Azithromycin Resistance

Susceptibility Testing

When possible, antibacterial substances, such as azithromycin, 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 bacterial pathogens, 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 azithromycin, intermediate, or resistant to streptomycin (FDA 2011). These concentrations may not be the latest values approved by the US FDA.

Resistance Mechanism(s)

Azithromycin 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, ARO:3000158):
* Antibiotic target alteration through ribosomal methylation
* Antibiotic target alteration through mutations
* Antibiotic inactivation
* Antibiotic efflux

Antibiotic Target Alteration – Ribosomal Methylation

Ribosome modification via methylation was the first mechanism of macrolide resistance that was studied. A family of rRNA methyltransferases known as the Erm enzymes are produced by bacteria. There are two types of Erm methyltransferases, those with inducible expression and those that are constitutively expressed. Both of these enzymes types methylate (add a CH3 group) to A2058 in the 23S rRNA. The enzymes may even dimethylate this base leading to a high resistance phenotype (Svetlov et al., 2021). Since this base participates directly in the binding of azithromycin, the addition of a methyl group(s) significantly reduces the binding of the drug to its target site in the 50S subunit, making it resistant to azithromycin (Leclercq and Courvalin, 2002). In addition to macrolides, this methylation also confer resistance to lincosamides and streptogramin B (MLSB resistance).

Learn more about Erm methyltransferases.

Antibiotic Target Alteration – Mutations

Numerous 23S rRNA mutations are present in azithromycin-resistant bacteria. There is a wide range of rRNA modifications, but key ones include mutations of A2058, A2059, and U2611. These bases are crucial because azithromycin binds to them after entering the ribosome. If they are modified, however, azithromycin loses its binding affinity to the ribosome. As a result, protein synthesis is unaffected by the drug (Gomes et al., 2017). These mutations also confer resistance to lincosamides and streptogramin B (MLSB resistance).

Learn more about azithromycin binding to 23S rRNA.

Antibiotic Inactivation - Macrolide phosphotransferases

Many human pathogenic bacteria, Enterobacter, Klebsiella, Pseudomonas, and Serratia exhibit resistance due to the presence of enzymes that inactivate the antibiotic. For example, there are two classes of macrolide phosphotransferase (MPH) enzymes (Phuc Nguyen et al., 2009). Both classes phosphorylate the O2' in the amino sugar moiety at the C5 position of the lactone ring, inactivating the antibiotic. While the overall shapes of these enzymes are similar, there is only ~37% sequence identity between these two proteins. They both use guanosine triphosphate (GTP) as a source of the phosphate group. The structures of MPH(2′)- type I (PDB ID 5igi) and MPH(2′)- type II (PDB ID 5igv) (Figure 5, Fon et al., 2017) show that they are structurally related to the aminoglycoside phosphotransferases. A close-up of the antibiotic and cofactor (GTP) binding sites for MPH (2') - type II shows that the antibiotic binding pocket is lined on one side with aromatic residues (Tyr273, Tyr289, Phe280) and there are few specific gatekeeper residues (His205 and Asp200) positioned between the antibiotic (shown in magenta) and the GTP (shown in orange). A clear understanding of the structures and mechanism of action of the phosphotransferase enzymes can provide valuable insights for developing new macrolide antibiotics.

Figure 5. Structures of MPH(2′)- type I (PDB ID 5igi) and MPH(2′)- type II (PDB ID 5igv) in complex with Azithromycin and a cofactor analog (Fong et al., 2017). The antibiotic (azithromycin) is shown in magenta while the Guanosine/GDP is shown in orange. The target where the phosphate group is attached is marked with a yellow dot.

Learn more about other macrolide inactivation enzymes.

Antibiotic Efflux

Chromosomal efflux pumps move toxins from the cytoplasm into the extracellular environment. Since antibiotics are recognized as toxic, efflux pumps extrude these drugs from the cell. There are many chromosomal efflux pumps that reduce the intracellular concentration of azithromycin. Some examples of resistance to streptomycin involving bacterial efflux pumps are described below in Table 7.

Table 7. Examples of efflux pumps that confer resistance to azithromycin (CARD, ARO:3000158).

Learn more about efflux pumps.

Back to the article on azithromycin.

References

Fong, D. H., Burk, D. L., Blanchet, J., Yan, A. Y., Berghuis, A. M. (2017) Structural Basis for Kinase-Mediated Macrolide Antibiotic Resistance. Structure. 25(5):750-761.e5. https://doi.org/10.1016/j.str.2017.03.007

Gomes, C., Martínez-Puchol, S., Palma, N., Horna, G., Ruiz-Roldán, L., Pons, M. J., Ruiz, J. (2017) Macrolide resistance mechanisms in Enterobacteriaceae: Focus on azithromycin. Crit Rev Microbiol. 43(1):1-30. https://doi.org/10.3109/1040841x.2015.1136261

Leclercq, R., Courvalin, P. (2002) Resistance to macrolides and related antibiotics in Streptococcus pneumoniae. Antimicrob Agents Chemother. 46(9):2727-34. https://doi.org/10.1128/aac.46.9.2727-2734.2002

Phuc Nguyen, M. C., Woerther, P. L., Bouvet, M., Andremont, A., Leclercq, R., Canu, A. (2009) Escherichia coli as reservoir for macrolide resistance genes. Emerg Infect Dis. 15(10):1648-50. https://doi.org/10.3201/eid1510.090696

Svetlov, M. S., Syroegin, E. A., Aleksandrova, E. V., Atkinson, G. C., Gregory, S. T., Mankin, A. S., Polikanov, Y. S. (2021) Structure of Erm-modified 70S ribosome reveals the mechanism of macrolide resistance. Nat Chem Biol. 17(4):412-420. https://doi.org/10.1038/s41589-020-00715-0