Moxifloxacin Resistance

Susceptibility Testing

When possible, antibacterial substances, such as moxifloxacin, 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 (Table 6). A lower MIC value indicates that a lower concentration of the antibiotic is needed to inhibit the growth of the bacterial pathogen, meaning that the microorganism is susceptible to the drug. Therefore, using antibiotics with lower MIC values would result in more effective treatment of an infection.

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

Pathogen MIC (µg/mL) for Susceptible (S) strains MIC (µg/mL) for Intermediate (I) strains MIC (µg/mL) Resistant (R) strains
Streptococcus pneumoniae ≤ 1 2 ≥ 4

Resistance Mechanism(s)

Moxifloxacin 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 (CARD, 2017):
* Antibiotic target alteration through mutations
* Antibiotic efflux

Antibiotic Target Alteration – Mutations

The most common mechanism of resistance is chromosomal mutations in the genes encoding GyrA/ParC and GyrB/ParE at conserved positions. These mutations have been seen in Escherichia coli, Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis, and many other bacteria.

Examples of such mutations have been reported in the E. coli DNA gyrase (Michalczyk et al., 2024), Acinetobacter baumannii Topoisomerase IV (Wohlkonig et al., 2010), and Streptococcus pneumoniae Topoisomerase IV (Laponogov et al., 2016). In all cases, the moxifloxacin molecule makes direct contact with a Mg2+ ion which is coordinated directly by a serine or via a water and a serine (Figure 5). In addition in most cases, the antibiotic also interacts with a nearby arginine. When these Ser and Arg residues are mutated, moxifloxacin binding is affected.

Figure 5. Moxifloxacin (MFX) bound to type II topoisomerase from different organisms show similar interactions. A. MFX bound to E.coli DNA gyrase, PDB ID 9ggq; B. MFX bound to Acinetobacter baumannii Topoisomerase IV, PDB ID 2xkk; C. MFX bound to Streptococcus pneumoniae Topoisomerase IV, PDB ID 4z3o.
Figure 5. Moxifloxacin (MFX) bound to type II topoisomerase from different organisms show similar interactions. A. MFX bound to E.coli DNA gyrase, PDB ID 9ggq; B. MFX bound to Acinetobacter baumannii Topoisomerase IV, PDB ID 2xkk; C. MFX bound to Streptococcus pneumoniae Topoisomerase IV, PDB ID 4z3o.

Antibiotic Efflux

Fluoroquinolones, including moxifloxacin, penetrate into the cytoplasm of bacteria. However, some resistant bacterial cells quickly remove the drug through efflux pumps. As a result, the bacteria become less susceptible to the antibacterial agent (Table 6).

Table 6. Examples of efflux pumps that confer resistance to moxifloxacin (CARD, ARO:0000074).

Cause of Resistance Description
NorB This major facilitator superfamily (MFS) multidrug efflux pump in Staphylococcus aureus confers resistance to fluoroquinolones and other structurally unrelated antibiotics such as tetracycline. It is a structural homolog of Blt in Bacillus subtilis. (ARO:3000421)
QepA2 This MFS multidrug efflux pump was found in an Escherichia coli isolate from France. It confers resistance to several classes of antibiotics including fluoroquinolones, tetracyclines, oxazolidinones, macrolides, and lincosamides. (ARO:3004103)

Learn more about efflux pumps.

Back to the article on moxifloxacin.

References

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

Laponogov, I., Veselkov, D. A., Pan, X. -S., Selvarajah, J., Crevel, I. M. -T., Fisher, L. M., Sanderson, M. R. (2016) https://doi.org/10.2210/pdb4Z3O/pdb

Michalczyk, E., Pakosz-Stępień, Z., Liston, J.D., Gittins, O., Pabis, M., Heddle, J.G., Ghilarov, D. (2024) Structural basis of chiral wrap and T-segment capture by Escherichia coli DNA gyrase. Proc Natl Acad Sci U S A. 121(49):e2407398121. https://doi.org/10.1073/pnas.2407398121

Wohlkonig, A., Chan, P. F., Fosberry, A. P., Homes, P., Huang, J., Kranz, M., Leydon, V. R., Miles, T. J., Pearson, N. D., Perera, R. L., Shillings, A. J., Gwynn, M. N., Bax, B. D. (2010) Structural basis of quinolone inhibition of type IIA topoisomerases and target-mediated resistance. Nat Struct Mol Biol. 17(9):1152-3. https://doi.org/10.1038/nsmb.1892