Trimethoprim Resistance

Susceptibility Testing

When possible, antibacterial substances, such as trimethoprim, 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).

Table 6.Minimum inhibitory concentrations (MIC) that would classify the pathogenic bacterial strain as susceptible to trimethoprim, intermediate, or resistant to trimethoprim (FDA, 2016).

Resistance Mechanism(s)

Trimethoprim 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. These mechanisms include (Jia et al., 2017):
* Antibiotic efflux
* Modification of Target

Antibiotic Efflux

Trimethoprim penetrates into the cytoplasm of bacterial cells where it then binds to DHFR. However, some resistant bacteria quickly extrude the drug which prevents it from exerting an inhibitory effect on folate synthesis. As a result, the bacteria become less susceptible to the antibacterial agent. Some examples of resistance to trimethoprim involving bacterial efflux pumps are described in Table 7.

Table 7. Examples of efflux pumps that confer resistance to trimethoprim (Jia et al., 2017).

Learn more about antibiotic efflux pumps.

Modification of Target

One of the most common mechanisms of Trimethoprim resistance is through resistant DHFR enzymes. These enzymes may appear due to mutations in the bacterial chromosomal gene or by acquisition of plasmids for resistance dfr genes through horizontal transfer.

Mutations of chromosomal DHFR genes can lead to trimethoprim resistance. Several resistant bacteria strains have been discovered with mutations close to the trimethoprim binding pocket. By changing the chemical properties of this region, the drug loses its binding affinity to it. One mutation in S. aureus (F98Y) causes low-level trimethoprim resistance by inducing a conformational change in the trimethoprim binding pocket which prevents trimethoprim binding (Vickers et al., 2009).

Resistance to trimethoprim is often acquired through horizontal acquisition (plasmid-mediated or conjugation) of dfr genes (i.e. antibiotic target replacement). These genes encode resistant DHFRs whose amino acid sequences are distinct from chromosomal DHFRs (Bergmann et al., 2014). For instance, in S. aureus, one plasmid-encoded form of DHFR shares only 80% sequence homology with the chromosomal DHFR (Heaslet et al., 2009). There are many types of horizontally transmissible dfr genes in gram-positive bacteria that each code for a different DHFR such as dfrA, dfrK, and dfrG. In 2010, 30 different dfr genes had been described (Bergmann et al., 2014).

Here we compare the structures of Klebsiella pneumoniae DfrA1 (a resistant DHFR) and Escherichia coli Dfr, each bound to the antibiotic trimethoprim (Krucinska et al., 2022). The antibiotic adopts a different conformation in these structures. Note that the substitutions Asp27 (in DfrA1) for Glu28 (in Dfr) and Gln29 (in DfrA1) for Leu28 (in Dfr) result in weaker interactions of the antibiotic and incomplete inhibition of the DfrA1 enzyme. This in turn leads to resistance to the antibiotic (Figure 6).

Learn more about the modification of antibiotic targets.

Figure 6: Comparison of structures of DfrA1 and Dfr bound to trimethoprim. a. Klebsiella pneumoniae DfrA1, PDB ID 7myl ; b. Escherichia coli Dfr, PDB ID 7nae. The antibiotic is colored magenta in both the panels.

Back to the article on trimethoprim.

References

Bergmann, R., van der Linden, M., Chhatwal, G., Nitsche-Schmitz, D. (2014). Factors That Cause Trimethoprim Resistance in Streptococcus pyogenes. Antimicrobial Agents And Chemotherapy, 58(4), 2281-2288. https://doi.org/10.1128/aac.02282-13

Heaslet, H., Harris, M., Fahnoe, K., Sarver, R., Putz, H., Chang, J. et al. (2009). Structural comparison of chromosomal and exogenous dihydrofolate reductase from Staphylococcus aureus in complex with the potent inhibitor trimethoprim. Proteins: Structure, Function, And Bioinformatics, 76(3), 706-717. https://doi.org/10.1002/prot.22383

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., 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

Krucinska, J., Lombardo, M. N., Erlandsen, H., Estrada, A., Si, D., Viswanathan, K., Wright, D. L. (2022) Structure-guided functional studies of plasmid-encoded dihydrofolate reductases reveal a common mechanism of trimethoprim resistance in Gram-negative pathogens. Commun Biol. 5, 459. https://doi.org/10.1038/s42003-022-03384-y

Vickers, A., Potter, N., Fishwick, C., Chopra, I., O'Neill, A. (2009). Analysis of mutational resistance to trimethoprim in Staphylococcus aureus by genetic and structural modelling techniques. Journal Of Antimicrobial Chemotherapy, 63(6), 1112-1117. https://doi.org/10.1093/jac/dkp090