao, R., Su, H., Wei, Z., He, Z., Pan, T., Li, Y., Sun, B. (2024) An induced mutation of ABC-transporter component VraF(K84E) contributes to vancomycin resistance and virulence in Staphylococcus aureus strain MW2. Int J Med Microbiol. 315, 151624. https://doi.org/10.1016/j.ijmm.2024.151624
Courvalin, P. (2006) Vancomycin Resistance in Gram Positive Cocci. Clinical Infectious Diseases 42, S25-S34. https://doi.org/10.1086/491711
FDA - Vancocin (2017), https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/060180s047lbl.pdf
