Tetracycline Resistance

Susceptibility Testing

When possible, antibacterial substances, such as tetracycline, 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 the antibiotic (See Table 6).

Table 6. Minimum inhibitory concentrations (MIC) that would classify the pathogenic bacterial strain as susceptible to, intermediate, or resistant to tetracycline (FDA, 2016). These values may not be the latest approved by the US FDA. These values may not be the latest approved by the US FDA.

Resistance Mechanism(s)

Tetracycline 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:
* Antibiotic efflux
* Antibiotic inactivation
* Antibiotic target alteration through mutations
* Antibiotic target protection

Antibiotic Efflux

Tetracycline penetrates the cytoplasm of bacterial cells where it binds to ribosomes. However, some resistant bacteria quickly extrude the drug which prevents it from exerting an inhibitory effect on protein synthesis. As a result, the bacteria become less susceptible to the antibacterial agent. This antibiotic efflux is perhaps the most common reason for tetracycline resistance.

Acquisition of mobile tetracycline resistance (tet) genes, introduces various mechanisms of resistance. The genes tet A-H, J-L, V, Y, Z, and more code for efflux pumps that expel tetracyclines from the cytoplasm, its site of action (Markley and Wencewicz, 2018). In addition, overproduction of the AcrAB TolC multidrug efflux pump also plays a key role in resistance (Peterson 2008). Some examples of efflux pumps that lead to tetracycline resistance are listed in Table 7.

Table 7. Examples of efflux pumps that confer resistance to tetracycline (CARD, ARO:0000051).

Learn more about efflux pumps.

Antibiotic Inactivation

A series of NADPH-dependent monooxygenases, the TetX series of enzymes, can modify tetracyclines by adding a hydroxyl group between the B and C rings of the antibiotic. Additional homologs Tet47-56 have also been found to carry out similar modifications of tetracycline. Learn about the modification of tetracycline, e.g., by TetX.

Antibiotic Target Alteration – Mutations

Mutations in the 16S rRNA can lead to tetracycline resistance. The first reported mutation was found to have a cytosine instead of a guanine at position 1058 (Chopra & Roberts, 2001). Some Helicobacter pylori strains have been found with mutations in bases 965-967. The locations of all these mutations are significant because they either directly interact with the drug molecule, or are located close to bases that do so. By changing the chemical properties of the tetracycline binding pocket, the drug loses its binding affinity for it (Nguyen et al., 2014).

Antibiotic Target Protection

There are 12 classes of ribosome protection proteins (RPPs) that confer tetracycline resistance. The Tet(M) and Tet(O) are the most extensively studied RPPs. After tetracycline binds to the A-site, the RPPs appear to release the drug from the ribosome. A comparison of the structures of ribosomes bound to tetracycline (PDB ID 4v9a, Jenner et al., 2013) and TetM (PDB ID 3j9y, Arenz et al., 2015) shows that they bind to the same location (Figure 6). The binding of TetM to the ribosome protects the antibiotic target site. Without a drug molecule bound, the A-site of mRNA is able to accommodate the tRNA and continue translation (Chopra & Roberts, 2001).

Figure 6. Comparing the structures of 16S rRNA bound to tetracycline or TetM. A. Structure of the 70S ribosome from Thermus thermophilus with bound tetracycline (PDB ID 4v9a, Jenner et al., 2013). B. Structure of tetracycline resistance protein TetM bound to a translating E. coli ribosome (PDB ID 3j9y, Arenz et al., 2015).

Back to the article on tetracycline.

References

Arenz, S., Nguyen, F., Beckmann, R., Wilson, D. N. (2015) Cryo-EM structure of the tetracycline resistance protein TetM in complex with a translating ribosome at 3.9-Å resolution. Proc Natl Acad Sci U S A. 112(17):5401-6. https://doi.org/10.1073/pnas.1501775112

Chopra, I., Roberts, M. (2001) Tetracycline antibiotics: mode of action, applications, molecular biology, and epidemiology of bacterial resistance. Microbiol Mol Biol Rev. 65(2):232-60 ; second page, table of contents. https://doi.org/10.1128/mmbr.65.2.232-260.2001

Markley, J. L., Wencewicz, T. A. (2018) Tetracycline-Inactivating Enzymes. Front Microbiol. 9:1058. https://doi.org/10.3389/fmicb.2018.01058

Nguyen, F., Starosta, A. L., Arenz, S., Sohmen, D., Dönhöfer, A., Wilson, D. N. (2014) Tetracycline antibiotics and resistance mechanisms. Biol Chem. 395(5):559-75. https://doi.org/10.1515/hsz-2013-0292

Peterson, L. R. (2008) A review of tigecycline--the first glycylcycline. Int J Antimicrob Agents. 32 Suppl 4:S215-22. https://doi.org/10.1016/s0924-8579(09)70005-6