Sulfamethoxazole Resistance

Susceptibility Testing

When possible, antibacterial substances, such as sulfamethoxazole, 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 (FDA 2013, Table 6). Trimethoprim and sulfamethoxazole is a commonly used synergistic antimicrobial combination. The MIC values for this combination are reported here.

Table 6. Minimum inhibitory concentrations (MIC) that would classify the pathogenic bacterial strain as susceptible, intermediate, or resistant to Trimethoprim Sulfamethoxazole (FDA, 2013). These values may not be the latest approved by the US FDA.

Resistance Mechanism(s)

Sulfamethoxazole resistance occurs when the antibiotic is not able to treat the infections it is intended to because the bacterial strains causing these infections have developed mechanisms to prevent the drug from functioning. The main mechanisms of resistance (Jia et al., 2017) include:
* Antibiotic target mutations
* Antibiotic target alteration
* Host-dependent nutrient acquisition

Antibiotic Target Mutations

DHPS mutations can reduce the binding affinity of sulfamethoxazole and cause resistance (Levy et al., 2008). It has been observed that common sites of resistance are located beyond the pABA binding site. Residues such as Phe33 and Pro69 are positioned near the heterocyclic ring of sulfamethoxazole (Figure 6), which has no counterpart in pABA. Mutations at Phe33 and Pro69 can therefore reduce drug binding affinity without significantly altering pABA affinity (Yun et al., 2012). Without sulfamethoxazole bound to DHPS, folic acid biosynthesis is unaffected.

Figure 6. Structure of the Yersinia pestis Dihydropteroate Synthase in complex with sulfonamide (PDB ID 3tzf, Yun et al., 2012). Note that the numbering used in the figure corresponds to that of Bacillus anthracis.

Antibiotic Target Replacement

In sulfa-insensitive bacteria, a series of plasmid-borne sul genes code for modified DHPS enzymes that can select against the antibiotic sulfamethoxazole and bind the natural substrate (Venkatesan et al., 2023). These genes are likely to have evolved from folP (the DHPS gene), in environmental bacteria. The overall shapes of these enzymes (e.g., Sul1, Sul2, Sul3) are similar (with rmsd values of ~2Å over ~230 amino acid residues), but the amino acid sequence identity between these proteins is only ~30% (see Figure 7). Due to specific differences in these proteins, Sul enzymes are able to discriminate against sulfamethoxazole (the competitive inhibitor of the enzyme) and bind to pABA (the substrate) rendering the bacteria resistant to the antibiotic.

Figure 7. Pairwise structure comparison of the structures of DHPS (Yun et al., 2012) with Sul enzymes (Venkatesan et al., 2023). a. DHPS enzyme, PDB ID 3tyz; b. Sul1 protein, PDB ID 7s2i; c. Sul2 protein, PDB ID 7s2k; d. Sul3 protein, PDB ID 7s2m.

Learn more about other examples of antibiotic target alterations.

Host-dependent nutrient acquisition

An energy coupled factor (ECF) transporter component gene (thfT) was identified in Group A Streptococcus that helps bacteria acquire extracellular reduced folate compounds. Access to folate in this manner allows bacteria to survive without any dependence on folate biosynthesis (Rodrigo et al., 2023). Although ThfT facilitates eukaryote-like folate uptake and confers high levels of resistance to sulfamethoxazole, it remains undetectable when these bacteria are grown in the absence of reduced folates.

Learn more about Folate biosynthesis.

Learn more about other global cell adaptations leading to antimicrobial resistance.

Back to the article on sulfamethoxazole.

References

FDA (2013), https://www.accessdata.fda.gov/drugsatfda_docs/label/2013/017377s068s073lbl.pdf

Jia, B., Raphenya, A. R., Alcock, B., Waglechner, N., Guo, P., Tsang, K. K., Lago, B. A., Dave, B. M., Pereira, S., Sharma, A. N., Doshi, S., Courtot, M., Lo, R., Williams, L. E., Frye, J. G., Elsayegh, T., Sardar, D. Westman, E. L., Pawlowski, A. C., Johnson, T. A., Brinkman, F. S., Wright, G. D., and McArthur, A. G. (2017) CARD 2017: expansion and model-centric curation of the Comprehensive Antibiotic Resistance Database. Nucleic Acids Research 45, D566-573. https://doi.org/10.1093/nar/gkw1004

Levy, C., Minnis, D., Derrick, J. P. (2008) Dihydropteroate synthase from Streptococcus pneumoniae: structure, ligand recognition and mechanism of sulfonamide resistance. Biochemical Journal, 412, 379-388. doi: 10.1042/BJ20071598 https://hal.science/hal-00478924v1

Rodrigo, M. K. D., Saiganesh, A., Hayes, A. J., Wilson, A. M., Anstey, J., Pickering, J. L., Iwasaki, J., Hillas, J., Winslow, S., Woodman, T., Nitschke, P., Lacey, J. A., Breese, K. J., van der Linden, M. P. G., Giffard, P. M., Tong, S. Y. C., Gray, N., Stubbs, K. A., Carapetis, J. R., Bowen, A. C., Davies, M. R., Barnett, T. C. (2022) Host-dependent resistance of Group A Streptococcus to sulfamethoxazole mediated by a horizontally-acquired reduced folate transporter. Nat Commun. 13,6557. https://doi.org/10.1038/s41467-022-34243-3

Venkatesan, M., Fruci, M., Verellen, L. A., Skarina, T., Mesa, N., Flick, R., Pham, C., Mahadevan, R., Stogios, P. J., Savchenko, A. (2023) Molecular mechanism of plasmid-borne resistance to sulfonamide antibiotics. Nat Commun. 14, 4031. https://doi.org/10.1038/s41467-023-39778-7

Yun, M. K., Wu, Y., Li, Z., Zhao, Y., Waddell, M. B., Ferreira, A. M., Lee, R. E., Bashford, D., White, S. W. (2012) Catalysis and sulfa drug resistance in dihydropteroate synthase. Science, 335, 1110-4. https://doi.org/10.1126/science.1214641